Part III
Papers and activities of each experiment in 2000-2004

E246 J. Imazato
Search for T violation in the K+ → π0µ+ν decay
E470 J. Imazato
Measurement of direct photon emission in K+ → π+π0γ
Y. H. Shin et al.
[attached]A kinematically complete measurement of K+ → π+π0π0 decays
Eur. Phys. J. C12 (2000) 627.
● S. Shimizu et al.
[attached]
Test of exotic scalar and tensor couplings in K+ → π0e+γ decay
Phys. Lett.. B495 (2000) 33.
● K. Horie et al.
[attached]
Measurement of Γ(Kµ3)/Γ(Ke3) ratio using stopped positive kaonsPhys. Lett. B513 (2001) 311.
● M. A. Aliev et al.
[attached]
Measurement of direct photon emission in K+ → π+π0γ decay using stopped kaonsPhys. Lett. B554 (2003) 7.
● V.V. Anisimovsky et al.
[attached]First measurement of the T-violating muon polarization in the decay K+ → µ+νγ
Phys. Lett. B562 (2003) 166.
● M.Abe et al.
[attached]
A new limit of T-violating transverse muon polarization in the K+ → π0µ+ν decay
submitted to Physcal Review Letters.
● S. Shimizu et al.
[attached]
Measurement of K+ → π0π0 e+ν decay using stopped positive kaons
to be published in Phys. Rev. D.
○ D.V. Dementyev et al.
[attached]CsI(Tl) photon detector with PIN photodiode readout for a Kμ3 T-violation experiment
Nucl. Instr. Method A440 (2000) 151.
○ J.A. Macdonald et al.
[attached]
Apparatus for a search for T-violating muon polarization in stopped-kaon decays
Nucl. Instr. Method A506 (2003) 60.
★ O.V. Mineev
Development and design of analogue electronics and trigger system in the experiment to measure Tviolating muon polarization in positive kaon decays (in Russian);
Institute for Nuclear Research, Russian Academy of Sciences, 2000.
★ K. Horie
Measurement of Γ(Kµ3)/Γ(Ke3) ratio using stopped positive kaons
Osaka Univesity, 2002.
★ M. Khabibullin
Measurement of T-violating muon polarization in the decay of positive kaons and limits on theparameters of non-standard models of CP violation (in Russian)Institute for Nuclear Research, Russian Academy of Sciences, 2003.
◇ C. Rangacharyulu
Search for T-violation in stopped K+ decay
17th International Conference on Weak Interactions and Neutrinos (WIN99),
Capetaown, South Africa, January 24-30, 2000.
◇ P. Depommier
Search for non-standard time-reversal violation in K+µ3 decayInternational Conference on Quark Nuclear Physics, Adelaide, Australia, Feb.21-25, 2000. [to be
published in Nuclear Physics A].
◇ P. Depommier
Search for time-reversal violation in the K+ → π0µ+ν decay●________________________________________________________________________● Physics papers published in refereed journal.
○★
◇＊

Technical papers.
PhD theses.
Conference and Symposium.
Internal Report and others.

VIth International Conference on Few-Body Problems in Physics, Taipei, Taiwan, March 6-10,
2000. [to be published in Nuclear Physics A].
◇ M. Khabibullin et al.
New limit on T-violating muon polarization in the decay K+ → π0µ+ν
XXXVth Recontres de Moriond, Electroweak Interaction and Unified Theories, Les Arcs, France,
March 11-18, 2000.
◇ Y. Kuno
Test of Time Reversal Invariance in K+µ3
3rd International Symposium on Symmetries in Subatomic Physics, Adelaide, Ausrtalia,, March 1317, 2000.
◇ M. Hasinoff
A Test of Time Reversal Invariance in Stopped K+ Decay
7th Conference on Intersections Between Particle and Nuclear Physics, Quebec, Canada, May 22-28,
2000, AIP conference Proceedings, Vol. 549, 969 (2002).
◇ M. Aoki et al.
Search for T-violating Muon Polarization in K+ → π0µ+ν decay
XXXth International Conference on High Energy Physics, Osaka, Japan, July 27- Aug.2, 2000.
◇ J. Imazato
Branching ratio measurement of K+ → π0µ+ν direct emission
Kaon Decay Workshop for Young Physicists, Tsukuba, Japan, February 14-16, 2001.
◇ J. Imazato
Search for T Violation K+ → π0µ+ν in at KEK E246
International conference on CP Violation (KAON2001), Pisa, Italy, June 12-17, 2001. Frascati
Physics series (2001, to appear).
◇ M. Khabibullin et al.
New limit on T-violating Parameters in Kaon Decays
International Europhysics Conference on high Energy Physics, Budapest, Hungary, July 2001.
◇ J. Imazato
Kaon Physics at KEK
Fifth KEK topical conference -Frontier of Flavor Physics-, Tsukuba, Japan, November 19-22, 2001.
◇ Yu.Kudenko
Further search for T violation in the deacy K+ → π0µ+ν
16th International Conference on Particles and Nuclei, Osaka, Japan, September 30-October 4,
2002.
◇ K. Horie
Search for T-violating transverse muon polarization in K+→ π0µ+ν decay using stopped kaons
KEK phenomenology workshop, March 1-3, 2004, KEK.
◇ J. Imazato
Result of T-violating Muon Polarization Measurement in the K+→ π0µ+ν Decay
XXXIX Rencontres de Moriond – March 21-28, 2004. (Electroweak Interactions).
＊ T. Baker et al.
KEK-PS Proposal: Branching ratio of K+ → π+π0γ direct emission, June 2000.
＊ J. Imazato
Search for Time Reversal Invariance Violation in the K+→ π0µ+ν (in Japanese).
High Energy News Vol.22 Number 5 , April /May 2004________________________________________________________________________● Physics papers published in refereed journal.
○★
◇＊

Technical papers.
PhD theses.
Conference and Symposium.
Internal Report and others.

Eur. Phys. J. C 12, 627–631 (2000)
Digital Object Identifier (DOI) 10.1007/s100520000291

THE EUROPEAN
PHYSICAL JOURNAL C

A kinematically complete measurementof K + → π +π 0π 0 decays
The KEK-PS E246 Collaboration
Y.-H. Shin1 , M. Abe2 , M. Aoki3 , I. Arai4 , Y. Asano2 , T. Baker3,5 , M. Blecher6 , M.D. Chapman3 ,D. Dementyev7 , P. Depommier8 , M. Grigorjev7 , P. Gumplinger9 , M. Hasinoff10 , R. Henderson9 , K. Horie4,16 ,W.S. Hou11 , H.C. Huang11 , Y. Igarashi3,4 , T. Ikeda4,17 , A. Ivashkin7 , J. Imazato3 , J-M. Lee1 , K.S. Lee12 , G.Y. Lim3 ,J.H. Kang1 , W. Keil9 , M. Khabibullin7 , A. Khotjantsev7 , Y. Kudenko7 , Y. Kuno3 , J.A. Macdonald9 , C.R. Mindas13 ,O. Mineev7 , C. Rangacharyulu5 , S.K. Sahu11 , S. Sekikawa4 , H.M. Shimizu3,17 , S. Shimizu14,16 , K. Shibata4 ,Y.M. Shin5 , K.S. Sim12 , A. Suzuki4 , T. Tashiro4 , A. Watanabe4 , D. Wright9 , T. Yokoi15
1

Department of Physics, Yonsei University, Seoul 120-749, Korea
Institute of Applied Physics, University of Tsukuba, Ibaraki 305-0006, Japan
3
IPNS, High Energy Accelerator Research Organization (KEK), Ibaraki 305-0801, Japan
4
Institute of Physics, University of Tsukuba, Ibaraki 305-0006, Japan
5
Department of Physics, University of Saskatchewan, Saskatoon SK S7N 5E2, Canada
6
Department of Physics, Virginia Polytechnic Institute and State University, VA 24061-0435, USA
7
Institute for Nuclear Research, Russian Academy of Sciences, Moscow 117312, Russia
8
Laboratoire de Physique Nucleaire, University de Montreal, Montreal H3C 3J7, Canada
9
TRIUMF, Vancouver, British Columbia V6T 2A3, Canada
10
Department of Physics and Astronomy, University of British Columbia, Vancouver V6T 1Z1, Canada
11
Department of Physics, National Taiwan University, Taipei 106, Taiwan
12Department of Physics, Korea University, Seoul 136-701, Korea
13
Department of Physics, Princeton University, NJ 08544, USA
14
Department of Applied Physics, Tokyo Institute for Technology, Tokyo 152-0033, Japan
15
Department of Physics, University of Tokyo, Tokyo 113-0033, Japan
16
Department of Physics, Osaka university, Osaka 560-0043, Japan
17
RIKEN, 2-1, Hirosawa, Wako, Saitama, 351-0198, Japan
2

Received: 16 October 1999 / Published online: 27 January 2000 – c Springer-Verlag 2000
Abstract. In a kinematically complete setting, the K + → π + π 0 π 0 decays of stopped K + were measured. In
the frame work of the Weinberg polynomial expansion, the coefficients up to quadratic terms were deduced.
The results are in good agreement with previous measurements and consistent with CP conservation
expectations. The empirical ∆I = 1/2 rule is violated.

1 Introduction

X=

The non-leptonic decays of K-mesons have attracted considerable attention in the literature [1]. In particular, theK → 3π(Kπ3 ) decays provide information on various topics such as CP violation, ∆I = 1/2 rule, and chiral symmetry breaking, etc. There are six distinct modes of Kπ30decays: K ± → π ± π ± π ∓ , K ± → π ± π 0 π 0 and KL,S
→
(±)

(±)

(L,S)

π + π − π 0 , herein labeled as K±±∓ , K00± and K0+−0 , respectively.
Weinberg [2] proposed a polynomial expansion of the
decay matrix elements, written in the form
|M |2 ∝ 1 + gY + hY 2 + jX + kX 2
where the kinematic variables are defined as(1)s1 − s2m2π

and Y =

s3 − s0.
m2π(2)with si = (pK − pi )2 , i= 1, 2, 3 corresponding to the threepions with s3 referring to the odd pion. Also, s0 = (s1 +s2 + s3 )/3. A few physics principles become transparent
from this expansion.
(±)
For K00± decays, Bose symmetry requires that j=0,
since s1 and s2 refer to identical pions. While the CP
conservation requires that g++− = g−−+ and g00+ = g00− ,the empirical selection rule ∆I = 1/2 stipulates thatg±,±,∓ = 12 g00± .
In this paper, for the first time, the results of a kine+
matically complete measurement for the K00+
are presented. The aim was to determine the coefficients of thepolynomial expansion.

628

The KEK-PS E246 Collaboration: A kinematically complete measurement of K + → π + π 0 π 0 decays

2 Experimental

TOF2

Muon holes

The measurement was performed using the apparatus constructed to search for T-violation in K + → π 0 µ+ ν decay [3, 4] at the 12 GeV proton synchrotron facility of theHigh Energy Accelerator Research Organization (KEK),
Tsukuba, Japan. The experimental arrangement is shownin Fig. 1, and is described in detail elsewhere (see for example, [4,5]). Briefly, 660 MeV/c K + are slowed in a BeO
degrader and stopped in an active target of 256 scintillating fibers each of 5 × 5 mm2 cross section. Charged
particles are momentum analyzed in the 12 sector superconducting toroidal spectrometer of solid angle of ∼ 1 sr.
The energies and angles of the photons from π 0 decays
are measured by the CsI calorimeter, the details of which
are shown in the inset of Fig. 1. It is an assembly of 768
CsI(Tl) crystals, each about 13 radiation lengths in thickness [5]. It covers a solid angle of 3π steradians, with openings for the beam entry and exit and gaps for the charged
particles entry to the spectrometer.
The Kπ3 decay study required a trigger condition different from that of the main experiment. We employed as
trigger:
12
X(Fidi ⊗ TOF2i ) ⊗ nγ .
(3)
CK ⊗

2
4
5
6
7
8

4

3

2

1

3

C3 C4

Cryostat

4

1

5
6

7

π+

8
9
10

K

9

10

+

CsI crystals

C2

Cerenkov

Q5
K

CsI(Tl)Lead
Shield

γ

γ

+

γ

γ

Ring
TargetFiducial(TOF1)

B0
Degrader
Iron
Pole

0

0.5 m

i=1

The Cerenkov condition CK ensures that the beam particle is a K + meson, rather than a π + meson. The latter
outnumber the former by a factor of 7. The coincidence
between CK , delayed by 3 ns, and the fiducial counters(Fidi ) for the charged particles eliminates the triggers due
to decays in flight. The time of flight signal from the corresponding gap (TOF2i ) completes the charged particle
trigger condition. In addition, the trigger requires n ≥ 2
photons with nγ = 2 for Kπ2 measured for calibration
purposes and nγ > 2 enhances the Kπ3 events in the data
sample and suppresses triggers from other decays such as
Kµ3 .
The Kπ3 data were collected for three spectrometerfield settings B = 0.45, 0.65 and 0.9 Tesla. It was intended
to serve as a consistency check with regard to spectrometer
acceptance and the dependences on the beam stopping
position and energy loss in target, etc.

3 Data analysis
The Kπ2 (K + → π + π 0 ) channel, a two body final statewith mono-energetic decay products, was used for the calibration purposes and also to verify the reliability of theMonte Carlo program. For photon energy reconstructions,the crystals with energy deposits of more than 10 MeV
were included. The photon energy was determined by adding the energy deposits in a cluster of crystals (3 × 3
in size) from the center crystal. The Monte Carlo code
is a part of the elaborate simulation program developed
for the main T-violation experiment. It incorporated thegeometry of the experimental arrangement and the physical interactions in the beam, the target and the detector

Fig. 1. Cross sectional view of the E246 setup. The layout of
the CsI(Tl) assembly is shown in the inset

system. The measured invariant mass and energy distributions of the π 0 s are found to be in good agreement with
the Monte Carlo simulation results.
The Kπ3 data analysis proceeds in two stages. The
first stage is a selection of good events. For this purpose,
we require a π + meson in coincidence with four photons(from the decays of two π 0 s) with the followings conditions, which are specific to the decay at rest: a) the total
emitted energy is equal to the K mass, mK ,ETOT = Eπ+ +4XEγi = mK ,

(4)

i=1

and b) for the decay at rest the total momentum PTOT iszero, i.e.
4X+pγi = 0,(5)
PTOT = pπ +i=1which can equivalently be written as
P4∆P = |pπ+ | − | i=1 pγi | = 0
θπ+ ,4γ = 180◦ ,(6)where θπ+ ,4γ is the opening angle between pπ+ and theP4
net momentum vector of four photons, i.e. i=1 pγi .
After the π + mesons were selected by the particle identification scheme using particle tracking and time of flightCounts

CountsThe KEK-PS E246 Collaboration: A kinematically complete measurement of K + → π + π 0 π 0 decays
70

(a)

60
50

Table 1. The g and k parameters for different field settings

(b)

600

B (T)
0.9
0.65
0.45
weighted mean

500

π+

40

700

400

30

300

µ+

20

200

10

100

0

0

20000
2

2

0

40000

4

0

20000
2

m (MeV /c )

2

(a)

140

CountsCounts

160

100

100

80

80

40000

(b)

60

60
40

40

20

20
400

500

600

Counts

0
-200

-100

0

100

200

∆P (MeV/c)

ETOT (MeV)
180

(c)

160
140
120

tail. Our Monte Carlo routines include this leakage effectand simulations are in good agreement with the experimental data. Fig. 3c shows the opening angle distributionof the π + with the resultant of the four photon direction. A sharp peak at cos θπ+ ,4γ = −1 is observed as required for Kπ3 decays. The final event selection satisfied−50 < ∆P < 70(M eV /c) and cos θπ+ ,4γ < −0.8. These
cuts leave 143, 503 and 169 events for 0.45T, 0.6T and
0.9T, respecitvely.
If one is satisfied with the determination of the g andh parameters of the polynomial expansion, (1), which depend on the momenta of π + mesons only, this analysis
would have been sufficient. However, in order to determine the k parameter, it is necessary to determine whichpairs of photons, out of the three possible combinations
in each event, constitute the π 0 s. To this end, we define aparameter Q as
Q = (mγi γj − mπ0 )2 + (mγk γl − mπ0 )2 ,

100
80
60
40
20
0

k
0.064 ± 0.056
0.034 ± 0.023
0.073 ± 0.054
0.043 ± 0.020

4

120

120

300

g
0.521 ± 0.088
0.477 ± 0.055
0.584 ± 0.070
0.518 ± 0.039

m (MeV /c )Fig. 2a,b. Mass squared plots for the charged particles, whichwere calculated from time of flight and momentum at the spectrometer, for the measured a and the simulation b

0
200

-1

-0.8

629

-0.6

cosθπ,4γFig. 3a–c. Kinematics of the Kπ3 decays at rest. a Total energy ETOT of π + and 4γs, b Momentum difference ∆P =pπ+ − p4γ between π + and 4γs, c opening angle of pπ+ with
p4γ . Solid lines are the experimental data and dashed lines are
the Monte Carlo simulationsin the spectrometer gap, the Kπ3 events were selected fromthe above two criteria.
Figure 2 shows the mass squared plots for the charged
particles, which were calculated from time of flight andmomentum in the spectrometer, for both the measured(a) and the simulated (b) data, respectively. Fig. 2a clearlyshows the separation of π + and µ+ . The Monte Carlo forKπ3 events were generated for 3-body phase space.
Figure 3 shows ETOT [Fig. 3a] and ∆P [Fig. 3b] plots
for the selected events. It is observed that ETOT peaks
slightly lower than the kaon mass. Also, the momentum
balance condition with the peak centered at zero momentum is satisfied, although the distribution is slightly asymmetric. The widths of the distributions are understood as
being due to the finite resolution of the detector elements.
The shift in ETOT and the asymmetry in ∆P are dueto the electromagnetic shower leakage in our CsI crystal
arrangement. For our detector, this leakage typically underestimates photon energy by 5% with some low energy(7)

where mγi γj and mγk γl are the invariant masses of γi ,γj , and γk , γl pairs (i 6= j 6= k 6= l = 1, 4), respectively and mπ0 = 135 MeV/c2 is the rest mass of pion.
The combination with the minimum value for Q is considered as the correct pairs. Figures 4a and b show theinvariant masses of the two π 0 s thus identified. The π 0with higher energy is labeled #1 and the other #2. Theyshow smooth distributions with peaks at mπ0 . Also shown
are the Monte Carlo results as dotted lines, subject tothe same criterion as the experimental data. Good agreement between the experiment and Monte Carlo is evident.
The same aspect is further examined from 2-D plots ofmass spectra of the two π 0 s. Figure 4c shows a plot of
mπ0 (1) versus mπ0 (2). There is a clear clustering of eventsat mπ0 (1) ∼ mπ0 (2) ∼ 135 MeV/c2 . For comparison, theplot of invariant masses of the rejected combinations areshown in Fig. 4d. The Figs. 4e and 4f show the corresponding plots from the Monte Carlo simulation. Again,
one sees that Monte Carlo and experiment are in very
good agreement. This gives confidence in the pairing criterion for selecting the two π 0 s on an event by event basis.
From the distribution of unselected pair candidates, it isestimated that the residual bias due to wrong pairing isless than 2%.

4 Results
To deduce the physics parameters of interest, the experimental data (EXP) is compared with the Monte Carlo

60

60

(b)
40

0.25

Arbitrary(a)Arbitrary

Counts80

CountsThe KEK-PS E246 Collaboration: A kinematically complete measurement of K + → π + π 0 π 0 decays

630

(a)

0.2

0.18

(b)

0.16
0.14
0.12

0.15

0.1

40

0.08

0.1

20

0.06

20

0.04

0.05

0.02

0

50

100

150

0

200

0

Mπ0 (MeV/c2)

50

100

150

0
-0.03

200

200

(c)

0
-0.03

0

(d)

0.1

-0.02

-0.01

0

(s3-s0) (MeV2)

(c)

0.08

2

Mπ0 (MeV/c )150

Mπ0 (MeV/c2)

150

-0.01

(s3-s0) (MeV )Mπ0 (MeV/c2)

200

-0.02

2

Arbitrary

0

100

50

0

0.06

100
0.04

50

0

50

100

150

0

200

0.02

0

2

50

100

150

0
-0.03

200

Mπ0 (MeV/c )Mπ0 (MeV/c )

200

200

(e)

-0.01

0

Fig. 5a–c. dR(s3 − s0 ) = dN EXP /d(s3 − s0 )/dN MC /d(s3 − s0 )distributions for spectrometer field settings a 0.9 T; b 0.65 T;
and c 0.45 T. The straight lines are the best fits to the data

100

50

0

50

100

150
2

Mπ0 (MeV/c )

200

0.35
0.3

0.18
0.16
0.14

0.25

50

0

Arbitrary100

Arbitrary2Mπ0 (MeV/c )150

Mπ0 (MeV/c2)

150

(f)

-0.02

(s3-s0) (MeV2)

2

0

0

50

100

150

200

0.1

0.15

0.08
0.06

0.1

Mπ0 (MeV/c2)

0.04

0.05

0

0

0.02
0

0.02

0.04

0.06
2

0

0

0.02

0.04

0.06

0.08

(s2-s1) (MeV2)(s2-s1) (MeV )Arbitrary

Fig. 4a–f. Invariant mass plots for the selected π pair in Kπ3events. a is invariant mass plot of the γ pair which has larger
energy than that of the ones in b. c shows the scatter plot of theinvariant masses of best π 0 pair. d shows the same plot for theunselected π 0 pair candidates. e and f are the plots of Monte
Carlo simulations with the criteria of c and d, respectively

0.12

0.2

0.08
0.07
0.06
0.05

(MC) events for the Kπ3 decays. Figure 5 shows the plotsof yield ratio dR(s3 −s0 ) = dN EXP /d(s3 −s0 )/dN MC (s3 −s0 ) along with a straight line fit. In each plot, the data at
extreme ends have larger errors, because they are at theedge of available phase space. Table 1 shows the deduced’g’ parameter for each field setting and the results agree
within one standard deviation. It is quite heartening tonote that there is no systematic change with spectrometer field, which indicates that the errors due to the beam
stopping distribution, energy losses of π + particles in thetarget etc., are quite small. The narrow region of (s3 − s0 )available in this experiment did not permit investigationof the weak quadratic term (h). The weighted mean of
the three field setting measurements for the coefficientg = 0.518 ± 0.039.
As mentioned before, it is known that the coefficient’j’ of the linear term in (s2 − s1 ) should be zero due toBose symmetry condition. We, therefore, fit the dR(s2 −

0.04
0.03
0.02
0.01
0

0

0.02

0.04

0.06
2

(s2-s1) (MeV )Fig. 6a–c. dR(s2 − s1 ) = dN EXP /d(s2 − s1 )/dN MC /d(s2 − s1 )distributions for each field settings fitted with linear functional
form in (s1 − s2 )2 . a for 0.9 T; b for 0.65 T; c for 0.45 Ts1 ) = dN EXP /d(s2 − s1 )/dN MC /d(s2 − s1 ) result for aquadratic term(k). Figure 6 shows the plots of the yieldratio dR(s2 − s1). It is worth noting that the range of(s1 − s2 ) covered by the current experiment is very close
to the threshold. At each field setting, the data is well
reproduced by a quadratic dependence on (s1 − s2 ). The
fit results are also tabulated in Table 1. A non-zero value

The KEK-PS E246 Collaboration: A kinematically complete measurement of K + → π + π 0 π 0 decays

631

Table 2. Summary of the measurements of g,h, k parameters of Wein+berg’s expansion in the Kπ3
DecaysRef.
Present
Batusov et al. [6]Braun et al. [7]Bertrand et al. [8]Sheaff [9]Smith et al. [10]Aubert et al. [11]Davison et al. [12]World average aWorld averagebg
0.518 ± 0.039
0.704 ± 0.012
0.670 ± 0.054
0.806 ± 0.220
0.630 ± 0.038
0.510 ± 0.060
0.67 ± 0.06
0.544 ± 0.048
0.673 ± 0.010
0.587 ± 0.019

h–0.104 ± 0.014
0.152 ± 0.082
0.164 ± 0.121
0.041 ± 0.030
0.009 ± 0.040-0.01 ± 0.08
0.026 ± 0.050

k0.043 ± 0.020
0.0197 ± 0.0045-0.02 ± 0.04
—
—
—
—
—

a

This estimate is the weighted mean of all values quoted above.
The results of [6] are not included in this world average estimate.
See text for details.
bk = 0.043 ± 0.020, is obtained from the weighted mean of
the three measurements.
Table 2 presents the results of K + → π + π 0 π 0 measurements available to date, including the present results. It
is to be remarked that ’g’ values of all experiments, withthe exception of Batusov et al.,[6] are in agreement. As
Batusov et al. measured the in flight decays of 10 GeV/ckaons, it is tempting to attribute the discrepancy to a possible energy dependence of ’g’ parameter. This may not,however, be entirely correct because the result of Smithet al. [10], which employed in flight decay of 5 GeV/ckaons, is in excellent agreement with the present result.
The world average from all the data, except that of Batusov et al. [6], gives g = 0.587 ± 0.019. This result isin very good agreement with the g = 0.582 ± 0.021 for
the CP symmetry partner K − → π − π 0 π 0 [13]. If we include Batusov et al’s result in the world average estimate,
we get g = 0.673 ± 0.010, about three standard deviations
away from the CP symmetry expectation. The ’g ’ parameters for the decays are g++− = −0.2154 ± 0.0035, andg−−+ = −0.217 ± 0.007, which predict, from ∆I = 1/2
rule, g+00 = 0.43 for the decay mode of our interest.
Clearly, this rule is not obeyed.
Prior to our work, there were only two other measurements which attempted to determine the parameter ’k’ inthis channel. The result of Braun et al. [7] was obtained
from a simultaneous fit of three parameters, without π 0
reconstruction. The only other work is due to Batusov etal [6]. Within the errors, all these results are compatible.
Both our result and that of Batusov et al show a non-zero’k’ parameter, while that of Braun et al is consistent withk=0.

5 Summary and conclusions
We have measured, in a kinematically over determinedsetting, the K + → π + π 0 π 0 decay parameters. The linear

term in Weinberg expansion is determined and the world
data now fixes it to g = 0.587 ± 0.019, comparable inprecision and in good agreement with the CP symmetry
partner decay of K − → π − π 0 π 0 with g = 0.582 ± 0.021.
It is found that the quadratic term in (s2 − s1 ) is k =0.043 ± 0.020, a small but non-zero term.
Acknowledgements. We would like to acknowledge financial
support from several granting agencies (Ministry of Education,
Science, Sports, and Culture-Japan, JSPS-Japan, Ministry of
Science and Technology of Russia, Russian Foundation for Basic Research, NSERC-Canada, KOSEF-Korea, NSF-USA and
DOE-USA). We would also like to thank Professors K. Nakai
and K. Nakamura as well as the technical and support staff of
KEK-PS.

References
1. L. Maiani and N. Paver, in The Second DAΦNE Physics
Handbook, eds. L. Maiani, G. Pancheri and N. Paver(INFN, Italy 1995).
2. S. Weinberg, Phys. Rev. Lett. 4 (1960) 87.
3. J. Imazato et al., “Search for T-violating Muon Polarization in K + → π 0 µ+ ν Decay”, KEK-PS Research Proposal, KEK Report 91-8 (1991).
4. Y. Kuno, Nucl.Phys. A629 (1998) 445c.
5. D.V. Dementiev et al., Nuclear Instruments and Methods,
A379 (1996)499.
6. V.Y. Batusov et al., in Proc. 28th Int. Conf. on High Energy Phys., Warsaw, Poland, July, 1996, edited by Z. Ajduk and A.K. Wroblewski (World Scientific, 1997).
7. H. Braun et al., Lett. Nuovo Cimento 17 (1976) 521.
8. D. Bertrand et al., Nucl. Phys. B114 (1976) 387.
9. M. Sheaff, Phys. Rev. D12 (1975) 2570.
10. K.M. Smith et al., Nucl. Phys. B91 (1975) 45.
11. B. Aubert et al., Il Nuovo Cimento 12A (1972) 509.
12. D. Davison et al., Phys. Rev. 180 (1969) 1333.
13. V.N. Bolotov et al., Sov. J. Nucl. Phys. 44 (1986) 73.

7 December 2000

Physics Letters B 495 (2000) 33–41
www.elsevier.nl/locate/npe

Test of exotic scalar and tensor couplingsin K + → π 0e+ν decay
KEK-E246 Collaboration
S. Shimizu a,b,∗,1 , K. Horie b , M. Abe c , M. Aoki d , I. Arai e , Y. Asano c , T. Baker d ,M. Blecher f , M.D. Chapman d , P. Depommier g , M.P. Grigorjev h , M. Hasinoff i ,H.C. Huang j , Y. Igarashi e,d,1 , T. Ikeda e,k,1 , J. Imazato d , A.P. Ivashkin h , J.H. Kang l ,M.M. Khabibullin h , A.N. Khotjantsev h , Y.G. Kudenko h , Y. Kuno d , J.-M. Lee l ,K.S. Lee m , A.S. Levchenko h , G.Y. Lim d , J.A. Macdonald n , C.R. Mindas o ,O.V. Mineev h , S. Sekikawa e , H.M. Shimizu k , Y.-H. Shin l , Y.-M. Shin n , K.S. Sim m ,A. Suzuki e , T. Tashiro e , A. Watanabe e , D.H. Wright i,n , T. Yokoi p,d,1
a Department of Applied Physics, Tokyo Institute of Technology, Tokyo 152-0033, Japanb Department of Physics, Osaka University, Osaka 560-0043, Japanc Institute of Applied Physics, University of Tsukuba, Ibaraki 305-0006, Japand IPNS, High Energy Accelerator Research Organization (KEK), Ibaraki 305-0801, Japane Institute of Physics, University of Tsukuba, Ibaraki 305-0006, Japanf Department of Physics, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0435, USA
g Laboratoire de Physique Nucléaire, Université de Montréal, Montréal, Québec, H3C 3J7, Canada
h Institute for Nuclear Research, Russian Academy of Sciences, Moscow 117312, Russia
i Department of Physics and Astronomy, University of British Columbia, Vancouver, V6T 1Z1, Canada
j Department of Physics, National Taiwan University, Taipei 106, Taiwan
k The Institute of Physical and Chemical Research, Saitama 351-0106, Japanl Department of Physics, Yonsei University, Seoul 120-749, South Korea
m Department of Physics, Korea University, Seoul 136-701, South Korea
n TRIUMF, Vancouver, British Columbia, V6T 2A3, Canadao Department of Physics, Princeton University, Princeton, NJ 08544, USAp Department of Physics, University of Tokyo, Tokyo 113-0033, Japan

Received 14 September 2000; accepted 16 October 2000
Editor: L. Montanet

Abstract+
The form factors of the decay K + → π 0 e+ ν (Ke3) have been measured with stopped kaons. A Dalitz plot containing 4.1 ×
4
10 events was analyzed. A comparison of the observed Dalitz plot with a Monte Carlo simulation determined the ratio of thestrengths of possible scalar and tensor couplings to that of the vector coupling, fS /f+ (0) = −0.002 ± 0.026(stat) ± 0.014(syst)* Corresponding author.

E-mail address: suguru@phys.wani.osaka-u.ac.jp (S. Shimizu).
1 Second institutions are present addresses.

0370-2693/00/$ – see front matter  2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 0 - 2 6 9 3 ( 0 0 ) 0 1 2 3 5 - 1

34

KEK-E246 Collaboration / Physics Letters B 495 (2000) 33–41

and fT /f+ (0) = −0.01 ± 0.14(stat) ± 0.09(syst). These values are consistent with the standard model prediction of zero, anddisagree with a previous experiment. The analysis also yielded the coefficient of the q 2 term in the vector form factor to be
λ+ = 0.0278 ± 0.0026(stat) ± 0.0030(syst).  2000 Elsevier Science B.V. All rights reserved.

1. Introduction
Weak interaction theory based on pure V–A coupling has been remarkably successful in explaining
experimental observations. The semi-leptonic decay±), which proceeds via W exchange
K ± → π 0 e± ν (Ke3in the standard model, is also believed to have only a
V–A contribution. However, a previous experiment using K + decay in-flight reported anomalous scalar andtensor couplings [1]. The PDG’s combined results forthe scalar and tensor form factors differ from zero by
3.6 and 3.4 standard deviations, respectively [2]. The
0 form factors recently measured at KTeV also inKe3
dicate non-zero values for these couplings [3]. The existence of these exotic interactions is in distinct disagreement with the standard model and thus requires
an accurate experimental check.
In this Letter, we present a new measurement of+form factors. The experiment used a stopped K +Ke3beam in conjunction with a 12-sector iron-core superconducting toroidal spectrometer [4]. Because of
the rotational symmetry of 12 identical gaps in the
spectrometer and the large directional acceptance ofthe π 0 detector [5], distortions due to detector acceptance are cancelled and systematic errors are greatly
suppressed. In addition, the kinematical resolution in
the C.M. frame is better for stopped kaons.

slowed down by a degrader and stopped in an active
target, which consisted of 256 5 × 5 mm2 scintillatingfibers, located at the center of the detector.
Ke3 events with a branching ratio of 4.8% wereidentified by analyzing the e+ momentum with thespectrometer and detecting the two photons in theCsI(Tl) calorimeter. Charged particles from the target
were tracked and momentum-analyzed using multiwire proportional chambers (MWPCs) at the entrance(C2) and exit (C3 and C4) of the magnet gap, as
well as by the active target and an array of ringcounters [7] surrounding the target. The active target
and ring counter signals were used for a consistency

2. Experiment
The experiment was performed at the KEK 12 GeV
proton synchrotron. The experimental apparatus wasconstructed for a T-violation search in K + → π 0 µ+ ν(Kµ3 ) decay [6]. A schematic cross sectional sideview of the detector is shown in Fig. 1. A separated
660 MeV/c K + beam (π + /K + ∼ 4) was discriminated from pion background by a Fitch type Čerenkov
counter with a multiplicity trigger. An efficiency ofmore than 99.0% was achieved for K + identification.
The typical K + beam intensity was 1 × 105 in a 0.6 s
spill duration and 3 s repetition rate. The kaons were

Fig. 1. Cross sectional view of the experimental setup. Assembly
detail of the active target, TOF1, and ring counter is shown in theinset.

KEK-E246 Collaboration / Physics Letters B 495 (2000) 33–41

35

2
Fig. 2. Mass squared spectra MTOF
obtained in TOF analysis for all momenta at (a) 0.65 and (b) 0.90 T. The solid line is the experimental data
and the dotted line is the Monte Carlo simulation. The exact values of the Ke3 , Kµ3 branching ratios, and π + decay in-flight from Kπ 2 decays
were taken into account in the simulation. The e+ selection region is indicated by the dashed lines.

check on the reconstructed trajectory. The momentum
resolution obtained after energy loss correction in theactive target was σp = 2.6 MeV/c at 205 MeV/c.
The e+ s were discriminated from the µ+ in Kµ3 decay by time-of-flight (TOF) between TOF1 and TOF2
scintillation counters. TOF1 surrounds the active target and TOF2 is located at the exit of the spectro2 ) of the charged partimeter. The mass squared (MTOF
cle, obtained from the TOF and momentum, is shownin Fig. 2 integrated over the entire momentum region.
The timing resolution of σt = 270 ps provides good
separation of the Ke3 and Kµ3 events. The Ke3 events
collected for two values of central magnetic field,
B = 0.65 and 0.90 T, yielded a consistency check withregard to the spectrometer acceptance and energy loss
in the target.
The π 0 detector, an assembly of 768 CsI(Tl) crystals, covers 75% of the total solid angle. There are 12
holes for outgoing charged particles to enter the spectrometer gaps and 2 holes for beam entrance and exit,
as shown in Fig. 1. Each crystal has a length of 25 cm
and covers 7.5◦ in both the polar and azimuthal directions. Since photons produce electromagnetic showers, their energy was shared among several crystals.
The photon energy and hit position were obtainedby summing the energy deposits and energy-weighted
centroid, respectively. A photon energy resolution of
σEγ /Eγ = 2.8% at 200 MeV and position resolution

Fig. 3. Invariant mass Mγ γ spectrum for Ke3 decay from
B = 0.65 T data. The solid line histogram is experimental data andthe dotted line is simulation data. The spectrum from B = 0.90 T
data is very similar. The Mγ γ selection region is indicated by thedashed lines.

of σpos = 7.6 mm were achieved. Timing information from each module was used to identify a photon
cluster and to suppress accidental backgrounds due tobeam particles. The two-photon invariant mass (Mγ γ ),π 0 energy and direction, were obtained from the photon momentum vectors. Fig. 3 shows the Mγ γ spectrum for Ke3 events. The resolutions of the π 0 energy

36

KEK-E246 Collaboration / Physics Letters B 495 (2000) 33–41

and direction at 246 MeV were σEπ 0 /Eπ 0 = 5.5% andσθ = 3.4◦ , respectively.
3. Ke3 event selection
Ke3 decays at rest were obtained from the following procedure. The K + decay time, defined as the e+signal at the TOF1 counter, was required to be morethan 5 ns later than the K + arrival time measured by
the Čerenkov counter. This reduced the fraction of K +decays in-flight to the 5 × 10−4 level. Events with
π + decays in-flight and scattering of charged particles from the magnet pole faces were eliminated bya track consistency cut in the ring counters. The cutoff gate for the ring counters, 20 mm, was determinedfrom a Monte Carlo simulation (see below). The se2<
lection of positrons, identified by −6000 < MTOF
2
44000 MeV /c , was also determined from the simulation, as shown in Fig. 2. Events with two clusters in
the CsI(Tl) calorimeter were selected as π 0 decays andevents with other cluster numbers were rejected. The
acceptance cut on the invariant mass was 80 < Mγ γ <140 MeV/c2 (Fig. 3). The opening angle θl + π 0 < 166◦
between a charged particle and the π 0 (θl + π 0 ) removed
the Kπ2 back-to-back events. The photon conversion
events (γ → e+ e− ) in the active target system were
rejected by selecting events in which the charged particle hit one of the TOF1 and ring counters.
The solid line in Fig. 4 (a), (c) shows the e+energy spectrum while (b), (d) shows the opening
angle (θe+ π 0 ) distribution of accepted Ke3 events
for B = 0.65 and 0.90 T, respectively. The energy
spectra have not been corrected for energy loss in thetarget. The Dalitz density plots are shown in Fig. 4(e), (f). The Ke3 acceptances, 3.1 × 10−3 (0.65 T) and3.6 × 10−3 (0.90 T), were obtained from the Monte
Carlo simulations. The remaining backgrounds, after
applying the above criteria, were estimated to be
0.14 ± 0.03% of the accepted Ke3 events for Kµ3
and 0.27 ± 0.03% for Kπ2 decays, respectively. The
present result is based on 2.4 × 104 (0.65 T) and1.7 × 104 (0.90 T) Ke3 events.

4. Analysis
The most general Lorentz invariant form of thematrix element for the semi-leptonic K + → π 0 l + ν+(Kl3) decay is given by [2,8]M ∝ f+ q 2 (PK + Pπ 0 )ūl γλ (1 + γ5 )uν+ f− q 2 ml ūl (1 + γ5 )uν+ 2mK fS ūl (1 + γ5 )uν+ (2fT /mK )(PK )λ (Pπ 0 )µ ūl σλµ (1 + γ5 )uν ,(1)where PK and Pπ 0 are the four-momenta of the K +and π 0 , respectively. f± (q 2 ) are the usual vector
form factors, which are functions of the momentum
transfer squared q 2 = (PK − Pπ 0 )2 to the leptons. fS
and fT are the strengths of the potential scalar andtensor couplings, assumed to be independent of q 2 .
In the case of Ke3 decay, the f− (q 2 ) term appearing
in M is proportional to the small e+ mass and is
negligible. The vector form factor, assumed to depend
linearly on q 2 as f+ (q 2 ) = f+ (0)[1 + λ+ (q/mπ )2 ],and exotic form factors fS and fT could be extracted
from the Dalitz plot distributions; parameters (λ+ ,fS /f+ (0), fT /f+ (0)) were determined by minimizing
the difference between the observed Dalitz plot and theMonte Carlo simulation. Using a GEANT [9] — basedMonte Carlo code, the simulation data were generated
according to Braun [10],ρ org (Ee+ , Eπ 0 ) ∝ F+2 q 2 A + BΞ + CΞ 2 ,
(2)
where
A = mK 2Ee+ Eν − mK Eπ0 0+ m2e+ Eπ0 0 /4 − Eν ,
B = m2e+ Eν − Eπ0 0 /2 ,C = m2e+ Eπ0 0 /4,

Eπ0 0 = Eπmax
0 − Eπ 0= m2K + m2π 0 − m2e+
2mK − Eπ 0 ,
F+ q 2 = f+ q 2 + (me+ /mK )RT , Ξ q 2 = f− q 2 /f+ q 2 + (2mK /me+ )RS+ me+ /mK + 2(Eν − Ee+ )/me+ RT
1 + (me+ /mK )RT ,

RT = fT /f+ q 2 .
RS = fS /f+ q 2 ,Here Eπ 0 , Ee+ , and Eν are, respectively, the pion,
positron, and neutrino energies in the K + C.M. frame.
The trigger requirement, detector resolution, and background caused by photon conversion, bremsstrahlung,KEK-E246 Collaboration / Physics Letters B 495 (2000) 33–41

37

Fig. 4. Experimental spectra (solid line) of Ee+ (a), (c) and θe+ π 0 (b), (d) for Ke3 decay, and Monte Carlo simulation (dotted line) with thebest fit λ+ , fS /f+ (0), and fT /f+ (0) values for the settings of 0.65 and 0.90 T. The Dalitz density plots (e), (f) are also shown.

38

KEK-E246 Collaboration / Physics Letters B 495 (2000) 33–41

Fig. 5. (a) π + momentum spectrum of Kπ 2 decay without energy loss correction in the target (b) opening angle between π + and π 0 for Kπ 2
decay. The solid line is experimental data and the dotted line is simulation data.

and other K + decay modes, were included in the calculation. The radiative correction following the Ginsberg procedure [11] was also taken into account. The
simulation data were analyzed in the same manneras the experimental sample, yielding Dalitz plot distributions ρ obs (Ee+ , θe+ π 0 ) in the C.M. frame. Although it is possible to analyze the distribution ofρ obs (Ee+ , Eπ 0 ), instead of ρ obs (Ee+ , θe+ π 0 ), the former is not used because it is not insensitive to CsI(Tl)energy calibration and gain shifts. The measurementof the π 0 direction is independent of the absolute energy calibration of the CsI(Tl) calorimeter, and is morereliable, while Eπ 0 is strongly affected by the accuracyof the calibration.
In order to check the validity of the simulation, theKπ2 spectra were compared to the experimental ones.
Fig. 5 (a), (b) show the distributions of the π + momentum without the energy loss correction in the target
and opening angle (θπ + π 0 ), respectively, for the experimental (solid line) and simulation (dotted line) data.
The χ 2 values are 114 (107 bins) and 183 (191 bins)
for the π + momentum and θπ + π 0 spectra, respectively,
showing that the simulation correctly reproduced theexperimental spectra. Also, the validity of the simulation was confirmed by comparing the obvious photon
conversion events, which were identified by 3 clusters,
in both the experiment and simulation data.

The fitting of the Ke3 Dalitz plot was performed
via the maximum likelihood method with a Poissondistributed function, in which χ 2 is defined as,
χ2 =

n
X

i

2(αNsim,i − Nexp,i )+ 2Nexp,i × ln(Nexp,i /αNsim,i ) ,where n is the number of bins in the Dalitz plot and α
is introduced for normalization. The bin size is set to
4 MeV for Ee+ and 3.6◦ for θe+ π 0 . A program basedon MINUIT [12] minimized χ 2 and estimated the error of the parameters. The dotted line in Fig. 4 (a)–(d)shows the simulation spectrum using the best fit parameters. The original ρ org (Ee+ , Eπ 0 ) distributions accepted by the present experimental apparatus are thus
obtained, as shown in Fig. 6 (a), (b), together with the
Ke3 kinematical boundary. The accepted events covera wide region of phase space and extend to the boundaries where fS and fT have high sensitivity.

5. Results
When a V–A form is assumed, λ+ is the only freeparameter in the matrix element of Eq. (2). The valuesof λ+ obtained for B = 0.65 and 0.90 T are shown inKEK-E246 Collaboration / Physics Letters B 495 (2000) 33–41

39

Fig. 6. The original ρ org (Ee+ , Eπ 0 ) distributions accepted by the present experimental apparatus. The solid line corresponds to the Ke3
kinematical boundary.

Table 1Fitting results of the Ke3 form factor parameters
Data

Analysis

Reduced χ 2

λ+0.65 T

V–A

1.25

0.0288 ± 0.0033

0.90 T

V–A

1.10

Combined

V–A

fS /f+ (0)fT /f+ (0)0.0266 ± 0.0038
0.0278 ± 0.0025

0.65 T

V–A, S, T

1.25

0.0288 ± 0.0033−0.002 ± 0.033−0.02 ± 0.15

0.90 T

V–A, S, T

1.10

0.0266 ± 0.0039−0.001 ± 0.040

0.01 ± 0.24

Combined

V–A, S, T

0.0278 ± 0.0026−0.002 ± 0.026−0.01 ± 0.14

Akimenko et al. [1]0.0284 ± 0.0029

0.070 ± 0.016

0.53 ± 0.14

World average [2]0.0276 ± 0.0021

0.084 ± 0.023

0.38 ± 0.11

Table 1 together with those of the previous experiment[1] and the world average quoted in PDG [2]. The
reduced χ 2 is also given. The combined value forthe two magnet settings, λ+ = 0.0278 ± 0.0025,
calculated as the error-weighted average, is consistent
with the world average. The relative amplitude of theform factors (fS /f+ (0), fT /f+ (0)) were then derived
using Eq. (2) with scalar and tensor couplings as wellas λ+ treated as free parameters. Fig. 7 shows the χ 2
contour plots for the best fit values of λ+ = 0.0288(0.65 T) and 0.0266 (0.90 T). They are consistent withpure V–A coupling. The combined values from the twomagnet settings are fS /f+ (0) = −0.002 ± 0.026 andfT /f+ (0) = −0.01 ± 0.14. It should be noted the χ 2
fitting for λ+ with and without the exotic interactions
has the same values for both B = 0.65 and 0.90 T.
The major systematic errors in the form factor determination come from imperfect reproducibility ofthe experimental conditions in the simulation. They
were estimated by a simulation study and are summarized in Table 2 in three categories: e+ detection,
π 0 detection, and Monte Carlo simulation. In contrast to the in-flight experiment, the decay particles
can be affected by interaction with the target. In thepresent analysis, the ambiguity of the bremsstrahlung
and photon conversion processes in the simulation

40

KEK-E246 Collaboration / Physics Letters B 495 (2000) 33–41

Fig. 7. The χ 2 contour plots for the best fit values of λ+ = 0.0288 and 0.0266 for 0.65 and 0.90 T, respectively. Solid, dashed, and dotted lines
correspond to 1-, 2-, and 3-σ constraint levels, respectively.

Table 2
Summary of systematic errors
Error source

1λ+

1fS

1fT

0.0014< 0.001< 0.01[π 0 detection]
Misalignment of the CsI(Tl) barrel
Cut point dependence

0.0002< 0.001< 0.01

Beam background0.0010< 0.001< 0.01

Misalignment of the MWPCs

0.0007< 0.001< 0.01Chamber spatial resolution

0.0005< 0.001< 0.01

Field strength of the toroidal spectrometer

0.0009< 0.001< 0.01e+ identification

0.0002< 0.001< 0.01

Cut point dependence for the tracking

0.0006< 0.001< 0.01

Cut point dependence for e+ identification

0.0007< 0.001< 0.01

Photon conversion in the target

0.0004

0.004

0.03

e+ bremsstrahlung in the target

0.0018

0.013

0.08

Total

0.0030

0.014

0.09

[e+ detection][MC simulation]

dominated the systematic error. Since the angle of thebremsstrahlung photon in the target is nearly parallelto the e+ direction, the spurious photon clusters due
to bremsstrahlung were rejected by eliminating eventsin crystals surrounding the hole through which the e+passed. Therefore, the bremsstrahlung contribution to
the systematic error was taken into account only forthe e+ energy loss in the target by artificially varying
the bremsstrahlung probability and response. Changes
in the form factors obtained under this assumptionKEK-E246 Collaboration / Physics Letters B 495 (2000) 33–41

were evaluated as a systematic error. The effect of photon conversion was also estimated in a similar mannerby varying the conversion probability.
On the other hand, the systematic errors associatedwith instrumental misalignment, non-uniform detector
performance, and asymmetric beam stopping profile,
were reduced by integrating over all e+ and π 0 directions. The effects due to positions of the CsI(Tl)barrel, K + target, and MWPCs were estimated byassuming the maximum conceivable misalignments.
The contribution from a non-uniform efficiency anda non-linearity for detector elements were also estimated. Background contaminations from other K +decay modes and K + decay-in-flight did not con2
andtribute to the systematic error. Changing the MTOF
ring counter consistency cut values did not bias theresults. The total systematic error was evaluated byadding all these contributions in quadrature, as shownin Table 2. It is similar in size to the statistical error
for 1λ+ = 0.0030 and smaller for 1fS = 0.014 and
1fT = 0.09.

41

The present value of the λ+ parameter is consistent
with the current world average [2] within experimental
errors. fS /f+ (0) and fT /f+ (0) are consistent with theV–A coupling, which is not the case for the previous
experiment [1] or the world average [2].

Acknowledgements
This work was supported in Japan by a Grant-in-Aid
from the Ministry of Education, Science, Sports andCulture, and by JSPS; in Russia by the Ministry of Science and Technology, and by the Russian Foundationfor Basic Research; in Canada by NSERC and IPP, andby the TRIUMF infrastructure support provided under its NRC contribution; in Korea by BSRI-MOE andKOSEF; in the USA by NSF and DOE; and in Taiwan by NSC. The authors gratefully acknowledge theexcellent support received from the KEK staff.

References
6. Conclusion+
form factors have been measured using a
The Ke3
stopped K + beam in conjunction with a superconducting toroidal spectrometer. Due to the higher kinematical resolution and rotational symmetry of the experimental apparatus, many systematic errors are drastically reduced. By minimizing the difference between
the observed Dalitz plot and the Monte Carlo simulation, the following parameters are obtained:

λ+ = 0.0278 ± 0.0026(stat) ± 0.0030(syst),
fS /f+ (0) = −0.002 ± 0.026(stat) ± 0.014(syst),
fT /f+ (0) = −0.01 ± 0.14(stat) ± 0.09(syst).

[1] S.A. Akimenko et al., Phys. Lett. 259B (1991) 225.
[2] Particle Data Group, Eur. Phys. J. C 15 (2000).
[3] R.J. Tesarek, hep-ex/9903069.
[4] J. Imazato et al., in: Proc. 11th Int. Conf. on Magnet Technology, Elsevier Applied Science, London, 1990, p. 366.
[5] D.V. Dementyev et al., Nucl. Instrum. Methods A 440 (2000)
51.
[6] M. Abe et al., Phys. Rev. Lett. 83 (1999) 4253.
[7] A.P. Ivashkin et al., Nucl. Instrum. Methods A 394 (1997) 321.
[8] H. Steiner et al., Phys. Lett. B 36 (1971) 521.
[9] R. Brun et al., GEANT3 User’s Guide, CERN-DD/EE/84-1,
September 1987.
[10] H. Braun et al., Nucl. Phys. B 89 (1975) 210.
[11] E.S. Ginsberg, Phys. Rev. 162 (1967) 1570.
[12] F. James, supplement to CERN D506 MINUIT, 1978;
F. James, M. Roos, CERN D506 MINUIT, 1989.

2 August 2001

Physics Letters B 513 (2001) 311–318
www.elsevier.com/locate/npe

Measurement of Γ (Kµ3)/Γ (Ke3) ratio using stopped
positive kaons
KEK-E246 Collaboration
K. Horie a,∗ , S. Shimizu a , M. Abe b , M. Aoki c , I. Arai d , Y. Asano b , T. Baker c ,M. Blecher e , M.D. Chapman c , P. Depommier f , M. Hasinoff h , H.C. Huang i ,Y. Igarashi d,c,1 , T. Ikeda d,j,1 , J. Imazato c , A.P. Ivashkin g , M.M. Khabibullin g ,A.N. Khotjantsev g , Y.G. Kudenko g , Y. Kuno c,a,1 , J.-M. Lee k , K.S. Lee l ,A.S. Levchenko g , G.Y. Lim c , J.A. Macdonald m , C.R. Mindas n , O.V. Mineev g ,Y.-H. Shin k , Y.-M. Shin m , A. Suzuki d , A. Watanabe d , T. Yokoi o,c,1
a Department of Physics, Osaka University, Osaka 560-0043, Japanb Institute of Applied Physics, University of Tsukuba, Ibaraki 305-0006, Japanc IPNS, High Energy Accelerator Research Organization (KEK), Ibaraki 305-0801, Japand Institute of Physics, University of Tsukuba, Ibaraki 305-0006, Japane Department of Physics, Virginia Polytechnic Institute and State University, VA 24061-0435, USA
f Laboratoire de Physique Nucléaire, Université de Montréal, Montréal, Québec, H3C 3J7 Canada
g Institute for Nuclear Research, Russian Academy of Sciences, Moscow 117312, Russia
h Department of Physics and Astronomy, University of British Columbia, Vancouver, V6T 1Z1 Canada
i Department of Physics, National Taiwan University, Taipei 106, Taiwan
j The Institute of Physical and Chemical Research, Saitama 351-0106, Japank Department of Physics, Yonsei University, Seoul 120-749, South Korea
l Department of Physics, Korea University, Seoul 136-701, South Korea
m TRIUMF, Vancouver, British Columbia, V6T 2A3 Canadan Department of Physics, Princeton University, Princeton, NJ 08544, USAo Department of Physics, University of Tokyo, Tokyo 113-0033, Japan

Received 31 May 2001; accepted 11 June 2001
Editor: L. Montanet

Abstract++) and K + → π 0 e+ ν (Ke3
) decay widths, Γ (Kµ3 )/Γ (Ke3 ), has been measured
The ratio of the K + → π 0 µ+ ν (Kµ3++
with stopped positive kaons. Kµ3
and Ke3
samples containing 2.4 × 104 and 4.0 × 104 events, respectively, were analyzed.
The Γ (Kµ3 )/Γ (Ke3 ) ratio was obtained to be 0.671 ± 0.007(stat.) ± 0.008(syst.) calculating the detector acceptance by
a Monte Carlo simulation. The coefficient of the q 2 dependent term of the f0 form factor was also determined to be+
decay. The agreement of our result
λ0 = 0.019 ± 0.005(stat.) ± 0.004(syst.) with the assumption of µ–e universality in Kl3* Corresponding author.

E-mail address: kate@phys.wani.osaka-u.ac.jp (K. Horie).
1 Second institutions are present addresses.

0370-2693/01/$ – see front matter  2001 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 0 - 2 6 9 3 ( 0 1 ) 0 0 7 5 5 - 9

312

KEK-E246 Collaboration / Physics Letters B 513 (2001) 311–318+
with the λ0 value obtained from Kµ3
Dalitz plot analyses supports the validity of the µ–e universality.  2001 Elsevier Science
B.V. All rights reserved.

1. Introduction
The spectroscopic studies to determine form factors of the K + semi-leptonic decays, K + → π 0 l + ν+(Kl3), are of importance both in studying low energy
properties of the strong interaction in terms of effective theories [1,2], and also in studying fundamental
interactions. In our previous work [3], we reported aresult testing the exotic couplings in K + → π 0 e+ ν+
) decay, showing the non existence of scalar and(Ke3tensor interactions, contradicting the current world
average adopted by Particle Data Group [4]. In the+) events, which
present work, the K + → π 0 µ+ ν (Kµ3were collected simultaneously, were analyzed to de++and Ke3
decay widths
termine the ratio of the Kµ3
Γ (Kµ3 )/Γ (Ke3 ). This quantity is one of the most im+
portant observables to evaluate the Kl3form factors.
Assuming that only the V−A interaction contributes to the Kl3 decay, the decay amplitude can be
described by two dimensionless form factors, f+ (q 2 )and f0 (q 2 ), which are functions of the momentum
transferred to the leptons q 2 = (PK − Pπ 0 )2 , where
PK and Pπ 0 are the four momenta of the K + and π 0 ,respectively. They are given as
f+ (q 2 ) = f+ (0) 1 + λ+ (q/mπ )2 ,
f0 (q 2 ) = f0 (0) 1 + λ0 (q/mπ )2 .
Assuming µ–e universality, the form factors between
Kµ3 and Ke3 decays are identical and Γ (Kµ3 )/Γ (Ke3 ) can be written as [5]Γ (Kµ3 )/Γ (Ke3 ) = 0.6457 − 0.1531λ+ + 1.5646λ0+ O λ2+ + λ+ λ0 + λ20 .
(1)This equation cannot determine the λ+ and λ0 parameters uniquely but simply fixes a relationship between
them. However, if the λ+ value derived from Ke3 data
analyses is assumed, the λ0 parameter can be obtained
from Eq. (1). In this analysis, our Ke3 result for the λ+parameter, λ+ = 0.0278 ± 0.0040, was employed [3].
It should be noted that the λ0 parameter can be also
determined by studying the Dalitz plot distribution ofKµ3 decay or the muon polarization in Kµ3 decay [4].

The determination of the λ0 parameter from theΓ (Kµ3 )/Γ (Ke3 ) ratio is based on the assumptionof µ–e universality. This is not the case for theother two methods. Thus, µ–e universality in Kl3
decay can be tested by comparing the λ0 parameterobtained from a branching ratio measurement λbr
0with that obtained from a Dalitz plot measurement
pol
+
λdal
0 and/or µ polarization measurement λ0 . In thiscase, however, a precise acceptance function has to becarefully determined over the whole Dalitz space forthe Kµ3 Dalitz plot analysis.
In this Letter, we present a new precise measurement of the Γ (Kµ3 )/Γ (Ke3 ) branching ratio andthe λ0 parameter. The experiment used a stopped
K + beam in conjunction with a 12-sector iron-coresuperconducting toroidal spectrometer [6]. Γ (Kµ3 )/Γ (Ke3 ) was obtained by measuring the ratio of thenumber of accepted Kµ3 and Ke3 events corrected for
detector acceptance.

2. Experiment
The experiment was performed at the KEK 12
GeV proton synchrotron. The experimental apparatus
was constructed for a T -violation search in Kµ3
decay [7]. A schematic cross sectional side viewof the detector is shown in Fig. 1. We detectedboth of the π 0 decay gammas which enabled us toreconstruct the complete kinematics of a Kl3 event incontrast to the previous experiment [8]; this increased
the reliability of the event selection. Because of
the rotational symmetry of the 12 identical gaps inthe spectrometer and the large directional acceptanceof the π 0 detector [9], distortions due to detector
acceptance were drastically reduced. Moreover, thesimilarity of the Kµ3 and Ke3 kinematics reduced thesystematic error due to the imperfect reproducibilityof the experimental conditions in the simulation of
the ratio of the calculated acceptance for the Kµ3
and Ke3 decays. The measurement was carried out
for two values of the central magnetic field strength,
B = 0.65 and 0.90 T, yielding a consistency checkKEK-E246 Collaboration / Physics Letters B 513 (2001) 311–318

Fig. 1. Cross-sectional side view of the E246 setup. Assembly detailof the active target, TOF1, and ring counter is shown in the inset.

with regard to the spectrometer acceptance and energy
loss estimation in the target.
Kµ3 (Ke3 ) events were identified by analyzing theµ+ (e+ ) momentum with the spectrometer and detecting the two photons in the CsI(Tl) calorimeter.
Charged particles from the target were tracked andmomentum-analyzed using multi-wire proportional
chambers (MWPCs) at the entrance (C2) and exit (C3and C4) of the magnet gap, as well as by the active target and an array of ring counters [10] surrounding thetarget. Particle identification between the µ+ s and e+ s
was carried out by time-of-flight (TOF) between TOF1
and TOF2 scintillation counters. TOF1 surrounds theactive target and TOF2 is located at the exit of thespectrometer. The π 0 detector, an assembly of 768
CsI(Tl) crystals, covers 75% of the total solid angle.
Since photons produce electromagnetic showers, theirenergy was shared among several crystals. The photon
energy and hit position were obtained by summing theenergy deposits and energy-weighted centroid, respec-

313

tively. Timing information from each module was used
to identify a photon cluster and to suppress accidental
backgrounds due to beam particles. The two-photon
invariant mass (Mγ γ ) and the π 0 energy and direction
were obtained from the photon momentum vectors.
Kµ3 and Ke3 decays at rest were selected by thefollowing procedure, which is similar to our Ke3
study. The K + decay time, defined as the charged
lepton signal at the TOF1 counter, was required tobe more than 5 ns later than the K + arrival timemeasured by the Čerenkov counter to remove inflight K + decays. Events with π + decays in-flightand scattering of charged particles from the magnet
pole faces were eliminated by a track consistency cut
in the ring counters. Events with two clusters in theCsI(Tl) calorimeter were selected as π 0 decays andevents with other cluster numbers were rejected. The
acceptance cut on the invariant mass was 50 < Mγ γ <140 MeV/c2 . Requirements for the charged particle
momentum corrected for the energy loss in the target(Pcor < 175 MeV/c) plus opening angle between acharged particle and the π 0 (θl + π 0 < 154◦ ) removed
the Kπ2 events.
2 ) of the charged particles,
The mass squared (MTOF
obtained from the TOF and momentum, are shownin Fig. 2 integrated over the entire momentum region. The µ+ and e+ selection regions are also indicated in the figure. The timing resolution of σT =270 ps provides a good separation between the Kµ3
and Ke3 events. It should be emphasized that only
these requirements were imposed for the Kµ3 andKe3 event extraction. The numbers of good events after those cuts mentioned above are 12882(10704) and
23122(16850) at 0.65(0.90) T for Kµ3 and Ke3 decays, respectively, where small fractions due to thebackground events are still included. Figs. 3 and 4
show the Kµ3 and Ke3 spectra for the setting of B =0.65 and 0.90 T, respectively.

3. Monte Carlo simulation
In order to obtain the detector acceptance and estimate the background fraction, the Monte Carlo simulation was carried out for both the charged particle
measurement by the spectrometer and the π 0 measurement by the CsI(Tl) detector. The initial Dalitz distributions were generated with the values of λ+ = 0.0278

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KEK-E246 Collaboration / Physics Letters B 513 (2001) 311–318

much higher statistical accuracy for the Monte Carlosimulation is necessary to determine the form factors
from the Kµ3 Dalitz plot analysis.
By using the simulation data, the detector acceptance (Ω) is calculated asΩ=

2Fig. 2. Mass-squared spectra MTOF
obtained in TOF analysis for
all momenta at (a) 0.65 and (b) 0.90 T. The solid line is theexperimental data and the dotted line is the Monte Carlo simulation.
The Monte Carlo data are scaled by the obtained Γ (Kµ3 )/Γ (Ke3 )value. The µ+ and e+ selection regions are indicated by the dashedlines.

and the current world average λ0 = 0.006, while theDalitz distribution of Ke3 decay is insensitive to the λ0
parameter. The radiative corrections were taken into
account by following the Ginsberg procedure [11].
The simulation data were analyzed in the same manner as the experimental sample. The dotted lines inFig. 3 and Fig. 4 show the simulation spectra for thesetting of B = 0.65 and 0.90 T, respectively, using theassumed λ+ and λ0 parameters. These spectra are normalized so that the total number of events is the sameas the experimental one. Here, it is to be noted thatNacc
(λ+ , λ0 ) ≡ Ω 0 (λ+ , λ0 ),Ngenwhere Nacc and Ngen are the number of events accepted by our event selection requirements and thenumber of generated K + events, respectively. Ω 0 denotes the detector acceptance at the assumed form factor for the production of the simulation samples. They
were Ω 0 (Kµ3 ) = (1.790 ± 0.014) × 10−3, Ω 0 (Ke3 ) =(2.161 ± 0.014) × 10−3 for the setting of B = 0.65 Tand Ω 0 (Kµ3 ) = (1.669 ± 0.014) × 10−3 , Ω 0 (Ke3 ) =(1.756 ± 0.013) × 10−3 for the setting of B = 0.90 T.
The detector acceptance depends only slightly on theform factor (i.e., the shape of the Dalitz distribution),
therefore a correction factor (λ+ , λ0 ) is introduced.
 was calculated by taking into account the event se2
lection requirements such as Pcor , θl + π 0 , and MTOF
cuts. As shown in Fig. 5(a), (b), the  distribution
for Kµ3 decay was obtained as a contour plot in the(λ+ , λ0 ) space, while  for Ke3 decay was obtained asa function of the λ+ parameter.
2
MTOF
spectra were calculated by assuming a TOF
resolution of σT = 270 ps. They are shown as dottedlines in Fig. 2. Since the charged particles were identi2
measurement, the choice of the TOF refied by MTOF
sponse function was important for the determination of
the background contamination. We took into accountKπ2 , Ke3 , Kµ3γ decays for the Kµ3 background andKπ2 , Kµ3 , Ke3γ decays for the Ke3 background. The
background fraction depends on the magnetic settingof the spectrometer, as summarized in Table 1. The
most dominant background is due to Kπ2 decay inflight. The accidental background fraction due to beam
particles was estimated to be 0.3% for both Kµ3 andKe3 samples.

4. Results
The Γ (Kµ3 )/Γ (Ke3 ) ratio can be written as
Γ (Kµ3 ) N(Kµ3 ) Ω(Ke3 )=,Γ (Ke3 )N(Ke3 ) Ω(Kµ3 )KEK-E246 Collaboration / Physics Letters B 513 (2001) 311–318

315

Fig. 3. Spectra of the selected events (solid line): (a) θµ+ π 0 , (b) Pµ+ for Kµ3 decay and (c) θe+ π 0 , (d) Pe+ for Ke3 decay, and Monte Carlosimulation (dotted line). They were obtained for the setting of B = 0.65 T.

Table 1
Background fractions included in the Kµ3 and Ke3 samples
Background item

Kµ3

B = 0.65 T

B = 0.90 T

Kπ 2

0.6%

0.9%

Ke3

0.1%

0.1%

Kµ3γ

< 0.1%< 0.1%

0.3%

0.3%

Accidental
Ke3

Background fraction

Kπ 2

0.4%

0.3%

Kµ3

< 0.1%< 0.1%

Ke3γ< 0.1%< 0.1%

0.3%

0.3%

Accidental

where N is the number of accepted events after subtracting backgrounds. The λ0 parameter can be determined by substituting the obtained Γ (Kµ3 )/Γ (Ke3 )value and λ+ = 0.0278 into Eq. (1). However, the sit-uation was a little more complicated because the Kµ3
acceptance also depends on the λ0 parameter. Therefore, it was derived by iteration to minimize the difference between the λ0 parameter for the acceptance determination (λacc
0 ) and the λ0 parameter obtained fromobt
obt
Eq. (1) (λ0 ). Fig. 6 shows the |λacc
0 − λ0 | plot asacc
a function of λ0 for both the 0.65 and 0.90 T data.
The value of λacc
0 , which satisfied the condition of
obt | = 0, was adopted as our final result. The
|λacc
−
λ
0
0
λ0 parameter and associated Γ (Kµ3 )/Γ (Ke3 ) value
are shown in Table 2 together with those of the world
average quoted in PDG. The results of B = 0.65 and0.90 T data were then combined by calculating theerror weighted average as, λ0 = 0.019 ± 0.005 andΓ (Kµ3 )/Γ (Ke3 ) = 0.671 ± 0.007.
The systematic errors, which have been categorized
into background contamination and inaccurate detector acceptance, are summarized in Table 3. The ambiguity of the Kπ2 fraction introduces a significant systematic error, while the other channels are negligible.
If the Kπ2 background in the Kµ3 sample is not cor-

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KEK-E246 Collaboration / Physics Letters B 513 (2001) 311–318

Fig. 4. Spectra of the selected events (solid line): (a) θµ+ π 0 , (b) Pµ+ for Kµ3 decay and (c) θe+ π 0 , (d) Pe+ for Ke3 decay, and Monte Carlosimulation (dotted line). They were obtained for the setting of B = 0.90 T.

Table 2
Results of Γ (Kµ3 )/Γ (Ke3 ) and λ0 parameterΓ (Kµ3 )/Γ (Ke3 )λ00.65 T

0.673 ± 0.010

0.020 ± 0.006

0.90 T

0.668 ± 0.011

0.017 ± 0.007

Combined

0.671 ± 0.007

0.019 ± 0.005

No µ–e universality assumption 0.669 ± 0.007
World average [4]0.680 ± 0.013−
0.006 ± 0.007

rectly evaluated, the results could strongly depend on
the cut points of θlπ 0 and Pcor . The dependence on
both cut points was treated as a systematic error. Also,
an imperfect TOF response function in the simulation
could introduce a systematic error, which would be2
peaks.
concentrated around the tail part of the MTOF
This contribution was also studied by the cut point dependence of the TOF window positions. The accidental backgrounds could be neglected because their frac-

tions were small and common to both the Kµ3 and theKe3 events.
The systematic errors associated with the measurement, namely, instrumental systematic errors, are negligible. The effects due to misalignment of the CsI(Tl)barrel, K + target, and MWPCs and the contributionof misunderstanding of the energy loss of the charged
particles in the target were estimated and found to be
negligible. Differences in the detection efficiency fore+ and µ+ in the MWPCs would introduce a systematic error, which was studied by the sector number dependence of the spectrometer. The results obtained in
the various sectors were distributed within statisticalerror, and this contribution was also estimated to be
negligible. Since the detector acceptance depends onthe λ+ parameter, as well as the λ0 parameter, the ambiguity of the λ+ parameter introduced a systematic
error. This effect was estimated using the  distributions shown in Fig. 5. Using λ+ = 0.0278 ± 0.0040,
the change of the (Kµ3 )/(Ke3 ) value in this region
was considered to be the systematic error. If the cur-

KEK-E246 Collaboration / Physics Letters B 513 (2001) 311–318

317

rent world average λ+ = 0.031 ± 0.008 for Kµ3 decay [4] is used instead of taking the µ–e universality
value of 0.0278, Γ (Kµ3 )/Γ (Ke3 ) is shifted to 0.669 ±0.007 through the modification of the Kµ3 acceptance.
Although this shift (−0.002) is much smaller than thestatistical error (0.007), the acceptance deformation
due to the ambiguity of the Kµ3 λ+ was included as
an additional systematic error. These errors, regarding
them as one standard deviation errors, are summarizedin Table 3. The total size of the systematic errors was
obtained by adding each item in quadrature. The total systematic errors for [Γ (Kµ3 )/Γ (Ke3 )] and λ0
are 0.008 and 0.004, respectively, which are basicallyequal to the statistical error.

5. Conclusion

Fig. 5. The correction factor for the detector acceptance for thesetting of B = 0.65 T; (a)  contour in terms of the λ+ and λ0
parameters for Kµ3 and (b) dependence on the λ+ parameter forKe3 . The correction factor for the setting of B = 0.90 T is similar.

+
The ratio of the K + → π 0 µ+ ν (Kµ3
) and K + →+π 0 e+ ν (Ke3 ) decay widths, Γ (Kµ3 )/Γ (Ke3 ), hasbeen measured for stopped positive kaons. Assuming+
decay, the coefficient of theµ–e universality in Kl3
2
q dependent term of the f0 form factor was determined from the measured Γ (Kµ3 )/Γ (Ke3 ) ratio. In
contrast to the previous experiments, a large detector
acceptance and its symmetrical structure enabled us toreduce the statistical errors while suppressing the systematic errors. Our results areΓ (Kµ3 )= 0.671 ± 0.007(stat.) ± 0.008(syst.),Γ (Ke3 )λ0 = 0.019 ± 0.005(stat.) ± 0.004(syst.).

The λ0 parameter obtained from the present work is+
Dalitz plot analyconsistent with that from recent Kµ3
ses [13,14], which supports the validity of µ–e uni+versality in Kl3
decay. If the assumption of µ–e universality is removed, Γ (Kµ3 )/Γ (Ke3 ) can be written
as [5]
µ
gµ f+ (0) 2
Γ (Kµ3 )=Γ (Ke3 )ge f+e (0)µ× 0.6457 + 2.2342λ+
(2)− 2.3873λe+ + 1.5646λ0 ,obt
acc
Fig. 6. |λacc
0 − λ0 | plot as a function of λ0 . The solid and dashedlines correspond to 0.65 and 0.90 T data, respectively. The value of
acc
obt
λacc
0 , which satisfied the condition of |λ0 − λ0 | = 0, was adopted
as the final result.

where g is weak coupling constant for the lepton current. Substituting Γ (Kµ3 )/Γ (Ke3 ) = 0.669 ± 0.011(present result with no µ–e universality assumption),µλ+ = 0.031 ± 0.008 (the current world average),

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KEK-E246 Collaboration / Physics Letters B 513 (2001) 311–318

Table 3
Systematic errors
[Γ (Kµ3 )/Γ (Ke3 )]λ0Background contamination
Decay-in-flight from Kπ 2 decay

0.004

0.003

TOF response

0.004

0.003

Instrumental misalignment< 0.001< 0.001

Energy loss estimation in the target< 0.001< 0.001

Experimental error of the Ke3 λ+ parameter

0.001

0.001

Choice of the Kµ3 λ+ parameter

0.005

−

Total

0.008

0.004

Detector acceptanceλe+ = 0.0278 ± 0.0040 (our Ke3 result), and λ0 =0.039 ± 0.011 (error weighted average of Refs. [13]µand [14]) into Eq. (2), gµ f+ (0)/ge f+e (0) is derived tobe 0.971 ± 0.019 which is consistent with unity within
the experimental error. From the viewpoint of the the+form factors are of impororetical framework, the Kl3tance in studying low energy properties of the strong
interaction in terms of effective theories. The predictedvalues of the λ0 parameter are λ0 = 0.017 ± 0.004 [1]and λ0 = −0.03 [2]. The present result is consistent
with the former and inconsistent with the latter. Also,the I = 1/2 rule leads to identical λ0 parameters+
0 decays [12]. The λ paramebetween Kl3
and Kl300 analyses has been determined to be
ter from the Kl3
λ0 = 0.025 ± 0.006 [4], which is consistent with thepresent result.

Acknowledgements
This work has been supported in Japan by a Grantin-Aid from the Ministry of Education, Science, Sports
and Culture, and by JSPS; in Russia by the Ministryof Science and Technology, and by the Russian Foundation for Basic Research; in Canada by NSERC and
IPP, and by the TRIUMF infrastructure support provided under its NRC contribution; in Korea by BSRI-

MOE and KOSEF; in the USA by NSF and DOE; andin Taiwan by NSC. The authors gratefully acknowledge the excellent support received from the KEK
staff.

References[1] J. Gasser, H. Leutwyler, Nucl. Phys. B 250 (1985) 517.
[2] D.I. Kazakov, V.N. Pervushin, M.K. Volkov, Phys. Lett. B 64
(1976) 201.
[3] S. Shimizu et al., Phys. Lett. B 495 (2000) 33.
[4] Particle Data Group, Eur. Phys. J. C 15 (2000);
Particle Data Group, Rev. Part. Phys. (2000).
[5] H.W. Fearing, E. Fischbach, J. Smith, Phys. Rev. D 2 (1970)
542;
H.W. Fearing, E. Fischbach, J. Smith, Phys. Rev. Lett. 24(1970) 189.
[6] J. Imazato et al., in: Proceedings of the 11th International
Conference on Magnet Technology, Elsevier Applied Science,
London, 1990, p. 366.
[7] M. Abe et al., Phys. Rev. Lett. 83 (1999) 4253.
[8] J. Heintze et al., Phys. Lett. B 70 (1977) 482.
[9] D.V. Dementyev et al., Nucl. Instrum. Methods A 440 (2000)
151.
[10] A.P. Ivashkin et al., Nucl. Instrum. Methods A 394 (1997) 321.
[11] E.S. Ginsberg, Phys. Rev. 42 (1966) 1035.
[12] L.B. Auerbach et al., Phys. Rev. 155 (1967) 1505.
[13] R. Whitman et al., Phys. Rev. D 21 (1980) 652.
[14] V.M. Artemov et al., Phys. At. Nucl. 60 (1997) 2023.

Physics Letters B 554 (2003) 7–14
www.elsevier.com/locate/npe

Measurement of direct photon emission in K + → π + π 0γ decay
using stopped positive kaons
KEK-E470 Collaboration
M.A. Aliev a , Y. Asano b , T. Baker c , P. Depommier d , M. Hasinoff e , K. Horie f,1 ,Y. Igarashi c , J. Imazato c , A.P. Ivashkin a , M.M. Khabibullin a , A.N. Khotjantsev a ,Y.G. Kudenko a , A.S. Levchenko a , G.Y. Lim c , J.A. Macdonald g , O.V. Mineev a ,C. Rangacharyulu h , S. Sawada c , S. Shimizu fa Institute for Nuclear Research, Russian Academy of Sciences, Moscow 117312, Russia
b Institute of Applied Physics, University of Tsukuba, Ibaraki 305-0006, Japanc Institute of Particle and Nuclear Studies (IPNS), High Energy Accelerator, Research Organization (KEK), Ibaraki 305-0801, Japand Laboratoire de Physique Nucléaire, Université de Montréal, Montréal, PQ, Canada H3C 3J7
e Department of Physics and Astronomy, University of British Columbia, Vancouver, BC, Canada V6T 1Z1
f Department of Physics, Osaka University, Osaka 560-0043, Japan
g TRIUMF, Vancouver, BC, Canada V6T 2A3
h Department of Physics, University of Saskatchewan, Saskatoon, Canada S7N 5E2

Received 13 December 2002; accepted 17 December 2002
Editor: L. Montanet

Abstract
The radiative decay K + → π + π 0 γ (Kπ2γ ) has been measured with stopped positive kaons. A Kπ2γ sample containing
4k events was analyzed, and the Kπ2γ branching ratio of the direct photon emission process was determined to be[6.1 ± 2.5(stat.) ± 1.9(syst.)] × 10−6 . No interference pattern with internal bremsstrahlung was observed.
 2003 Elsevier Science B.V. All rights reserved.

1. Introduction
The chiral anomaly [1], which is a basic featureof quantum field theory, has been the subject of exE-mail address: suguru@phys.wani.osaka-u.ac.jp(S. Shimizu).
1 Present address: Institute of Particle and Nuclear Studies(IPNS), High Energy Accelerator Research Organization (KEK),
Ibaraki 305-0801, Japan.

tensive theoretical investigations. There are a number of calculations for decays of the pseudoscalarmesons—π , η, and K [2]; however, few experimental studies of this anomaly have been performed so far.
With the increased interest in chiral perturbation theory (ChPT) [3], the chiral anomaly has attracted renewed attention. In particular, non-leptonic kaon decays are well-suited for the study of ChPT. It is known,
however, that the genuine manifestation of the chiral anomaly in non-leptonic decays is restricted to

0370-2693/03/$ – see front matter  2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0370-2693(02)03236-7

8

KEK-E470 Collaboration / Physics Letters B 554 (2003) 7–14

the radiative decay of K + → π + π 0 γ (Kπ2γ ) for the
charged kaon [4].
The K + → π + π 0 (Kπ2 ) decay is hindered because
it violates the I = 1/2 rule. As a consequence,
the internal bremsstrahlung (IB) contribution to Kπ2γ
decay is suppressed, although it is still dominant.
This feature, in turn, could enhance the direct photon
emission (DE) which is sensitive to meson structure.
Here, the chiral anomaly appears as the magnetic
component of DE. In terms of ChPT, it enters themagnetic amplitude at O(p4 ), while IB arises fromO(p2 ) [4]. Also, the electric amplitude of DE may
arise from O(p4 ), which one can identify through theinterference pattern (INT) with IB. Thus the Kπ2γ
decay channel is one of the most interesting andimportant channels in determining the low energy
structure of QCD.
Three in-flight-K + experiments [5–7] and one
stopped-K + experiment [8] have reported on the DEbranching ratio in the π + kinetic energy region of
55 < Tπ + < 90 MeV. These experiments restricted
the Tπ + region to avoid backgrounds from K + →π + π 0 π 0 (Kπ3 ) decay, although the DE component
is significantly enhanced with decreasing π + energy.
The weighted average of the in-flight K + experiments
is Br(Kπ2γ , DE) = (18 ± 4) × 10−6 [9] which is muchlarger than the result from the stopped K + experimentBr(Kπ2γ , DE) = (4.7 ± 0.8 ± 0.3) × 10−6 [8]. Therefore, further experimental studies are necessary to settle this discrepancy of the DE branching ratio. It is also
desirable to check the existence of the INT component
which has not yet been observed.
In this Letter, we present a new measurement of direct photon emission in Kπ2γ decay. The experimentused a stopped K + beam in conjunction with a superconducting toroidal spectrometer with a 12-sectoriron-core [10]. We measured the Kπ2γ events by extending the Tπ + region (Tπ + > 35 MeV) below theKπ3 upper threshold (Tπmax+ = 55 MeV).

spectroscopic studies for K + → π 0 e+ ν (Ke3 ), Kµ3 ,and Kπ3 decays have been carried out using the samedetector system [12–14]. A schematic cross sectionalview of the detector is shown in Fig. 1. Because of
the rotational symmetry of the 12 identical gaps inthe spectrometer and the large directional acceptanceof the photon detector [15], distortions of the Kπ2γ
spectra due to instrumental misalignment, if any, weredrastically reduced.
A separated 660 MeV/c K + beam (π + /K + ∼ 7)
was used. Each K + was discriminated from pion
background by a Čerenkov counter with an efficiency
of more than 99%. The typical K + beam intensity
was 1.5 × 105 over a 2 s spill duration with a 4 s
repetition rate. The kaons were slowed down by adegrader and stopped in an active target located at
the center of the detector system. The Kπ2γ eventswere identified by analyzing the π + momentum withthe spectrometer and detecting three photons in the

2. Experiment
The experiment was performed at the KEK 12 GeV
proton synchrotron. The experimental apparatus was
based on the E246 experiment searching for T -violating transverse muon polarization in K + → π 0 µ+ ν(Kµ3 ) decay [11]. Besides the T -violation search,

Fig. 1. Cross sectional side view of the E470 setup. Assembly detailof the active target, TOF1, and ring counters is shown in the inset.

KEK-E470 Collaboration / Physics Letters B 554 (2003) 7–14

CsI(Tl) calorimeter. Charged particles from the target
were tracked and momentum-analyzed using multiwire proportional chambers at the entrance (C2) andexit (C3 and C4) of the magnet gap, as well as anarray of ring counters [16] surrounding the active
target system. The π + s were discriminated from e+2 ) ofand µ+ by calculating the mass squared (MTOF
the charged particles from the time-of-flight between
TOF1 and TOF2 counters. Counter TOF1 surrounded
the active target and counter TOF2 was located at
the exit of the spectrometer. The measurement was
carried out for the central magnetic field strength B =0.65 T which was chosen to match the spectrometer
acceptance to the π + momentum distribution due tothe DE process.
The photon detector, an assembly of 768 CsI(Tl)crystals, covered 75% of the total solid angle. There
were 12 holes for outgoing charged particles to enter the spectrometer and 2 holes for beam entrance
and exit, as shown in Fig. 1. The photon energy andhit position were obtained, respectively, by summing
the energy deposits and taking the energy-weighted
centroid of the crystals sharing a shower. Timing information from each module was used to identify
a photon cluster and to suppress accidental backgrounds due to beam particles. A Pb-plastic sandwich detector with 2.6 radiation length was set at theouter radius of the magnet pole to monitor photons
passing through the photon detector holes, as shownin Fig. 1.

9

3. Kπ2γ event selection
Kπ2γ decays at rest were obtained by the following procedure which was carefully studied in a Monte
Carlo simulation (see below). The K + decay time, defined as the π + signal at the TOF1 counter, was required to be more than 1.4 ns later than the K + arrival time measured by the Čerenkov counter. This
reduced the fraction of K + decays in-flight contamination to the level of 10−3 . Events with π + decays in-flight and scattering of charged particles fromthe magnet pole faces were eliminated by requiring the hit position in ring counters to be consistent
with the charged particle track. The selection of π + ,2< 30000 MeV2 /c4 , rejected e+ s
by 15000 < MTOF+
and µ s. To remove Kπ2 events, the reconstructed
π + momentum pπ + (Fig. 2(a)), corrected for the energy loss in the target, was required to be pπ + <175 MeV/c. Events with three clusters in the CsI(Tl)calorimeter were selected by assigning two photons
from π 0 → γ1 + γ2 and a radiated photon (γ3 ), while
events with other cluster number were rejected. The
opening angle between the two photons was required
to be larger than 37◦ to eliminate photon conversion
backgrounds in the active target system. Also, the photon conversion events could be removed by selecting only events in which there was one charged particle hit in TOF1 as well as in ring counters. The total
K + energy MK + = Eπ + + 3i=1 Eγi and momentum

vector p K + = p π + + 3i=1 pγi were used to rejectFig. 2. (a) pπ + distribution and (b) correlation plot of pπ + and Q2min without pπ + and Q2min cuts. To demonstrate background rejection
performance by the Qmin cut, the pπ + cut is not applied here.

10

KEK-E470 Collaboration / Physics Letters B 554 (2003) 7–14

background contaminations. The windows of 420 <MK + < 500 MeV/c2 and |(pK + )x,y,z | < 50 MeV/c
were imposed.
The final step in reconstructing Kπ2γ was todetermine which photons were a pair from a π 0 ,because there are three possible combinations to form
the π 0 . The IB events with incorrect pairing mimic the
DE decay and might introduce a systematic error. In
order to find the correct pairing, a quantity Q2 defined
as,
  2

π 0 2 σM
Q2 = Mγ1 γ2 − M
π0
  2

K + 2 σM+ Eπ 0 + Eγ 3 + Eπ + − MK+2  2CALC
MEAS
σπ + γ3+ cos θπ + γ3 − cos θπ + γ3 − δ1



2
MEAS
σπ2+ π 0 , (1)+ cos θπCALC+ π 0 − cos θπ + π 0 − δ2
was calculated for the three combinations, and the pairwith the minimum Q2 , Q2min , was adopted as the π 0pair. Here, Mγ1 γ2 is the invariant mass of the selected
pair and Eπ 0 is the π 0 energy obtained as,Eπ 0 = Eγ21 + 2Eγ1 Eγ2 cos θγ1 γ2 + Eγ22 + m2π 0 , (2)where θγ1 γ2 is the opening angle of the selected pairand mπ 0 is the rest mass of π 0 . The angles θπ + γ3
and θπ + π 0 are that between π + and γ3 and thatbetween π + and π 0 , respectively. The superscripts
MEAS and CALC stand for the measured angles andthe calculated angles using the Kπ2γ kinematics as,cos θπCALC= m2K + + m2π + − m2π 0 − 2mK + Eπ ++γ3− 2mK + Eγ3 + 2Eγ3 Eπ +
−1

(3)
× 2|pπ + ||pγ3 | ,
 2
CALC
2
2
cos θπ + π 0 = mK + + mπ + + mπ 0 − 2mK + Eπ +− 2mK + Eπ 0 + 2Eπ 0 Eπ +
−1

(4)
× 2|pπ + ||pπ 0 | ,where m, E, and p denote the rest mass, energy, andmomentum vector, respectively, of the particle shownin the subscript. The σ and offset values for each term
were σMπ 0 = 11.4 MeV/c2 , σMK + = 10.5 MeV/c2 ,π 0 = 116.0 MeV/c2 ,σπ + γ3 = 0.48, σπ + π 0 = 0.06, M
K = 471.6 MeV/c2 , δ1 = 0.35, and δ2 = 0.06. The
M
choice of Eq. (1) and these parameters was determined
to get the highest probability for the correct pairingby using a simulation with the IB component. The
correct pairing probability was estimated to be 85%
for IB, 79% for DE, and 83% for INT. Since Kπ3 andKπ2 decays do not satisfy the Kπ2γ kinematics, theQ2min values of the Kπ3 and Kπ2 events are essentiallylarger, as shown in Fig. 2(b). Therefore, the cut of
Q2min < 4.3 drastically reduced these contaminations.
The number of good Kπ2γ events is 4434 after theabove standard selection conditions. The backgroundfraction of the Kπ3 decay was estimated to be 1.2%
from the simulation. The solid lines in Fig. 3 show
the experimental Kπ2γ spectra after subtracting thebackgrounds: (a) π + momentum, (b) opening angle
between π + and γ3 , (c) opening angle between π 0
and γ3 , and (d) γ3 energy. These are compared withthe calculated ones (see below).

4. Monte Carlo simulation
In order to obtain the detector response function, a Monte Carlo simulation was carried out for
both the π + measurement by the spectrometer andthe photon measurement by the CsI(Tl) calorimeter using a GEANT-based Monte Carlo code. In thissimulation, Kπ2γ events due to the IB, DE, and
INT processes were generated with the corresponding matrix elements [17] and analyzed in the samemanner as the experimental data. The accuracy ofthe simulation was carefully checked by comparing
the Kπ2 spectrum with the experimental one. The
Kπ3 decay was also used to check the reproducibility of the experimental conditions in the simulation.
The detector acceptance ratios were obtained tobe Ω(DE)/Ω(IB) = 2.63 and Ω(INT)/Ω(IB) = 2.56
which were used to determine relative decay strengthof the DE and INT components normalized to the
IB component. Here it should be noted that eventswith incorrect pairing were included in the Kπ2γ sample, and this effect was also treated in the simulation. The pairing probability could be controlled byrequiring extra cuts in addition to the standard cutsat the cost of good Kπ2γ events. Two cut conditions, (I) Q2s − Q2min > 1 and (II) cos θγ1 γ2 > 0 orcos θπ 0 γ3 < 0, were individually or simultaneously imposed, where Q2s is the second smallest Q2 value
among the three combinations. The pairing probabil-KEK-E470 Collaboration / Physics Letters B 554 (2003) 7–14

11

Fig. 3. Kπ 2γ spectra: (a) π + momentum, (b) opening angle between π + and γ3 , (c) opening angle between π 0 and γ3 , (d) γ3 energy. The
solid lines are the experimental data. The dotted and dashed lines show the fitted spectra and the sole DE components, respectively, under theassumption of no INT component. The bump structure in (b) is due to the CsI(Tl) assembly structure with 12 holes.

Table 1
Summary of the detector acceptance Ω and correct pairing probability P for IB and DE
ConditionΩ(IB) −3 10Ω(DE) −3 
10

P (IB)(%)

P (DE)(%)

Standard
Standard ⊗ (I)Standard ⊗ (II)Standard ⊗ (I) ⊗ (II)

0.24
0.20
0.22
0.18

0.63
0.53
0.53
0.46

85
91
92
95

79
84
80
85

ity and good event acceptance under these conditions
are summarized in Table 1. It is possible to increase
the probability up to 95% (IB) and 85% (DE), although 23% of the Kπ2γ events are rejected. These
cuts were used for the estimation of systematic errors.

The Kπ3 background was also estimated by thesimulation and subtracted from the experimental data.
Since the present spectrometer acceptance overlapped
the Kπ3 region, the estimation of the Kπ3 fraction was
important. The obvious Kπ3 events were identified
as 3 photon clusters with one escaping photon using
cuts of Tπ + < 55 MeV (pπ + < 135 MeV/c), 350 <2
MK + < 440 MeV/c2 , 15000 < MTOF
< 35000 MeV2 /42c , and Qmin > 10. These events were compared withthe simulation to confirm the accuracy of the simulation and to check the correctness of the backgroundestimation. Fig. 4(a), (b) show the Q2min distributionsof the Kπ2γ and Kπ3 events, respectively, for the experimental data (solid line) and the simulation (dotted line). They are in excellent agreement, supportingthe correct estimation of the Kπ3 fraction in the Kπ2γ
sample. The dashed line in Fig. 4(a) shows the simu-

12

KEK-E470 Collaboration / Physics Letters B 554 (2003) 7–14

Fig. 4. Q2min distributions of (a) Kπ 2γ and (b) Kπ 3 events for the experimental data (solid line) and the simulation (dotted line). Dashed linein (a) shows the simulated Kπ 3 background survived after the Kπ 2γ selection.

lated Kπ3 background which survived after the Kπ2γ
selection.

5. Results
The DE branching ratio was determined using
the three observables, cos θπ + γ3 , cos θπ 0 γ3 , and Eγ3which have characteristic spectra corresponding to
the decay processes. The experimental distribution
ρ(cos θπ + γ3 , cos θπ 0 γ3 , Eγ3 ) of Kπ2γ data from thestandard cut conditions was fitted to a simulation
spectrum of IB[1 + α(DE/IB) + β(INT/IB)] with
α and β being free parameters. The DE and INTcomponents were obtained to be α = (0.92+0.44−0.38 )%and β = (−0.58+0.91)%,
respectively.
The
reducedχ2−0.83
is 1.27. Fig. 5 shows the χ 2 contour plot in the (α, β)space. β is consistent with zero and we conclude that
there is no significant INT component observed within
the experimental accuracy. By assuming β = 0 in thefitting, the DE component was determined to be α =(0.76 ± 0.31)% with a reduced χ 2 = 1.28. The DEbranching ratio was derived to be Br(Kπ2γ , DE) =(6.1 ± 2.5) ×10−6 by normalizing it to the theoreticalvalue of the IB branching ratio [17]. The dottedand dashed lines in Fig. 3 show the fitted spectra
and the sole DE components, respectively, under theassumption of β = 0. It can be seen that 26% of DE
comes from the region Tπ + < 55 MeV. Fig. 6(a), (b)show the cos θπ + γ3 and cos θπ 0 γ3 spectra, respectively,

Fig. 5. χ 2 contour plot in the (α, β) space. Contour levels of 1-, 2-,
and 3σ constraints are drawn.

of the DE component obtained by subtracting the fitted
IB component from the experimental data (dots) andthe fitted DE component (histogram).
The major systematic errors in the DE measurement come mainly from imperfect reproducibility ofthe experimental conditions in the simulation as follows. The γ mispairing effect was studied by varying the pairing probability. In addition to the standard
Kπ2γ selection conditions, extra cuts given in Table 1
were imposed. The DE branching ratio was found to
be consistent within α < 0.17% which was regarded
as the upper bound of this systematic error. The uncertainty due to the calibration error of the CsI(Tl)calorimeter was evaluated by introducing the maxi-

KEK-E470 Collaboration / Physics Letters B 554 (2003) 7–14

13

Fig. 6. (a) cos θπ + γ and (b) cos θπ 0 γ spectra of the DE component obtained by subtracting the fit IB component from the experimental data33
(dots) and the fitted DE component (histogram).
Table 2
Summary of estimated systematic uncertainty. All items are addedin quadrature to get the total
Error source

Uncertainty of α (%)γ mispairing
γ calibration
Kπ 3 background
Accidental background
Spectrometer field ambiguity< 0.17
0.13
0.03
0.08
0.02

Total systematic error< 0.23bound down to 35 MeV. The detector response andacceptance functions were evaluated by a Monte Carlosimulation based on GEANT. The reproducibility ofthe experimental conditions in the simulation was
carefully checked using Kπ2 and Kπ3 decays. The
Kπ3 background contamination in the Kπ2γ data
sample was also estimated by the simulation.
Comparing the experimental data with the simulation, the DE component was obtained to be
α = 0.76 ± 0.31(stat.) ± 0.23(syst.) %,mum conceivable errors in the simulation, and foundto be α = 0.13%. The Kπ3 fraction in the Kπ2γ
data depends slightly on the Kπ3 decay form factors, however this effect is negligibly small. The systematic error due to the ambiguity of the Kπ3 fraction was obtained as α = 0.03% by rejecting eventswith Tπ < 55 MeV. The contribution from the accidental background was estimated to be α = 0.08%
by changing its fraction varying the TDC gate widths.
The uncertainty due to the spectrometer field ambiguity was obtained to be α = 0.02%. As shown in Table 2, the total systematic error was estimated to beαtotal < 0.23% by adding all the item in quadrature.
However, this upper bound was regarded here as one
standard deviation of the systematic error.

6. Conclusion
We have performed a new measurement of the
K + → π + π 0 γ decay by extending the Tπ + lower

under the assumption of no INT component. The DEbranching ratio was then determined to beBr(DE, total)= 6.1 ± 2.5(stat.) ± 1.9(syst.) × 10−6 ,by normalizing it to the theoretical value of the IB
branching ratio [17]. In order to compare this resultwith the previous ones, the partial branching ratio in
the region of 55 < Tπ + < 90 MeV was also calculatedasBr(DE, partial)= 3.2 ± 1.3(stat.) ± 1.0(syst.) × 10−6 ,which is consistent with the previous stopped experiment [8]. We did not observe any significant effectdue to the INT component, indicating the pure magnetic nature of the direct photon emission process in
Kπ2γ decay.

14

KEK-E470 Collaboration / Physics Letters B 554 (2003) 7–14

Acknowledgements
This work has been supported in Japan by aGrant-in-Aid from the Ministry of Education, Culture,
Sports, Science and Technology, and by JSPS; in Russia by the Ministry of Science and Technology, and by
the Russian Foundation for Basic Research; in Canada
by NSERC and IPP, and by the TRIUMF infrastructure support provided under its NRC contribution. The
authors gratefully acknowledge the excellent support
received from the KEK staff.

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[8] S. Adler, et al., Phys. Rev. Lett. 85 (2000) 4856.
[9] Particle Data Group, Eur. Phys. J. C 15, Rev. Particle Phys.,
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Laboratori Nazionali di Frascati, Frascati, 1992.

Physics Letters B 562 (2003) 166–172
www.elsevier.com/locate/npe

First measurement of the T-violating muon polarizationin the decay K + → µ+ νγ
KEK-PS E246 Collaboration
V.V. Anisimovsky , A.N. Khotjantsev a , A.P. Ivashkin a , M. Abe b , M. Aliev a ,M. Aoki c , Y. Asano b , M. Blecher d , P. Depommier e , M. Hasinoff f , K. Horie g ,H.C. Huang h , Y. Igarashi g , J. Imazato g , M.M. Khabibullin a , Yu.G. Kudenko a ,Y. Kuno c , A.S. Levchenko a , G.Y. Lim g , J.A. Macdonald i , O.V. Mineev a ,N. Okorokova a , C. Rangacharyulu j , S. Shimizu c , Y.-M. Shin i ,N.V. Yershov a , T. Yokoi ga,∗a Institute for Nuclear Research RAS, 117312 Moscow, Russia
b Institute of Applied Physics, University of Tsukuba, Tsukuba 305-0006, Japanc Osaka University, Osaka 560-0043, Japand Virginia Polytechnic Institute and State University, VA 24061-0435, USAe Université de Montréal, Montréal, H3C 3J7 Canadaf University of British Columbia, Vancouver, V6T 2A3 Canada
g High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Japanh Department of Physics, National Taiwan University, Taipei 106, Taiwan, ROC
i TRIUMF, Vancouver, V6T 2A3 Canada
j University of Saskatchewan, Saskatoon, S7N 5E2 Canada

Received 4 April 2003; accepted 9 April 2003
Editor: L. Montanet

Abstract
We present the result of the first measurement of the T-violating transverse muon polarization PT in the decay K + → µ+ νγ .
This polarization is sensitive to new sources of CP-violation in the Higgs sector. Using data accumulated in the period 1996–
1998 we have obtained PT = (−0.64 ± 1.85(stat) ± 0.10(syst)) × 10−2 which is consistent with no T-violation in this decay.
 2003 Elsevier Science B.V. All rights reserved.
PACS: 11.30.Er; 12.60.-i; 13.20.Eb
Keywords: Kaon decays; Muon polarization; T-violation* Corresponding author.

E-mail address: valera@al20.inr.troitsk.ru (V.V. Anisimovsky).
0370-2693/03/$ – see front matter  2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0370-2693(03)00576-8

KEK-PS E246 Collaboration / Physics Letters B 562 (2003) 166–172

1. Introduction
The decay K + → µ+ νγ (Kµ2γ ) is one of thekaon decays which could probe new physics beyond
the Standard Model (SM). However, the decay isrelatively poorly measured and there have been only a
few experiments that determined the Kµ2γ branchingratio [1,2] and the structure-dependent component [3].
Other interesting observables for this decay are thecomponents of the muon polarization: PT which is
the T-odd polarization component normal to the decay
plane defined as PT = sµ · (pγ × pµ )/|pγ × pµ |, andPN -normal muon polarization (T-even) in the decay
plane which is sensitive to the values and signs of
the kaon vector FV and axial-vector FA form factors,
as shown in [4]. The contribution to PT from theStandard Model is known to be very small 10−7 [5].
The T-even transverse polarization due to final stateinteractions (FSI) can mimic the T-odd effect at the 10−3 level [6–8]. However, it can be reliably
calculated. Therefore, any non-zero value of PT above
the level of FSI or different from the FSI values,
if the experimental sensitivity to the polarization isbetter than 10−3 , would indicate new physics. Several
non-SM models predict non-zero values for PT in
Kµ2γ decay [9–11] and from two of them (SUSY
with R-parity and multi-Higgs models without natural
flavor conservation) large values of PT  10−2 are
expected.
In this Letter we present the first results on PT obtained in the analysis of the Kµ2γ data accumulated
in the KEK E246 experiment for the 1996–1998 datataking period.

2. Experiment
The measurement was performed using the detector constructed to search for T-violating muon polarization in the K + → π 0 µ+ ν (Kµ3 ) decay. The apparatus is shown in Fig. 1, and described in detailelsewhere [12–15]. A separated K + beam (π/K 6)
of 660 MeV/c was used with a typical intensity of
3.0 × 105 kaons per 0.6-s spill duration with a repetition of 3 s. A Čerenkov counter with a multiplicity trigger distinguished K + s from π + s with an efficiency of more than 99%. Kaons then were slowed

167

in a Al + BeO degrader, and stopped in an active
target made of 256 scintillating fibers located at thecenter of a 12-sector superconducting toroidal spectrometer. A charged particle (µ+ , e+ or π + ) fromthe kaon decay hit one of the twelve fiducial counters surrounding the target and was bent through oneof the spectrometer sector gaps and detected by the
TOF and µ+ counters at the entrance of the polarimeter. Tracking of charged particles was provided by
the target fibers, a scintillation ring hodoscope surrounding the target and multiwire proportional chambers at the entrance (C2) and exit (C3 and C4) ofeach magnet sector gap. The momentum resolution of
the whole tracking system was σp = 2.6 MeV/c at
205 MeV/c.
The energies and angles of the photons were
measured by the CsI photon calorimeter consistingof 768 CsI(Tl) modules [15]. The CsI(Tl) barrel has
twelve holes for the charged particle entry to thetoroidal spectrometer and covers a solid angle of about
0.75 × 4π sr. Energy resolution of σE /E = 2.7%
at 200 MeV and angular resolutions of σθ = 2.3 ◦were obtained. Good time resolution of 3.5 ns (rms)
at 100 MeV allowed us to use the fast CsI signal
at the trigger level and thereby effectively suppress
accidentals in the calorimeter.
Muons entering the polarimeter (Fig. 1(c)) weredegraded by an Al+Cu block and stopped in a stack ofpure Al plates. The stopper material (Al) was chosenbecause there is no depolarization of muons stopped
in aluminum. Positrons from µ+ → e+ ν ν̄ decays of
stopped muons were detected by 12 positron countersconsisting of three layers of plastic scintillators whichwere located azimuthally between the muon stoppers.
The time spectra of the positrons were recorded bymulti-stop TDCs.
A three-level trigger for selection of events wasused. At the first level, a signal from the kaonring in the Čerenkov detector followed by a hit
in at least one fiducial counter in coincidence withsignals from the TOF and µ+ counters of a corresponding magnet sector were required. The second level trigger required at least one hit in theCsI(Tl) calorimeter, and the third level trigger required the triple coincidence signal from the positroncounter arrays adjacent to the corresponding magnet sector within 20 µs after the first level trigger.

168

KEK-PS E246 Collaboration / Physics Letters B 562 (2003) 166–172

Fig. 1. The layout of the KEK E246 detector: (a) side view, (b) end view and (c) one sector of the polarimeter.

The T-violating asymmetry was extracted using adouble ratio as:
1 (Ncw /Nccw )fwd
AT =
(1)
−1 .
4 (Ncw /Nccw )bwd

where α is the analyzing power of the polarimeter, f
is an angular attenuation factor and β is the overall
fraction of all backgrounds.

Here, Ncw and Nccw are the sums over all 12 sectors ofcounts of clockwise (cw) and counter-clockwise (ccw)emitted positrons. Indices ‘fwd’ and ‘bwd’ denote twoclasses of events: forward events (fwd) when the angle
between the photon and the beam direction (z-axis)
was less than 70◦ and backward events (bwd) when theangle between the photon and the beam direction was
more than 110◦ . The signal values Ncw and Nccw wereextracted by integrating the positron time spectrum inthe polarimeter after subtraction of the background.
The signal integration region was chosen to be from
20 ns to 6 µs and the background fitting region for the
background extraction was from 6.0 to 19.5 µs.
The sign of AT for events with forward-going
photons is opposite to that for events with backwardgoing photons. This allows us to employ a double ratio
method which is a powerful technique for reductionof most systematic errors. Moreover, considerable
reduction of systematic effects was achieved from theazimuthal symmetry of the 12-gap detector and the useof the same positron detector as a ‘cw’ for one gap anda ‘ccw’ detector for the adjacent gap.
The value of PT is related to AT by

3. Analysis

PT =

AT
,
αf (1 − β)(2)The main problem of the Kµ2γ analysis is therejection of the predominant Kµ3 background with
only one photon (1γ ) from the π 0 decay detected by
the CsI calorimeter. The extraction of Kµ2γ events was
performed in two stages.
The first stage was mostly identical to that of
the main (Kµ3 ) E246 analysis and was used tosuppress Kµ2 , Kπ2 , Ke3 , and K + → π + π + π − ,K + → π + π 0 π 0 (Kπ3 ) background decays.
The Kπ2 and Kµ2 decays were rejected using a cuton the muon momentum pµ  190 MeV/c selecting
the Kµ2γ muons in the region of 100–190 MeV/c,
as illustrated in Fig. 2. Most of the muons from Kπ2
events, in which π + decays in-flight, were rejectedby applying the tracking cut χ 2 < 10. These events
had kinks, i.e., the tracks did not point back to thekaon vertex. The Kπ3 backgrounds were negligible
due to the combination of several suppressing factors:
acceptance of the magnet spectrometer (the endpointsof charged particle momentum spectra for both decays are quite low: 125 and 133 MeV/c, respectively),
the low probability of detecting these decays as onephoton events and, finally, most of π + s were stopped
in the Cu degrader and did not reach the polarime-KEK-PS E246 Collaboration / Physics Letters B 562 (2003) 166–172

Fig. 2. Momentum spectrum of charged particles for events with one
photon detected by the CsI calorimeter. The double-hatched region
shows the events with momentum less than 190 MeV/c.

ter. The Ke3 events were removed using a time-offlight method by imposing a geometric cut on the twodimensional spectrum of a charged particle mass calculated from time-of-flight vs. the energy deposited in
the TOF counter (Fig. 3). The background from accidental photons in the CsI(Tl) detector was suppressedusing a photon energy threshold of 50 MeV and by requiring a coincidence between a photon signal in the
CsI and a signal in the fiducial and TOF counters froma charged particle within a window of ±15 ns. Background associated with the beam was suppressed by aveto counter system surrounding the beam region.
The second stage of the analysis was aimed at
the extraction of Kµ2γ events and the rejection ofKµ3 decays. Due to the precise measurement of
the muon momentum and the photon energy anddirection, the kinematics of Kµ3 and Kµ2γ decays
can be reconstructed completely [16]. The kinematic
2 , theparameters such as missing mass squared Mmiss
angle between the muon and the photon, Θµγ , andthe neutrino momentum, pν , can be efficiently used
to suppress the Kµ3 background. The missing masssquared for 1γ events is
2
2
2
Mmiss
= Emiss− pmiss= (mK − Eµ − Eγ )2 − (pK − pµ − q)2 ,

(3)

169

Fig. 3. Rejection of Ke3 using the time-of-flight technique. The“cloud” in the bottom-left corner corresponds to positrons, theevents inside the rectangle are muons.

where q is the photon momentum. For Kµ2γ events,
the neutrino is the only missing particle, therefore,2pν = pmiss , Eν = Emiss and Mmiss= Eν2 − pν2 = 0,2of Kµ3 events is distributed overwhile the Mmissa wide range with the maximum at about 2 × 104
MeV2 /c4 . The reconstructed neutrino momentum of
1γ events from experimental and Monte Carlo (MC)data after imposing the cuts −0.7 × 104 MeV2 /c4 <2Mmiss< 1.5 × 104 MeV2 /c4 and Θµγ  90 ◦ , and accepting events with Eγ > 50 MeV is shown in Fig. 4.
The peak at pν ∼ 220 MeV/c in MC data corresponds
to Kµ2γ events. In the region with pν > 200 MeV/c,
the contamination of the Kµ3 events is estimated tobe17%. Fig. 5 shows the experimental and MC
spectra of neutrino missing mass after applying the cutpν > 200 MeV/c.
After applying the cuts pν > 200 MeV/c, −0.7 ×
2< 1.5 × 104 MeV2 /c4 and104 MeV2 /c4 < Mmiss◦Θµγ  90 , the accepted Kµ2γ events are concentrated in the Dalitz plot region, as shown in Fig. 6
where the inner bremsstrahlung (IB) component andinterference of the IB and structure-dependent withpositive-helicity photon (SD+ ) term dominate.
A good test of the quality of detected Kµ2γ eventsis the value of the normal asymmetry, AN . Its value
is proportional to the T-even muon polarization, PN ,which is the in-plane component of the muon polar-

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KEK-PS E246 Collaboration / Physics Letters B 562 (2003) 166–172

Fig. 4. Momentum spectra of missing neutrino. The black dotswith error bars show the experimental data. MC simulation: dotted
line—Kµ2γ , dashed line—Kµ3 , solid line—Kµ2γ + Kµ3 . Cuts:
2−0.7 × 104 MeV2 /c4 < Mmiss< 1.5 × 104 MeV2 /c4 , Θµγ < 90◦ .

Fig. 5. Missing mass spectra for the following cuts:pν > 200 MeV/c, Θµγ < 90◦ . The black dots with error bars show
the experimental data. MC simulation: dotted line—Kµ2γ , dashedline—Kµ3 , solid line—Kµ2γ + Kµ3 .

ization normal to the muon momentum
PN =sµ · (pµ × (pγ × pµ )).
|pµ × (pγ × pµ )|(4)Fig. 6. Contour lines of the normal polarization PN over theKµ2γ Dalitz plot. The dots represent the experimental data after2
tight cuts: −0.7 × 104 MeV2 /c4 < Mmiss< 1.0 × 104 MeV2 /c4 ,◦pν > 220 MeV/c, Θµγ  75 , Eγ > 50 MeV; x = 2Eγ /mK and
y = 2Eµ /mK .

AN can be measured if the accepted events are
separated into two classes: events where the photon
moves into the left hemisphere with respect to themedian plane of the given magnet sector and events
where the photon moves into the right hemisphere.
The values of AN for these two classes should be
the same but opposite in sign. For Kµ2γ the normal
polarization has a positive sign in the region where
the selected events are located [10], as seen from
Fig. 6, and its average value is about 0.2, while forKµ3 , the normal muon polarization calculated using
the formulae of Ref. [17] has a negative sign over thewhole Dalitz plot region, as shown in Fig. 7.
From this measurement, the value of AN for the
one-photon Kµ3 events is AN (Kµ3 ) = (−3.87 ± 0.06)
× 10−2 and for selected Kµ2γ events with tighter cuts2
< 1.0 × 104 MeV2 /c4 ,−0.7 × 104 MeV2 /c4 < Mmisspν > 220 MeV/c and Θµγ  75◦ the normal asymmetry is AN (Kµ2γ ) = (3.59 ± 0.56) × 10−2 . These
values are both non-zero and have opposite signs consistent with the expectation. That indicates a strong
signature of the correct Kµ2γ selection and the suppression of one-photon Kµ3 events.

KEK-PS E246 Collaboration / Physics Letters B 562 (2003) 166–172

Fig. 7. Contour lines of PN for the Kµ3 Dalitz plot.

In Monte Carlo simulations the polarization value
and corresponding asymmetry for Kµ2γ was obtainedusing the form factors from Chiral Perturbation Theory (ChPT) FV = −0.095, FA = −0.043 [18] andfK = 159 MeV. After applying the same cuts to the
MC data we obtained AN (Kµ2γ ) = (4.06 ± 1.14) ×
10−2 . These simulations were used to check the polarimeter sensitivity to muon polarization, i.e., thevalue of the analyzing power α, and the geometric attenuation factor f . The α value obtained from Kµ2γ
is in a good agreement with α = 0.289 ± 0.015 [19]extracted from Kµ3 MC simulation. The attenuation
factor f = 0.80 ± 0.03 was obtained.
From the experimental data collected during the
1996–1998 beam cycles, a total number of 1.14 × 105
fwd + bwd Kµ2γ events were extracted. The value ofthe transverse asymmetry measured for the collected
events is AT = (−0.099 ± 0.320(stat)) × 10−2 . The
contamination of the beam accidental backgrounds in
Kµ2γ events was estimated to be not higher than 8%.
The constant background in the polarimeter was 11–12%. Both these backgrounds only dilute the sensitivity to PT , but they do not produce any spurious T-violating asymmetry. Using MC data the backK
ground decay fractions were estimated to be βbgrµ3
K

17% and βbgrπ 2 < 0.5%. Thus, the total backgroundcontamination is β 25% which is used in the denominator of Eq. (2).

171

Using the values of AT , α, f and β given above weobtained the transverse polarization PT = (−0.57 ±1.85(stat)) × 10−2 . The mean value of transverse polarization due to the final state interactions for the
Dalitz plot region in which the selected Kµ2γ events
reside is PT (FSI) = 0.7 × 10−3 which is much lessthan the statistical error of PT . The theoretical uncertainty of the PT (FSI) value is about 15% and is connected with the uncertainties of FV and FA values. For
the final result we subtract PT (FSI) from the measured
PT value which gives PT = (−0.64 ± 1.85(stat)) ×
10−2 and as the PT (FSI) uncertainty is more than twoorders of magnitude less than the statistical error we
do not include that uncertainty in the PT error.
The main systematic error contributions to PT
come from the presence of the two large componentsof the muon in-plane polarization, PL which is parallel to the muon momentum and PN (PT  PN,L  1).
However, most of their contribution is canceled by
the azimuthal symmetry of the detector as well as by
the fwd/bwd ratio. The largest instrumental systematic errors are due to the misalignment of the polarimeter, the asymmetry of the magnetic field distribution,and the asymmetric kaon stopping distribution. The
fwd/bwd ratio efficiently reduces these contributions.
Additionally, a non-zero value of PT for the Kµ3 background would contribute to the systematic error of PT
for Kµ2γ , but that was measured in the main E246
analysis with much higher accuracy and is consistent with zero [19] PT (Kµ3 ) = (−1.12 ± 2.17(stat) ±0.90(syst)) × 10−3 . All possible sources of systematic
errors were considered in [20] for much higher statistics of 1γ Kµ3 events as well as for Kµ2 and Kπ2
events. The total systematic error of PT for Kµ3 eventswas found to be 0.9 × 10−3 [20], while for Kµ2γ weestimated the systematic error of PT to be less thansystδPT = 1.0 × 10−3 . The absence of significant systematic errors (comparable to the statistical error) is
confirmed by measuring the dependence of the transverse asymmetry AT on the magnet sector number as
seen in Fig. 8.

4. Results
We have performed the first measurement of the
T-violating muon polarization in Kµ2γ decay basedon about 1.14 × 105 good fwd + bwd Kµ2γ events

172

KEK-PS E246 Collaboration / Physics Letters B 562 (2003) 166–172

NSERC and IPP, and by the TRIUMF infrastructure
support provided under its NRC contribution; in the
USA by the NSF. The authors gratefully acknowledge
the support received from the INR and KEK staff.

References[1] Y. Akiba, et al., Phys. Rev. D 32 (1985) 2911.
[2] V.S. Demidov, et al., Sov. J. Nucl. Phys. 52 (1990) 1006.

Fig. 8. The dependence of the transverse asymmetry (AT ) on
the sector number. The rightmost point represents the sum of theasymmetries over all 12 sectors. The error bars show the statisticalerrors.

collected in 1996–1998. These events are locatedin the Dalitz plot region where the IB and INT+
components dominate. The value obtained for PT is(−0.64 ±1.85(stat)±0.10(syst))×10−2 including thetheoretical background from final state interactions. At
the present level of experimental sensitivity this value
is consistent with no T-violation in this decay.

Acknowledgements
This work has been supported in Japan by aGrant-In-Aid from the Ministry of Education, Science,Sports and Culture, and by JSPS; in Russia by the
Ministry of Science and Technology, and by the
Russian Foundation for Basic Research; in Canada by[3] S.C. Adler, et al., Phys. Rev. Lett. 85 (2000) 2256.
[4] F.L. Bezrukov, D.S. Gorbunov, Yu.G. Kudenko, hepph/0302106.
[5] I.I. Bigi, A.I. Sanda, CP Violation, Cambridge Univ. Press,
New York, 2000;
E. Golowich, G. Valencia, Phys. Rev. D 40 (1989) 112.
[6] V.P. Efrosinin, Y.G. Kudenko, Phys. At. Nucl. 62 (1999) 987,
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[7] V.V. Braguta, A.E. Chalov, A.A. Likhoded, Phys. Rev. D 66
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[8] R.N. Rogalev, Phys. Lett. B 521 (2001) 243.
[9] M. Kobayashi, T.-T. Lin, Y. Okada, Prog. Theor. Phys. 95(1995) 361.
[10] C.H. Chen, C.Q. Geng, C.C. Lih, Phys. Rev. D 56 (1997) 6856.
[11] G.-H. Wu, J.N. Ng, Phys. Rev. D 55 (1997) 2806.
[12] M.P. Grigorev, et al., Instrum. Exp. Tech. 41 (1998) 803, Prib.
Tekh. Eksp. 41 (1998) 65 (in Russian).
[13] A.P. Ivashkin, et al., Nucl. Instrum. Methods A 394 (1997) 321.
[14] M. Abe, et al., Nucl. Instrum. Methods, in press, hepex/0302001.
[15] A.D. Dementjev, et al., Nucl. Instrum. Methods A 440 (2000)
151.
[16] Y.G. Kudenko, Phys. At. Nucl. 65 (2002) 244, Yad. Fiz. 65(2002) 269 (in Russian), hep-ex/0103007.
[17] N. Cabibbo, A. Maksymowicz, Phys. Lett. 9 (1964) 352;
N. Cabibbo, A. Maksymowicz, Phys. Lett. 11 (1964) 360,
(Erratum);N. Cabibbo, A. Maksymowicz, Phys. Lett. 14 (1965) 72,(Erratum).
[18] J. Bijnens, G. Ecker, J. Gasser, Nucl. Phys. B 396 (1993) 81.
[19] M. Abe, et al., hep-ex/0211049.
[20] M. Abe, et al., KEK-E246 Collaboration, Phys. Rev. Lett. 83(1999) 4253.

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Nuclear Instruments and Methods in Physics Research A 440 (2000) 151}171

CsI(Tl) photon detector with PIN photodiode readout fora K T-violation experimentl3
D.V. Dementyev!, M.P. Grigoriev!, A.P. Ivashkin!, M.M. Khabibullin!,
A.N. Khotyantsev!, Yu.G. Kudenko!,*, O.V. Mineev!, M. Aoki", J. Imazato",
Y. Kuno", T. Baker#, M. Blecher$, P. Depommier%, M. Hasino!&,Y. Igarashi1,' , T. Ikeda2,' , J.A. Macdonald), C.R. Mindas*, C. Rangacharyulu#,
S. Shimizu3,+ , T. Yokoi4,,!Institute for Nuclear Research RAS, 60-th October Revol. Pr. 7a, 117312 Moscow, Russia"Institute of Particle and Nuclear Studies, High Energy Accelerator Research Organization (KEK),
Tsukuba, Ibaraki 305-0801, Japan#Department of Physics, University of Saskatchewan, Saskatchewan S7N 0W0, Canada$Department of Physics, Virginia Polytechnic Institute and State University, VA 24061-0435, USA%Laboratoire de Physique Nucleaire, University de Montreal, Montreal H3C 3J7, Canada&Department of Physics, University of British Columbia, British Columbia V6T 1Z1, Canada'University of Tsukuba, Institute of Physics, Tsukuba, Ibaraki 305-0006, Japan)TRIUMF, British Columbia V6T 2A3, Canada*Princeton University, Princeton, NJ 08544,USA+Tokyo Institute of Technology, Tokyo 152-0033, Japan,University of Tokyo, Tokyo 113-0033, Japan
Received 31 March 1999; accepted 10 July 1999

Abstract
The p0 detector constructed for the T-violation search experiment E246 at KEK is described. It consists of 768 CsI(Tl)crystals with PIN photodiode readout. The crystals are assembled in a tight barrel with holes for muons entering themagnetic spectrometer. The design and performance of the calorimeter are presented. The average light yield of
11 000 p.e./MeV and average equivalent noise level of 63 keV (bench test) were obtained for the CsI(Tl) modules used in
the calorimeter. The coherent noise was found to be about 11 keV per a module. A readout electronics has been
speci"cally developed for the high counting rate environment of the experiment. A p0 invariant mass resolution of* Corresponding author. Fax: #7(095)334-01-84.
E-mail address: kudenko@wocup.inr.troitsk.ru (Yu.G. Kudenko)
1 Present address: Institute of Particle and Nuclear Studies, High Energy accelerator Research Organization (KEK), Tsukuba, Ibaraki
305-0801, Japan.
2 Present address: Institute of Physical and Chemical Research, Hirosawa 2-1, 351-0106, Japan.
3 Present address: Department of Physics, Faculty of Science, Osaka University, Osaka 560-0043, Japan.
4 Present address: Tanashi Branch, High Energy Accelerator Research Organization, Tokyo 188-0002, Japan.
0168-9002/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 8 9 0 - 6

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D.V. Dementyev et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 151}171

9.9 MeV (p) was obtained for the whole detector, including regions around the muon holes. A time resolution of 3.8 ns(rms) was achieved in the photon energy range of 10}200 MeV. ( 2000 Elsevier Science B.V. All rights reserved.
PACS: 29.40.Vj; 29.40.Mc
Keywords: CsI(Tl) detector; PIN photodiode readout

1. Introduction
1.1. Principles of the experiment E246
The main goal of the experiment E246 at KEK
12 GeV proton synchrotron is a search for T-violating transverse muon polarization (PT ) in the
decay K`Pp0l`m(Kl3 ) [1]. Since no CP-violation appears in this decay in the standard model
which predicts PT of about 10~6, this experiment isable to search for additional or alternative sourcesof CP-violation postulated in various extensions tothe standard model. In particular it is sensitive to
the CP-violation in the three-Higgs doublet modeland the leptoquark model [2,3], where predicted

PT to be as high as few]10~3. The experiment was
designed to reach a sensitivity to PT at the level ofabout 10~3.
In contrast to all previous experiments whichwere performed with in-#ight kaon decay, the E246
uses stopped kaons. This stopped beam method
allows us to perform a measurement covering almost all phase space of decay particles. Also isotropic decay of stopped kaons reduces a potential
source for a systematic error associated with thekaon beam asymmetries in determining the decay
plane of the Kl3 event.
In this experiment, the Kl3 decay is identi"ed bydetecting the p0 as well as the l` from the decay.
The layout of the E246 setup is shown in Fig. 1.

Fig. 1. The layout of the E246 setup.

D.V. Dementyev et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 151}171

A 660 MeV/c kaon beam is slowed down in a BeO
degrader and stopped in an active target, made of2565]5 mm2 scintillating "bers. The energy anddirection of the p0 from the Kl3 decay are measured by a segmented CsI(Tl) photon detector installed in the central region of the toroidal magnet.
A muon from the Kl3 decay at rest is momentumanalyzed by one of the 12 magnet gaps of thesuperconducting toroidal spectrometer using
tracking performed by the stopping target, a scintillating ring hodoscope surrounding the target, a cylindrical drift chamber and three MWPCs. The
muon exiting the spectrometer is stopped ina polarimeter in which the decay positron asymmetry AT is measured in order to obtain PT . The
polarimeter consists of 12 azimuthally arranged Al
stoppers, aligned with the magnet gaps, with scintillator counter system located between the stoppers.
A positron from the decay of l`Pe`mm tends to be
emitted in the same direction as the muon spin, andis detected by plastic scintillator counters. In thisdetector AT appears as a di!erence in the counting
rate between clockwise (cw) and counter-clockwise(ccw) emitted positrons. Summing of the cw (N#8 )and ccw (N##8 ) positron counts over all 12 sectors
PT is derived from
1 N !N
#8
##8 .
PT "
a f N #N#8##8where a is the analyzing power of the polarimeterand f is an attenuation factor which re#ects the factthat the decay plane of the Kl3 event determinedfrom the l` and p0 momenta is not parallel to themedian plane of the gap in which the muon momentum is measured.
The Kl3 events are separated into two classes:
events with the pion moving along the kaon beam
(forward direction) and those where the pion moves
in the backward direction. The sign of PT for theforward-going p0 events is opposite to that for
backward-going p0 events. A double ratio between
the forward- and backward-going p0 events cancels
out most spurious instrumental sources of false
polarization, which are likely independent of thep0 direction. This method is an important feature of
E246 experiment, crucial for suppression of most
systematic errors. This paper describes the design

153

and performance of the CsI(Tl) detector speci"callyconstructed for this experiment.
1.2. Requirements for the photon detector
CompletekinematicreconstructionofKl3 events using high resolution p0 measurement isalso needed for clear separation of the Kl3 decay
from other K` decay modes and for a measurement of the in-plane normal muon polarization.
For these reasons, the acceptance for p0's needed toful"ll following several conditions:(1) The photon detector must provide near 4p
photon coverage while allowing a maximum
acceptance for Kl3 events by optimizing thesize of the 12 holes for muons to enter a sectorof the toroidal magnet.
(2) The detector parameters must be optimized todetect photons over an energy range fromabout 10 MeV to 250 MeV of the Kl3 decay
with high energy resolution and good position
resolution in order to reconstruct the p0 kinematics.
(3) Good timing is needed for e$cient backgroundrejection and neutral pion identi"cation. It also
allow us to reject the copious Kl2 events at thetrigger level.
(4) Limited space in the central region of the magnet imposes geometrical restrictions on the detector size. A fringing magnetic "eld makes
detection of the scintillation light with photomultipliers impractical.
(5) A forward/backward symmetric shape andhigh precision in assembly.
In addition to the physics requirements of theexperiment, the environment of a stopping kaon
beam was taken into consideration for the design ofthe photon detector. The intensity of about2]105/s of 660 MeV/c kaon beam with stopping
e$ciency of about 40% must be compatible withthe photon detector. Moreover, a higher intensityof pions (the p/K-ratio is about 6) and an asymmetrical pion halo which contains about 30% of pions
are essential factors which must also be taken into
account.
Basing on these requirements we havechosen thallium-doped CsI crystals with a direct

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D.V. Dementyev et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 151}171

photodiode readout. In order to handle the background rate, and achieve good time and energy
resolutions the electronics for the calorimeter was
specially designed.
The design of the photon detector is given in
Section 2. In Section 3 a detailed description of theCsI(Tl) modules with PIN-diode readout is presented. Detector electronics and stabilization
system are described in Sections 4 and 5, respectively. In Section 6 we present the performance of thedetector in the experiment.

2. Overall design of the detector
The photon detector consists of 768 CsI(Tl) crystals each with a PIN photodiode readout. They
form a barrel structure that covers about 75% of 4p
with 12 symmetrically spaced holes for l`'s going
into the magnetic spectrometer (Fig. 2). The detector is capable detecting photons emerging fromthe target in the polar angle range from 153 to 1653
and in 2p of the azimuthal angle except for themuon holes. The muon hole sizes and azimuthal
size of the crystals between the holes were optimized by a Monte-Carlo simulation to reach thehighest detection e$ciency of the Kl3 decay. In thesimulation, a realistic "nite size distribution ofFig. 2. The CsI(Tl) p0 detector.

K` stopping in the target was assumed. The azimuthal aperture of the holes was just determined to
optimize the fraction of l` accepted by the spectrometer gap from this K` stopping distribution.
The maximum acceptance of the set-up to theKl3 decay of stopped kaons is about 7%, if themuon hole size is about $22.53 relative to central
axis of the toroidal magnet, as shown in Fig. 3. The
p0 detection e$ciency for such a geometry is about
70% while the probability for muon to passthrough the toroidal magnet and being detected by
MWPCs is 10%.
The barrel inner radius of 20 cm was chosen tohave a space around the active target enough forthe installation of a cylindrical drift chamber used
for muon tracking. The outer radius of the barrel of
52 cm was limited by the available space in thecentral region of the toroidal magnet. The crystal
length of 250 mm (13.5 r.l.) was chosen in order toleave about 4 cm of space available radially tomount a PIN-diode and preampli"er. A MonteCarlo simulation for such a crystal length showsthat #uctuations of the longitudinal energy

Fig. 3. Monte-Carlo optimization of the muon holes.

D.V. Dementyev et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 151}171

155

leakages are comparable with #uctuations of thetransverse leakages or energies absorbed in thepassive materials in the c energy range of below
250 MeV (see Fig. 4).
The optically isolated CsI modules point towards the center of the active target. An individual
crystal covers 7.53 in both polar and azimuthal
angles, except for 48 crystals near to the beam axis,
where the azimuthal angle was doubled to 153, andthe muon hole region. The crystals are of 10 di!erent dimensions and are shaped like pyramidal sectors with trapezoidal basis as shown in Table 1. The
average transverse dimensions of the segments areclose to 3]3 cm2 for front end and 6]6 cm2 forrear end. The crystals installed in between themuon holes have a narrower front end than standard crystals. The choice of crystal dimensions wasa trade o! between the spatial resolution and thenumber of channels (cost of the detector). The speci"cation of the crystal barrel calorimeter and main
parameters are presented in Table 2.

Fig. 4. Energy resolution as a function of photon energy forcrystal length of 20, 25 and 30 cm.

Table 1
Dimensions of CsI(Tl) crystals in mm
Type number!

A

B

H

a

b

h

1
2
3
4
5
6
7
8
9
10

61.14
59.76
57.42
53.30
50.92
48.10
42.26
35.77
28.52
42.15

61.98
62.31
61.71
56.87
55.61
54.01
49.16
43.69
37.51
62.21

57.76
58.46
59.69
61.58
64.35
68.37
70.05
72.64
76.42
82.42

5.63
5.64
5.66
24.86
24.87
24.88
22.27
19.35
15.96
25.08

6.00
6.77
7.60
26.53
27.17
27.95
25.94
23.70
21.09
37.12

24.99
25.68
26.92
28.81
31.58
35.60
37.28
39.87
43.65
49.46

!See Fig. 2 for numbering.

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D.V. Dementyev et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 151}171

Table 2
Speci"cation of the CsI calorimeter
CrystalSegmentation
Number of crystals
Length of a crystal
Readout
Total crystal weight
Inner diameter
Outer diameter
Detector length
Solid angle coverage

CsI(Tl)*h"*/"7.53
768
25 cm (13.5X )0
one PIN diode per crystal
1700 kg
40 cm
100 cm
141 cm75% of 4p

The calorimeter support structure is designed tobear the considerable weight of the CsI modules(1700 kg) while minimizing the amount of material
in front of the active volume to avoid substantial
conversions of photons. The modules are stacked ina barrel with no support material between them.
Some details of the holder mechanics are shown in
Fig. 5. The longitudinal and azimuthal aluminumribs form a rigid outer cylindrical lattice, coveredby 3 mm aluminum skin panels and capped by disk

plates at each end. The self-supporting barrel geometry provides a minimum load on the inner aluminum cylindrical holder, which could be reduced
to 3 mm in thickness. To keep the crystals ina place, especially during assembling, slotted screws
press the modules from the back side throughthreaded holes made in the structure elements. The
detector was assembled on a specially designed jig,
which provided for the rotation of the detector
during crystal installing and the lift to insert thedetector into the magnet. Gaps between the
crystals, which degrade energy resolution and reduce the e$ciency, are kept to a minimum by
precise installation of the modules. Accurate positions were provided by small reference "xtures,
welded on the outer surface of the inner holder.
A small aligning tip is glued on the forward face ofthe module to facilitate the assembling procedureby aiming this tip toward this corresponding reference "xture. The mechanical design allows for removal of units for repair. The detector is mounted
on rails installed inside the toroidal magnet andcan be rolled into and out of the magnet for servicing purposes. The rails are electrically isolated

Fig. 5. A quadrant of the detector support structure.

D.V. Dementyev et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 151}171

from the detector frame to avoid induced noise andground loops.
The overall gain stability of the calorimeter ismonitored by a Xe-lamp light pulser system with
optical "bers which distribute the light from thelamp to the crystals. Since the CsI(Tl) crystals areslightly hygroscopic, #owing cold dry air is used tokeep humidity inside the barrel at the level of below
10%. This air #ow serves also to dissipate heat ofabout 200 W inside the barrel during operation
keeping the maximum temperature to (25$1)3C.
Air is dried and pumped into the detector byrecycling it from the setup room.

3. CsI(Tl) modules
3.1. Properties of cesium iodide
A magnetic "eld and limited space in the central
region of the toroidal magnet stipulate using silicon
photodiodes to detect the scintillation light. Thenthe choice of thallium-doped cesium iodide CsI(Tl)is a natural consequence from the point of view ofspectral matching and light yield. The spectral
emission of NaI(Tl) and fast crystals (pure CsI,
BaF ) makes them inadequate to couple with the2photodiodes. Another scintillating material BGO,
which is widely used in particle physics experiments, has the relatively low light output of around
10% of CsI(Tl), although BGO has a number ofdesired characteristics, such as a very short radiation length of 1.13 cm, a decay time of 300 ns, anda rather good matching to silicon diode. If we wereto use these materials we could face a problem ofdetecting low-energy photons due to the increased
in#uence of electronic noise. CsI doped with Na isclose to CsI(Tl) in terms of the number of photons
produced per MeV of energy loss in the crystal.
However the spectral emission of CsI(Na) is also
shifted in the short wave region.
Compared with NaI(Tl), CsI(Tl) scintillator isonly slightly hygroscopic and has better mechanical properties that greatly simplify the construction of a mechanical support. CsI(Tl) can behandled in a normal atmospheric humidity forhours although for long term it must be kept ina dry environment. A humidity below 20% is con-157

sidered to be safe for the crystals. The radiation
length of 1.86 cm is shorter than for NaI(Tl)(2.59 cm) and thus a detector made of CsI(Tl) crystals requires one-third of the volume of a NaI
detector of the comparable e$ciency and is thus
well suited to our case of con"ned geometry. The
higher light output of the NaI(Tl) measured withphotomultipliers is o!set by better spectral matching of silicon photodiodes to CsI(Tl) emission
spectrum. The long decay time of 900 ns appears
to be the most serious disadvantage of thescintillator, however using a charge-sensitive
preampli"er and a shaping ampli"er with longshaping times to minimize photodiode noise`hidesa this problem.
The long decay time causes dead time of a crystal
and reduces the timing resolution that could becritical for an experiment with high counting rates.
However, for the expected rate in E246, the "nesegmentation of the detector allows us to use thethallium doped CsI.
3.2. PIN photodiode readout
In general, PIN photodiodes (PD) have the following advantages: (1) immunity from magnetic"elds; (2) matched spectral response to CsI(Tl) witha quantum e$ciency of 75% at the peak wave
length of 550 nm; (3) compact size; (4) low operating
voltage; (5) inherent stability and long lifetime.
However, PD readout also leads to complicationsof the electronics connected with: (1) electronicnoise of the PD/preampli"er system, which dominates the energy resolution at low energies; (2)a problem in accepting high counting rate; (3) poor
timing resolution. Since a photodiode has no gaina low-noise charge-sensitive preampli"er is required. The photodiode performance, such as thejunction capacitance and leakage current, dominate the overall noise in the case of the large-area
photodiodes.
A study was carried out to "nd the bestCsI(Tl)}PD con"guration for the photon detector[4]. Light yield measurements clearly demonstrated that the direct coupling of a PD to the
crystal provides a much higher yield of photoelectrons in a comparison with using wavelength
shifter (WLS) plates to concentrate the scintillating

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D.V. Dementyev et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 151}171

Table 3
Performance of di!erent CsI(Tl) crystal/photodiode assemblies
PD type size(mm2) ) thickness

S
PD
(mm2)

p.e. yield ENC(N /MeV) (N )
e
e

ENL
(keV)

10]10 (300 lm)
18]18 (300 lm)
18]18 (500 lm)
28]28 (300 lm)
28]28 (500 lm)

100
324
324
784
784

3300
8800
9100
15 90015 900

380
540
670
1150
840

116
61
75
72
53

3.4]30 with
R-3 WLS plate

102

2800

370

140

light on a PD. The photoelectron yield fora 28]28 mm2 sensitive area PD coupled witha large size CsI(Tl) crystal (see Table 3) was 5.6
times greater than the largest value of a light yieldobtained with Y-8 and R-35 WLS plates of di!erent
thicknesses and #uor concentrations. The resultspresented in Table 3 were obtained under thefollowing conditions: (a) CsI(Tl) crystal was30]30 mm2 in the front, 60]60 mm2 in the backside, and 250 mm in length; (b) all of the crystal
surfaces were polished, the re#ector used was
300 lm Millipore "lter6 sprayed with Kodak
paint7 (c) the 4 mm R-3 WLS plate totally covered
the rear surface of the crystal; (d) an optical grease
was used as an interface between the crystal and
PD and the WLS plate; (e) shaping time constantwas 2 ls. All tested photodiodes were produced byHamamatsu.
The diodes are operated with a reverse bias voltage which lowers the junction capacitance andraises leakage current so some optimum value must
be chosen to minimize the noise. The measurementsof the noise charge in the bias range of 20}130 V
revealed that the noise is practically constant from40 to 100 V for S3204 and S3584 diodes.8 Long-

term reliability argues for a low bias so we set thebias at 45 V taking into account some voltage drop
due to the leakage current at a "lter resistor ina preampli"er. S3584-05 (28]28 mm2, 500 lm wafer) provided the best signal-to-noise ratio and also
reduced the pick-up noise interference due to large
output signal. In the actual p0 detector, however,they are used only for the 48 largest crystals due tocost considerations.
Although WLS readout gives less light output
and a higher equivalent noise level (ENL), the socalled nuclear counter e!ect seems to be a strong
argument for using the WLS instead of a directly
coupled PIN diode with a large sensitive area. This
e!ect occurs when a minimum ionizing particle
from a shower leakage creates up to 4.2]104 e}hpairs in a Si diode of 300 lm thickness, that corresponds an energy deposit of 4 MeV in the CsI
crystal. The e!ect was investigated using an electromagnetic shower simulation program to estimate
the rear leakage of e~(e`) in the 25 cm length
crystal for the incident c energy range of50}200 MeV. Taking into account that a PIN
diode covers only part of the crystal rear surface, we
concluded that the probability of the nuclear
counter e!ect (e~(e`) leakage event) at 100 MeV
is below 3]10~3 and 1.2]10~3 for the
28]28 mm2 PD and 18]18 mm2 PD, respectively. Low energy shower photons ((5 MeV) escapea crystal with a higher probability and thus increase the e!ect. In Ref. [5] the nuclear counter
e!ect was measured under a beam using 30 cm
length CsI(Tl) crystals with 20]20 mm2 PD
readout. The observed hit rate of the PD was foundto have a negligible e!ect on the performance ofa crystal calorimeter in the energy range below
1 GeV. Even when the electromagnetic shower escapes through the photodiode, the e!ect is relatively small because of the very high light yield of theCsI(Tl) crystal.
3.3. Optimization of the CsI(Tl) module parameters

5 From Kuraray Co. Ltd., Tokyo, 104 Japan.
6 From Millipore corporation, VGVHP00010.
7 From Eastman Kodak Co. Kodak white re#ectance coating(Chemical d6080).
8 Hamamatsu S3584-05 (28]28 mm2 sensitive area,
C "200 pF, I (100 nA) and S3204-03 (18]18 mm2,
PD
D
C "140 pF, I (20 nA).
PD
D

3.3.1. Crystal light output uniformity
The CsI(Tl) crystals were manufactured by
Monocrystalreactive factory (Kharkov, Ukraine).
The Tl concentration in the crystal was chosen to
be 0.06}0.10% (molar fraction), since theD.V. Dementyev et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 151}171

luminescence intensity of the CsI(Tl) is nearly constant at the concentrations of Tl more than 0.06%[6]. A study of the light collection with surfaces ofdi!erent quality for a truncated pyramid shape
crystal shows that the best light yield is achieved
when all sides, including both bases, are "nelypolished. However, the longitudinal light output
gradient of a totally polished crystal is strongly
negative: light yield at the point nearest to a PD
was higher by 7}11% than at the far end. To obtain#at dependence and to optimize the spectrometriccharacteristics of the crystals a special treatment
was necessary. Several methods were tested to reduce the spatial nonuniformity of the totally
polished crystals. One way was to cover a part ofthe crystal surface with black tape. In this case, thenonuniformity can be reduced to a level of 2}3%,
but the light yield drops by 25% as well. The more
appropriate way is to sand a part of the crystal sidesurface near the photodiode. The matted part of
4 cm length improves the light collection
nonuniformity to 1}2% at the expense of 5}10%
reduction in the light yield, on average. The
nonuniformity was inspected with a 22Na c-sourceby measuring pulse height for 6 positions along the
crystal axis for all the crystals used in the p0 detector. The data were approximated with a straightline and the longitudinal gradient G was comz
puted from the slope of this line, as the relativechange in light yield from the front to the rear endof the crystal. A module is considered to satisfy theselection criteria if the measurement results for all
6 points are within $2.5% deviation from theaverage value, i.e. G must not exceed 5%. In Fig. 6z
the distribution of G for all modules (includingzspare ones) is shown. Only 3% of the preparedmodules do not satisfy the nonuniformity criteria
and were rejected or sanded again.
3.3.2. Choice of reyector
The number of photons collected on a PD depends strongly on the optical characteristics of there#ector. Several widely used di!use re#ectors werestudied. A photodiode with a sensitive area of18]18 mm2 and a crystal with the front size of30]30 mm2 and rear size of 60]60 mm2 anda length of 250 mm were utilized in this test. The
results are shown in Table 4. The highest light yield

159

Fig. 6. Distribution of the light yield gradient G , relative lightz
yield di!erence between front and rear ends of the crystal, for all
799 prepared modules, obtained from 22Na c-source scanningalong the crystal axis. Positive gradient means larger yield near
the rear end.

was obtained using two layers, each 100}120 lm
thick, of the Millipore GSWP00010 di!use white"lter paper. According to data from Ref. [7],the re#ection coe$cient (in the CsI(Tl) emission
Table 4
Light yield output of a CsI(Tl) crystal with di!erent re#ectors
measured with a 22Na source
Re#ector typeLight yield (p.e./MeV)

3 layers of Te#on tape,
150 lm thickness

5700

Millipore "lter VGVHP00010,
2 layers

9000

Millipore "lter VGVHP00010,
2 layers#Kodak paint!

9500

Millipore "lter GSWP00010,
1 layer,

9700

Millipore "lter GSWP00010,
2 layers

11500

Millipore "lter GSWP00010,
2 layers # Kodak paint

11000!From Eastman Kodak Co. Kodak white re#ectance coating(Chemical d6080).

160

D.V. Dementyev et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 151}171

spectral range) for two layers of the Millipore "lteris equal to 98%, while the value for the Te#on was85}90%. Re#ector quality is especially important
for a large crystal where the scintillation light suffers multiple re#ections before being detected. Polishing the crystal surface increases the light yield
almost twice compared to the module with sanded
surface, where a signi"cant number of photons
escaped instead of being re#ected.
3.3.3. Optical couplant
A moderate power dissipation of 0.25 W pera single preampli"er (200 W for the whole detector)
causes a temperature gradient of 83C over the ambient air temperature in a con"ned space arounda photodiode. Relatively small heat cycles may leadto a degradation of the mechanical and opticalcontact between a PD and the crystal surface forhard optical glues. To "nd an appropriate couplantthe following tests were applied: a crystal witha glued PD was heated to 603C and then cooled
down to room temperature. After a heating cycle
the light output was measured. A crystal underwent
up to 7 heating cycles to see the resultant light yield
drop. As seen in Fig. 7, the hard epoxy glues NE
581 (NE Technology, Scotland) and Eccobond 24(Grace Co., Japan) did not provide the stablecontact with the well polished CsI surface in theunstable temperature environment. The third
tested epoxy resin Epotec 301-2 (Epoxy Technology, USA) reduced the light yield by 35% after
a single heating cycle. Elastic silicone glues KE-103(Shin-Etsu Chemical, Japan) and SE7779 showedno degradation and appeared to be the most appropriate couplants to provide the crystal}PD interface. SE777 has been chosen because of its fastercuring time. It is a colorless single component glue,easy to employ. After three years of detector operation no problem with the crystal}PD coupling havebeen encountered.
3.4. The CsI(Tl) module assembly
The crystal surfaces were polished using a mix of
distilled water and "ne Al O powder to the speci2 3
9 Toray Dow Corning Silicone, Japan.

Fig. 7. The dependence of a light output of the crystal}PD
detector on the number of heating cycles for di!erent opticalglues tested. One cycle includes a heating to 603C and cooling
down to the room temperature. Epotec which light yield dropped by 35% after a single cycle is not shown in this "gure.

"ed dimension with a precision of about 150 lm
and then partially sanded to control the light collection uniformity. Each prepared crystal was
wrapped with two layers of di!use Millipore "lter
GSWP00010 (120 lm thick) and enclosed in analuminum can of 0.1 mm thickness. The can provides mechanical protection, optical isolationand strong suppression of electromagnetic noiseinterference to the PD. The outside of the canwas painted with a black paint to electricallyisolate the modules in the stack in order to minimize correlated noise. One PIN photodiode S3204-03 (18]18 mm2, 300 lm wafer)or S3584-05 (28]28 mm2, 500 lm wafer) for
the 48 largest crystals was glued directly to the rearface of the crystal using the SE777 silicone glue.
Right behind the PD a preampli"er was mounted
into an aluminum frame as shown in Fig. 8. An
optical "ber from the Xe-lamp monitoring system
was inserted through a hole in the frame and "xedby a plastic screw. The re#ector paper covers theentire crystal surface except the area under the PD
and the "ber.

D.V. Dementyev et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 151}171

Fig. 8. View of the CsI(Tl) module.

3.5. Light yield and timing
The total number of CsI(Tl) crystals manufactured is about 850. Of these, 799 modules wereselected for use in the detector after preliminary
tests, including spares. The distribution of average
light yield, obtained from the 22Na c-source tests, isshown in Fig. 9(a). Measurements were done witha shaping time of 1 ls. Almost all the preparedsegments have light yield more than 8000 p.e./MeV
with the average value of about 11 000 p.e./MeV for
S3204-03. The small peak in the region of
16 000}20 000 p.e./MeV corresponds to the 48
modules with S3584-05.
According to Ref. [8] the absolute light yield ofCsI(Tl) is around 52 000 photons/MeV. Taking
80% as the average quantum e$ciency of a photodiode for CsI(Tl) emission spectrum we can roughly
estimate the light collection e$ciency in the mod-161

ules to be approximately 26% for the 18]18 mm2
PD and 43% for the 28]28 mm2 photodiodes.
The energy resolution of the modules at
E "1.27 MeV was 13}14% (FWHM). The averc
age equivalent noise level (ENL) of 63 keV was
obtained for all crystals as shown in Fig. 9(b). It isa factor of 4}8 lower than the ENL reported fromlarge CsI calorimeters with a PIN diode readout[9,10].
Cosmic rays were employed to measure the time
resolution for a single typical crystal in the appropriate energy range. A plastic scintillator(100]200 mm) provided the start pulse, while thestop was generated by the CsI(Tl) module. The CsI
pulse, integrated in a preampli"er, was shaped bya timing "lter ampli"er and sent to a constantfraction discriminator. The di!erential time
resolutions were measured within several energy
windows, and the results are listed in Table 5.
3.6. Radiation damage
Radiation damage in a CsI(Tl) crystal leads tothe formation of color centers that reduce the lighttransmission, and also destroys the luminescentcenters in the material. The former is dependent
on the purity of material used for crystal growth.
Radiation hardness is inconsistent and very much

Fig. 9. (a) Distribution of light yield. The shadowed region of 16 000}20 000 p.e./MeV corresponds to the modules which have a PD
with 28]28 mm2 sensitive area. (b) Distribution of equivalent noise level. Both histograms are plotted for prepared 799 modules.

162

D.V. Dementyev et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 151}171

Table 5Time resolution of one module obtained in the cosmic ray test
within di!erent energy windows.
Energy window (MeV)

FWHM (ns)

5}9
9}13
15}19
24}28
32}36
40}44
49}53
Integral (5}100)

59
40
25.3
16.7
13.8
11.0
9.6
12.6

depends on crystal quality and the growth method.
To check the radiation hardness a typical CsI(Tl)
module with a glued PD was exposed to a 550 MeV
pion beam for 10 days. In front of the crystal a fast
plastic counter was used to count the beam particles.
Taking into account the counter and crystal dimensions and average energy deposit in the crystal, theabsorbed dose was roughly 100 rad. The "rstmeasurement of the light yield change was done
2 days after the beam exposure, and resulted in:(1) Photoyield drops by 23%.
(2) Light output nonuniformity deteriorated fromless than 1% to 4%.
A second radiation test was performed two weeks
later. The dose received by the same crystal during
this test was 27 rad. No change in the light yield

was observed after the second test. The result isclose to one reported in Ref. [11], where the photoyield of the CsI(Tl) crystal with PD readout dropped by 13% after a 50 rad dose. Radiation resistant
crystals of similar size with PD readout, describedin Ref. [12] had reduced light output by 5% after
100 rad, however their light yield before irradiation was 2 times less (5000 p.e./MeV) than for our
modules.
The radiation #ux in the E246 experiment is not
su$cient to cause any signi"cant radiation damage
for the CsI calorimeter, although the crystals
located close to the beam axis are exposed toa beam halo. For these crystals the absorbed dose isestimated to be around 100 rad (upper level) after
3 years of operation.

4. The electronics for the calorimeter
A detailed description of the electronics for theCsI detector is given in Ref. [13]. The systemfor each channel (Fig. 10) consists of a low-noise
preampli"er mounted directly behind the crystal,the main ampli"er with low-gain, high-gain andtiming outputs, a constant-fraction discriminator(CFD), a common stop TDC (0.7 ns resolution),a time digitizer (TD) and a peak-sensitive ADC.
The 32-channel FASTBUS TD module is based ona switched capacitor array integrated circuit, whichallows waveform recording to be performed with
12-bit accuracy at a sampling rate of 10 MHz [14].

Fig. 10. Electronics block diagram for the CsI detector.

D.V. Dementyev et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 151}171

163

Fig. 11. Preampli"er schematic.

In general, the electronics for the photon detector
must provide high energy resolution over a wide
range of 5}300 MeV and maintain the resolution
under a counting rate up to 30 kHz per crystal.
Electrical noise is the main factor which deteriorates the resolution in the low-energy region((10 MeV). Taking into account that an electron}photon shower spreads over several crystals,
the noise charge is summed by an algorithm which
depends on the origin of the noise. Thermal andcurrent shot noise are added quadratically, while
pick-up noise contributes linearly in the total noise.
For the optimum signal-to-noise ratio the signal
should be processed by a pulse-shaping circuit
which, however, has to constrain the pulse toa "nite duration in order to avoid any pileup. In
order to achieve good energy resolution in thephoton detector, a low-noise preampli"er andshaping ampli"er have been designed and tested.
A charge-sensitive preampli"er (Fig. 11) has
been developed for our purpose by Clear Pulse
Co.,10 basing on a commercial product CS507. It is
assembled with two hybrid ICs (a charge-sensitive
preamp and a bu!er ampli"er) into a compact
Al-box of 33]40]20 mm3, thereby minimizing
the possibility of noise pickup. To keep a small sizeand save space, no connector was used in the pre-10 Clear Pulse Co., Ohta-ku, Tokyo 143, Japan.

ampli"er. A 8-wire shielded cable was soldered directly inside the preamp case and drawn out of thedetector to a distributor board mounted on thefront and rear end plates of the barrel crystal
holder. The cable lengths vary from 250 to 800 mm,
depending on the crystal position in the barrel. The
preampli"er features an integration time constantof 660 ls (330 M)]2 pF). The feedback gain was
optimized taking into account the average energy
deposit and anticipated maximum counting rate so
as not to reach the saturation. A pole-zero cancelation network (PZC) shortens the long fall time of
the integrated pulse to 49 ls just before the output
ampli"er stage (gain of 8), which drives a 25 m long
twisted-pair cable. The overall gain of the preampli"er is 2 V/pC at 100 ) load, resulting in aconversion coe$cient of about 2.4 mV/MeV fora single CsI(Tl) module. Using an Ortec 572 ampli"er with a shaping time of 2 ls, the equivalent noise
charge (ENC) at room temperature has been foundto be 640$10 and 840$10 electrons for S3204-03
and S3584-05 photodiodes, respectively. For
S3204-03 the noise rises to 690 electrons at a reduced shaping time of 1 ls. A typical CsI(Tl) crystal(photoyield of 104 p.e./MeV), attached to S3204-03
and the preampli"er displays an excellent noiselevel of ENL"70 keV, in spite of such a shorter
than optimum shaping time.
A 16-channel main ampli"er module incorporates the shaping ampli"er (SA) with two unipolar

164

D.V. Dementyev et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 151}171

outputs and a timing "lter ampli"er (TFA). The
low- and high-gain outputs ("xed gain di!erence of10) of the SA allow it to work over a large dynamic
range. The high-gain output facilitates a calibration
procedure with low-energy c-sources without any
gain adjustment. The main parts of the SA are
a di!erential receiver, a variable PZC circuit, a second-order active "lter for the low-gain output anda fourth-order "lter for the high-gain output,
a baseline restorer and output bu!ers. The baseline
restorer (BLR) is employed to compensate the e!ectof a baseline shift caused by uncanceled AC-interstages in the preampli"er and undercompensationof the PZC network. A random pulse generator
reproduced the real waveform input signals for the
SA to evaluate the BLR performance. If the measured pulse amplitude was shifted by morethan 5% of its spectrum peak value, we counted
such an event as lost. While the losses exceed 40%
even at 10 kHz rate in the tested ampli"er withouta BLR, the baseline restoration reduces the losses
to 8% at 10 kHz rate. The "lter parameters correspond to 1 ls shaping time and were chosen in an
empirical way to optimize the energy resolution

while keeping the pulse width as short as possible inorder to handle a high counting rate. The output
waveforms of the CsI signal for several shaping
times are shown in Fig. 12.
The TFA forms a timing pulse in an optimum
way to attain the best available rise time whilerelaxing the e!ect of the pulse front jitter caused bynoise #uctuations. While the risetime of the signal
after the preampli"er is about 2 ls, the TFA
shortens it to 450 ns. The TFA has variable gain inorder to allow individual adjustment of each channel, so that a single common threshold can be used
for all discriminator channels.
The CsI(Tl) detector identi"es p0 using the time
coincidence between two decay photons. Good
time resolution provides suppression of accidental
events, which can seriously deteriorate pion identi"cation under an intense background. A 16-channel CFD discriminator was designed to accept longrisetime signals in a wide range of amplitudes corresponding to energies from 5 to 300 MeV. We had
no motivation to discriminate energies below
5 MeV, where few signal photons and mainly
shower leakages and accidentals are concentrated.

Fig. 12. Ampli"er output waveforms of CsI(Tl) signals at di!erent shaping times of 0.5, 1.0 and 2.0 ls.

D.V. Dementyev et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 151}171

Exploiting the timing performance obtained, theCsI signals from all CFD outputs were used in thesecond level trigger logic as the OR strobe in coincidence with "rst level fast trigger. A 150 ns discriminator pulse width is su$cient to accept 100%of valid events, while reducing the recorded trigger
rate by a factor of 2 under the current experimental
condition.

5. Monitoring of gain stability
A schematic of the light #asher system used tomonitor electronic channels is shown in Fig. 13.
The light source is a xenon lamp Hamamatsu
L4647 operating at a repetition rate of 10 Hz. Itsemission spectrum is close to the CsI(Tl) emission,and long-term stability and high intensity are suitable for a few years of operation. The #asher light isdistributed through optical glass "bers and mixer
fan-out boxes to each CsI(Tl) crystal. The typicalresponse of a crystal corresponds to an energy
deposit of around 50}100 MeV with pulse height#uctuations of 2% (fwhm) in a short period. Aftera trigger pulse is applied to the Xe-lamp, the maindischarge takes place after a certain delay. The
delay time varies from 2 to 6 ls depending on theoperational parameters. Unstable preliminary discharge characteristic of the sparker causes delay
time jitter of a few hundred nanoseconds, making
the trigger pulse from a driving power module
di$cult to use for readout synchronization. For
this purpose the Xe-lamp light is fed to a photomultiplier, which produces the synchronization signal.
The optical distributor of 1-to-34 has beendeveloped by Mitsubishi Gas Chemical Co. It con-165

sists of an input optical connector, and a glass
mixer/lightguide fused with a spliced bundle of
100 lm output "bers. The distributor introduces
a light intensity splitting loss of 18 dB. The output"bers give uniform light within $12% from average. However the coupling to the crystals greatly
increased the nonuniformity between channels.
The #asher system is useful in monitoring electronic channels and detecting failed photodiodes
and gives a fast diagnosis of the state of the crystal
readout electronics. While the Xe-lamp light output
is rather stable, we found it unrealistic to monitor
the overall long term gain stability of the calorimeter with the #asher system. As seen in Fig. 8, theoptical "ber is "xed to the crystal by a small screw.
Nevertheless, the inner lightguide core of 100 lm
diameter can slide inside the protective isolation of
2 mm outer diameter, so that touching the "bercauses a shift of the core, and correspondently
a change in the Xe-light entering the crystal (seeFig. 14). The transparency of the unprotected CsI
surface where the #asher light enters the crystal
also a!ects the response of the slightly hygroscopic
crystals over time.
The failure rate of the photodiodes and preampli"ers is an important "gure of merit for overallcalorimeter performance, because these parts cannot be replaced without disassembling of a substantial part of the whole detector apparatus. The CsI
detector went into operation in October 1995,
when E246 "rst took data. During the subsequent
3 years period of operation, none of the 768 photodiodes failed. One preampli"er failed soon after
detector operation, and another one failed in 1998,
making the preampli"ers the most unreliable element of the readout electronics.

Fig. 13. Schematic of the Xe-lamp monitoring system.

166

D.V. Dementyev et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 151}171

Fig. 14. The Xe-lamp light output change in each crystal for
1.5 years. While the average shift is negligible, an individual
crystal response has #uctuated in a random mode. The grossstructure with $7% is due to a mechanical instability of the
optic "bers before the second distribution branch. One channel
at 443 is dead.

6. Performance of the p0 detector
6.1. Electronic noise
The electronic noise was determined from thewidths of the pulse height distribution in pedestal
runs, where there were no energy signals presented,
and can be separated into coherent (correlated) and
incoherent contributions. The incoherent contributions from a cluster of crystals sum in quadrature,
whereas the coherent ones combine linearly. The
coherent and incoherent contributions were extracted by "tting the dependence of the pedestal width
on the number of crystals summed up. The mostprobable source of the coherent contribution is
pickup noise of a PD, which serves as a receiver ofelectromagnetic interference. To suppress thissource, the PDs were shielded individually by theAl crystal cans, and a proper grounding scheme
was applied. The detector mechanics is electricallyisolated from the magnet and connected to theground bus of the CsI electronics in the counting
house. The signals are transmitted from the preampli"ers to the main ampli"ers by twisted-pairunshielded cables, which have high immunity to the
induced noise. The noise contribution to the energy
resolution obtained for a sum of nine CsI crystals is
250 keV (p). The incoherent contribution to thenoise from each crystal does not exceed 75 keV (p).
This value is quite consistent but slightly higher
than was obtained for the individual PD}preampli"er assemblies in bench tests (63 keV) using thehigh gain output of the shaping ampli"er. The
di!erence may be explained by the di!erent
measurement method, by higher temperature inside
the barrel and a stronger e!ect of ADC resolution
for the low-gain small signals. The coherent noise
was found to be about 11 keV (p) per a module, andincreases the noise of a 9 crystal cluster by 10%.
For energies of photons detected in the experiment (5}250 MeV), the noise contribution to theenergy resolution was negligible, and the main factors which determined the energy spread are the#uctuations of shower leakages through the crystal
rear sides and the lateral sides of crystals placedaround the muon holes of the calorimeter.
6.2. Energy resolution and calibration of the photon
detector
6.2.1. Test in a tagged photon beam
To estimate the characteristics of the calorimeter,
we tested a prototype of the detector on the electron and tagged photon beams of the INS electron
synchrotron in Tokyo. The prototype consisted of
30 CsI(Tl) crystals assembled in a matrix of 5]6.
Each individual crystal had a length of 25 cm andthe shape of a truncated pyramid with bases of 3]3
and 6]6 cm2, that represented typical dimensionsof crystals to be used for the calorimeter. PD ofsensitive area 28]28 mm2 (S3584-05) were glued
on the rear faces. The results of the beam test arepresented in Ref. [15], where the energy resolution
was obtained to be p /E"2.8% for a 200 MeV
Eelectron beam. A spatial accuracy of the coordinate
measurement by `center-of-gravitya method at thesame electron energy was p "7.6 mm. For photoX
ns, the energy resolution was 4.3 and 2.8% (p /E) at
E
energies of about 100 and 200 MeV, respectively.
6.2.2. Calibration in kaon beam
During the actual experiment in order to obtain
the gain coe$cients for the CsI detector, special
calibration runs are carried out in beam byusing muons from the dominant decay mode
K`Pl`m(K ). These muons have a kineticl2

D.V. Dementyev et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 151}171

energy of 152.4 MeV and are stopped in the CsI
crystals. The events in which no energy was detected in the neighboring crystals were selected toobtain the individual gains of CsI modules. The
kaon decay vertex was reconstructed froml` tracking and muon energy losses in the target
were corrected using the target "ber ADC information. Fig. 15(a) shows the energy spectrum of single
crystal events after adjusting the coe$cients. The
resolution p /E"1.7% of the peak at 152 MeV isE
due to the intrinsic resolution of one crystal andenergy loss correction uncertainty. Using the gain
coe$cients obtained from K decay, the positivel2
pion energy from K`Pp`p0(K ) was measured
p2
to be 112.0 MeV; this value is very close to a sum of
the kinetic energies of p` (108.5 MeV) andl` (4.2 MeV) from the pion decay. Fig. 15(b) shows
the spectrum of the energy sum of two photon
clusters from the p0. The peak value 229 MeV corresponds to about 93% of the E 0 "245.6 MeV,
pthat is consistent with the result of Monte Carlocalculation for the cluster size of 3]3 crystals. The
low energy tail is due to the shower leakage into themuon holes and beam holes and the rear sides of
the crystals. The resolution p /E obtained for theE

167

energy sum of two photons from K decay is 5.5%.
p2
Since the p` and p0 in the K decay are emitted in
p2
back-to-back directions, i.e. the angle between
them is 1803, this can be used to determine theaccuracy of the p0 direction measurement. The distributions of the opening angle between charged
and neutral particles in K and K decays are
p2
l3
shown in Fig. 16. In the case of the K distribup2
tion, the photon angular resolution determined to
be about pH "2.33, is consistent with the photon
spatial resolution of 7.6 mm.
The calibration runs were performed at intervalsof a few months. A long-term drift in electronics
gains and light yields of CsI crystals over a one year
interval resulted in an overall gain decrease ofabout 3%, which was compensated in calibration
coe$cients.
6.3. Time resolution
Good time resolution is an important factor for
reliable identi"cation of neutral particles and suppression of the background. The high light yield of
the CsI(Tl) crystals is the main factor which determines the timing performance of the calorimeter.

Fig. 15. (a) Energy spectrum of single crystal events corrected by the energy losses in the target. All the crystals were histogrammed after
adjusting the gain coe$cients. The high energy tail up to 200 MeV corresponds to the Michel spectrum of undetected positrons from themuon decays in the crystal. Small bump around 112 MeV is due to the p` from K . (b) Energy sum of two photon clusters from K inp2
p23]3 analysis. The peak value 229 MeV, about 93% of the E 0 "245.6 MeV, is consistent with the expected shower leakage.
p

168

D.V. Dementyev et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 151}171

Fig. 16. The distribution of opening angle between charged particle and p0 for K and K decays.
p2
l3

The calorimeter timing was determined bymeasuring the time di!erence between a muon signal from K decay produced by one of the 12
l3"ducial plastic counters surrounding the target,
and a CFD signal from a CsI module. Fig. 17(a)shows the time spectrum of the CsI crystals which
are the centers of the gamma clusters. The resolution of 3.8 ns (rms) was obtained for the energy
range of 10}200 MeV. Some asymmetry of the timing peak shape was observed at online monitoringfor small pulse amplitudes (low energies). This kindof the residual walk is removed in o%ine analysis
by the time-amplitude correction. As seen in
Fig. 17(b), the time resolution of 3.5 ns (rms) at
100 MeV deteriorates to 16.8 ns in the low energy
range of 10}20 MeV. These results were derived
from the summed time distributions of all 768 crys-tals, where a component from the incorrect time
peak alignment between the di!erent modules is
included, that widens the total spectrum. The cosmic ray test of an individual CsI module showed
the resolution of 4.1 ns at 50 MeV while the calorimeter resolution at the same energy is 5.8 ns(see Fig. 17).
6.4. Pileup ewect study
The slow scintillation decay time of CsI(Tl) andthe long "lter time constant can lead to possible
losses of detected events under a high intensity
beam due to pileup of the CsI pulses. In the actual
experimental conditions of E246 the counting rate
in individual crystals depends on their position
relative to the beam axis and is dramaticallyD.V. Dementyev et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 151}171169

Fig. 17. (a) Time spectrum of all CsI crystals (centers of clusters) with aligned peak positions for K events. (b) Time resolution (rms) ofl3
the calorimeter per an energy window, *E"10 MeV, as a function of the photon energy.

increased for the crystals closest to the beam which
su!er also from the high-energy pion halo. The
maximum rate per crystal can reach 20 kHz for
signals above a threshold level of &10 MeV. If
there is a #uctuation in the beam intensity the
instantaneous rate might exceed this level.
The #asher system was used to test the baseline
stability and pileup e!ect at high counting rates.
The Xe-lamp pulse height peak position wasmonitored at di!erent beam intensities and thelosses were calculated by counting the Xe-lamp
events, whose energy shift from the peak was morethan 8 MeV. Comparing this number with the total
number of triggers generated by the #asher, we
obtained the fraction of real events lost due topileup. The Xe-lamp energy spectrum was distorted
from the true Gaussian shape by piling up of the
Xe-pulses with randomly occurring scintillating
pulses from the beam. The peak position and resolution were preserved at a maximum measured
counting rate of 32 kHz per crystal, where counts
were registered if the energy deposit in a crystal
exceeded 10 MeV. The fraction of lost events due topile-up as a function of the counting rate is shownin Fig. 18. The so-called lost fraction can be reduced if pileup is accepted by increasing thehigh-energy shift cut. The limit of 8 MeV has beenchosen rather arbitrarily for estimation purposes
only. In o%ine analysis an event is rejected at more

Fig. 18. Relative pileup losses of the Xe-lamp signals vs. thecounting rate in a crystal for two shaping times of 1 and 0.5 ls.
The Xe-lamp rate was "xed at 10 Hz. The losses are counted if
the Xe-pulse height shift from the peak value was more than
8 MeV. The inset shows the Xe-lamp pulse waveform for a 1 ls
shaping time.

than 20 MeV pileup in the p0 invariant mass. Since
the average counting rate per a crystal in the experiment is under 8 kHz, the pileup events do notcreate any signi"cant problem for the experiment.
6.5. Clustering and p0 reconstruction
The p0 reconstruction begins with a search fora crystal with the highest energy deposit above
10 MeV and a time within certain time windows.

170

D.V. Dementyev et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 151}171

Such a crystal is considered to be the center ofa cluster. Then we sum the in-time energy deposits of
8 crystals surrounding the center crystal within theappropriate time windows at energies exceeded the
CFD threshold of 5 MeV. The energy sum of the
9 crystals (3]3 clustering) is required to be at least
20 MeV. Other algorithms of collecting the cluster
energy were also studied both with and without anenergy threshold. The position of each cluster isdetermined using the `center-of-gravitya method:h"+ E h /+E
i i
i
i
i
u"+ E u /+ E ,
i i
i
i
i

where E is the energy deposited in ith crystal of theicluster and h and u are the polar and azimuthaliiangles of the ith crystal, respectively. The sum isperformed over nine (or 25) crystals in each cluster.
Fig. 19a shows the opening angle distribution in
p0Pcc decay from K decays in the case of 5]5
p2
clustering. For nearly symmetrical p0Pcc decay,
i.e. (E !E )/(E #E )40.1, where E and
1
2
1
2
1
E are the photon energies, the opening angle dis2
tribution is shown in Fig. 19(b). The reconstructed
minimum opening angle of 68.03 is obtained, that isclose to the value of 66.73 of kinematical minimum
opening angle. The photon angular resolution
obtained from Fig. 19(b) is 2.23.

Fig. 19. The measured distribution of an opening angle between two photons from p0 in K decays: (a) all K events are shown; (b)p2p2events with (E !E )/(E #E )40.1 are presented. The minimum opening angle of 68.03 is obtained.
12
12

Fig. 20. The invariant mass of c}c events from K decays using 5]5 clustering; (a) all events reconstructed in the p0 detector;
p2(b) events detected away from the muon holes area.

D.V. Dementyev et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 151}171
Table 6
Parameters of the CsI p0 detector
Average light yield
Equivalent noise level
Correlated noise level
Energy resolution p /E
E
Spatial resolution (rms)Angular resolution (rms)Time resolution (rms)p0 mass width (rms)

11 000 p.e./MeV
75 keV per a module
11 keV per a module
4.3% at 100 MeV
2.8% at 200 MeV
7.6 mm at 200 MeV
2.2}2.43
3.5 ns at 100 MeV
49.9 MeV

The invariant mass of two photons can be thendetermined using the coordinate of the stopped
kaon in the active target and the coordinates in thecalorimeter of both photons from p0Pcc. If morethan two clusters are found, we select a pair withinvariant mass within 90}140 MeV. Inside this region the best pair combination is chosen by timing,
i.e. those clusters are considered to be produced by
a p0 decay whose time signals are closest to thepeaks in time distributions. Fig. 20 shows the invariant mass spectrum of c}c events fromK decays in the case of 5]5 clustering. Shower
p2
leakages near the muon holes deteriorate the invariant mass resolution of the whole detector to
8% (rms) at 135 MeV. The resolution is 7% forevents which occurred away from the muon holes
area, however these events make up only about
25% of the total. Using the cluster size of 3]3slightly deteriorates the invariant mass resolution
to about 7.5% for these events. The main detector
parameters are presented in Table 6.

7. Summary
The CsI(Tl) p0 detector has been built for the
E246 experiment at KEK to search for T-violationin the decay K`Pp0l`m and is being used successfully in this experiment since October 1995.
About 800 CsI(Tl) modules with PIN photodiode readout were carefully assembled to optimize
their parameters, such as light yield, nonuniformityof the light yield along the crystal axis, noise level
and time resolution. The average light yield of
11 000 p.e./MeV and average equivalent noise level171

of 75 keV were obtained for the modules used in thecalorimeter. The coherent noise in the calorimeter
was found to be about 11 keV per module. The
readout electronics system was developed speci"cally for the needs of the experiment to optimize
energy, timing and pileup performance.
The p0 invariant mass resolution of p"9.9 MeV
was obtained in the experiment for K decay.
l3
A time resolution of 3.8 ns (p) for the wide gamma
energy range (10}220 MeV) was achieved. The detector has demonstrated good stability, which ismonitored by a Xe-lamp system and beam calibration runs, and good reliability during the
3 years period of operation.

Acknowledgements
We would like to thank all members of E246
collaboration for help and useful discussions. This
work was supported in part by the Russian Foundation for Basic Research Grant No. 96-02-16081
and by the Grant-in-Aid for Scienti"c Research of
the Japan Ministry of Education, Culture andScience.

References[1] J. Imazato et al., KEK Report 91}8, 1991.
[2] S. Weinberg, Phys. Rev. Lett. 37 (1976) 657.
[3] G. BeH langer, C.Q. Geng, Phys. Rev. D 44 (1991) 2789.
[4] Yu. G. Kudenko, J. Imazato, KEK Report 92}15, October
1992.
[5] A. Satpathy et al., Nucl. Instr. and Meth. A 391 (1997)
423.
[6] B.C. Grabmaier, IEEE Trans. Nucl. Sci. NS-31 (1984)
372.
[7] P. Coyle et al., Report SLAC-PUB-5594(E), September
1991.
[8] I. Holl et al., IEEE Tranc. Nuc. Sci. NS-35 (1988) 105.
[9] Y. Kuboto et al., Nucl. Instr. and Meth. A 320 (1992) 66.
[10] E. Aker et al., Nucl. Instr. and Meth. A 321 (1992) 69.
[11] S. Schlogl et al., Nucl. Instr. and Meth. A 242 (1985) 89.
[12] K. Kazui et al., Nucl. Instr. and Meth. A 394 (1997) 46.
[13] Yu.G. Kudenko, O.V. Mineev, J. Imazato, Nucl. Instr. andMeth. A 411 (1998) 437.
[14] R.L. Wixted et al., Nucl. Instr. and Meth. A 386 (1997)
483.
[15] J. Imazato et al., Proceedings of PANIC XIII, Perugia,
Italy, June 28}July 2, 1993, p. 791.

ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research A 506 (2003) 60–91

Apparatus for a search for T-violating muon polarization in
stopped-kaon decays
J.A. Macdonalda,{, M. Abeb,1, M. Aokic,2, I. Araib, Y. Asanob, T. Bakerd,
M. Blechere, M.D. Chapmanc, P. Depommierf, P. Gumplingere, M. Hasinoffg,
R. Hendersona, K. Horieb,1, Y. Igarashib,1, T. Ikedab,3, J. Imazatoc,*,
A.P. Ivashkinh, A. Kagai, J.H. Kangj, M.M. Khabibullinh, A.N. Khotjantsevh,
E.J. Kimk, K.U. Kimk, Y.G. Kudenkoh, Y. Kunoc,2, J-M. Leej,4, K.S. Leek,
G.Y. Limk,1, D.R. Marlowl, C.R. Mindasl, O.V. Mineevh, C. Rangacharyulud,
S. Sekikawab, K. Shibatab, H.M. Shimizuc,5, S. Shimizum, Y.-M. Shind,
Y.H. Shinj, K.S. Simk, A. Suzukib, T. Tashirob, A. Watanabeb,
D. Wrighta, C.H. Yimk, T. Yokoica
TRIUMF, Vancouver, Canada V6T 2A3
University of Tsukuba, Tsukuba 305-0006, Japanc
High Energy Accelerator Research Laboratory (KEK), Tsukuba 305-0801, Japand
University of Saskatchewan, Saskatoon, Canada S7N 5E2eVirginia Polytechnic Institute and State University, Blacksburg, VA 24061-0435, USAf
Universit!e de Montr!eal, Montr!eal, Canada H3C 3J7
g
University of British Columbia, Vancouver, Canada V6T 1Z1
hInstitute for Nuclear Research of RAS, 117312 Moscow, Russiai
Kanagawa University, Hiratsuka 259-1293, Japanj
Yonsei University, Seoul 120-749, South Korea
kKorea University, Seoul 136-701, South KorealPrinceton University, Princeton, NJ 08544, USAm
Osaka University, Osaka 560-0043, Japanb

Received 25 January 2003; accepted 18 February 2003
The E246 KEK-PS Collaboration

Abstract
The detector built at KEK to search for T-violating transverse muon polarization in Kþ -p0 mþ nm ðKm3 Þ decay of
stopped kaons is described. Sensitivity to the transverse polarization component is obtained from reconstruction of the*Corresponding author.
E-mail address: jun.imazato@kek.jp (J. Imazato).
{
Deceased 22 May 2003.
1
Present address: High Energy Accelerator Research Laboratory (KEK), Tsukuba 305-0801, Japan.
2
Present address: Osaka University, Osaka 560-0043, Japan.
3
Present address: The Institute of Physical and Chemical Research, Saitama 351-0198, Japan.
4
Present address: Korea Research Institute of Standards and Science, Taejon 305-346, Korea.
5
Present address: Fermilab Computing Division, Batavia, IL 60510-0500, USA.
0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0168-9002(03)01363-9

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61

decay plane by tracking the mþ through a toroidal spectrometer and detecting the p0 in a segmented CsI(Tl) photon
calorimeter. The muon polarization was obtained from the decay positron asymmetry of muons stopped in apolarimeter. The detector included features which minimized systematic errors while maintaining high acceptance.
r 2003 Elsevier Science B.V. All rights reserved.
PACS: 29.27.Eg; 29.27.Fh; 29.30.Aj; 29.40.Cs; 29.40.Gx; 29.40.Ka; 29.40.Mc; 29.40.Vj; 13.88.+e; 07.05.Hd; 11.30.Er; 12.60.i;
13.20.Eb; 13.35.Bv
Keywords: Separated stopped K beams; Km3 Decays; T-violation; Transverse muon polarization; Toroidal spectrometer; Muon
polarimeter; Scintillation ﬁber target; CsI(Tl) photon detector

1. Introduction
Violation of time-reversal invariance (T) could
be inferred from a weak interaction induced
transverse polarization ðPT Þ of muons normal tothe decay plane in the decays Kþ -p0 mþ n ðKm3 Þand Kþ -mþ ng ðKm2g Þ: Standard Model (SM)contributions to PT are at about the 107 level[1] and ﬁnal state interactions in Km3 contribute at
the o105 level [2]. A measurement in excess of
this has the potential to reveal new CP violation
physics, given CPT invariance. The value of PT
can be as large a 103  102 in models with multiHiggs doublets, leptoquarks, left–right symmetryor SUSY [3]. Motivation to search for additionalsources of CP violation arises from the observed
baryon asymmetry in the universe, which cannot
be explained by the CP violation in the SM alone
such as the SM T-violation observed in the neutralkaon system [4]. At the High Energy Accelerator
Research Organization (KEK) in Japan, the E246
collaboration is completing a search for PT in Km3
at the B103 level.
In 1999, the ﬁrst result, PT ¼ 0:004270:0049(stat) 70:0009 (syst), was published [5], based on
B3:9  106 good Kþ
m3 events from the data taken
during 1996 and 1997, indicating there is noevidence for T violation. The T-violating parameter Imx was then extracted as Imx ¼
0:01370:016 (stat) 70:003 (syst). The 90%
conﬁdence limits are given as jPT jo0:011 andjImxjo0:033: Further data were collected during
1998–2000, and the ﬁnal E246 sensitivity to PT in
Km3 is expected to be B2  103 corresponding todImðxÞ of about 7  103 : Since statistics will
dominate the sensitivity, the detector will have acapability to improve the sensitivity by a factor of
3–4 using a more intense kaon beam in the future.
In this paper we give a description of theapparatus.

2. Experimental method and detector overview
2.1. Principle of measurement
The measurement of the T-violating transverse
muon polarization PT requires ﬁrst that thedetectable constituents of the Km3 decay with abranching ration of 3.2% be distinguished andthat the decay plane be established from thekinematic parameters. Secondly, a polarimetermust be sensitive to the polarization component
normal to the decay plane. For example, if events
are selected whose decay plane includes the beamaxis, then PT would appear as an azimuthalcomponent of the polarization around the beamaxis. The muon polarization is measured from theasymmetry of the decay positron angular distribution relative to the polarization axis.
2.2. Stopped beam method
In our experiment, we used a stopped kaon
beam in contrast to the previous search [6] whichused the in-ﬂight decay of a 4-GeV kaon beam.
Although a low-momentum stopped-kaon beamrisks a potential disadvantage of reduced kaon ﬂux
compared to the high momentum beam used forin-ﬂight measurements, the stopped method
offers several potential advantages in minimizing
backgrounds and systematic uncertainties which

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62

compensate the reduced beam intensity, and canresult in a measurement with improved overallsensitivity.
In a stopped-kaon decay, the kinematics can be
determined with high resolution which allows a
precise deﬁnition of the decay plane as well as
powerful tools to reject backgrounds and reduce
systematic instrumental sources of asymmetry. In
our apparatus, muons are accepted in a range ofpolar angles around 90 with respect to the beamaxis and are momentum analyzed in one of the 12
gaps of a superconducting toroidal spectrometer
magnet. The decay vertex is determined bytracking in scintillators and/or wirechambers of
the charged incoming kaon and outgoing muon.
The muon momentum is obtained with adequate
resolution to reject the dominant decay modesKþ -pþ p0 ðKp2 Þ and Kþ -mþ n ðKm2 Þ: The p0
momentum is obtained from a high-solid-angle
segmented CsI(Tl) calorimeter, and the kaon
momentum is, of course, zero.
Good determination of the p0 decay kinematics
in the segmented calorimeter allows one to identify
and include events in the muon polarimetercorresponding to both forward (downstream)
and backward (upstream) going p0 ’s, fwd andbwd events, respectively. The two classes of events
give opposite asymmetries in the polarimeter due
to PT as illustrated in Fig. 1. That is, a properly
normalized sum of fwd- and bwd-type events must
give a zero asymmetry. In the previous in-ﬂight
experiment only a forward-decaying ðfwdÞ p0 was
inferred from the forward boost from the K rest

PTΤ

PTΤ

frame and an energy threshold on one of the decay
photons.
From the kinematics it is also possible tooptimize the instrumental sensitivity to the physics
parameter of T violation in this decay, Imx: The
value of /PT =ImxS varies by an order ofmagnitude over the mþ ; p0 phase space. A ﬁgureof merit combining the statistical sensitivity of the
decay rate and /PT =ImxS peaks for mþ energies
above 50 MeV and p0 energies below B80 MeV;
well away from the kinematic peak. The muon
acceptance in particular can therefore be optimized to enhance the sensitivity to Imx from PT :2.3. Longitudinal field and double ratio method
Instead of using the transverse ﬁeld method
which was adopted in the previous experiment [6],we adopted the longitudinal ﬁeld conﬁguration. In
this case the transverse component PT of the muonpolarization is held, and the non-T-violating inplane components precess, while in the transverse
ﬁeld method the in-plane component PL parallel tothe muon momentum is held and the T-violating
component was sought as a possible phase shift ofthe precessing component. The comparison isshown in Fig. 2.
In our apparatus the polarimeter is similar ingeneral concept to that used in Ref. [6], except thatthe fringe magnetic ﬁeld from the toroid is
shimmed to produce an azimuthal ﬁeld at themuon stoppers. Muons are stopped in an annularlongitudinal field
R

L
PL,N

transverse field
R

L
PL,N

PT

π forward00

π backward

Fig. 1. Comparison of PT for fwd and bwd going p0 ’s.

PT

B

B

L−R
L+R

cos (ωt+ δT)

Fig. 2. Comparison of longitudinal and transverse holding ﬁeld
methods.

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array of aluminum stoppers surrounding the beamaxis. Positrons from muon decay are detected in
scintillator arrays located azimuthally between themuon stoppers so that an asymmetry between
clockwise, cw-decaying muons and counter-clockwise, ccw-decaying muons can be measured. In ourcase, the 12-segment polarimeter accepts muons
exiting the gaps of the toroid and is located wellremoved from both the stopping target and beamaxis where background rates can be reduced to alow level. This minimizes the problems of accidental rates in detecting the decay positrons andallows a long acceptance time for the muon decay
ðB20 msÞ in order obtain higher statistical sensitivity and detailed determination of the background.
One advantage of using the longitudinal ﬁeld
method in the present experiment is higher
statistical signiﬁcance from a given numberpofﬃﬃﬃ
positron events compared to a reduction of 1= 2
in the case of the precession measurement.
Although usually it is problematic how to remove
instrumental misalignment which gives rise to a
spurious asymmetry effect in the longitudinal ﬁeld
method, in the present experiment such effects canbe canceled out by taking the ratio of fwd and bwd
pions.
PT would be indicated by an asymmetry
between cw and ccw decay positrons asNðcwÞC172a/cos yT SPTNðccwÞ

ð1Þ

where a is the effective analyzing power and/cos yT S is the kinematical attenuation relating
the polarization axis to the decay plane.
Furthermore, it is possible to obtain thepolarization from the double ratio½NðcwÞ=NðccwÞ fwd
C1 þ 4a/cos yT SPT½NðcwÞ=NðccwÞ bwd

ð2Þ

with a doubling of the sensitivity to PT : Several
sources of potential systematic errors due to
instrumental misalignments cancel out to ﬁrst
order in this double ratio. A direct measurementof the analyzing power can be obtained from the
PN component, which is the in-plane component
parallel to the pion momentum, by selecting eventswith a transverse p0 decay. The ﬁeld requires good63

mapping and stability, but it does not need to bereversed as did the axial ﬁeld in the previous
experiment [6]. This avoids potential systematiceffects due to the reversing ﬁeld itself as well as its
effect on photomultiplier tube (PMT) gains.
2.4. Overall assembly
General assembly views of the apparatus areshown in Fig. 3 as end and side views. Ourdetector’s three main analytical elements are thetoroidal spectrometer, the CsI(Tl) photon calorimeter [7], and the polarimeter.
The space available for the CsI(Tl) array islimited by the 1-m inner diameter of the toroidal
magnet and cryostat. The crystal length of 250 mm(13.5 radiation lengths) is adequate to keep shower
leakage for photons up to 250 MeV acceptable
while still allowing sufﬁcient space for the PIN
diode and preampliﬁer readout system. The crystal
array has 12 holes aligned with the magnet gaps
which provide the acceptance for charged particles
in the spectrometer.
Two additional holes in the crystal array wererequired on the beam axis: The upstream holeallowed for the incoming kaon beam and instrumentation which included a Cherenkov detector, ascintillator hodoscope and a degrader. This hole
had to accommodate not only the kaon beamphase space but also the six-times-higher pion
contamination including an asymmetric haloaround the kaon beam, which could result in
unacceptable rates in the crystal elements near the
beam axis. This problem was partially alleviatedby a Pb and W collimator upstream of theapparatus. Downstream, the beam-axial holeprovided access for the support and readout of
the segmented stopping target assembly and innertracking elements. The latter include the 12 closepacked ﬁducial counters used in the event trigger
and for muon time-of-ﬂight (TOF) measurements,
and a stack of ring scintillators and a cylindricaldrift chamber, both used for muon tracking.
The target size was matched to the measured
kaon beam proﬁle at the stopping location at thecenter of the toroid. It is necessary to minimize thetarget size to avoid excess mass in the path of
exiting muons and photons from p0 ’s, as well as toARTICLE IN PRESS
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64

gap
gap

iron
pole
iron
pole

coil
coil
CryostatPolarimeterµ stopperµ stopper
+

0.9 Τ

+

counter
e+ecounterµ+

Pb collimator

TOF2
Cherenkov

CsI(Tl)Fiducial (TOF1)
Targetγ+Κγ

W collimatortargetTargetCsI
CsI
counterdetector

Ring Hodoscope

DegraderBeO

MWPCside viewend
end view
view

0

0.5

1m

Fig. 3. General assembly side and end views of the detector.

economize on the number of segmented elementsin the target to be instrumented.
The polarimeter was mounted immediately
downstream of the spectrometer to maximize
acceptance for Km3 mþ ’s in the 12 pure aluminum
stopping arrays, and to take advantage of theshimmed fringe ﬁeld of the toroid which serves as aholding ﬁeld for PT : A wedge-shaped copper
degrader preceded each aluminum muon stopping
array, and compensated for the momentum
dispersion from the analyzing magnet so that allmuons would come to a near uniform stopping
depth. In addition to the stoppers and positrondetectors, the polarimeter included several vetocounters used to help identify good stoppingmuons and valid decays while rejecting backgrounds.
2.5. Acceptance optimization
The ﬁrst design goal was to simultaneously
optimize the acceptance for mþ ’s from Km3 in the
12 magnet gaps, and to maximize the acceptancefor p0 ’s in the CsI(Tl) calorimeter. This required

careful matching of the 12 holes in the CsI(Tl)array to allow muons from the distribution of
stopped kaons in the central stopping target toenter the magnet gaps and be accepted in thepolarimeter at the magnet exit. A Monte Carlostudy [7] lead to a design with 10% acceptance formþ ’s from Km3 ; while maintaining 70% coveragefor g detection. Optimum overall acceptance of
8% was obtained for the angle a of 22:5 as
illustrated in Fig. 4.
2.6. Systematic error reduction by symmetry
Our apparatus is constructed with 12-foldazimuthal symmetry set by the 12-gap toroidal
spectrometer magnet. The summation of eventsfrom the 12 spectrometer gaps contributes to theacceptance, and also allows additional capabilities
to reduce systematic asymmetry effects due to the
instrumentation and the kaon beam phase space.
For example, each positron detector in thepolarimeter serves as a cw counter for one gap
and a ccw counter for the neighboring gap, and an
apparent asymmetry due to variations in counter

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65

2α

K+

TargetCsI barrel

8.0

Acceptance (%)

7.0
6.0
5.0

- All events- F+B events

4.0

Fig. 5. Low-momentum kaon beam channel K5.

3.0
2.0

Table 1Main parameters of the K5 beam channel

1.0

7.5

15

22.5

30

37.5

45

α (degree)
Fig. 4. Monte Carlo optimization of the muon holes. F+B
refers to events with forward- and backward-going pions.

efﬁciencies cancel. Similarly, asymmetries induced
by an asymmetric kaon stopping distribution also
cancel in the azimuthal sum over 12 gaps. In thiscase also, since the decay vertex is determined
event-by-event, residual higher order asymmetry
can be tested for and corrected.

3. Beamline and beam instrumentation
3.1. Low momentum Kþ beam line K5
The experiment was set up at the low-momentum separated kaon beam line K5 [8] in the North
Hall of the KEK proton synchrotron. The layoutof the beam line is shown in Fig. 5 and the main
parameters are listed in Table 1. This channel was
originally designed for 550 MeV=c maximum
momentum, but it was upgraded to 660 MeV=c
for this experiment. It is equipped with a singlestage electrostatic separator (DCS) with an electric

Maximum momentum
Production target
Take-off angle
Solid angle
Channel length
Momentum bite
Beam size on target
Dispersion on targetKþ beam intensity
pþ =Kþ ratio

660 MeV6ðHÞ  10ðW Þ  60ðLÞ mm3
073 degrees
10 msr ð7180 mrðHÞ  mrðVÞÞ
12:5 m73%
sx ¼ sy ¼ 2:5 cm
0:28 cm=% Dp=p
1:0  105 =1012 protons
6–7

ﬁeld of 50 kV cm1 and can deliver a Kþ beamwith p:K ratio of 5–10 making a nearly achromatic
spot on the experimental target.
Secondary particles were produced at a 6ðHÞ 
10ðW Þ  60ðLÞ mm3 Pt target in the 12-GeV proton beam with a typical intensity of 3  1012
protons per pulse of 0.7-s duty with 3.0-s repetition. The secondary beam was collected at 0 witha large solid angle of B10 msr: Two bends D1 andD2 and a horizontal acceptance slit at theintermediate point selected the 73% beam momentum bite. The slit serves also to reduce theintensity if necessary. The mass separation is done
after the DCS at the vertical mass slit after theintermediate quadrupole doublet of Q2 and Q3.
The last doublet of Q4 and Q5 focuses the beamon the target with a round spot. The momentumARTICLE IN PRESS
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J.A. Macdonald et al. / Nuclear Instruments and Methods in Physics Research A 506 (2003) 60–91

dispersion at the target is 0:28 cm=ð%Dp=pÞ: The
total length of the channel is 12:5 m:
In a Monte Carlo simulation it was shown thatthe optimum momentum to obtain the maximum
kaon stopping rate in the ﬁducial region of ourtarget was 700–800 MeV=c taking into account the
momentum dependence of the kaon production
cross-section and scattering of the beam from thedegrader. Accordingly, the maximum channel
momentum of 660 MeV=c was chosen for theexperiment.
Beam tuning was done to maximize both thenumber of stopped kaons and the K : p ratio as
well as to realize a symmetric kaon stopping
distribution in the target. Since the channel has
only one stage of DCS, it was difﬁcult to reducepion contamination which arises from Ks decay
near the proton target. In addition, a pion beam
halo made background hits in the CsI(Tl) calorimeter. In order to remove this halo a ray-tracing
study was performed for kaons and pions identiﬁed separately by a Cherenkov counter and backtracking the beam by means of an MWPC at thetarget position and a horizontal counter hodoscope placed after the mass slit. Rejection of thepion halo was realized by installing a beam slit
inside the second bend D2. The typical beam
intensity at 660 MeV=c of Kþ is 105 per 1012
proton with a pþ =Kþ ratio of 6–7.
3.2. Beam collimation and degrader
Between the last quadrupole of the K5 channel
and the beam Cherenkov counter a collimatorsystem was inserted. It consists of a 45-cm thicklead wall with a rectangular hole with anadditional insert made of Hevimet for shaping
the beam proﬁle. The thickness was determined so
as to stop the 660-MeV=c halo pions. The shape of
the insert was designed to transmit the kaon proﬁlewhile also blocking the pion halo in the CsI(Tl)region. This collimator system cut the pion halobackground by 70% while sacriﬁcing only 15% of
the kaon yield, and reduced the singles counting
rate of the affected CsI(Tl) crystals drastically.
After passing through the Cherenkov counterthe beam was slowed by a degrader system
consisting of 65 mm of Al followed by 242 mm

of BeO located close to the stopping target. The
100-mm diameter was enough to cover the beam
proﬁle. The use of light-element materials isessential to achieve high stopping power withminimum multiple scattering; in addition, using
BeO near the target helped to reduce the interaction of photons from the target in the degraderwhile the use of Al further upstream served toreduce the cost. The combination of lengths was
optimized for a stopping distribution centered in
the 20-cm long ﬁducial region of the target. The
broadening of the beam through the degrader was
not negligible but acceptable.
3.3. Beam detectors
3.3.1. Cherenkov
In order to trigger on kaons in the beam, aFitch-type differential Cherenkov counter was
employed [9]. Fig. 6 shows the cross-section sideview and end view of the counter. Incident pions
and kaons emit Cherenkov light in a 4-cm thickacrylic radiator with a velocity-dependent emission
angle. The index of refraction of the radiator issuch that at our chosen momentum the kaon lightemerges from the rear surface, while the pion lightis internally reﬂected and transmitted to emerge
from the circumference of the radiator. Two
circular arrays of PMTs are arranged so that the
inner circle detects the pion light from the radiatorcircumference, and the outer circle detects thekaon light which is reﬂected by a curved mirror
behind the radiator. Each array comprises 14
PMTs with a Winston cone in front.
The conﬁguration of the radiator, mirrors, andPMTs was optimized to maximize the total
number of PMT-sensitive photons for incident
beam momenta in the range 620–700 MeV=c:Additionally, the design was constrained to minimize the thickness of the radiator to avoid
excessive multiple scattering of the beam, tooptimize its effective aperture to cover the wholebeam, and to minimize overall dimensions in order
to ﬁt inside the entrance aperture of the CsI(Tl)array.
The number of PMTs was determined by theavailable room. The parabolic kaon mirror andkaon Winston cones were made of acrylic with aARTICLE IN PRESS
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67

Fig. 6. Cross-section side view and end view of the beam Cherenkov counter.

reﬂective coating of evaporated Al. The pion
mirror was made of aluminized Mylar and thepion ring Winston cones were coated with white
BaSO4 : Preceding the experiment, the basic
characteristics of the counter were tested at the
KEK 12-GeV synchrotron using a pion beam ofthe same velocity as the pions and kaons in theexperiment. The effective radius with more than
90% detection efﬁciency was measured to be 6 cm:
The main parameters of the counter are summarized in Table 2.
For the online trigger, the multiplicity of each
PMT ring was used. The trigger condition wasinvestigated in a test experiment by identifying
particles using TOF. Fig. 7 shows the typicalmultiplicity spectra of both rings for both kaonsand pions incident. In normal operation, the kaon
trigger threshold was set to multiplicity of X6 witha trigger efﬁciency of more than 99%. The
probability of mis-identiﬁcation of a pion as akaon was less than 1% under the typical beam
intensity conditions.
3.3.2. Hodoscope
The B0 hodoscope shown in Fig. 8 was placeddownstream of the Pb collimator, and is an
assembly of 22 plastic scintillating counters. Kþ

Table 2Main parameters of the beam Cherenkov counter
Kaon momentum range
Radiator material
Radiator thickness
Effective aperture
Beam angular tolerance
Kaon ring
PMTs
Winston cone
Pion ring
PMTs
Winston cone

620–700 MeV=c
Acrylic ðnD ¼ 1:49Þ
4:0 cm12-cm diameter73 for ro4 cm
Hamamatsu R580UV  14
5:3 cm + with Al coating
Hamamatsu R1398  14
3:8 cm + with BaSO4 coating

and pþ beam proﬁles were obtained in coincidencewith the Cherenkov counter. The beam proﬁle wasmonitored to check the stability of both theprimary proton beam and the secondary particle
beam. The trajectories of beam particles were also
obtained by using the Kþ entrance position
determined by the active target.
Timing data of charged particle hits in the B0
hodoscope were recorded by multi-hit TDCs
during the eþ gate for the mþ decay. Hits in thepolarimeter positron counters in time with B0
formed part of the constant background in the mþ
decay spectra related to beam particles and wereARTICLE IN PRESS
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J.A. Macdonald et al. / Nuclear Instruments and Methods in Physics Research A 506 (2003) 60–91

x 10

5

x 10

10

counts / bincounts / bin

5

2.5

5

0

(a)
x 10

5

0

K-ring multiplicity

π-ring multiplicity

(b)

5

x 10

5

counts / bincounts / bin

8

4

0

0

K-ring multiplicity

(c)

2.5

(d)

π -ring multiplicity

Fig. 7. Multiplicity spectra of the kaon ring (a,c) and pion ring (b,d). The upper spectra are for kaon incident and the lower spectra arepion incident. Particle identiﬁcation was done by TOF.

140 mmBeamx
Pb
Collimater
Beam

90 mm

PMT
Side view

End view

Fig. 8. Schematic diagram of the B0 beam counter and its location relative to the collimator.

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rejected by vetoing on coincident events in the B0
hodoscope.

4. Charged particle measurement
4.1. Component overview
Several detector elements were used to measurecharged particles from the decay of stopped kaons.
The main functional goals of this system areparticle identiﬁcation and kinematic reconstruction using TOF and tracking through the magnetic
spectrometer before entering the polarimeter.
In the central region, the segmented active target
gave the lateral coordinates ðx; yÞ for the stopping
position and decay vertex of the kaon as well as
the initial track of exiting charged particles. The
ﬁducial counters surrounding the target provided
the initial trigger information and the start for the
TOF measurement. Immediately outside the ﬁducial counters, the ring hodoscope gave the ﬁrst
axial ðzÞ track coordinate before passing throughthe gaps in the CsI(Tl) calorimeter to enter thetoroidal spectrometer. A cylindrical drift chamber(C1) just outside the ring hodoscope gave additional tracking information. At the entrance andexit of each spectrometer gap were sets of planar
wire chambers (C2, C3, C4) which gave thenecessary coordinates for momentum analysis.
Finally, the charged particles entered the polarimeter by passing ﬁrst through the second timeof-ﬂight counter (TOF2) and then a deﬁning
polarimeter trigger counter (PL).

69

Based on studies of scintillating ﬁbers of various
sizes as well as from the experience with the target
used in the E787 experiment at Brookhaven [10],we chose an array of 5  5 mm2 Bicron BCF-12
ﬁbers.
The radial extent of the target was optimized tomatch the kaon beam proﬁle, while minimizing thematerial to be traversed by exiting decay muonsand photons from p0 ’s. The optimum was a B9cm diameter array of 256 ﬁbers shown in Fig. 9.
The 45 -diagonal orientation was chosen to
maximize the vertical moment of inertia of theindividual ﬁbers to resist sagging under gravity,
since the array is cantilevered from its support
downstream of the active center region of thedetector.
In order to determine the target stopping
efﬁciency with respect to incident kaons, several
measurements with a special trigger consisting
only of the kaon Cherenkov counter were carriedout. The efﬁciency was 38% for kaons identiﬁed in
the target as events with a large energy deposit in
one ﬁber ðEfiber > 5 MeVÞ normalized by the
number of triggers. The same efﬁciency was
obtained when the three-cluster events in theCsI(Tl) detector were taken into account. The
stopping efﬁciency of 38% was consistent with

4.2. Target and ring counters4.2.1. Active target
A segmented active target identiﬁed the stopping kaon and its decay vertex, and provided theinitial tracking information for the decay muon as
well as an energy-loss correction to the momentum
measurement in the spectrometer. The segmentation level is a trade-off between position resolution
and rate handling from ﬁne segmentation on the
one hand, and energy and timing resolution fromminimum ionizing decay muons and lower costfrom coarse segmentation on the other hand.

Fig. 9. Cross-sectional view of the active target taken from theon-line event display, showing the elements hit by the stoppingkaon and the exiting charged particle (muon). The ﬁducial
counter hit contributing to the event trigger is shown at the top.
The shading roughly indicates the energy loss in each element.

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preliminary estimations ðB40%Þ based on the
beam proﬁle and target dimensions.
After selection for cross-section dimensions andlack of ‘‘pin cushion’’ distortion, one end of each
2-m long BCF-12 ﬁber received from Bicron wasmachined with a diamond cutter to be coupled to a
PMT in a test setup where scintillation light output
and attenuation lengths were measured. The
measured attenuation lengths of most ﬁbers fellin a range 200–400 cm; however, the parameter ofpractical concern is the light output in photoelectrons per MeV (p.e./MeV) measured at a ﬁxed
distance of 1:6 m from the PMT, which approximated the conditions in the actual target. The
distribution is shown in Fig. 10 for the 282 ﬁbers
measured. Fibers with a minimum light yield of
14 p:e:=MeV were accepted for the target with anaverage for the 256 target ﬁbers of 17:3 p:e:=MeV:
It was found that changing the PMT couplingfrom an air gap to a silicone gel coupling increased
the light output by 12%. Nevertheless, since thelight output was adequate with an air gap, thissimpler arrangement was adopted in the target
assembly.
The selected ﬁbers were then diamond machinedto the ﬁnished length of 185 cm; and spiralwrapped with 6.4-mm aluminized Mylar. The main
purpose of the Mylar wrapping was to protect theFig. 10. The distribution of light yields in photo-electrons per
MeV of ﬁbers tested for the target, showing the criterion for the256 ﬁbers selected.

ﬁbers in the target array and to avoid thepossibility of light cross-talk between ﬁbers.
Wrapping with white teﬂon tape was also investigated but neither material produced a signiﬁcant
improvement in the light output. This can belargely attributed to the air gap coupling to the
PMT which preselects light rays with near normal
exit angles and thus shallow angles of reﬂection
propogating in the ﬁber, where the dominant effectis internal reﬂection at the ﬁber cladding and not
the outer surface.
Fig. 9 also shows the close-packed ﬁducial
counters surrounding the ﬁber array. These
counters will be discussed further in Section 4.5.
The array of target ﬁbers and ﬁducial scintillators/
lightguides was assembled in a jig and the active
end was wrapped to form a rigid light tight ‘‘log’’.
The position of each ﬁber in the array was
documented so that balancing of higher-gain
PMTs with lower-light-yield ﬁbers was possible.
The ‘‘log’’ was then clamped in a support collar
from which the active end of the target array wascantilevered in the ﬁnal assembly. The opposite
ends of the ﬁbers were free to fan out to individual
PMTs (Hamamatsu H3164PX MOD).
Two support assemblies are attached to the
support collar. One extends below to a base whichslides on a track allowing the target to bepositioned and aligned in the detector and easily
moved for access to other systems. The othersupport consists of six struts which hold the PMTs
mounted on a facetted plate approximating asection of a spherical surface, where the ﬁbers are
fanned out to the PMTs. The facetted plate is also
mounted on the sliding base.
The 256 PMT signals from the target ﬁbers were
fed to custom-made postampliﬁer-discriminator
modules housed in a TKO crate. The postampliﬁers had a 10-fold gain output delayed by a builtin hybrid circuit for 100 ns: The analog output fed
the ADC and the output of an incorporated
leading-edge discriminator was used for the TDC.
Fig. 9 illustrates the on-line display of a typicalKm3 event showing the cluster of ﬁbers hit by the
stopping kaon, and the track from the exiting
decay muon. The Kþ decay vertex in the plane
perpendicular to the beam axis is obtained fromthe target scintillating ﬁbers with an accuracy of

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Table 3Main parameters of the target array and ring counters
Target parameters
Number of elements (ﬁbers)
Scintillator type
Fiber dimensions
Array ‘‘diameter’’Light yield (av.)
PMTs
PMT–ﬁber coupling
Ring parameters
Number of elements (rings)
Scintillator type
WLS type
Ring dimensions
Light yield (typ.)
PMTs
PMT–WLS coupling

256
Bicron BCF125  5  1850 mm39373 mm
17:374 p:e:=MeV (MIP)
Hamamatsu H3164 PX
MODAir gap
32
Bicron BC408
Kurraray Y11 (1-mm diam.)
ID 118 mm; OD 128 mm; zthickness 6 mm
46 p:e:=MeV (MIP)
Hamamatsu R580-17
Optical greases ¼ 1:4 mm: The main parameters of the target aresummarized in Table 3.
4.2.2. Ring counters
The coordinate along the beam axis (z-coordinate) was recovered with the hodoscope surrounding the target and ﬁducial counter assembly. The
design is a cylindrical array of plastic scintillator
rings employing wavelength-shifting ﬁber readout[11]. The hodoscope parameters were optimized toobtain high efﬁciency for detection of minimum
ionizing particles (MIPs) in the high rate environment, and to achieve better than 2-mm spatial
resolution. The main parameters are summarizedin Table 3.
A schematic view of the ring system is shown in
Fig. 11. The rings of 118-mm inner and 128-mm
outer diameters were cut from 6-mm thick BC408
scintillator sheet using a diamond cutter. The 5mm radial ring thickness was a compromise
between high light output and minimal additional
material causing particle scattering and photon
conversions.
Scintillation light is trapped inside the ring byusing a highly reﬂective wrapping until it iscaptured by the WLS ﬁber. Since the light issusceptible to inevitable losses before entering the71

target fibers

Kring scintillators
fiducial countersFig. 11. Isometric view showing the 32 ring scintillators
surrounding the active target and ﬁducial counters. The WLS
ﬁbers, not shown, were bent and exited to the right to the PMTs
located downstream.

ﬁber, the optimization requires high reﬂectivity at
the scintillator–wrapping boundary, high transmittance at the scintillator–ﬁber boundary, andlarge geometrical acceptance of the ﬁber. The ﬁrst
factor is dependent on the wrapping and scintillator surface treatment, while the second is determined by the optical couplant, the surface quality
in the contact area, and the ratio of refractive
indices between the scintillator and ﬁber.
The spectral matching of multi-clad Kuraray
Y11 WLS ﬁber in combination with Bicron BC408
scintillator provides high light yield [12]. The
Y11(K27) emission peak of 495 nm is shifted to
510 nm after the light travels 1:5 m through theﬁber to the PMT. A ﬁber with a diameter of
1:0 mm was chosen as a reasonable compromise
between light output and ﬁber ﬂexibility. To
increase the geometrical acceptance, the ﬁbers areplaced in spiral grooves machined into the outercircumference of the scintillator rings and glued
using a colorless silicone adhesive SE777.6
The two B1:3-m long ends of the ﬁber wereinserted into a light-tight ﬂexible tube and the tips
were glued into an acrylic holder. After gluing theﬁber into the holder its ends were cut off and
6

Toray Dow Corning Silicone, Japan.

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J.A. Macdonald et al. / Nuclear Instruments and Methods in Physics Research A 506 (2003) 60–91

polished. The holder provides a mechanical ﬁber–
phototube interface and accurately controls theperpendicular alignment between the ﬁber axis andthe PMT window. The supporting acrylic also
prevents any erosion or chipping of the ﬁber edge.
The 32 ring-ﬁber assemblies were positioned ona support cylinder made of 0.5-mm thick G10
sheet. The rings are interleaved with aluminized
Mylar to prevent light cross-talk. The wrapping material causes a 0.30-mm gap between
neighboring rings which resulted in a dead fraction
of 4% for the hodoscope. The hodoscope outer
diameter is limited by the available space requiringthat the ﬁbers be bent and held with adhesive tapeto the detector surface using a minimum 2-cm
bending radius to route the ﬁbers downstream of
the target to the PMTs. The assembled detector
was shielded by a protective outer cylinder
wrapped with black light-tight tape. After installing the hodoscope and ﬁber readout ‘‘tails’’ onto
the target, the acrylic holders were attached to theHamamatsu R580-17 phototubes using high-viscosity optical grease.
4.3. Superconducting spectrometer magnet
The heart of the muon spectrometer is asuperconducting toroidal magnet with 12 ironsectors separated by 12 gaps [13]. Each iron sector

is magnetized by a superconducting coil, and aﬁeld up to 1:8 T can be excited across the 20-cm
uniform gaps. At a ﬁeld strength of 0:9 T used forthis experiment, the ﬁeld is nearly completely
dipole with a slight toroidal component superposition. Charged particles from the target locatedin the center of the magnet are bent by B90 andtracked by MWPCs at the entrance and exit of
the gap. One sector of the magnet is illustrated inFig. 12.
In manufacturing the magnet special care wastaken to assure the dimensional accuracy necessary for high precision experiments. Each iron core
was machined with a precision of 50 mm: The
positioning of the superconducting coil relative tothe core to form a sector was achieved within an
accuracy of 2 mm: Assembly of the entire structure
was performed using a special positioning device
ensuring a rotational symmetry of 30 : The 12
median planes of the gaps converge to a virtual
central axis with an accuracy of 0:3 mm: The
difference of the diameters in the horizontal andvertical directions is less than 2 mm over adiameter of about 4:0 m:
As a superconducting magnet this device isunique in its structure. The coil windings are NbTi
monolith stabilized by Cu operated at about halfof the critical current density at 4:5 K at the
maximum excitation. The windings are cooled

GAP

COIL

IRON
CORE
0

1.0m

Fig. 12. One sector of the superconducting toroidal magnet.

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indirectly by two-phase He ﬂow through one turnof cooling channel around the windings.
The 12 coils are connected in series and equalﬁeld distributions are generated in the 12 gaps. A
power supply with stability of 5  105 is used.
Because high-quality magnet steel was used for thecores, there is no signiﬁcant hysteresis in magnetization. Nevertheless, a deﬁnite excitation cycle from0 T up to 0:9 T and from 0:9 T down to 0 T-has
been followed for the E246 experiment in order toassure the reproducibility of the ﬁeld strength.
The ﬁeld map necessary for tracking was
calculated by using a three-dimensional code
TOSCA [14]. The validity of the map could bechecked from the measured monochromatic momentum spectra of muons from Km2 and pionsfrom Kp2 :4.4. Tracking chambers
Charged-particle tracking through the spectrometer is crucial in rejecting other kaon decay
modes such as Kp2 and Kþ -pþ pþ=0 p=0 (Kp3 ) by
momentum measurement, and in ﬁxing the kinematics of Km3 for the determination of the decay
plane.
To meet these objectives the tracking chambers
needed to have position resolution better than 500
mm in sigma in the direction parallel to the magnet
pole. They must also be able to measure theposition of particles with steep incident angles(maximum 40 ), and have the capability to operate
in the non-uniform magnetic fringe ﬁeld of thetoroid up to 4 kG:
4.4.1. C1
Before the ring hodoscope was installed, the ﬁrst
tracking element outside the active target/ﬁducial
counters was an annular cylindrical drift chamber,
C1. It consisted of four layers of drift cells with acentral anode wire in each, between cylindricalcathode planes at the inner and outer radii of theannular cylinder. The anode signals from the drift
cells provided additional ðx; yÞ coordinates outside
the target, and were also able to detect photon
conversion in the outermost layers of the target.
The cathode planes were etched to form 45
spiral strips, with the inner and outer planes of73

opposite pitch, so that a measure of the axial track
coordinate z was obtained from the induced
charge distribution from the inner- and outermost
drift cell layers. Under the conditions of highintensity beam operation, the high rate experienced so near the stopping target proved the ringcounter hodoscope (Section 4.2.2) to be morereliable in extracting the z coordinate rather thanthe C1 cathodes.
4.4.2. C2, C3 and C4
Sets of three planar multiwire proportional
chambers (MWPC) were installed at each magnet
gap. One MWPC was installed at the entrance ofthe magnet gap (C2) and two were installed at theexit (C3 and C4).
These MWPCs utilize cathode readout in orderto save readout space around the chamber and tooffer high position resolution in the non-uniform
magnetic ﬁeld. Each chamber has two sets ofcathode planes: one for the azimuthal (or y)coordinate and the other for the polar (or radial)
coordinate. The 20 mm-diameter Au-coated W
anode wires run with 2-mm spacing parallel to themagnet gap median plane in order to provide
higher position resolution for the cathode planemeasuring the most sensitive coordinate for the
momentum reconstruction. The cathode strips
running perpendicular to the anode wires, are 9mm wide, spaced 1-mm apart, and consist of an
18-mm thick Cu coating on Kapton foil. The halfgap between the anode and the cathode planes is
6 mm:
A schematic view of the MWPC is shown in Fig.
13. Given the spatial constraints near the gap, one
important feature (in particular for C2) is the thin
Al frame which maximizes the sensitive area in thespace available.
The chambers were operated in proportionalmode with a 50:50 Ar:ethane gas mixture. The
three chambers in each gap are connected in series
with a gas ﬂow of 30 cm3 min1 from the gas
circulating system. The main parameters are
summarized in Table 4.
Signals from each cathode strip were ampliﬁed
in two stages. The ﬁrst was the preampliﬁer
mounted on the MWPC which drove the cablegoing into the counting house where the signal was

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1mm

9mm
HV.(+)

anode wire ( W: φ = 20µm )pre_amp.

6mm
post_amp.

2mm

cathode strip (kapton 25 µm: Cu 18 µm)
total 56 strips

total 16 strips

Fig. 13. Schematic view of the MWPC construction showing
the cathode strip orientation with respect to the anode wires.

Table 4Main parameters of the planar MWPCs
Anode wires
Anode pitch
Half gap
Cathode strips
Effective area
C2
C3
C4
Gas mixture
Readout

20-mm-diameter Au-coated W
2 mm
6 mm
9-mm width with 1-mm spacing
16  56 cm2
20  64 cm220  72 cm2
Ar:ethane 50:50
x; y cathode strips

further ampliﬁed by the main ampliﬁer. After themain ampliﬁer, charge information of each cathode was digitized in an ADC, after being multiplexed with signals from six chambers into one
output in order to reduce the number of ADC
channels.
The MWPC hit position was obtained using the‘‘charge-ratio method’’ [15]. In this method,
the analog signals of three strips—the strip withthe maximum charge and the two adjacent strips—
are used. Using data, the value
qmax  qmaxþ1
R¼
ð3Þ
qmax  qmax1
is calculated, where qmax ; qmaxþ1 ; qmax1 are themaximum induced charges in the three strips, andtabulated. The hit position is then estimated bytable lookup of values of R determined for 2-mm

steps across the 10-mm wide strip pitch using theinduced charge distribution as in Eq. (3).
Using the charge-ratio method there are two
major problems which cause systematic shifts of
the reconstructed position. One is caused byoblique incident particles. In our case the incident
angle could be as large as 40 : In such a steep
incident angle, the gas multiplication is no longer
point-like. Rather, it is distributed on the anodewire along a limited path. As a result, the induced
charge distribution differs from that in normal
incident tracks. To manage the oblique incident
case, charge-ratio tables for various incident
angles were prepared and used in the analysis.
This treatment reduced the systematic position
shift from 7600 to 750 mm: The position resolution ðsÞ was measured as a function of incident
angle and is shown in Fig. 14.
The other problem is caused by non-uniformityof the ampliﬁer gains. From test results the
maximum ﬂuctuation of the gains was as large as
20% and would result in a B500 mm systematic
shift in the reconstructed position. To minimize
this effect, gain calibration using ADC pedestal
information was applied for each ampliﬁer channel[16]. Using the calibration, the hill-valley structure
seen in the reconstructed position caused by nonuniform ampliﬁer gain was removed and theoverall position resolution was improved about afactor of two.
4.4.3. Gas recycler
The recycler was designed to provide gas ﬂows
up to 3:5 l min1 with recycle ratios from 0 to
100%. Reusable active ﬁlters in the system reduce
1.2
Resolution (mm)

74

1.0
o
0.8 400.6 30 o
0.4

20 o

0.2 10 o
0

0o

0

2

4
6
Position on the strip (mm)810Fig. 14. MWPC position resolution ðsÞ as a function of
position on the cathode strip for different particle incident
angles.

ARTICLE IN PRESS

SW1

MFC1
Ar

Partial Exhaust
MFM

PD

PD
MFM MFM

Buffer Tank

MFC3

MFC2
C2H6

75

Chambers

Ethanol
Bubbler

FilterFilter

J.A. Macdonald et al. / Nuclear Instruments and Methods in Physics Research A 506 (2003) 60–91

Pump
MixingBuffer

PID
Controller

MFC4
Storage
Buffer Tank

Pump
BufferFig. 15. Schematic diagram of the MWPC gas system.

water vapor and oxygen concentrations to below
5 ppm: A schematic diagram of the system isshown in Fig. 15.
Mass ﬂow controllers (MFCs) are set to deliverargon and ethane to prepare the required mixture
at a high ﬂow rate (5 l min1 Ar/ethane 50:50).
The ﬂow of the mixture is monitored by a mass
ﬂow meter (MFM) and the mixed gas ﬂows into amixing buffer. The pressure of the mixing buffer
measured by a pressure transducer (PD) controls
the MFCs, i.e., the MFCs are turned on whenever
the pressure in the buffer falls below the lower set
point and are turned off when the pressure rises
above the upper set point.
The gas supplied from the mixing buffer
combines with the recirculated gas from the pumpand ﬂows into one of two ﬁltration cartridges.
( molecular
Each cartridge contains one-third 4-A
sieve to remove water, and two-thirds BASF R311-activated copper for removal of oxygen. A
particulate ﬁlter removes all particulates larger
than 2 mm: After ﬁltration, the gas ﬂows throughthe temperature-controlled (typically 15 C) ethanol bubbler which adds ethanol if there is too little
in the gas stream and removes ethanol if there is
too much.
The gas then ﬂows through a manual valve anda solenoid valve to the distribution rack. To
protect the chambers from excessive over or underpressure, the solenoid valve will automaticallyclose if the pressure measured by the pressuretransducer moves beyond the preset upper andlower set points.

The returned gas from the chamber distribution
system ﬂows to a return buffer tank. The pressure
with respect to atmosphere at this point is
controlled by a pressure transducer, PID controller, and mass ﬂow controller and remains constantat the preset value of the PID controller.
The distribution rack allowed the gas supplied
from the recycler to be partitioned into 17 separate
gas circuits, then recombined into a single return
ﬂow back to the recycler. Manual ﬂowmeters(rotameters) and mass ﬂow controllers were used
to control and measure supply ﬂows in eachcircuit, and mass ﬂow meters measured returnﬂows from the chambers.
Gas contamination was checked periodicallywith a gas chromatograph.7 When a ﬁlter was
saturated and no longer able to absorb oxygen, aregenerated one was put into service by redirecting
the gas, and the saturated one was removed forregeneration in a controlled heating cycle in an
H2 =N2 gas ﬂow.
4.5. TOF counters
Charged particles were identiﬁed by measuring
TOF between the ﬁducial counter next to thetarget (see Fig. 9) and the TOF2 counters placed at
the exit of the magnet gaps, just behind the C4chambers, as shown in Fig. 3. Ideally for TOF
measurement, scintillation light should be collected from both ends of the scintillator to cancel
7

Shimazu Model GC-4C, Kyoto, Japan.

ARTICLE IN PRESS76

J.A. Macdonald et al. / Nuclear Instruments and Methods in Physics Research A 506 (2003) 60–91

the time jitter due to the light propagation time.
However, the ﬁducial counters are restricted to
one-side readout because of limited space aroundthe target. Therefore, we had to correct for thetime walk related to the particle hit position. The
20-cm axial length of the ﬁducial counters waschosen to match acceptance of the spectrometerand the stopping distribution of the 660-MeV/ckaon beam with momentum spread of B73%:
The main TOF system parameters are summarizedin Table 5.
The size of the TOF2 counter was 20 cm inwidth, 2 cm in thickness, and 80 cm in length. The
scintillation light was read from both ends. Since
there was a relatively strong fringing ﬁeld at theexit of the toroidal magnet, the phototubes had tobe located remotely (B50 cm from the scintillators). The scintillation light was reﬂected once andtransported to the phototube (Hamamatsu H1161)
through acrylic light guides. Output signals of the
ﬁducial counters and TOF2 counters were split byan analog divider to feed the leading edge
discriminator and the charge ADC. For timing
adjustment, the discriminator output signal wasdelayed and fed to a 25-ps bin TDC.
The hit time for the TOF2 counter ðtTOF2 Þ wasdeﬁned as the mean time measured by bothphototubes, while the time for the ﬁducial counterðtf Þ was corrected to giveTOF ¼ tTOF2  tTOF1 þ Dt :The offset term, Dt ; was calibrated using themono-energetic charged particles of pþ and mþfrom Kp2 and Km2 decay. In these decays, theparticle identiﬁcation could be done by analyzing
their momenta, and therefore an actual TOF was
calculated by using the information on ﬂight path
length and momentum. Then, the velocity ratio ofb ¼ v=c and mass squared of the charged particle
ðM 2 Þ were deduced asb¼

L;
TOF  cDimensions

counts / bin+

Scintillator
type
PMT

ð6Þ

e

where x is the particle hit position and cn is the
speed of light in the plastic counter used to apply
the photon propagation time correction. The value

Flight path
TOF resolution

M 2 ¼ p2 ð1=b2  1Þ

where L is the ﬂight length and p is the momentumof the charged particle. Fig. 16 shows a typical M 2
spectrum integrated for all momentum regions.
The timing resolution is approximately 270 ps
which is adequate to separate Km3 andKþ -p0 eþ n ðKe3 Þ decays.

ð4Þ

Table 5Main parameters of the TOF measurement system

ð5Þ

+

tTOF1 ¼ tf þ x=cn

of cn ¼ 18 cm ns1 was obtained in a test experiment. A time-walk correction of the leading-edge
discriminator was also applied. The actual TOF
between the ﬁducial and TOF2 counter was
obtained using an offset term to account for ﬁxed
relative time differences between the counters.

µ

2 m (typical)
270 ps ðsÞ
Fiducial-TOF1 (start)

TOF2 (stop)

Bicron BC408

Bicron BC408
2

Hamamatsu H3171-03 2  Hamamatsu
H1161
27n  10n  200 mm3 200  20  800 mm3nIrregular (see Fig. 9)

MTOF (MeV 2 / c 4 )

x 10

3

Fig. 16. Mass squared spectrum M 2 obtained in the TOF
analysis. The solid line is the experimental data and the dottedline is a Monte Carlo simulation.

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J.A. Macdonald et al. / Nuclear Instruments and Methods in Physics Research A 506 (2003) 60–91

5. Photon detector

Fig. 17 shows the crystal arrangement in thedetector. It has a barrel structure with two holeson the beam axis, and 12 azimuthal holes formuons to enter the spectrometer magnet gaps. The
optimization of the crystal array to maximize
overall acceptance was described in Section 2.5,
and the diameter of the beam-axis holes wasdetermined so as to accommodate beam instrumentation upstream, and the target and central
tracking assembly downstream.
The whole structure is upstream–downstream
symmetric. Around the target in the center of thebarrel, the inner diameter is 40 cm and the outer
diameter is 100 cm: There are 10 different crystal
shapes (see Fig. 17) used as a function of the polar
angle position. The angular segmentation is 7:5 inboth the polar and azimuthal directions except forthe crystals nearest the beam axis (Type 10). The
axis of each crystal points to the center of thetarget and has a length of 25 cm corresponding to13.5 radiation lengths. The main parameters of thebarrel are summarized in Table 6.
The crystal assembly is mounted in a cylindricalhousing with high mechanical precision. An
aluminum support structure bears the total weightof 1700 kg while minimizing material between the

Muon holes+µThe photon calorimeter was segmented into 768
CsI(Tl) crystals covering a large solid angle
surrounding the kaon stopping target of approximately 70% of 4p sr in order to measure thep0 -gg from Km3 decay with high acceptance. The
detector was optimized for the photon energy
range of 10–250 MeV and the high degree ofsegmentation provided good kinematic resolution
for the p0 ’s. In order to reject background fromother beam-associated accidentals, good timing
resolution is necessary. The very limited space inthe central part of the spectrometer magnet
required a compact design with a read-out
technique immune to the signiﬁcant fringe magnetic ﬁeld. Details of the system have been
described in Refs. [7,17]. In this section the basic
features are summarized.
5.1. Barrel structure

77

2
4
5
6
7
8
9
10

4

3

2

1

3

4

1

5
6

7
8

9

10

+
K

CsI crystals

Fig. 17. Barrel structure of the CsI(Tl) detector. The numbers
1–10 refer to the crystal types described in the text.

Table 6
Speciﬁcations of the CsI(Tl) photon detector
BarrelSegmentation
Total crystal weight
Inner diameter
Outer diameter
Detector length
Solid angle coverage
Modules
Crystal length
Surface treatment
Reﬂector
Mean light yield (Type 1–9)
Mean light yield (Type 10)
Container
PIN diodePower consumption

Dy ¼ Df ¼ 7:51700 kg
40 cm
100 cm
141 cm75% of 4p

25 cm ð13:5X 0Þ
Mirror polished (partiallysanded)Two layers of 120-mm
thick GSWP00010
10920 p:e:=MeV
17400 p:e:=MeV
100 mm thick Al can
18  18 mm2 ; 300 mm(Type 1–9)
25  25 mm; 500 mm(Type 10)
0:25 W per preampliﬁer

ARTICLE IN PRESS
78

J.A. Macdonald et al. / Nuclear Instruments and Methods in Physics Research A 506 (2003) 60–91

stopping target and the CsI(Tl) volume. Eachcrystal module is suspended from an outer rib
structure avoiding a load on the inner 3-mm thickAl cylinder. Care was taken in the precise
mounting to minimize gaps between crystal
modules which would degrade energy resolution.
The total assembly rolls on four small wheels on
two rails into position in the inner bore of thetoroidal spectrometer magnet.
5.2. CsI(Tl) modules
The crystals were grown at the Institute ofSingle Crystals in Kharkov, Ukraine, and shapedwith a precision of 150 mm for each type. The
structure of a crystal module is shown schematically in Fig. 18. The typical size of the modules is3  3 cm2 at the front end and 6  6 cm2 at theback end. Each crystal was wrapped with twolayers of 120-mm thick white diffuse ﬁlter paper8
and contained in a 100-mm thick Al shell. To
achieve good light-yield uniformity along the
crystal axis, before wrapping the mirror-polishedsurface of the crystals was ‘‘tuned’’ in the region
nearest the readout by roughening by sanding. The
light-yield uniformity is as good as 72% which is
sufﬁcient to provide linearity in energy measurement.
PIN photo-diodes of the type S3204-03 ð18 
18 mm2 ; 300-mm thick wafer) and S3584-05 ð28 
28 mm2 ; 500-mm thick wafer) were chosen forType 1–9 crystals and the larger Type 10 crystals,
respectively. 9 They were glued directly to the rearface of the crystal using a single-component type
SE777 silicone glue (see footnote 6) with shortcuring time and excellent adhesion between the
crystal and diode. A charge-sensitive preampliﬁer
was mounted just behind the diode attached to an
Al frame. Power consumption is 0:25 W for eachpreampliﬁer for a total detector heat load of 200
W, which is cooled by temperature-controlled dry
air ﬂow into the barrel cylinder. The outside ofeach Al shell was painted black to obtain electricalisolation and absorb any stray reﬂected light. An
8Millipore Corporation, GSWP00010.
Hamamatsu S3584–05 (CPD ¼ 200 pF; ID o100 nA) andS3204–03 (CPD ¼ 140 pF; ID o20 nA).
9

Reflector paper
Al-container

PIN diodeoptical fiber

CsI(Tl)PA

Positioning tip

Fig. 18. CsI(Tl) crystal module assembly.

optical ﬁber feeds calibration light to the back end
from a Xe-lamp monitoring system.
Average light yields for the modules are
summarized in Table 6 and almost all the modules
have light yield more than 8000 p:e:=MeV: Theenergy resolution of the modules was measured
using a 1.27-MeV22 Nag source to be 13–14%(FWHM), and the average equivalent noise level(ENL) was determined to be 63 keV:
5.3. Readout electronics
Each low-noise pre-ampliﬁer feeds a main
ampliﬁer (MA) with low-gain and high-gain outputs and incorporated timing-ﬁlter-ampliﬁer(TFA). The low-gain signal is fed to a peaksensing ADC as well as to a transient digitizer(TD) for energy measurement. The TFA provides
a 450-ns wide pulse to a constant-fraction discriminator (CFD) whose output signal is fed to a
0.7-ns bin TDC for time measurement. The gain
and time constant of the preampliﬁers were
optimized for the average energy deposit andcounting rate. The adjustable shaping time constant of the MA was typically 1:0 ms, and a polezero cancellation circuit and baseline restorer inthe main ampliﬁer allowed good high rate
performance; at 10 kHz average rate, energy
resolution did not degrade appreciably, keeping
92% counting efﬁciency.
The TD is based on a switched-capacitor-array(SCA) integrated circuit which allows waveformrecording with 12-bit accuracy at the maximum
sampling rate of 10 MHz [18]. It provided additional double-pulse resolving ability at higher rate,
supplementing the ADC and TDC systems. The
main characteristics are summarized in Table 7.

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J.A. Macdonald et al. / Nuclear Instruments and Methods in Physics Research A 506 (2003) 60–91
Table 7Main parameters of the transient digitizer

Table 8Performance of the CsI(Tl) photon detectorType
Bin size
Number of bins
Dynamic range
Conversion time
Linearity

Energy resolution s=E

Switching capacitor array + ADC
640 ns128
12 bit
820 ms for four multiplexed SCAs
o0:1%

Electric noise (incoherent)Electric noise (coherent)
Linearity
Spatial resolution (rms)Angular resolution (rms)Time resolution (rms)p0 mass resolution

79

4.3% at 100 MeV
2.8% at 200 MeV
250 keV ðsÞ for nine modules
11 keV per module
2% for 5–250 MeV
7:6 mm at 200 MeV
2.2–2:4
3:5 ns at 100 MeV
p7:5 MeV=c2

5.4. Xenon lamp monitoring system
The Xe-lamp monitor system provided test lightpulses to each module to detect failed photodiodes and to do other diagnosis of the readout
electronics. The light is distributed to each crystal
module through an optical glass ﬁber and a mixer
fanout box. The light signal is split in two stages,
24  32 to feed the 768 crystals. The distributors
provide uniform light to each crystal to within
712% with overall loss of 18 dB: The Xe lamp10
operates at a repetition rate of 10 Hz; and has anemission spectrum similar to CsI(Tl). The typicalresponse of a crystal corresponds to an energy
deposit of 50–100 MeV: The short-term stability ofpulse height was 2% (FWHM) which was monitored by a PMT which also provided the trigger
signal in the calibration measurement.
5.5. p0 detection performance
The CsI(Tl) calorimeter performance is summarized in Table 8. The energy resolution wasmeasured using tagged photon beams with variable energy in a prototype assembly of 30 similar
crystals assembled in a 5  6 matrix. Energy
resolutions of sE =E ¼ 2.8 and 4.3% were obtainedfor 200 and 100 MeV photons, respectively. In theactual detector with the 12 muon holes, however,
the resolution is dominated by lateral leakage of ashower resulting in a low-energy tail. Typicalresolution for the sum energy of two photons from
Kp2 decay is sE =E ¼ 4:1%: The single crystal
resolution of sE =E ¼ 1:7% was measured using
the 152-MeV mþ from Km2 decay by selecting
10

Hamamatsu L2189.

events with no energy deposit in the neighboring
crystals. The Km2 muons were used for the crystal
gain calibration. The electronic noise contribution
to the energy resolution was found to be negligibly
small; from an ADC pedestal measurement theincoherent contribution was sE ¼ 250 keV whensummed over nine crystals while the coherent
contribution was sE ¼ 11 keV per module.
For p0 kinematics determination, the photon
spatial resolution is essential. The directional
resolution of p0 ’s was sy ¼ 2:3 using a back-toback correlation between the p0 and pþ ofKþ -pþ p0 ðKp2 Þ decay, corresponding to a spatial
resolution of 7:6 mm: A typical time spectrum isshown in Fig. 19(a). For the energy range of 10–
200 MeV the resolution was sT ¼ 3:8 ns in theexperiment. Fig. 19(b) shows the energy dependence of the resolution.
Fig. 20 shows the invariant mass spectrum of
two photons from Kp2 decay using 5  5 crystal
clustering: (a) is the spectrum of the whole detector
with sMgg ¼ 6:7% at 129:6 MeV=c2 which includes
the deterioration due to shower leakage. The
resolution improves to 5.6% for events away fromthe muon holes (b) [19].

6. Muon polarimeter
By using the longitudinal ﬁeld method introduced in Section 2.2, the transverse polarization
component PT is preserved under the holding ﬁeld
and the in-plane components PL and PN are
precessed as illustrated in Fig. 2. The transverse
component is then detected as a left–right, orARTICLE IN PRESS
J.A. Macdonald et al. / Nuclear Instruments and Methods in Physics Research A 506 (2003) 60–91

80

E γ > 20 MeV

16.0

FWHM = 9.0 ns

Time resolution (ns)Counts

14.0
12.0
10.0
8.0
6.0
4.0
2.0
20

(a)

Time (ns)

(b)

40

60

80

100

120

140

160

180

Energy (MeV)

Fig. 19. Timing characteristics of the photon detector. (a) Time spectrum of all CsI crystals (center of clusters) with aligned peak
position for Km3 events. (b) Time resolution ðsÞ as a function of photon energy.

(a)

(b)

Fig. 20. Invariant mass spectra of two photons from Kp2 using 5  5 clustering: (a) all events reconstructed in the detector; (b) events
detected away from the muon hole area.

clockwise–counter-clockwise asymmetry of decay
positrons with respect to the muon stopper.
The structure of the polarimeter is shown in Fig.
21. The ﬁeld on the stopper was formed by ironshim plates which shaped the superconducting
magnet fringing ﬁeld. Its distribution on the muon
stopper was symmetric but not uniform as can beseen in Fig. 22.
To detect a tiny effect of PT ; the analyzing
power AT =PT should be as large as possible, andthere must be no depolarization of the stopped
muon spin. Any depolarization that could bematerial and ﬁeld strength dependent not only

reduces the effect but also has the potential tointroduce spurious effects.
6.1. Rotation of the polarization vector in flight
As the muon goes through the magnetic ﬁeld
in the magnet gap its spin rotates around the B
ﬁeld axis along with the trajectory. The effectof the spectrometer magnetic ﬁeld on the rotation of the muon polarization vector was studied
in a GEANT3 simulation [20] which included
the relativistic Bargmann–Michel–Telegdi equation [21] to treat polarization dynamics. Muon

ARTICLE IN PRESS
J.A. Macdonald et al. / Nuclear Instruments and Methods in Physics Research A 506 (2003) 60–91

81

On entering the Cu degrader and Al stopper,
depolarization takes place due to the relativistic
effect in multiple scattering. The degree of
depolarization can be evaluated both for longitudinal component ðPL Þ and transverse components (PN and PT ) [22] and was found to be
negligibly small. The spin dependence of Mott
scattering was also considered, since it may
generate left–right asymmetry in the muon stopping distribution giving an opposite effect forforward and backward pions potentially resulting
in a spurious T-violating asymmetry. However,this asymmetry is very small for small scattering
angles in multiple scattering where the crosssection is large.

6.2. Spin rotation field
Fig. 21. Schematic structure of the muon polarimeter.

110

Field distribution on y-z plane

105
100

z (cm)

95
90
+stopper

85
80

65

60
25 -20 -15 -10 -5

Fe shim70

Fe shim75

0

5

10 15 20 25

y (cm)
Fig. 22. Magnetic ﬁeld distribution on the muon stopper.

depolarization due to multiple scattering wasevaluated [22] separately and found to be unimportant. After being bent by about 90 through thegap, the projection of polarization on the medianplane of the gap is conserved as well as the medianplane perpendicular component. By integrating
forward or backward pion regions the net perpendicular component, which should be PT ; remains
the azimuthal polarization at the exit of the gap.

The conﬁguration of the muon degrader,
stopper, positron counters, and magnetic ﬁeld
shim plates was designed to optimize the polarization sensitivity and minimize spurious instrumental effects. This required a careful balance between
ensuring the necessary solid angle for positrondetection and avoiding asymmetries in the ﬁeld
distribution. The ﬁeld distribution the in r  fplane is shown in Fig. 22. If seen locally PN and PL
are admixed to the azimuthal component PT in thecourse of precession around the tilted ﬁeld line. It
was therefore essential to realize a perfect symmetry of the ﬁeld distribution across the magnetgap median plane so that any effect from the inplane components vanishes in total.
The position and structure of the shim plates
was designed using a three-dimensional ﬁeld
calculation code TOSCA [14]. The ﬁeld on themuon stopping distribution should be as high as
possible by guiding the fringing ﬁeld from thesuperconducting magnet so that it decouples thestray ﬁelds in the experimental area. In addition, apossible effect from coil misalignment in thecryostat should be negligible. There should be nosaturation in the iron in order to suppress
unwanted effects which might be material dependent and have hysteresis characteristics. Finally, asimple structure was preferred for ease of precise
installation.

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In the designed conﬁguration an average ﬁeld
strength in the region of stopping muons of 130 G
was realized for the 0.9-T magnet excitation. The
magnetization in the shim plates made of highquality magnet steel is 300 G maximum and well
below saturation.
Each plate was manufactured with a precision of
50 mm and the installation was done with anaccuracy of 200 mm relative to the magnet gap
median planes by using a specially prepared jig.
Field mapping was performed at 0.9-T excitation
using a three-dimensional Hall element mounted
on a specially made scanning device. The fourEuler-angle method was employed. Details of themeasurement in which the strength accuracy of
0:1 G and an angular accuracy of 1:0 mr areachieved was described in Ref. [23]. Three tilt
angles of the distribution were deduced fromthe measured ﬁeld maps, and are summarized inTable 9. These characteristics of the muon spinrotation ﬁeld are adequate for the present experiment.
6.3. Al stopper and material test
The muon stopper was required to be thickenough to stop muons, but it must also allow thepositrons to escape. An array of eight 6-mm thickpure aluminum slabs, 160-mm wide by 550-mm
long, with 8-mm spacing was used. The stopping
efﬁciency for Km3 muons was 78%. The material
we used was type 1N99 which corresponds to apurity of more than 99.99%. It is well known thatmuons imbedded in pure Al show neither initialloss of polarization nor spin relaxation [24]. Actual
material was studied in a mSR experiment withpolarized muons at the Meson Science Laboratoryof the University of Tokyo (UT-MSL). Muons
from the beam were stopped and allowed to decay
in the aluminum located in the center of two
Helmholtz coil magnets, allowing the applicationof both transverse and longitudinal magnetic ﬁelds
of variable strength. Located symmetrically upstream and downstream from the stopper were twoarrays of positron detectors.
The forward/backward asymmetry of muon
decay positrons was determined as the ratio
A¼

Nb  Nf
Nb þ Nfð7Þ

where Nb and Nf are the time-dependent positroncounts in the forward and backward counters,
respectively. After subtracting constant background components in Nb and Nf and normalization, the asymmetry A was ﬁt to a function
F ðtÞ ¼ A0 elt cos ðot þ f0 Þ

ð8Þ

where A0 is the initial asymmetry, l is the damping
rate determined by spin relaxation and ﬁeld
inhomogeneity, o is the precession angular velocity, and f0 is the small initial phase shift. Fig. 23
shows a typical asymmetry spectrum for a 30-G
ﬁeld. The measured damping rates for threedifferent ﬁeld strengths are listed in Table 10.

0.4

0.2

A

82

0

Table 9Magnetic ﬁeld characteristics on the muon stopper
Average ﬁeld strength
Accuracy of ﬁeld mapping
Field strength
Field vector angle
Asymmetry of the distribution
Offset in y direction
Rotation around z-axis
Rotation around x-axis

130 G
0:07 G
1:0 mr
0:9 mm0:15 mr
1:58 mr

-0.2

0

5

10

15

time (µs)

Fig. 23. The mSR spectrum in a 30-G transverse ﬁeld from asample of type 1N99 pure aluminum used for the polarimetermuon stoppers. The vertical axis is the asymmetry after
subtraction of the muon decay lifetime.

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Table 10
Measurement of mSR amplitude damping rate for Type 1N99
Al (> 99:99% pure) in various transverse ﬁeld strengths
Field (Gs)

Initial asymmetry

Damping rate ðms1 Þ

30
50
70

0:20870:001
0:20570:001
0:19970:001

0:006270:0012
0:008170:0010
0:005170:0010

83

Table 11
Maximum alignment deviation of polarimeter elements

Muon stopper
Positron counter

Dx (mm)Dy (mm)Dz (mm)

71
70:5

70:2
70:2

70:2
70:2

They are all small enough to indicate that spin
relaxation in this Al material is negligible.
6.4. Positron counter system
6.4.1. Structure
Two adjacent muon stopper arrays share one
positron counter assembly between them, being
the clockwise counter with respect to one gap andthe counter-clockwise counter of the neighboring
gap. Each of the 12 positron counter assemblies
comprises a three-fold segmented positron countersandwiched between two coincidence counters,
depicted as (B) and (A,C), respectively, in Fig.
21. Each segment of the positron counter is 10-mm
thick, 100-mm wide, and 840-mm long, so that the
total width is 300 mm: Each segment is read by
two mesh-type PMTs (Hamamatsu H6154), which
retain high gain in a magnetic ﬁeld. The3  380  920-mm3 coincidence counters were
designed so that their solid angles from any pointin the muon stopper are larger than that of thepositron counter. The triple coincidence among all
three layers conﬁrmed a positron free from thebackground, mainly photons and neutrons. The
geometrical acceptance of the positron counter
assembly is about 13% of 4p solid angle.
Each element in the polarimeter was accurately
positioned to better than 1 mm in order tominimize false asymmetry in the positron counting
due to instrumental mis-alignment. The maximumalignment deviations of the positron counters andmuon stoppers are listed in Table 11, and aresmaller than the calculated limitations.
6.4.2. Coincidence
A block diagram of the positron detection logic
is shown in Fig. 24. In order to obtain the time
spectrum of the positron from Km3 muons, theFig. 24. Block diagram of the positron detection logic.

coincidence information between the three-foldsegmented positron counter and the coincidence
counters was used as the TDC stop signal. The
TDC start was generated by the coincidence of
TOF2, the polarimeter trigger counter and theﬁducial counters. To reject other decay modes,
additional signals from other detectors wereavailable. The discriminator thresholds of thecounters were set to be sufﬁciently low (o200mV) in order to achieve close to 100% efﬁciency
for positrons. The triple coincidence reduced thecount rate to about 40% of that without thecoincidence counters by suppressing the background of mainly neutral particles. The PMT gain
drifts were monitored using the MIP peak positionin the ADC spectra.
Fig. 25 shows a typical ADC spectrum of theenergy deposition of minimum ionizing cosmic
rays in one positron counter, corresponding to a

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J.A. Macdonald et al. / Nuclear Instruments and Methods in Physics Research A 506 (2003) 60–91

Fig. 25. Positron counter ADC spectrum from cosmic rays.

light yield at the peak of 290 photoelectrons. An
additional calibration system using a xenon lamp
was also employed. Optical ﬁbers distributed thelight to all 36 scintillators of the three-segmented
positron counters. The system supplied a stable
photon yield to each PMT larger than that for
MIP, with a ﬂuctuation of less than 10%.
6.5. Asymmetry measurement and polarization
The measurement of muon polarization Pm isbased on the asymmetry of positrons emitted inmuon decays with a lifetime of tm ¼ 2:2 s: In thelongitudinal ﬁeld method, the angular distributionof the energy integrated positron counting rate,Neþ ðy; tÞ is expressed asNeþ ðy; tÞ ¼ N0 expðt=tm Þ½1 þ aPm cos y :ð9ÞHere, a is the analyzing power, Pm is the longitudinal muon polarization, and y is the emission
angle relative to the axis. (For an ideal case of nointeraction with matter of the emitted positron andno trajectory distortion, a is 1/3.) In the present
conﬁguration, the longitudinal component is theaverage transverse polarization in the azimuthal
direction, which can be determined from cw/ccw
asymmetry of time integrated positron counting
rates asNcw  Nccw
PT ¼ ða/cos yT SÞ1:
ð10Þ
Ncw þ Nccw
The analyzing power, a; included three experimental effects from (1) the ﬁnite solid angle of thecounters, (2) scattering and absorption in the Al
stopper, and (3) spin precession around themagnetic ﬁeld which is not everywhere parallel tothe azimuthal direction (see Fig. 22). The analyzing power a could be optimized
pﬃﬃﬃﬃﬃﬃﬃﬃﬃ to provide themaximum ﬁgure of merit, a Neþ : Experimentally,
it was checked using the Km3 data. By selecting p0 ’s
normal to the beam on the left side and right side,
one orients the in-plane mþ polarization PN
azimuthally in the stopper rather than PT in thecase of fwd or bwd p0 ’s. The positron asymmetry
AT was measured as the cw and ccw difference ofthe counting rate as in Eq. (10). The average PN
was calculated from a Monte Carlo simulation. ais then deduced as a ¼ AT =/PT S to be typically
0.28–0.30.
6.6. Veto counter system
One of the advantages of the present experimentis a relatively low background level at thepolarimeter compared with the previous in-ﬂight
experiments. However, the inﬂuence of the backgrounds due to beam particles cannot be eliminated completely. In order to reduce thesebackgrounds and study their characteristics, anarray of veto counters was installed, as shown in
Fig. 26.
The main components are the B-veto counters,
which cover the inner-radius region of thepolarimeter to isolate the polarimeter from thebeam. The P-veto and U-veto counters locatedbehind the muon stopper and positron counter,
respectively, were aimed at rejecting events such as
muons passing through or scattering off the muon
stopper. The S-veto counters, covering the outerradius region of the polarimeter, rejected muons
exiting the muon stopper to the outside of thepolarimeter. About 30% of the accidental background in the positron time spectrum could be
rejected by these veto counters. Also, by using
calibration data which were collected with a
special trigger, we found that 10% of Km3 decaymuons escaped the polarimeter and were rejected.

7. Trigger
The overall trigger scheme is shown in Fig. 27.
The ﬁrst-level trigger requires an incoming kaon

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crystals surrounding the muon hole corresponding
to the ﬁrst-level trigger vetoed the event in order tosuppress muons scattering off the crystals aroundthe muon hole. A rejection factor at the second
level trigger was about 1.7.
The third-level trigger required hits in the triple
coincidence of the positron counter arrays adjacent to gap i within 20 ms after the ﬁrst-level
trigger. The rejection factor of the third-level
trigger is about 13. Events which did not satisfy
either the second or third-level triggers by 20 ms
after the ﬁrst-level trigger were cleared. As aconsequence, about 110 events/pulse survived, and100 events were recorded to tape for analysis
ofﬂine. The breakdown of the trigger rates issummarized in Table 12.

magnet gapµ+
S-vetoP-vetoe+ counterU-vetoX

85

beam vetoYZ

beam axis

8. Data acquisition
8.1. Hardware

Fig. 26. The array of veto counters in the polarimeter.

followed by its decay daughter particle hitting aﬁducial counter and passing through a toroidal
magnet gap to the TOF2 counter. The coincidence
logic is deﬁned as
CK #

12
X

ðFIDi #TOF2i #PLi Þ

ð11Þ

i¼1

where CK is the kaon Cherenkov counter withmultiplicity larger than six PMTs hit, FIDi is a hitof the ith ﬁducial counter or its immediateneighbors, and TOF2i and PLi are hits in the
TOF2 and polarimeter trigger counters, respectively, in the ith gap.
The ﬁrst-level trigger rate was about 200/beamspill per gap, i.e. 2:4  103 =pulse in total, for 3 
105 incident kaons/beam-spill with a p=K ratio of
B6: The second-level trigger required hits in theCsI(Tl) photon detector. Analog signals from theCsI(Tl) crystals were discriminated by constantfraction discriminators (CFD) with an energy
threshold of 5 MeV; and then the existence of atleast one discriminated signal within a coincidence
time window of 150 ns was required. Hits in the

The E246 DAQ system was designed as aparallel processing system based on a VME bus,a TKO bus, and standard Fastbus and CAMAC
systems linked with workstations. The VME bus
was used as the main back-end system whichcollected all the data using a single-board computer on the VME bus.
The TKO system was developed and has beenused as a standard at KEK [25]. It was used toread data from ADC and TDC modules connectedto the scintillation counter systems and the
MWPCs.
Fastbus was used to read data from multi-hit
TDC modules fed from the polarimeter positroncounters and from the high rate beam counters.
The SCA transient digitizers for the CsI(Tl)photon counters were also mounted and read out
in Fastbus crates. CAMAC was used only for slow
monitoring and for trigger control. Workstations
were used for on-line monitors. The conﬁgurationof the DAQ system is shown in Fig. 28.
TKO and Fastbus have intelligent controllers
and buffer memories and work independently
crate by crate. The KEK-PS beam has a spill
structure that is 0.7-s beam on and 2.3-s beam off
and the DAQ system took advantage of thisARTICLE IN PRESS
J.A. Macdonald et al. / Nuclear Instruments and Methods in Physics Research A 506 (2003) 60–91

86
GAPS

THRESHOLD

TOF

1 o’clockFidoverlap

10 ns

Fid :

TOF

25 ns

TOF : 80 mV

PolTr

30 ns

PolTr : 100 mV

PolTr

AND
Fid

Out

VETO

overlap

TOF

2 o’clock

25 mV

PolTr

AND

10 ns

Fid

OR

VETO

Gap Selector

PolTr

Veto Reset

Veto
Generator

40 ns

Veto

AND

Veto Start

in ADC

Fid
Fid

Delay

ChP

Delay

AND
DISCR

35 ns

MULTIPLISITYTHRESHOLD

CherK :

>6

CherK :

60 mV

CherPi :>5

CherPi :110 mV

60 ns

8 mV

Delay
Generator

10 ns

Out

40 ns

Trigger is bound to Fid pulses.
1-st level trigger: CherK*Fid*TOF*PolTrig

CsI pulse

clock

gate

150 ns2-nd level trigger: 1-st*CsI pulse*Positron

Gate
Generator

1-st level
delayed trigger

gate

1-st trig
Gate OutCsI pulse

:

10 ns

CherK

1-st level

B0

Fid

21uS

clock

Gate Out

Delay

Delay Generator

Trigger Module

ChK

1-st level trigger

Veto

Computer Veto

60 ns

Gated Positron

VETO

delayed trigger

12 o’clock

TOF

150ns

Discriminator

overlap

PosTDCClear

DAQ ACPT
PosTDC Stop

Clear

Fig. 27. Block diagram of the trigger logic.

structure. During the spill beam-on period, thesystem only read data from the front-end modules
and stored the data into local memories in eachcrate. During the between spill beam-off period,
the stored data were transferred to the back-end
system, and gathered by the managing process.

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J.A. Macdonald et al. / Nuclear Instruments and Methods in Physics Research A 506 (2003) 60–91
Table 12
Break down of the total 12 gap trigger rate

The gathered data were stored in a large main
buffer which has typically about 32 Mbytes
capacity and then recorded onto DLT tape. A
part of the data were distributed to analysis
processes via ethernet for on-line monitoring.

3  1012 Protons/spill
1st level trigger
CK
CK #FID
CK #FID#TOF
CK #FID#TOF#PL

300 k/spill
125 k/spill
7.7 k/spill
2.4 k/spill

1st level # photon
1st level # photon # positron

1.4 k/spill
110 /spill

Live time fraction = 90%

100 /spill87

8.1.1. TKO system
The TKO intelligent sequencer, the SCH (VME
version of the control header, CH [26]) has many
functions for reading modules and works as abridge between TKO and the VME bus. On the
VME bus side, a partner module called the

X-terminal
Ether net

Work Station
DLT Tape Drive

Triggerlogic

HP-743
SMP
DPM

Trigger

Trigger control

VME

CAMAC

Condition
Monitor

ADC-module
TDC-module

TKO
Chambers
Photon detectors
Polarimeters
Counters

Multistop TDCTD, DSP sub system

8

FastBus

Active Target
Polarimeter

Fig. 28. A schematic view of the data acquisition (DAQ) conﬁguration.

4

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J.A. Macdonald et al. / Nuclear Instruments and Methods in Physics Research A 506 (2003) 60–91

SMP (S Memory Partner [26]) functions as a local
buffer memory for the TKO data and forcontrolling the SCH. A total of eight TKO crates
are used to read out the ADCs and TDCs for theplastic scintillator PMTs and the CsI(Tl) PIN
diodes, as well as the ADCs for the MWPCs.
8.1.2. Fastbus system
Fastbus was used to read multi-hit LeCroy
1877/1879 TDC modules with a time range longenough to cover several 2-ms lifetimes of the muon
decay. The LeCroy 1877/1879 has local buffer
memory in each module which was used in pipeline
mode. The conversion time of the multi-hit TDC
could be ignored in this experiment, which wasdone at a trigger rate of a few hundred Hertz.
To communicate between Fastbus and the VME
bus, we used a master processor unit in each of thefour Fastbus crates, which controlled the 1877 and1879 modules. This unit has a dual-port memory(DPM) which is connected to both the inner bus
and the VME bus to handle communication
between the two.
A command interpreter for controlling the
Fastbus system running on each unit received acommand from the VME system via the DPM toacquire data independently. During beam-off thiscommand interpreter initiated the data transferfrom each of the local memories to the VME bus
system also via the DPM.

RISC, 64 Mbytes memory) was used as the main
DAQ controller, running HP-RT, a real-time
operating system based on LynxOS. This module
was connected to a DLT tape drive for data
logging.
The slave modules for the other bus systems
that communicated with the VME bus masters
were eight SMP modules for the TKO system andfour AMSK DPM modules for the Fastbus
system. A second HP-753 module on the VME
bus running the UNIX-based HP-UX operating
system provided the development environment for
the HP-RT system. When the DAQ system wasrunning, this module was available to run on-line
analysis.
8.1.5. Data recording and on-line analysis
Two major features of the DLT tape format arehigh recording speed ð1:5 Mbytes s1 Þ and large
capacity (20 Gbytes). Our data rate was typically
2:4 Mbytes in each 0.7-s long spill every 3:0 s foran average 0:8 Mbytes s1 ; well within the capability of the DLT.
The on-line analysis mainly for monitoring data
quality used a distributed system over ethernet. A
fraction of the data were transferred via TCP/IP totypically three workstations—Sun ipx, Sun ipc,
and intel-PC/Linux.
8.2. Software

8.1.3. CAMAC system
The CAMAC system was used as a trigger
control and condition monitor. In the CAMAC
system, output registers, scalers, input registers,
scanning ADCs, and a crate controller were used.
The Kinetic Systems 3922 crate controller received
commands from a partner Kinetics 2940 which
resided in the VME crate. After each beam-on
spill, the CAMAC system scanned the spill
information, trigger conditions, and counter voltages for monitoring.

The DAQ software was based on UNIDAQ[27], a UNIX-like OS which was developed at
KEK and the Tokyo Institute for Technology(TIT). It includes a buffer manager using eight 4Mbyte standard memory data buffers, and aprocess control system. The data acquired during
one spill were stored into one buffer. This DAQ
program is an ensemble of single-function modular
processes which work together:novad is a buffer manager which accepts
requests from the other processes and issues
buffer pointers.
* xpc manages a process database, and checks
process status, e.g., whether the processes are
stopped illegally or not.
*8.1.4. VME bus system
The VME bus system functioned as master ofthese systems, controlling the other systems andreading the data collected by each system during
the spill. An HP-753 VME master module (PA-ARTICLE IN PRESS
J.A. Macdonald et al. / Nuclear Instruments and Methods in Physics Research A 506 (2003) 60–91

89

Histgram Viewer
ViewerHistogram
(PAW)

Data
Data
recording
Recordeing
process
process

Data analysis
process

Buffer Manager

Histgram
HistogrambufferMain data
bufferData Collecting
Process
Communication
with
Fastbus system

CAMAC
control

Fastbus controler
TDcontroller
controler
TD
Fastbus controler
CommunicationbufferTD controller
controlerTKO
control

Fig. 29. Schematic view of the DAQ software.

collector is a data collector, which controls
many devices and reads data, assembles andpacks data, and writes data into a buffer.
* operator is a system control interface for users
through which runs are started and stopped.
* recorder is a data recorder, which writes datafrom a buffer onto tape.
* analyzer is an online analysis process, whichgets a part of the data and makes low-level andhigh-level histograms.
*

A schematic view of the DAQ software is shown in
Fig. 29.
8.2.1. Collector
The data collection system consists of two
programs, the collector working on the VME
HP-753, and the CIP working on the Fastbus
68020FPI in each crate. The collector communicates with the buffer manager, controls andcollects data from TKO, and communicates with

the CIP and gets data from Fastbus. The CIP
communicates with the collector, and handles
control and data collection in each Fastbus
crate.
8.2.2. On-line analysis
The on-line monitor system is also divided
between two programs. The analyzer gets datafrom the buffer manager via ethernet and forms
the histograms, and the other is a histogram
viewer. The analyzer was compiled to run on bothSun UNIX and Linux based computers on the
network, requiring Sun compatible IPC (Internal
Process Control) and socket libraries. The analyzer
and histogram viewer worked independently withshared memory.
Viewers based on PAW/Hbook, an analysis tool
developed by CERN11 and on Histo-Scope which
11
CERN Program Library, IT division/CERN, http://cernlib.web.cern.ch/cernlib/.

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J.A. Macdonald et al. / Nuclear Instruments and Methods in Physics Research A 506 (2003) 60–91

system typically operated with a live time efﬁciency
from 85% to 95%.

Live time efficiency

General Run condition

9. Summary
Special Run condition

0.5msec dead time

1.0 msec dead time

Trigger rate (Hz)Fig. 30. DAQ efﬁciency as a function of trigger rate.

was developed by FNAL12, were used. PAW
includes many powerful functions useful for online analysis, while Histo-Scope is convenient to
use interactively.
8.3. DAQ system performance
The detector has about 2000 output channels in
total. One event consisted of about 1600 32-bit
words, and at a typical trigger rate of about 100–
200 events per spill, the data size of one spill was
about 0.7–1:4 Mbytes:
The data taking speed differed in each crate of
the TKO system depending on module conﬁguration. The trigger timing and the wait for A/D
conversion were tuned individually. The slowest
crate needed 150 ms for conversion and 350 ms forreading for a dead time of 500 ms: The Fastbus
system used a pipeline mode so that A/D conversion time was negligible, but the readout time wasabout 500 ms; so that the dead time of this system
was also about 500 ms: The efﬁciency of the DAQ
system depends on the trigger rate and thecorrelation between the trigger rate and the live
time efﬁciency is shown in Fig. 30. The DAQ
12Fermilab Computing Div. Library, http://www-pat.fnal.gov/nirvana/histo.html.

The detector built for the search for a Tviolating transverse muon polarization in Km3
decay at rest included systems for identifying andstopping the low-momentum kaon beam, andprecise measurement of the kinematics of charged
decay daughters in a toroidal spectrometer and ofphotons in a segmented Cs(Tl) calorimeter. The
muon polarimeter used pure aluminum stopping
material to avoid depolarization, and used theshimmed fringe magnetic ﬁeld of the superconducting toroid to provide a longitudinal holdingﬁeld for the azimuthal polarization component.
Emphasis on precision component alignment anda high degree of detector symmetry allowed manysources of potential systematic errors to beeliminated or minimized.
For the primary motivating Km3 T-violation
search, systematic errors in the physics analysis
have been reduced below the 103 level [5].
Because of the versatility of the detector, it has
also been used to study other Kþ decay modes[28]. The detector as described here was dismantled
in early 2002.

Acknowledgements
This work was supported in Japan by a Grantin-Aid from the Ministry of Education, Science,Sports and Culture, and by JSPS; in Russia by the
Ministry of Science and Technology, and by the
Russian Foundation for Basic Research; in Canada by NSERC and IPP, and by infrastructure of
TRIUMF provided under its NRC contribution;
in Korea by BSRI-MOE and KOSEF; in the USA
by NSF and DOE.
The authors thank S. Iwata, M. Kobayashi,
V.M. Lobashev, V.A. Matveev, K. Nakai, K.
Nakamura, J.-M. Poutissou, V.A. Rubakov, H.
Sugawara, E. Vogt, S. Yamada and T. Yamazaki
for encouragement in executing the present work.
They gratefully acknowledge the excellent support

ARTICLE IN PRESS
J.A. Macdonald et al. / Nuclear Instruments and Methods in Physics Research A 506 (2003) 60–91

received from several staff members of KEK, INR
and TRIUMF, in particular, T. Fujino, E.
Shabalin and N. Khan. They also recognize theearly contributions of P. Bergbusch, C. Chen, D.
Dementyev, M. Grigoriev, W. Keil and I.S. Park.

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E288/E337/E393(MULTI): T. Murakami
Target Multifragmentation by GeV Energy Proton
K. H. Tanaka et al.
Energy Spectra and Angular Distributions of IMFs Produced in 8 GeV Proton and 12 GeV Proton and
20
Ne Induced Multifragmentation Reactions
Proc. of 9th Int'l Conf. on Nuclear Reaction Mechanisms, Varenna, Italy, June 2000
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◇ T. Murakami et al.
Temperature Distributions in 12-GeV and 8-GeV Proton Induced Multifragmentations
Proc. of 9th Int'l Conf. on Nuclear Reaction Mechanisms, Varenna, Italy, June 2000
Universit 'a Degli Studi di Milano, Supplemento N.115, p.315.
◇ T. Murakami et al.
Comparative Study of Target Multifragmentation Induced by 12-GeV Proton and 12 GeV 20Ne
7th Int'l Conf. on Nucleus Nucleus Collisions (NN2000), Strasbourg, France, July 2000.
◇ T. Murakami et al.
Fragment Formation in GeV-Energy Proton and Light Heavy-Ion Induced Reactions
Proc. of Int'l Sympo. on Clustering Aspects of Quantum Many-Body Systems, Kyoto, Japan,
Nobember 2001
“Clustering Aspects of Quantum Many-Body Systems'' (World Scientific, Singapore, 2002) p.177.
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2002 Nuclear Chemistry GORDON CONFERENCE, New London, USA, June 2002.
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Multifragmentation Induced by Light Heavy-Ions with GeV energy
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◇________________________________________________________________________● Physics papers published in refereed journal.
○★
◇＊

Technical papers.
PhD theses.
Conference and Symposium.
Internal Report and others.

Multifragmentation Induced by Light Heavy-Ions with GeV Energy
Tetsuya MURAKAMI
Department of Physics, Kyoto University, Kyoto 606-8502 Japan
E-mail: murakami@nh.scphys.kyoto-u.ac.jp
andTHE KEK-PS E337/E393 and NIRS-HIMAC P052 COLLABORATION
M. Hagaa , M. Hasenob , Y. Hiraic , H. Itoc , M. Itohc , T. Kawabatac,
K. Kimurab , F. Kosuged , R. Kuboharad, S. Miharac, K. Miyazakic ,S. Morinobue , R. Muramatsuc , J. Muratac , Y. Nagasakab, K. Nakaid ,H. Ochiishie , Y. Ohkumaa , Y. Okunob , S. Otac , T. Shibataf ,Y. Shibatad , Y. Sugayag , E. Takadah , H. Takedac, K.H. Tanakaf ,Y. Tanakab , Y.J. Tanakac , S. Terashimac , M. Uchidac , K. Ushieb ,Y. Yamanoif , K. Yasudac , and Y. YasudacaInstitute of Physics, Tsukuba University, Tsukuba 305-8577, Japanb
Nagasaki Institute of Applied Science, Nagasaki 851-0193, Japanc
Department of Physics, Kyoto University, Kyoto 606-8502, JapandFaculty of Science and Technology, Science University of Tokyo, Noda 278-8510, Japane
Department of Physics, Kyushu University, Fukuoka 812-8581, Japanf
KEK: High Energy Accelerator Research Organization, Tsukuba 305-0801, Japang
Department of Physics, Osaka University, Toyonaka 560-0043, JapanhNational Institute for Radiological Science, Chiba 263-8555, Japan

ABSTRACT
We have extended our investigations on nuclear multifragmentations induced by
GeV-energy light projectiles. By using light heavy-ion beams (16 O, 20 Ne and28Si) provided from the Heavy-Ion Medical Accelerator in Chiba (HIMAC) at
the National Institute for Radiological Science (NIRS) we studied similarities
and diﬀerences among various fragment formation reactions in the same totalenergy domain. We found there are noticeable diﬀerences in spectrum shapesof Intermediate-Mass Fragment (IMF) and their magnitudes depending upon
the projectile mass, but all of their angular distributions clearly exhibit theexistence of sideward-peaked components, indicating fragment formations aremainly governed by a total energy of the projectile, not by a incident energy per
nucleon.

1. Introduction
Our collaboration, the MULTI, has been exploring thermo-dynamical nature ofhighly excited nuclear system produced in GeV-energy nuclear reactions for last few
years.1,2,3 We selected the GeV-energy domain as a region of interest, because possible precursor phenomena of liquid-gas phase transition in the nuclear matter havebeen reported for light-projectile induced reactions in this energy domain. For example, (i) the production cross section for an Intermediate Mass Fragment (IMF) as afunction of the beam energy clearly shows plateau above 10 GeV.4 (ii) The energy
541

spectra of IMFs qualitatively change their shapes at around 5 GeV.4,5,6 (iii) The IMF
angular distributions show the transition from forward to sideward-peaking in the energy domain of 3 to 11.5 GeV.7,8,9,10,11,12 (iv) The power-law index τ for the inclusive
element-distribution, σ(Z)∝Z−τ , as a function of the incident energy exhibits minimum at about 4 GeV.4 There is, however, no direct evidence that these phenomena
strongly link to the phase-transition.
In order to understand those phenomena several groups in the world have initiated research projects on the multifragmentation in the GeV-energy domain. Recently the ALADIN collaboration at GSI successfully extracted a so-called “nuclear
caloric curve,” namely temperature as a function of the average excitation energyof the decaying system, obtained from measurements of the isotope yield ratios forthe highly excited nuclear system produced through a Au + Au reaction at 600
MeV/nucleon. Their caloric curve behaves just like the one of water at normal pressure of 1 atmosphere13 . More recently the IUCF group demonstrated that nuclear
liquid-gas phase transition does occur in the GeV-energy light-projectile induced multifragmentation reactions14,15 by using 5.0-14.6 GeV/c proton and π − beams, and 8.0
GeV/c anti-proton and π − beams at AGS together with their powerful Indiana Silicon Sphere (ISiS) 4π array16 . Our group also extracted a caloric curve from 8 and
12 GeV proton induced target multifragmentation (TMF) data, which suggested thatthe existence of nuclear liquid-gas phase transition17 . All these ﬁndings seem tosuggest that the multifragmentation is a statistical decay process from a thermallyequilibrated system.
There are, however, still several unresolved questions. For example we still do notknow why the character of the IMF angular distributions evolves from forward-peaked
to sideways-peaked in the energy regime of 3 to 12 GeV. Appearance of sidewardpeaking angular distribution could be very disturbing because it might imply thatthe emission source of IMFs is not thermally equilibrated or not spherical as usuallyassumed in many theoretical models for the multifragmentation. Although a great
deal of eﬀorts has been made on explaining this phenomenon in the past (shock
wave formation18 , nuclear-doughnuts formation19 , signiﬁcant transverse momentumof remnant from intra nuclear cascade12 ), there is little agreement on the origin of the
sideward peaking. Considering the fact that 5.5 GeV proton induced TMF did notshow sideward peaked nature20 while our 20 GeV α data did exhibit clear sideward
peaked character similar to the one observed in the 12 GeV proton data1 , we suspectthat the interpretation proposed by Hsi et al.12 is not well-founded.
Last year a nuclear theory group at the Hokkaido University reported results of
their calculations based on both transport model and non-equilibrium percolation
model21 to account for our 12 GeV proton data. They suggested sideward peaking
angular distribution might arise from a “doughnut shape” of the IMF emission source.
Inclusion of Coulomb expansion seems to be essential to account for a sideward-peaked
angular distribution of IMFs. They also urged us to accumulate more systematic data542

on the incident energy dependence of the IMF angular distribution. In order to obtain data in the wider incident energy range and to investigate a possible scaling
rules22,23,24,25 and/or density eﬀect in the TMF reaction, we moved our entire experimental setup to the HIMAC after the completion of the experiments at the KEK-PS.
2. Experimental Procedure
New experiments were performed with 16 O and 20 Ne beams of 8 GeV, 28 Si beams
of 8.12 GeV, 20 Ne beams of 12 GeV, and 28 Si beams of 22.4 GeV accelerated by the
HIMAC at the NIRS. Typical beam intensity was about a few times 108 particles/sec.
with a beam-spot size of roughly 10 mm in diameter. A metallic Au target of about 0.6
mg/cm2 was mainly used for the measurements. Intermediate Mass Fragments (IMFs;
3≤Z≤20) were detected and particle-identiﬁed with the 37 Bragg Curve Counter(BCC) array, which was almost the same as the one used for the previous experimentswith 8- and 12-GeV proton beams at the KEK-PS. As counter gas we used the P10 (Ar
90% and CH4 10%) gas of 300 Torr instead of 200 Torr. In order to measure angular
distributions of the IMFs 12 BCCs were placed in the horizontal plane including thetarget and the other 25 were set in upper hemisphere in order to measure the IMF
multiplicity. They covered an angular range of θlab = 30◦ -150◦ in 20◦ steps and a solid
angle of nearly 20% of 4π. The technical details of our BCCs have been published inRef. 25. The data were taken under the minimum bias condition; i.e. a sum of 37
self-triggers from individual BCC.
Energy Spectra of IMFs at 90˚
20 GeV α

100

200

O
0

12 GeV 20Ne

O

250

F

200

Ne

200

0

50

F

100

Ne

100

Na

100

22.4 GeV 28Si

O

O

250

0

0

F

200

Ne

200

F

2

d σ/dEdΩ (µb/MeV sr)

12 GeV Proton

0

0

50

0
50

0

500

0

0
200

20
0

0

50

100

150

0

0
200

Mg

0

Al

100

Si

100

0

Al

100

Si

100

0

50

Energy (MeV)

Na

Mg

100

0

Si

20

0
200

0

Mg

50

0

50

100

Ne

Na

0

Al

0

Na

0
100

Mg

0

0

150

0

Energy (MeV)

Al

0

0

50

100

150

Energy (MeV)

0

Si

0

50

100

150

Energy (MeV)

Figure 1: Inclusive energy spectra of IMFs for 12 GeV proton, 20 GeV α, 12 GeV
20
Ne and 22.4 GeV 28 Si induced TMFs at 90◦ .
543

3. Experimental Results
Inclusive IMF energy spectra for the GeV-energy light heavy-ion induced TMFs
are shown together with those for the proton and α induced ones at the similar beamenergies in Fig. 1 and 2. In order to avoid a shape-distortion arising from a movementof emission-source we select the data at 90◦ in the laboratory system. As can be seenin the ﬁgures all spectra show usual Maxwell-Boltzmann distribution-like shapes, butabsolute cross sections for the 16 O, 20 Ne and 28 Si induced TMFs are about 4-5 times
larger than those for the proton induced ones. Coulomb peaks become wider andhigh energy tails become longer as the projectile mass increases.
Energy Spectra of IMFs at 90˚
8 GeV 16O

8 GeV 20Ne

8.1 GeV 28Si

50

O

O

200

0

0
200

0
200

200

F

O

200

0

F

25

O

200

F

F

2

d σ/dEdΩ (µb/MeV sr)

8 GeV Proton

0

2000

Ne

25

0

0

20

Na

100

Mg

100

0

0
20

0

50

100

150

Energy (MeV)

0

0

Mg

100

100

Si

100

0

Al

100

Si

100

0

0

50

100

150

0

Energy (MeV)

Mg

100

0

Al

Na

100

0

Mg

0
100

Si

0

Na

0

0
20

Ne

2000

100

Al

0
200

Ne

Na

0

20

0

0
200

Ne

Al

0

0

50

100

150

Energy (MeV)

0

Si

0

50

100

150

Energy (MeV)

Figure 2: Inclusive energy spectra of IMFs for 8 GeV proton, 8 GeV 16 O, 8 GeV 20 Ne
and 8.12 GeV 28 Si induced TMFs at 90◦ .
In Fig.3 inclusive angular distributions of Na for 8 GeV proton, 8 GeV 16 O, and 8
GeV 20 Ne induced TMFs are shown. The location of the Coulomb peaks shifts more
rapidly as a function of the emission angle for the 16 O and 20 Ne induced TMFs thanfor the proton induced one, indicating the emission sources of IMFs move faster forthe 16 O and 20 Ne projectile cases than for the proton beam case. In fact results ofthe moving source analysis suggest that the velocity of the moving source increases
as the mass of projectile increases. It was also found that the moving source model
always underestimates yields at sideward angles even for the 16 O and 20 Ne induced
TMFs, especially for heavy IMFs (Z≥10).
To see this underestimation more clearly, we integrated the IMF yields within a544

Inclusive Angular Distribution of Na
8 GeV

16

O induced TMF

8 GeV

20

Ne induced TMF

200
20

30deg.

100

50deg.

100

70deg.

100

90deg.

100

02030deg.

030deg.

0200
50deg.

50deg.

2d σ/dEdΩ (µb/MeV sr)

8 GeV Proton induced TMF

0
20

0

0
20

0

0
20

100

130deg.

100

0

0
200

150deg.

100

0
200

0

110deg.

0200130deg.

00 20 40 60 80 100120140
Energy (MeV)

90deg.

110deg.

002070deg.

90deg.

0110deg.

0
20

0
200
70deg.

130deg.

0200
150deg.

0 20 40 60 80 100120140
Energy (MeV)

150deg.

00 20 40 60 80 100120140
Energy (MeV)

Figure 3: Inclusive angular distribution of Na for 8 GeV proton, 8 GeV 16 O and 8
GeV 20 Ne induced TMFs.
certain dynamic range in the laboratory system, took a ratio to the integrated ﬁttings
and then obtained angular distributions of relative yield to the moving source model
for various IMFs of Z≤16. As can be seen in the Fig. 4, 5, and 6 the resultant ratios
show clear peaks at around 110◦ in the laboratory system for the 8 GeV 20 Ne and
8 GeV 16 O induced TMFs, which is slightly backward compared to the 70◦ for the
proton induced TMFs at the same energy. The maximum value in the ratio evidently
increases as a function of a mass of IMF and seems to decrease as a function ofa projectile mass for very light IMFs (lighter than F nucleus). This trend is also
visible and slightly stronger in the 12 GeV data. These facts probably indicate thatthough there might be some eﬀect arising from the nuclear density prior to the target
multifragmentation, a total beam energy plays a decisive role to govern a reaction
dynamics for the IMF emission.
4. Summary and Future Plan
We have studied similarities and diﬀerences among the IMF formation processes
in various light-projectile induced reactions in the GeV-energy domain. we foundthere is signiﬁcant diﬀerence in the IMF spectrum shapes and magnitudes but all of
their angular distributions show the existence of sideward-peaked components. This
fact probably suggests that the IMF formations are mainly dictated by a total energyof the projectile (i.e. the total energy scaling exists).
545

8GeV Ne Target=Au
2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

O

0

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

90

180

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

Mg

0

0

Angle (deg.)

90

180

180

90

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6
180

90

180

Na

0

Angle (deg.)

90

180

Angle (deg.)Si

inclusive0

Angle (deg.)

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

Ne

0

Angle (deg.)Al0Angle (deg.)90

E/A 2-3 MeV

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

F

90

multiplicity=2

180

Angle (deg.)

Figure 4: Angular distributions of relative yield to moving source ﬁt for 8 GeV 20 Ne
induced TMF on gold target.
8GeV O Target=Au
2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

O

0

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

90

180

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

Mg

0

0

Angle (deg.)

90

180

Angle (deg.)

90

180

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

90

180

Angle (deg.)

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

Ne

0

Angle (deg.)Al0

E/A 2-3 MeV

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

F

90

180

Angle (deg.)

Na

0

90

180

Angle (deg.)Si

inclusive
0

90

180

Angle (deg.)

multiplicity=2

Figure 5: Angular distributions of relative yield to moving source ﬁt for 8 GeV 16 O
induced TMF on gold target.
As for a near future plan, we would like to ﬁnalize our analysis on 28 Si induced
multifragmentation at 8.12 GeV and 22.4 GeV, and to compare their results withthose from light-ion induced multifragmentation in details. We also want to extract“isotope temperatures” for all light heavy-ion induced TMFs in order to investigate
a possible density eﬀect in an early stage of the nuclear collision.
Acknowledgements
The authors would like to thank Profs. M. Takasaki, K. Nakamura, S. Iwata,
S. Yamada and H. Sugawara of KEK and Dr. Takeshi Murakami of NIRS for theircontinuous supports. Accelerator crews and beam channel members of the KEKPS and the NIRS-HIMAC are highly appreciated for their excellent machine opera-

546

8GeV Proton Target=Au
2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

O

0

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

90

180

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

Mg

0

0

Angle (deg.)

90

180

Angle (deg.)

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

F

90

180

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

Al

0

90

180

Angle (deg.)

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

Ne

0

Angle (deg.)

E/A 2-3 MeV

90

180

Angle (deg.)

Na

0

90

180

Angle (deg.)Si

inclusive
0

90

180

Angle (deg.)

Figure 6: Angular distributions of relative yield to moving source ﬁt for 8 GeV proton
induced TMF on gold target.
tion. This work was supported in part by a Grant-in-Aid for Scientiﬁc Research (B)(No.1144078) and (B) (No.11440084) of the Ministry of Education, Science, Sports
and Culture.
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2. K.H. Tanaka et al. Proceedings of the 8th International Conference on Nuclear
Reaction Mechanisms, Varenna, Italy p.208, (1997).
3. K.H. Tanaka et al. Proceedings of the 9th International Conference on Nuclear
Reaction Mechanisms, Varenna, Italy p.323, (2000).
4. N.T. Porile et al., Phys. Rev. C 39, 1914 (1989).
5. S.J. Yennelo et al., Phys. Rev. Lett. 67, 671 (1991).
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7. L.P. Remsberg and D.G. Perry, Phys. Rev. Lett. 35, 361 (1975).
8. D.R. Fortney and N.T. Porile, Phys. Rev. C 21, 2511 (1980).
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10. U.A. Urbon et al., Phys. Rev. C 21, 1048 (1980).
11. K.H. Tanaka et al., Nucl. Phys. A 583, 581c (1995).
12. W.-c. Hsi et al., Phys. Rev. C 60, 034609 (1999).
13. J. Pochodzalla et al., Phys. Rev. Lett. 75, 1040 (1995).
14. M. Berkenbusch et al., Phys. Rev. Lett. 88, 022701 (2002).
15. J.B. Elliott et al., Phys. Rev. Lett. 88, 042701 (2002).
16. K. Kwiatkowski et al., Nucl. Instrum. Methods A 360, 571 (1995).
17. T. Murakami et al.,Proc. of Int. Sympo. on Clustering Aspects of Quantum
Many-Body Systems, p.177, ed. A. Ohnishi et al. (World Scientiﬁc, Singapore,2002).
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18. A.E. Glassgold, W. Heckrotte, and K.M. Watson, Ann. Phys. (N.Y.) 6, 1
(1959).
19. T. Maruyama and K. Niita, Prog. Theor. Phys. 97, 579 (1997).
20. A.M. Poskanzer et al., Phys. Rev. C 3, 882 (1971).
21. Y. Hirata et al., Nucl. Phys. A 707, 193 (2002).
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23. J.B. Cumming, P.E. Haustein, and R.W. Stoenner, Phys. Rev. C 33, 926(1986).
24. A. Schüttauf et al., Nucl. Phys. A 607, 457 (1996).
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548

1

Total Energy Dependence of Angular Distributions of IMFs Produced inProton and Heavy-Ion Induced Target Multifragmentation Reactions ataround 10 GeV
T. Murakamia and K.H. Tanakab for the MULTI Collaboration∗aDepartment of Physics, Kyoto University, Kyoto 606-8502, Japanb

High Energy Accelerator Research Organization(KEK), Tsukuba 305-0801, Japan

We have extended our investigations on nuclear multifragmentations induced by GeVenergy light projectiles using light heavy-ion beams (16 O, 20 Ne and 28 Si) provided fromthe Heavy-Ion Medical Accelerator in Chiba (HIMAC) at the National Institute for Radiological Science (NIRS). At a total beam energy of 8 GeV we found there are noticeable
diﬀerences in spectrum shapes of Intermediate-Mass Fragment (IMF) and their magnitudes depending upon the projectile mass, but all of their angular distributions clearly
exhibit the existence of sideward-peaked components, indicating fragment formations aremainly governed by a total energy of the projectile, not by a incident energy per nucleon.
1. Introduction
Our collaboration, the MULTI, has been exploring thermo-dynamical nature of highly
excited nuclear system produced in GeV-energy nuclear reactions for last few years[1–3].
We selected the GeV-energy domain as a region of interest, because possible precursor
phenomena of liquid-gas phase transition in the nuclear matter have been reported forlight-projectile induced reactions in this energy domain[4]-[12]. There is, however, nodirect evidence that these phenomena strongly link to the phase-transition.
Recently the ALADIN collaboration at GSI successfully extracted a so-called “nuclear
caloric curve,” namely temperature as a function of the average excitation energy of the
decaying system, obtained from measurements of the isotope yield ratios for the highly
excited nuclear system produced through a Au + Au reaction at 600 MeV/nucleon. Theircaloric curve behaves just like the one of water at normal pressure of 1 atmosphere[13].
More recently the IUCF group demonstrated that nuclear liquid-gas phase transition
does occur in the GeV-energy light-projectile induced target multifragmentation (TMF)
reactions[14,15] by using either 5.0-14.6 GeV/c proton and π − beams, or 8.0 GeV/c antiproton and π − beams at the AGS together with their powerful ISiS 4π array[16].
Though these ﬁndings seem to suggest that the multifragmentation is indeed a statistical decay process from a thermally equilibrated system, there are still several unresolved∗

Kyoto Univ., KEK, Kyushu Univ., NIAS, NIRS, Osaka Univ., Science Univ. of Tokyo and Tsukuba
Univ. Collaboration

2
questions. For example we still do not know why the character of the IMF angular distributions evolves from forward-peaked to sideways-peaked in the incident energy regime of
3 to 12 GeV. Appearance of sideward peaking angular distribution could be very disturbing because it might imply that the emission source of IMFs is not thermally equilibrated
or not spherical as usually assumed in many theoretical models for the multifragmentation. Although a great deal of eﬀorts has been made on explaining this phenomenon in
the past (shock wave formation[18], nuclear-doughnuts formation[19], signiﬁcant transverse momentum of remnant from intra nuclear cascade[12]), there is little agreement on
the origin of the sideward peaking. Considering the fact that 5.5 GeV proton induced
TMF did not show sideward peaked nature[20] while our 20 GeV α data did exhibit clear
sideward peaked character similar to the one observed in the 12 GeV proton data[1], we
suspect that the interpretation proposed by Hsi et al.[12] is not well-founded.
Last year a nuclear theory group at the Hokkaido University reported results of theircalculations based on both transport model and non-equilibrium percolation model[21]to account for our 12 GeV proton data. They suggested sideward peaking angular distribution might arise from a “doughnut shape” of the IMF emission source. Inclusion of
Coulomb expansion seems to be essential to account for a sideward-peaked angular distribution of IMFs. They also urged us to accumulate more systematic data on the incident
energy dependence of the IMF angular distribution. In order to obtain data in the wider
incident energy range and to investigate a possible scaling rules[22–25] and/or density
eﬀect in the TMF reaction, we moved our entire experimental setup to the HIMAC after
the completion of the experiments at the KEK-PS.
2. Experimental Procedure
New experiments were performed with 16 O and 20 Ne beams of 8 GeV, 28 Si beams of 8.12
GeV, 20 Ne beams of 12 GeV, and 28 Si beams of 22.4 GeV accelerated by the HIMAC at
the NIRS. Typical beam intensity was about a few times 108 particles/sec. with a beamspot size of roughly 10 mm in diameter. A metallic Au target of about 0.6 mg/cm2 wasmainly used for the measurements. Intermediate Mass Fragments (IMFs; 3≤Z≤20) weredetected and particle-identiﬁed with the 37 Bragg Curve Counter (BCC) array, which
was almost the same as the one used for the previous experiments at the KEK-PS. As
counter gas we used the P10 (Ar 90% and CH4 10%) gas of 300 Torr. In order to measure
angular distributions of the IMFs 12 BCCs were placed in the horizontal plane including
the target and the other 25 were set in upper hemisphere in order to measure the IMF
multiplicity. They covered an angular range of θlab = 30◦ -150◦ in 20◦ steps and a solid
angle of nearly 20% of 4π. The technical details of our BCCs have been published in Ref.
26. The data were taken with a sum of 37 self-triggers from individual BCC.
3. Experimental Results
Inclusive energy spectra of Na for 8 GeV proton, 8 GeV 16 O, and 8 GeV 20 Ne showed
usual Maxwell-Boltzmann distribution-like shapes[17]. The location of the Coulomb peaks
shifts more rapidly as a function of the emission angle for the 16 O and 20 Ne induced TMFs
than for the proton induced one, indicating the emission sources of IMFs move faster forthe heavier projectile. It was found that the moving source model always underestimates

3
8GeV Ne Target=Au
2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

O

0

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

90

180

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

Mg

0

0

Angle (deg.)

90

180

Angle (deg.)

90

180

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

90

180

Angle (deg.)90

8GeV O Target=Au

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

Ne

0

Angle (deg.)Al0

E/A 2-3 MeV

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

F

180

Angle (deg.)

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

Na

0

90

180

Si

inclusive
0

90

180

Angle (deg.)0

Angle (deg.)

multiplicity=2

Figure 1. Angular distributions of relative
yield to moving source ﬁt for 8 GeV 20 Ne
induced TMF on gold target.

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

O

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

90

180

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

Mg

0

0

Angle (deg.)

90

180

Angle (deg.)

90

180

90

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6
180

Angle (deg.)

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

Ne

0

Angle (deg.)Al0

E/A 2-3 MeV

2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6

F

90

180

Angle (deg.)

Na

0

90

180

Angle (deg.)Si

inclusive
0

90

180

Angle (deg.)

multiplicity=2

Figure 2. Angular distributions of relative
yield to moving source ﬁt for 8 GeV 16 O
induced TMF on gold target.

yields at sideward angles not only for the proton induced TMFs but also for the 16 O and20
Ne induced ones, especially for heavy IMFs (Z≥10).
To see this underestimation clearly, we integrated the IMF yields within a certain
dynamic range in the laboratory system, took a ratio to the integrated ﬁttings and thenobtained angular distributions of relative yield to the moving source model for various
IMFs of Z≤16. As can be seen in the Figs. 1 and 2 the resultant ratios show clear peaks
at around 110◦ in the laboratory system for the 8 GeV 20 Ne and 8 GeV 16 O induced
TMFs, which is slightly backward compared to the 70◦ for the proton induced TMFs atthe same energy. The maximum value in the ratio evidently increases as a function of amass of IMF and seems to decrease as a function of a projectile mass for very light IMFs
(lighter than F nucleus). This trend is also visible and slightly stronger in the 12 GeV
data. These facts probably indicate that though there might be some eﬀect arising fromthe nuclear density prior to the target multifragmentation, a total beam energy plays adecisive role to govern a reaction dynamics for the IMF emission.
4. Summary and Future Plan
We have studied similarities and diﬀerences among the IMF formation processes invarious light-projectile induced reactions in the GeV-energy domain. We found there issigniﬁcant diﬀerence in the IMF spectrum shapes and magnitudes but all of their angular
distributions show the existence of sideward-peaked components. This fact probably suggests that the IMF formations are mainly dictated by a total energy of the projectile (i.e.
the total energy scaling exists) and that the explanation of sideward peaking penomena
proposed by Hsi et al.[12] breaks down.
As for a near future plan, we would like to ﬁnalize our analysis on 28 Si induced multifragmentation at 8.12 GeV and 22.4 GeV, and to compare their results with those from4
light-ion induced multifragmentation in details. We also want to extract “isotope temperatures” for all light heavy-ion induced TMFs in order to investigate a possible density
eﬀect in an early stage of the nuclear collision.
Acknowledgements
The authors would like to thank Profs. M. Takasaki, K. Nakamura, S. Iwata, S. Yamada and H. Sugawara of KEK and Dr. Takeshi Murakami of NIRS for their continuous
supports. Accelerator crews and beam channel members of the KEK-PS and the NIRSHIMAC are highly appreciated for their excellent machine operation. This work was
supported in part by a Grant-in-Aid for Scientiﬁc Research (B) (No.1144078) and (B)(No.11440084) of the Ministry of Education, Science, Sports and Culture.
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26. H. Ochiishi et al., Nucl. Instrum. Methods A369 (1996) 269.

E289(SigN) M. Ieiri:
Hyperon-Nucleon Scattering Experiments (II)Y. Kondo et al.
[attached]Σ-p elastic scattering in the region of 400 MeV/c ≤ pΣ- ≤ 700 MeV/c with a scintillating fiber
active target
Nucl. Phys. A676 (2000) pp.371 – 387.
○ J. K. Ahn et al.
[attached]
Active target-detector with scintillating fibers for hyperon proton scattering
Nucl. Instr. Method A457 (2001) pp137-150.
○ J.K.Ahn et al.
[attached]
Active Target With Plastic Scintillating Fibers for Hyperon-Proton Scattering Experiments
IEEE Trans. NS 49 (2002) pp592-596.
★ Y. Kondo
Measurement of Σ-p Elastic Scattering Cross Sections with a Scintillating-Fiber Active Target
Kyoto University 2000.
◇ M. Ieiri
Hyperon-proton scattering experiment (KEK-PS E289 and plans)
7th International Conference on Hypernuclear and Strange Particle Physics; Torino, Italy, 23-27
October, 2000; Nucl. Phys. A691 (2001) pp.354c – 360c.
●________________________________________________________________________● Physics papers published in refereed journal.
○★
◇＊

Technical papers.
PhD theses.
Conference and Symposium.
Internal Report and others.

Nuclear Physics A 676 (2000) 371–387
www.elsevier.nl/locate/npe

6 −p elastic-scattering in the region of
400 < p6 − < 700 MeV/cwith a scintillating-fiber active target
KEK-PS E289 collaboration
Y. Kondo a , J.K. Ahn a,1 , H. Akikawa a , J. Arvieux b,2 , B. Bassalleck c ,M.S. Chung d,3 , H. En’yo a , T. Fukuda e,4 , H. Funahashi a , S.V. Golovkin f ,A.M. Gorin f , Y. Goto a,5 , M. Hanabata g , T. Hayakawa h , A. Ichikawa a ,M. Ieiri i , K. Imai a , M. Ishino a , H. Kanda a,6 , Y.D. Kim i,7 ,E.N. Kozarenko j , I.E. Kreslo j , J.M. Lee k,8 , A. Masaike a,9 , S. Mihara a,10 ,K. Nakai h , K. Nakazawa g , K. Ozawa a , A. Sato h , H.D. Sato a , K.S. Sim d ,T. Tabaru a , F. Takeutchi l , P. Tlustý e,11 , H. Torii a , K. Yamamoto a,12 ,S. Yokkaichi a , M. Yoshida a,5
a Department of Physics, Kyoto University, Kyoto 606-8502, Japanb Laboratory National Saturne, F-91191, Gif-sur-Yvette cedex, Francec Department of Physics, University of New Mexico, NM 87131, USAd Department of Physics, Korea University, Seoul 136-701, Korea
e Institute for Nuclear Study, University of Tokyo, Tanashi, Tokyo 188-0002, Japanf Institute for High Energy Physics, RU-142284, Protvino, Russia
g Physics Department, Gifu University, Gifu 501-1193, Japanh Department of Physics, Science University of Tokyo, Noda, Chiba 278-8510, Japani KEK, High Energy Accelerator Research Organization, Tsukuba 305-0801, Japanj Joint Institute for Nuclear Research, RU-141980, Dubna, Russia
k Department of Physics, Yonsei University, Seoul 120-749, Korea
l Department of Physics, Kyoto Sangyo University, Kyoto 603-8555, Japan

Received 1 March 2000; revised 29 March 2000; accepted 3 April 2000
1 Present address: Reseach Center for Nuclear Physics, Osaka University, Ibaraki, Osaka 567-0047, Japan.
2 Present address: Institut de Physique Nucléaire, Université Paris-Sud, 91406 Orsay, France.
3 Present address: Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
4 Present address: KEK, High Energy Accelerator Research Organization, Tsukuba 305-0801, Japan.
5 Present address: RIKEN, Institute of Physical and Chemical Research, Wako, Saitama 351-0198, Japan.
6 Present address: Physics Department, Graduate School of Science, Tohoku University, Sendai 980-8578,
Japan.
7 Present address: Physics Department, Sejong University, Seoul, 143-747, Korea.
8 Present address: Korea Research Institute of Standards and Science, Taejon, 305-600, Korea.
9 Present address: Fukui University of Technology, Fukui 910-8505, Japan.
10 Present address: International Center for Elementary Particle Physics, University of Tokyo, Tokyo 113-0033,
Japan.
11 Present address: Nuclear Physics Institute of the Academy of Science, 250 68 Řež, Czech Republic.
12 Present address: Deparment of Physics, Osaka City University, Osaka 558-8585, Japan.

0375-9474/00/$ – see front matter  2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 5 - 9 4 7 4 ( 0 0 ) 0 0 1 9 1 - 3

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Y. Kondo et al. / Nuclear Physics A 676 (2000) 371–387

Abstract
We have performed a hyperon-proton scattering experiment with a scintillating fiber active target.
The 6 − p, 3p and 6 + p scattering have been studied with the same experimental setup. In this paper,
we present the differential cross sections of the 6 − p elastic scattering in the momentum region from
400 to 700 MeV/c. This is the first measurement of the 6 − p elastic scattering in the momentum
region where the P- and higher waves contributions are important. The present data are in good
agreement with the one boson exchange model (Bonn–Jülich model A) and the quark cluster model(FSS of Kyoto–Niigata model).  2000 Elsevier Science B.V. All rights reserved.
PACS: 13.75.Ev; 21.30.Cb; 25.60.Bx; 25.80.Pw
Keywords: 6 − p elastic scattering; Hyperon–nucleon interaction; Differential cross sections; Scintillating fiber
active target

1. Introduction
The strong interaction in the perturbative region is well described by quantum
chromodynamics (QCD). However, it is still a quite difficult problem to derive the nuclear
interaction from QCD. Although the nucleon–nucleon (NN) interaction has been welldescribed by the meson exchange picture, there are various methods to treat the shortrange repulsive forces. It is believed that effects from the quark–gluon structure of nucleons
should appear in the short-range forces. In order to understand the strong interaction in
the nonperturbative region, studies of interactions between various baryons play a very
important role. The one boson exchange (OBE) models have been developed with theabundant experimental data of the NN scattering, and have been extended to describe thehyperon–nucleon (YN) interaction by the Nijmegen [1–3] group and Bonn–Jülich [4,5]group. In these models, the short-range forces have been treated with phenomenologicalmethods. On the other hand, the quark-cluster model (QCM) approaches, which describe
the short-range forces based on quark and gluon dynamics, have been developed
independently by the groups from Tokyo [6,7], Tübingen [8] and Kyoto–Niigata [9].
The parameters of these models were determined by fitting the existing experimental
data of the NN and YN scattering. However, as for the YN scattering, there are only
very limited data. Due to the short lifetime of the hyperons, a device which can observecomplicated reaction topologies in the region of approximately a cm about the reaction
vertex is needed to perform a YN scattering experiment in the low-momentum region(below 1 GeV/c). In this region, almost all experiments were performed with bubble
chambers in the 1960’s [10–17]. Since the bubble chambers required the low beam
rates, the stopping K− method was the most efficient way to produce hyperons. As for
6p scattering, because most experiments [10–12,16] used this method, the momentaof the 6’s were limited to be less than 200 MeV/c. In this region, S-waves are
expected to be dominant. For example, in Ref. [16], the allowed region for the 6 + p
scattering lengths was determined by applying the effective range approximation assuming
S-wave predominance. All models used only these low-momentum 6p scattering data toY. Kondo et al. / Nuclear Physics A 676 (2000) 371–387

373

determine their parameters. They have not been tested with experimental data at higher
momenta, where the contributions from the P- and higher partial waves are important.
It should be noted that important information of the NN interaction, such as the spindependent forces and hard core, was provided from NN scattering experiments up to
800 MeV/c. Experimental studies of YN scattering in momentum regions higher thanprevious experiments (up to 1 GeV/c) are urgently needed.
As a first step of these efforts, a 6 + p scattering experiment was performed at KEKPS (E251) [18]. The experiment E251 was aimed at measuring the differential crosssection of the 6 + p scattering with a new technique using a scintillating fiber (SCIFI)
active target. The SCIFI detector system had a better time resolution (∼ 2 µs) than thatof bubble chambers. It enabled us to use the 5×104 /s π + beam and to record only therelevant pictures event by event. With the SCIFI detector, the in-flight reactions could
be used to measure the 6 + p scattering at higher momentum, which was not possible
with bubble chambers. 6 + ’s of 300 to 700 MeV/c were produced through the in-flightp(π + , K+ )6 + reaction with a 1.6 GeV/c pion beam. Only the (π + , K+ ) events wererecorded by using an external trigger system. A plastic scintillating fiber block, which wasdeveloped for the H-dibaryon search [19], was used as a production target for hyperons,
a 6 + p scattering target, and a 4π detector for the related charged particles around thereaction vertices. The reaction topologies were recorded as digitized image data. As a next
step, we performed experiment E289 to measure the 6 − p and 3p scattering cross sections,and to obtain more statistics on the 6 + p scattering data. It is important to measure allchannels in order to check the assumption of flavor SU(3) symmetry in the YN interaction
models. We measured the three reaction channels with the same setup. The SCIFI target
system was improved regarding several points. The effective volume was larger than thatof the previous one to observe 3’s whose lifetime is longer than that of 6’s. Also, thinner
scintillating fibers were adopted to improve the spatial resolution.
In this paper, we present the result of the measurement of the differential cross sectionsfor 6 − p elastic scattering. The momentum region is from 400 to 700 MeV/c where nodifferential cross section data exist. In this region, contributions from P-waves and higher
partial-waves appear in the elastic cross sections. The momentum dependence is alsopresented.

2. Experiment
The experiment was carried out at the K2 beam line of the 12 GeV Proton Synchrotron(PS) at KEK. Fig. 1 shows a top view of the experimental setup. A 1.32 GeV/c π −beam was used to produce 6 − ’s through the (π − , K+ ) reaction. Outgoing K+ ’s wereidentified with a K+ spectrometer system. A scintillating fiber (SCIFI) target was usedas the production and scattering target. It also worked as a 4π tracking detector for the
charged particles and as a range counter for the charged particles stopped in the SCIFI
block. We could identify the hyperons, which may be scattered and decay inside the SCIFI
target.

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Y. Kondo et al. / Nuclear Physics A 676 (2000) 371–387

Fig. 1. Schematic view of the experimental setup.

2.1. The π − beam
The typical intensity of the separated π − beam was 1.5×105 /spill (∼ 2 s). The
momentum was 1.32 GeV/c with a spread of 1p/p = 0.7% (r.m.s.). The contamination ofelectrons and µ− ’s was 13% and that of p̄’s and K− ’s was less than 1%. The e− ’s and µ− ’s
were vetoed with a gas Čerenkov counter (GC), and the p̄’s and K− ’s were eliminated with
an aerogel Čerenkov counter (BAC). The contamination was reduced to be less than 0.1%.
The tracking of the beam particles was performed with multiwire proportional chambers(BPC3-5). To check the online trigger and to confirm the particle identification, the timeof-flight was measured with scintillation counters (T1,T2), where T1 was located 7 m
upstream of the SCIFI target. The TOF resolution (T2–T1) was 1t = 55 ps (r.m.s.).
2.2. The K + spectrometer
On-line and off-line particle identification (P.I.D.) of outgoing particles from the SCIFI
target were performed with the K+ spectrometer system. In order to avoid any overlappingof the K+ and 6 − tracks in the SCIFI pictures, the acceptance of the spectrometer did
not cover 0 degree in the horizontal direction; it covered from 4 to 24 degrees. The
pion background was rejected with the forward aerogel Čerenkov counter (FAC). The
momentum of an outgoing particle was approximately calculated with the combinationof a hodoscope (CH) located just before a dipole magnet (SP) and a TOF array (FTOF) at
the most downstream end of the spectrometer. This momentum information and the PID
information obtained from the Čerenkov counters were used for the first-level trigger. In
addition, by using the momentum information and the TOF information obtained from the
FTOF and T2, the mass of an outgoing particle was selected with a set of fast-encoding
electronics (Lecroy FERA–FERET system) to reduce the proton background at the secondlevel trigger (Mass Trigger). The decision times of the first- and second-level triggers
were 360 ns and within 15 µs, respectively. The typical trigger rate was 5 ∼ 8 /spill. The
momentum of an outgoing particle was calculated with use of the SP, MWPC’s (PC1-2)
and drift chambers (DC1-3) in the off-line analysis. The integrated magnetic field along theY. Kondo et al. / Nuclear Physics A 676 (2000) 371–387

375

tracks of outgoing particles in the SP was 0.94 T m in average. The momentum resolution
was (1p/p) = 1.0% (r.m.s.) at p = 0.7 GeV/c. To reconstruct the mass of an outgoingparticle, the velocity of the particle was also obtained by measuring the TOF between
T2 and FTOF. The TOF resolution (FTOF–T2) was 1t = 140 ps (r.m.s.) in the off-line
analysis.
2.3. The SCIFI active target system
The SCIFI block consisted of 300 µm × 300 µm square plastic scintillating fibers(KURARAY SCSF-78 [20]). The core material of the fiber was polystyrene ((CH)n ) andthe size was 288 µm × 288 µm. The refractive index of the core was 1.59 and the density
was 1.05 g/cm3 . The cladding was made of polymethylmethacrelate (PMMA, C5 H8 O2 )with refractive index of 1.49 and density of 1.19 g/cm3 . The attenuation length wasmeasured to be 114 cm with a 90 Sr β-ray source. The wavelength of the transmitted light
was peaked at 450 nm. The SCIFI block was assembled by stacking fiber sheets alternately,as shown in Fig. 2. Each fiber sheet consisted of about 330 fibers, which corresponded to
100 mm in width. The fiber sheets were bonded with white paint, which eliminated any
cross talk over the fiber sheets by diffused reflection of the untrapped photons. About 600
sheets were used to create an effective volume of 100 mm × 100 mm × 200 mm. The
fiber sheets of each direction were bundled at the readout arms to make readout surfacesof 100 mm × 100 mm. At the other ends of the fibers, aluminum evaporated lucite-plates
were attached as mirrors, which improved the effective attenuation length by a factor ofthree.
The output light from the SCIFI was amplified with two sets of image-intensifier (II)chains. Fig. 3 shows a schematic structure of the II chain. The II chain consisted of four
II’s. The first (HAMAMATSU V4440PX [21]) and second (DEP PP0030X [22]) stages
were electrostatic-type II’s. The diameters of the input and output windows of the first II
were 100 mm ∅ and 25 mm ∅, respectively. The quantum efficiency of the photo-cathodewas about 20%. In order to hold the images until the trigger would be fired, phosphors
were used at the output window of each II, except for the fourth stage. The phosphor of
the first II was PS-5, with a decay-time of which was 1.3 µs. This decay-time limited theFig. 2. Schematic view of the structure of the SCIFI block.

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Y. Kondo et al. / Nuclear Physics A 676 (2000) 371–387

Fig. 3. Schematic structure of the image intensifier (II) chain.

acceptable beam rate for the SCIFI target to less than a few ×105 /spill. The diameters ofboth the input and output windows of the second II were 25 mm ∅. The P46 phosphor witha decay-time of 300 ns was used for the second II. The first and second II’s were operated
in DC mode. The operating voltages for the first and second II were 20 kV and 15 kV, andthe photon gains were both ∼ 10.
The third stage (PROXITRONIC BV2563McG [23]) was a micro-channel–plate (MCP)
type. The diameters of the input and output windows were 25 mm ∅, and the decay-timeof the phosphor (P20) for the third II was 20 µs. The gain of the third II was ∼ 103 .
At the fourth stage, an electron bombarded CCD system (GEOSPHAERA EBCCD [24])was used. The third II and EBCCD were gated by the first-level and second-level triggers,
respectively. The output window of the third II and the input window of the EBCCD wereconnected through a tapered fiber-optic. On the input window of the EBCCD, the sizeof the image was reduced to 8 mm in diameter, which fitted the effective size of the CCD
chip. The intensified photon images were read out by the EBCCD and recorded as digitizedimage data. The photo-electrons were accelerated and bombarded the back side of the CCD
chip, and the EBCCD itself had a gain of 103 . The CCD chip consisted of 532 × 580 pixels,
where the size of one pixel was 17 µm × 23 µm. The effective area on the CCD chip was
9.04 mm × 13.34 mm. The frame transfer to read out the charge on the CCD was started
by the external trigger. The driving clock rate of the CCD was 10 MHz and the readout
time was 45 ms. The images were digitized with a 6-bit ADC. Fig. 4 shows an example ofimage data.
The performance of the SCIFI system was investigated with the 1.32 GeV/c minimumionizing π − ’s and slow 6 − ’s whose typical momenta were about 500 MeV/c. For the
1.32 GeV/c π − tracks, photons from the fibers are recognized as the cluster of active
pixels. The average number of hit fibers (hit density) along the track was 0.25 /sheet,and the residual of the cluster around the fitted line was 165 µm. This value is consistent
with the calculated one determined with the size of the fiber and CCD pixel, the distortionof the II and the error of position calibration. For the track of the 6 − , the clusters were
stuck together and it was difficult to recognize each cluster, since the energy deposit of

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377

Fig. 4. An example of a 6 − p scattering event observed with the SCIFI detector. The open squares
represent the input positions from the pointing process and the solid lines on the incoming π − andoutgoing K+ are the results of the tracking with the external wire chambers. The lines from the(π − , K+ ) vertices correspond to the predicted momentum vectors of the initial 6 − (see text).

the 500 MeV/c 6 − is about four-times larger than that of the 1.32 GeV/c π − . The track
width is, therefore, defined as the distribution of each active pixel around the track, and thetrack width of the 6 − track was 440 µm (r.m.s.).

3. Analysis
3.1. Identification of the (π − , K + ) reaction
The number of collected events was 4.3×106 . The (π − , K+ ) events were selected byreconstructing the mass of the outgoing particles with the K+ spectrometer. Fig. 5 shows
the mass spectrum of the outgoing particles. The particles whose reconstructed mass were
within 0.4 to 0.6 GeV/c2 were regarded as K+ ’s. The K+ mass resolution was 1M =15 MeV/c2 (r.m.s.) at a momentum of 0.7 GeV/c, and the background contamination
was estimated to be less than 1.3%. The vertex of the (π − , K+ ) reaction was obtainedby tracking the incoming and outgoing particles with wire chambers, and the vertex was
required to be in the region of −100 < z < 100 mm, where the direction of the z-axis wasalong the π − beam-line and the origin was the center of the SCIFI target. The tracking
efficiency was 91%.
3.2. Event categorization and pointing
The pictures of the selected (π − , K+ ) events were eye-scanned manually and categorized according to their image patterns on the video pictures. The number of scanned eventswas 2.8×105 . About 65% of these events were 6 − production events:

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Y. Kondo et al. / Nuclear Physics A 676 (2000) 371–387

Fig. 5. Mass spectrum of the outgoing particles. The hatched area was recognized as K+ ’s.

Fig. 6. Patterns for the event categorization. The event topology was recognized as a combination ofthese patterns.

π − + p → K+ + 6 − .

(1)The scanned events included other reactions, for example,
π − + p → K+ + 6 ∗− ,6 ∗− → 3 + π − ,(2)which are not discussed in this paper. Some of the 6 − ’s reacted with protons:
6 − + p → 6 − + p,−

6 + p → 6 + n,−0

6 + p → 3 + n.

(3)
(4)
(5)

In this paper, only the elastic scattering (3) is discussed.
The event topology was recognized as a combination of patterns. For example, theweak decay of the 3 → pπ − was recognized as a ‘V-pattern’, and the decay of the 6
was recognized as a ‘/-pattern’. The event patterns which were necessary to derive thecross section of the 6 − p elastic scattering are shown in Fig. 6. The (π − , K+ ) vertex was
obviously recognized with the help of the guide lines on the event display obtained fromthe wire chambers. If a track of an emitted particle from the (π − , K+ ) vertex kinked only
once, this track was recognized as the ‘/-pattern’. Even if the track was straight, a change
in the brightness of the track was regarded as a kink. In case there were more than two

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379

kinks, the track was recognized as the ‘L-pattern’. The events including the ‘/-pattern’or ‘L-pattern’ are 6 − beam candidates. The first kink of the ‘/-pattern’ and ‘L-pattern’corresponds to the decay of the 6 − , and the following kinks correspond to the decay orscattering of the π − which is a decay product of the 6 − ; the branching ratio of 6 − → π − nis 100%. As for the ‘L-pattern’, the first kink may be a 6 − scattering without a visible
recoil proton, but such a event is easily recognized by the brightness of the tracks, and was
not used for the analysis. The number of such events was negligible compared to the events
regarded as the 6 − beam. The ‘C-pattern’ consists of one straight track (prong) and one
kinked track. The events including the ‘C-pattern’ are candidates for the 6 − p scattering.
For the identification of the 6 − p elastic scattering event, one visible recoil proton track was
required, and the prong in the ‘C-pattern’ corresponds to this proton. The kink corresponds
to the decay of the scattered 6 − . The number of the 6 − beam candidates was 1.8×105
and the number of the 6 − p scattering candidates was 776.
After event categorization was completed, the reaction points were marked on a graphic
display with a pointing device. The reaction points were determined 3-dimensionally bythese marks. We call this process ‘pointing’. The pointing was done for 2947 samples of
the 6 − beam candidates and all events of the 6 − p scattering candidates.
The efficiencies of the eye-scanning and all other analyzing processes described below
were estimated by a set of Monte Carlo simulations. Simulated image data were generated
by the same method as described in Ref. [18], and the parameters to reproduce the picture
were tuned for the conditions of the present experimental apparatus. The generated image
data were analyzed through the same process as the real data, and the efficiencies were
evaluated from the surviving rates of the simulation events. The angular resolution and theaccuracy of the track-length measurements were also estimated using the simulation databy comparing the generated positions to the positions obtained by the pointing process.
For short tracks, the accuracies of the angle and length measurements and the scanning
efficiencies decrease, as shown in Fig. 7. Therefore, for the events including short tracks,
the event identification is ambiguous, and especially, the angular resolution becomes not
sufficient to distinguish the background, which is mainly from quasi-free scattering, as
described later. In order to avoid these ambiguities, a set of geometric criteria was required
for further analysis. For both the 6 − beam and 6 − p scattering candidates, track-lengthof the 6 − was required to be more than 5 mm, and for 6 − p scattering candidates, thetrack-lengths of the scattered 6 − and the recoil proton were required to be more than
3 mm. The production vertex of the (π − , K+ ) reaction was reconstructed more precisely
by using the image data in addition to the wire chambers. The vertex resolution along
the z-axis was estimated to be 0.6 mm from the simulation data. To avoid ambiguities in
the categorization for events which occurred at the upstream and downstream ends of the
SCIFI, the z-position of the vertex was required to be within −90 ∼ 60 mm. The recoil
protons for 6 − p scattering candidates were required to stop inside at least one of thefiducial areas of u−z and v−z views, where the definition of u−z and v−z views is shownin Fig. 4. The fiducial area was defined to be inside of the ellipse whose long axis was
180 mm in the beam direction and short axis was 90 mm, which correspond to the input

380

Y. Kondo et al. / Nuclear Physics A 676 (2000) 371–387

Fig. 7. Track-length dependence for the scattered 6 − track of the (a) angular resolution (r.m.s.),(b) deviation of the track length and (c) efficiency of the pointing process; (d), (e) and (f) are forthe recoil proton track. They were obtained from the analysis of the simulated image data. The lines
represent the threshold of the track-length cut.

window of the first II. The stopping point of the recoil proton was also required to be inside
the effective volume of the SCIFI target.
3.3. Total flight length of the Σ − ’s
The 6 − ’s are produced through both the reaction on free protons and the quasi-freeprocess on carbon nuclei, since the SCIFI target consists of (CH)n . Only the 6 − ’s producedon free protons were used as the 6 − beam, because the momenta of the 6 − ’s from carbon
nuclei are ambiguous due to the Fermi motion. The selection was done by applying angledifference and missing-mass cuts. The angle-difference for the 6 − production kinematics(1θprod) was defined as
E6 − prod · vE6 − prod
−1 p
,
(6)
1θprod = cos|pE6 − prod ||E
v6 − prod |where vE6 − prod is the measured direction of the 6 − obtained from the pointing process, andpE6 − prod denotes the predicted momentum vector of the 6 − calculated from the momentum
vectors of the π − and the K+ . The momenta of the π − and K+ were obtained from theanalysis of the spectrometer data, and the directions were measured with the SCIFI detector
and external wire chambers.

Y. Kondo et al. / Nuclear Physics A 676 (2000) 371–387

381

Fig. 8. (a) Scatter plot of 1θprod versus the flight length of the 6 − (fl6 − ). (b) Missing-mass spectrumof the p(π − , K+ )X reaction after the angle-difference cut was applied. The lines on the spectrum
are obtained with a two-Gaussian fitting. The events inside the region indicated by the arrows wereregarded as the 6 − produced on the free proton.

Fig. 8(a) shows the distribution of 1θprod versus the flight length of the 6 − (fl6 − ).
The 1θprod distributes around zero within the resolution of the SCIFI system for the
6 − ’s produced on the free protons. These events make a narrow peak, and which canbe distinguished from the broader distribution of the quasi-free events. The region inside
the lines in Fig. 8(a) was the accepted region for the 6 − ’s produced on the free protons.
The threshold was set to be
1θprod <1.35+ 0.03,fl6 −fl6 − > 5 mm,(7)by taking projections of Fig. 8(a) onto 1θprod in some slices of fl6 − . Fig. 8(b) shows
the missing-mass spectrum of the p(π − , K+ )X reaction after the angle-difference cut
was applied. The missing-mass resolution was 7.4 MeV/c2 (r.m.s.). The events whose
missing-mass were calculated to be from 1.174 to 1.218 GeV/c2 were regarded as the 6 − ’sproduced on free protons. The contamination from the quasi-free process was estimated tobe 10%.
The amount of the 6 − beam was defined as the total flight length of the 6 − ’s (FL6 − ),P
where FL6 − = (fl6 − − flth ). Here, fl6 − is the flight length for each event and the flthis the threshold length, which was set to be 5 mm, as shown in Section 3.2. Using withthe 2947 pointed data sample, the total flight length of the 6 − ’s produced on free protons
was measured to be 963 ± 33 cm. The efficiency for this length measurement was estimatedwith the simulation event being 0.827 ± 0.026. Taking into account this efficiency, for all of
the 6 − beam candidates, the number of which was 1.8×105 , FL6 − of the 6 − ’s producedon free protons was estimated to be (7.13 ± 0.33) × 104 cm.

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Y. Kondo et al. / Nuclear Physics A 676 (2000) 371–387

3.4. 6 − p scattering events
The number of 6 − p scattering candidates which satisfied the criteria described in
Section 3.2 was 311. Because the 6 − ’s were required to be produced on free protons,
the same cuts as described in Section 3.3 were applied, reducing the number of eventsto 88. Additionally, cuts on the angle-difference and missing-mass were also applied tothe scattering kinematics to eliminate the background, which was mainly from quasifree
scattering on the carbon nuclei. The angle-difference for the scattering kinematics (1θscat )was defined as
E6 − scat · vE6 − scat
−1 p
.
(8)
1θscat = cos|pE6 − scat ||Ev6 − scat |Here, vE6 − scat is the measured direction of the scattered 6 − by the pointing, and pE6 − scat
denotes the predicted momentum vector of the scattered 6 − calculated from the momentaof the initial 6 − and recoil proton. The momentum of the initial 6 − was determined fromthe kinematics of the (π − , K+ ) reaction, taking the energy loss in the SCIFI target into
account. The momentum of the initial 6 − just before scattering ranged from 400 MeV/c
to 700 MeV/c and the momentum resolution was 13 MeV/c (r.m.s.). The momentum of
the recoil proton was calculated from the range in the SCIFI.
Fig. 9(a) shows the distribution of 1θscat versus the flight length of the scattered 6 − , andthe region surrounded by the lines is the accepted region for elastic events. The accepted
region was parameterized as1θscat <1.0
fl6 − scat

+ 0.06,

fl6 − scat > 3 mm.

(9)Fig. 9. (a) Distribution of the angle-difference 1θscat versus the flight length of the scattered 6 − ’s.
The definition of 1θ is described in the text. The region inside the lines is the accepted region.
(b) Missing-mass spectrum of the 6 − p scattering before the angle-difference cut was applied.
(c) After the angle-difference cut was applied. The missing-mass resolution was 7.8 MeV/c2 . The
region between the arrows was accepted as 6 − p elastic scattering.

Y. Kondo et al. / Nuclear Physics A 676 (2000) 371–387

383This threshold was determined with the same distribution as in Fig. 9(a) obtained from thesimulation data of the 6 − p elastic scattering, because the statistics for real data was not
sufficient. Fig. 9(b) shows the missing-mass spectrum of the p(6 − , p)X reaction before
the angle-difference cut was applied and Fig. 9(c) is the one after the cut was applied. The
events whose missing-mass were 1.175 ∼ 1.221 GeV/c2 were recognized as elastic events.
An example of a picture of an elastic scattering event is shown in Fig. 4. Finally, 30 events
survived after all of the cuts were applied. The scattering angle in the center of mass (CM)system (θCM ) was calculated with the momenta and directions of the initial 6 − and recoil
proton. The accuracy of the scattering angle was 1(cos θCM ) = 0.09 (r.m.s.).
The amount of background contamination was estimated using the simulation data.
The simulation events of the quasi-free scattering were generated and were normalized
to reproduce the tail of the 1θscat distribution of the real data, as shown in Fig. 10. The
same cuts as for real elastic events were applied and the survived events were regarded as
the contamination from quasifree events. The error of the normalization was ±25%. This
caused a systematic error of ±40% for the cross section in the region −0.5 < cos θCM <−0.2; for other angular regions, however, these errors were estimated to be less than ±3%.
In addition, there was the contamination from the quasifree 6 − production. For the 6 − pscattering events, this contamination was 7%, and the systematic error for the cross section
was 3%, because the 6 − beam events also included this contamination of 10%. The total
analyzing efficiency was obtained from the analysis of the simulated data of the 6 − pelastic scattering. The yields, estimated backgrounds and efficiencies for each angularregion are summarized in Table 1. The errors of the track-length measurement cause an
uncertainty on the efficiency. The systematic deviation of the track-length was estimatedto be less than 0.3 mm with the simulated image data. The effect of this uncertainty on the
cross section was estimated to be ±7% by generating a systematic deviation of ±0.3 mm

Fig. 10. Distribution of 1θscat obtained from the real data and the simulation data of the elastic andquasi-elastic 6 − p scattering. The hatched band represents the simulation data.

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Y. Kondo et al. / Nuclear Physics A 676 (2000) 371–387

Table 1The yields of the 6 − p elastic events, estimated backgrounds, total efficiencies, and differential crosssections. Except the cross sections, only the statistical errors were displayed. The errors for no. of
events correspond to a probability of 68.3% for a Poisson distributioncos θCM−0.5−−0.2−0.2−0.1
0.1−0.4
0.4−0.7

Numberof events

Background

1+2.3
−0.8

0.62 ± 0.22

9+4.1
−2.9

11+4.4
−3.3
9+4.1
−2.9

0.69 ± 0.23
0.62 ± 0.22
0.85 ± 0.26

Backgroundsubtracted

Efficiency

0.4+2.3−0.9

0.20 ± 0.04

8.3+4.1−3.0

10.4+4.4−3.3
8.2+4.1−3.0dσdΩ ± stat. ± sys.

(mb/sr)0.43 ± 0.06
0.47 ± 0.06
0.34 ± 0.05

0.3+1.8−0.7 ± 0.1
3.0+1.5−1.1 ± 0.3
3.4+1.5−1.1 ± 0.3
3.7+1.9−1.4 ± 0.3

in the analysis of the simulation data. In the derivation of the cross section, the target
was assumed to be made of (CH)n , and the materials of the cladding and white paintwere neglected. If these materials are taken into account, the effect on the cross section
is about 2%. The systematic errors of the cross sections given in Table 1 include all of
those listed above.

4. Result and discussions
The differential cross sections were derived from
Nscat
dσ
=
.
dΩ
FL6 − · ρt · dΩ(10)Here, Nscat is the number of scattering events corrected by the efficiency factor. The FL6 −is the total flight length of the 6 − beam obtained in Section 3.3, and ρt represents thenumber density of the protons in the SCIFI target, which is 4.87×1022 /cm3 .
The experimental results are summarized in Table 1. The statistical errors include theerrors on the real data and the errors from the statistics of the simulated image data.
The systematic error for −0.5 < cos θCM < −0.2 is dominated by the error from thebackground subtraction, and the errors for other angular regions are from the errors onlength measurements, as described in Section 3.4. In this channel, since the difference
between the models is not so large, we compare our results to typical two calculations.
Fig. 11 shows a plot of the differential cross sections together with theoretical calculations
from the OBE model [25] and QCM [26]. The momenta of the 6 − ’s for the calculations
are 0.5 GeV/c. The error bars represent only statistical ones. In Fig. 11, Jülich means
the OBE model of the Bonn–Jülich group (model A in [4,5]). The Bonn–Jülich modelis based on the spin–flavor SU(6) symmetry and contains 2π -exchange, replacing thescalar meson exchange. Form factors, which were parameterized by the cutoff mass 3α ,are used at each vertex. The FSS is one of the QCM calculations of the Kyoto–Niigata
group [9]. They are using the resonating group method (RGM) with antisymmetrized sixquark wave functions. In both models, the forward-peaking structures appear, which are

Y. Kondo et al. / Nuclear Physics A 676 (2000) 371–387

385

Fig. 11. Differential cross section for 6 − p elastic scattering. The dashed line represents thetheoretical calculation by the Bonn–Jülich group with their OBE model (model A), and the solidline is by the Kyoto–Niigata group with their quark-cluster model (FSS).

Fig. 12. Integrated cross section for 6 − p elastic scattering. The solid line represents the theoretical
calculation with FSS and the dashed line is the Bonn–Jülich model A. (This model can be used in
the region less than about 0.58 GeV/c, i.e. below the threshold for pion production.)the contributions of P-waves. Both models are in reasonably good agreement with ourexperimental data.
The momentum dependence of the experimental data is shown in Fig. 12. The plottedvalues are the integrated cross sections in the angular region of −0.5 < cos θ < 0.7, they
+14
+23
are 19+10−5 mb (0.4 < p6 − < 0.5 GeV/c), 29−10 mb (0.5 < p6 − < 0.6 GeV/c), and 15−8
mb (0.6 < p6 − < 0.7 GeV/c). The errors are statistical ones. The momentum dependenceof the data is consistent with the FSS and the Bonn–Jülich model A within the statisticalerrors.

386

Y. Kondo et al. / Nuclear Physics A 676 (2000) 371–387

5. Summary
We have presented the results of a measurement of the differential cross sections for 6 − pelastic scattering in the momentum region from 400 to 700 MeV/c where no differential
cross section data existed. We have also measured the differential cross section for 6 + p
and 3p scattering, and these results will be presented in separate papers.
The experiment was carried out at the K2 beam line of KEK 12 GeV PS. A 1.32 GeV/c
π − beam was used to produce the 6 − ’s via the (π − , K+ ) reaction, and the scatteringand decay of 6 − were observed with a scintillating fiber (SCIFI) active target system.
The SCIFI target consisted of about 200000 scintillating fibers, and image data were readout with image intensifiers (II’s) as digitized image data. The 2.8×105 pictures were eyescanned, and 776 6 − p scattering candidates and 2947 samples of 6 − beam candidates
were analyzed. To select the 6 − p elastic scattering events, geometrical and kinematic cuts
were applied; 30 events survived as 6 − p scattering events.
The 6 − p elastic scattering differential cross sections have been compared with theBonn–Jülich model A and the quark-cluster model calculation of the Kyoto–Niigata group(FSS). The present result is in good agreement with both theoretical calculations. The
momentum dependence is also consistent with the theoretical calculations.

Acknowledgements
We wish to thank staffs of the KEK PS for their support during the experiment andanalysis. We thank Professors H. Sugawara, S. Yamada, K. Nakamura, Y. Yoshimura, and
J. Chiba for their continuous encouragement throughout this work. We would like to thankProf. J. Haidenbauer, who provided the theoretical calculation of Bonn–Jülich model.
We express our appreciation to Prof. Y. Fujiwara and Dr. T. Fujita for their theoretical
calculation of Kyoto–Niigata model. This work was supported partially by Grants-in Aid
for Scientific Research No. 06640425 of Japan Ministry of the Education, Science andCulture. Some of authors (H.K and Y.K) were supported by JSPS Research Fellowships
for Young Scientists.
References[1] M.M. Nagels, T.A. Rijken, J.J. de Swart, Phys. Rev. D 15 (1977) 2547; Phys. Rev. D 20 (1979)
1633.
[2] P.M.M. Maessen, T.A. Rijken, J.J. de Swart, Phys. Rev. C 40 (1989) 2226.
[3] T.A. Rijken, V.G. Stokes, Y. Yamamoto, Phys. Rev. C 59 (1999) 21.
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[5] A. Reuber, K. Holinde, J. Speth, Nucl. Phys. A 570 (1994) 543.
[6] M. Oka, K. Yazaki, in: W. Weise (Ed.), Quarks and Nuclei, World Scientific, 1984, p. 489.
[7] K. Yazaki, in: T. Yamazaki, K. Nakai, K. Nagamine (Eds.), Perspectives of Meson Science,
Elsevier, 1992, p. 795.
[8] U. Straub, Z.Y. Zhang, K. Bräuer, A. Faessler, S.B. Khadkikar, G. Lübeck, Nucl. Phys. A 483(1988) 686; Nucl. Phys. A 508 (1990) 385c.

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387[9] Y. Fujiwara, C. Nakamoto, Y. Suzuki, Prog. Theor. Phys. 94 (1995) 215; Prog. Theor. Phys. 94(1995) 353; Phys. Rev. Lett. 76 (1996) 2242; Phys. Rev. C 54 (1996) 2180.
[10] H.G. Dosch et al., Phys. Lett. 21 (1966) 236.
[11] R. Engelmann et al., Phys. Lett. 21 (1966) 587.
[12] H.A. Rubin et al., Phys. Rev. 159 (1967) 1149.
[13] B. Sechi-Zorn et al., Phys. Rev. 175 (1968) 1735.
[14] G. Alexander et al., Phys. Rev. 173 (1968) 1452.
[15] G.R. Charlton et al., Phys. Lett. B 32 (1970) 720.
[16] F. Eisele et al., Phys. Lett. B 37 (1971) 204.
[17] J.A. Kadyk et al., Nucl. Phys. B 27 (1971) 13.
[18] J.K. Ahn et al., Nucl. Phys. A 648 (1999) 263.
[19] J.K. Ahn et al., Phys. Lett. B 378 (1996) 53.
[20] KURARAY Co. Ltd., Maruzen Building, 3-10, 2-Chome, Nihonbashi, Chuo-ku, Tokyo 1030027, Japan.
[21] HAMAMATSU Photonics, 314-5, Shimokanzo, Toyooka-village, Iwata-gun, Shizuoka, 4380193, Japan.
[22] Delft Electronishe Producten (DEP), NL-9300 AB Roden, The Netherlands.
[23] PROXITRONIC, Robert-Bosch-Strasse 34, D 64625 Bensheim, Germany.
[24] GEOSPHAERA, Case Postale No. 6, Moscow, 117133, Russia.
[25] J. Haidenbauer, private communications.
[26] Y. Fujiwara and T. Fujita, private communications.

Nuclear Instruments and Methods in Physics Research A 457 (2001) 137}150

Active target-detector with scintillating "bers for
hyperon}proton scattering
J.K. Ahn , H. Akikawa , J. Arvieux , B. Bassalleck, M.S. Chung ,
H. En'yo , T. Fukuda, H. Funahashi , S.V. Golovkin, A.M. Gorin, Y. Goto ,
M. Hanabata, T. Hayakawa , A. Ichikawa , M. Ieiri *, K. Imai , M. Ishino ,H. Kanda , Y.D. Kim , Y. Kondo , E.N. Kozarenko , I.E. Kreslo , J.M. Lee ,
A. Masaike , S. Mihara , K. Nakai, K. Nakazawa, K. Ozawa , A. Sato,
H.D. Sato , K.S. Sim , T. Tabaru , F. Takeutchi , P. TlustyH , H. Torii ,K. Yamamoto , S. Yokkaichi , M. Yoshida 
Department of Physics, Kyoto University, Kyoto 606-8502, Japan
Laboratoire National Saturne, F-91191 Gif-sur-Yvette, Cedex, FranceDepartment of Physics, University of New Mexico, Albuquerque, NM 87131, USA
IPNS, KEK, High Energy Accelerator Research Organization, Oho 1-1 Tsukuba 305-0801, Japan
Institute for High Energy Physics, RU-142284 Protvino, Russia
Physics Department, Gifu University, Gifu 501-1193, JapanDepartment of Physics, Science University of Tokyo, Chiba 278-8510, Japan
Physics Department, Seoul National University, Seoul 151-742, South Korea
Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
Department of Physics, Yonsei University, Seoul 120-749, South Korea
Department of Physics, Korea University, Seoul 136-701, South Korea
Department of Physics, Kyoto Sangyo University, Kyoto 603-8555, Japan
Nuclear Physics Institute, 250 68 R[ ez\ , Czech Republic
Received 20 June 2000; accepted 25 June 2000

* Corresponding author.
E-mail address: masaharu.ieiri@kek.jp (M. Ieiri).
 Present address: Research Center for Nuclear Physics,
Osaka University, Ibakaki, Osaka 567-0047, Japan.
 Present address: Institut de Physique NucleH aire, UniversiteH
Paris-Sud, F-91406 Orsay-Cedex, France.
 Present address: Los Alamos National Laboratory, P-25,
MS H846, Los Alamos, NM 87545, USA.
 Present address: RIKEN, Institute of Physical and Chemical
Research, Wako, Saitama 351-0198, Japan.
 Present address: Central Japan Railway Company, Meieki
1-1-4, Nakamura, Nagoya, Aichi 450-6101, Japan.

 Present address: Physics Department, Graduate School of
Science, Tohoku University, Sendai 980-8578, Japan.
 Present address: Physics Department, Sejong University,
Seoul, 143-747, South Korea.
 Present address: Korea Research Institute of Standards andScience, 1 Toryong-dong, Yusong-gu, Taejoen 305-600, South
Korea.
 Present address: Fukui University of Technology, Fukui910}8505, Japan.
 Present address: International Center for Elementary Particle Physics, University of Tokyo, Tokyo 113-033,
Japan.

0168-9002/01/$ - see front matter  2001 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 7 6 8 - 3

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J.K. Ahn et al. / Nuclear Instruments and Methods in Physics Research A 457 (2001) 137}150

Abstract
An active target with scintillating "bers has been developed and constructed. This target-detector is designed formeasurements of hyperon}proton scattering for hyperon momenta of several hundred MeV/c. The detector serves as
a production target of hyperons as well as a hyperon scattering target on hydrogen. It works as a 4p-detector for the
production, scattering, and decay of charged particles in the target region. A spectrometer system for an incident beam
and outgoing particles is linked with this target-detector in order to produce triggers for the imaging device. Reactions
inside the "ducial volume of the detector are reconstructed kinematically by using image data together with spectrometer
information.  2001 Elsevier Science B.V. All rights reserved.
PACS: 29.40.Gx; 29.40.Mc; 13.75.Ev
Keywords: Scintillating "ber; Image intensi"er tube; Hyperon}nucleon scattering

1. Introduction
Scintillating "bers (SCIFI) have been developed
and used as a tracking device, a calorimeter or an
active target [1]. Furthermore, recent progress in
optoelectronics technology has made it possible todesign new types of visual detector assemblies. In
order to measure hyperon proton (Yp) scattering,
we have developed an active target. This target ismade of a SCIFI block viewed by two chains ofimage intensi"ers (IIs) to reconstruct three-dimensionally reaction kinematics of the event. In recent
nuclear physics experiments with strangeness,a precise tracking of hyperons decays plays anessential role in order to classify the event or to get
information about the reaction sequence.
Experimental studies of hyperon}nucleon (YN)
scattering will shed light on our understanding of
the strong interaction by an additional degree of
strangeness. However, there is not much experimental data on YN scattering, especially in the
momentum region below 1 GeV/c [2}8]. The major
di$culty of YN scattering experiments arises fromthe short lifetime of hyperons. The existing Yp data
were from bubble-chamber experiments in the
1960s. However, the statistics was limited due totheir data-acquisition rate. Because the nominal#ight length of hyperons in this momentum range isa few centimeters, an active target is required bothas a production target and a scattering target,
which provides a three-dimensional visual capability to recognize a complicated reaction sequence
event by event.

Scintillating "bers or tracking detectors withSCIFI have the following characteristics:
E Chemical composition is rather simple; hydrogen and carbon are the target material.
E It is easy to handle; bend and assemble in a small
space.
E A "ducial volume with dimensions of several
centimeters can be made by stacking "bers.
E Tracking resolution can be around a hundred
microns.
E An imaging system with SCIFI can be operated
at a fairly high counting rate. (Optoelectronic
imaging devices with phosphors for SCIFI havea repetition rate of about 10 Hz.)
Our target consists of an active gateable detector
with SCIFI. Hyperons, e.g. &! and ", produced in
SCIFI below 1 GeV/c have a #ight path of several
centimeters and may scatter o! another hydrogen
nucleus in SCIFI before they decay. Thus, a SCIFI
block with a "ducial volume of the order of 10 cm
and a readout system with a spatial resolution ofa few hundred microns can be the most suitabledetector for studying Yp scattering around several
hundred MeV/c.
A prototype of the SCIFI target-detector was
originally designed and employed for an Hdibaryon [9] and a Yp scattering experiment [10]to observe images of strange-particles decays. The
SCIFI target}detector described here has been improved in resolution, timing performance, and"ducial size compared with the original design. The

J.K. Ahn et al. / Nuclear Instruments and Methods in Physics Research A 457 (2001) 137}150

139

selection of SCIFI is described in Section 2. The
assembly of the SCIFI block target and the imaging
device are explained in Sections 3 and 4, respectively. The measurement of Yp scattering and its triggers are described in Section 5. The detector
performance is mentioned in Section 6. A summaryis presented in Section 7.

2. Scintillating 5ber
Several new "ber samples from KURARAY 
were tested in order to select a "ber which gives the
maximum photon yield. The scintillation process
was stimulated by a collimated beam of electrons
coming from the b-decay of a 3.3 MBq Sr source.
Each "ber sample with 70 cm in length anda 300 lm;300 lm square cross-section wasstretched horizontally. The size of the SCIFI crosssection was chosen based on a consideration of thesystem resolution, which is described in Section 6.1.
The source and a trigger counter were "xed toa small carriage which could be moved along the"ber on a rail. The whole set-up was mounted
inside a totally darkened box during the measurements.
Fig. 1 shows an example of an ADC spectrum
collected at a given position of the source along the"ber. The ADC spectrum was "tted by the following expression:





e\IkG
(x!x )
G
exp !
(2.1)
f(x)"
i!
2 p
p
(2p
G
G
G
where M is the maximum number of Gaussian
photoelectron peaks contributing to the measured
spectrum and, x and p are the parameters of theGGi-th photoelectron contribution. We assumedx!x "constant and p "(i p . These free
G>
G
G

parameters (x!x and p ) were determined in
G>
G

a particular ADC spectrum with a low light yield.
The mean value of the ADC spectrum was calculated including zero probability for a Poisson
distribution. All measurements on the "ber samples+

N KURARAY Co. Ltd., Kurashiki 2-28, Nakajo, Kitakanbara, Niigata 959-2653, Japan.

Fig. 1. Example of an ADC spectrum collected at a given
position of the source along the "ber. The crosses show theexperimental data and the solid line represents simulated data
based on Poisson distribution. The dotted lines are the 1 p.e.
peak, the 2 p.e. peak and the 3 p.e. peak contributions, and thethick solid line shows the dark current distribution which isarbitrarily generated with a factor of 1}3%.

were repeated at least three times to reproduce thelight yield. The absolute number of photoelectrons
was determined to 5% uncertainty, which mainly
came from the 1 p.e. peak position determination,
but the relative number of photoelectrons of eachpeaks was not changed much.
It can be seen in Fig. 2 that two "ber samples,
SCSF-50H and -78, show a higher light output
than others at a distance between 20 and 30 cm.
The SCSF-50H generates slightly more lighter thanthe others at short distances, but it is below SCSF78 at longer distances. SCSF-78 has a longer attenuation length (114 cm) due to the longer
wavelength of generated light. A geometrical consideration of our "nal block dimension suggests
choosing the SCSF-78 "ber because of its better
photon yield than SCSF-50H for a hit distance ofaround 30 cm.

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J.K. Ahn et al. / Nuclear Instruments and Methods in Physics Research A 457 (2001) 137}150

Fig. 2. Photoelectron yield as a function of distance for "bersamples of KURARAY with a 300 lm square cross-section.
SCSF-78 has a long attenuation length (114 cm).

3. Scintillating 5ber block target
The SCIFI block consists of plastic scintillating"bers, KURARAY SCSF-78. Each "ber is 30 cm
long and has a 300 lm;300 lm square cross-section. The core of the "ber is 288 lm;288 lm
square, which is coated with a 6 lm thick cladding.
The core material is polystyrene ((CH) ) with a reL
fractive index (n) of 1.59 and a density of 1.05 g/cm,while the cladding is made of polymethyl methacralate (PMMA, C H O ) with n"1.49 and  a density of 1.19 g/cm. The wavelength of the induced photons of SCSF-78 peaks at 435 nm at
short distances close to the interaction point, and at
450 nm at a distance of 30 cm.
The SCIFI block is assembled by stacking alternate "ber sheets to read out from two arms, as
shown in Fig. 3. Each "ber sheet consists of about
330 "bers, corresponding to 100 mm in width. The"ber sheets are bonded with white water paintwhich eliminates any cross-talk across the "bersheets by di!used re#ection of the untrapped
photons. The average thickness of the paint isabout 30 lm. The overlapping area between alternate "ber sheets is 100;100 mm. About 600
sheets are stacked to form an e!ective target thick-Fig. 3. Schematic views of (a) a cross section and a "ber sheet,and (b) stacking sheets. (c) A photo of the assembled SCIFI
block. The SCIFI block is formed by stacking alternate "bersheets. The SCIFI block is "nally covered by black acrylic
plates, as in (c).

ness of 200 mm along the beam direction. The "bersheets in each direction are bundled and polished at
the readout arms to make readout surfaces of100;100 mm. At the other end of the "bers, aluminum evaporated acrylic plates are attached as
mirrors, which improves the light output by50}60%. The e!ective volume serving as an active
target is the overlapping area of 100;100 mm
with 200 mm thickness in the beam direction, which
is su$cient to observe hyperon production, scattering, and decay.

J.K. Ahn et al. / Nuclear Instruments and Methods in Physics Research A 457 (2001) 137}150

Photons created by the interaction of a charged
particle in a "ber reach the end of each "ber, andthose end surfaces will make a line of bright points
as the projected track of a charged particle. By
using two sets of these projected tracks in the U}Zand V}Z plane of the SCIFI block, we reconstructthree-dimensional track(s) of charged particle(s) inthe target volume, and the kinematics of the reaction inside the SCIFI target is calculated together
with the information of incident pion and outgoingkaon obtained by the spectrometer.

4. Imaging device: image intensi5er tubes
Photons from the projected track of charged
particles in the SCIFI block are ampli"ed by IIs.
Two sets of the II chain are attached at the readoutarms for the U}Z and V}Z planes of the SCIFI
block. The chain of IIs is shown in Fig. 4 and theircharacteristics are listed in Table 1. The "rst(V4440PX: HAMAMATSU) and the second(PP0030X: DEP) stages are of the electrostatic
type. The third stage (BV2563McG: PROXITRONIC) is of the micro channel plate (MCP)
type. At the fourth stage, an electron bombarded
CCD (EBCCD: GEOSPHAERA) is employed.
A tapered "ber optic (HOYA-SCHOTT ) is used
between the third stage and the fourth stage tomatch the image size to the sensitive area of the
CCD.
The "rst and second stages are operated in the
DC mode. The cathode voltages of the "rst II is
operated at 20 kV. Since the "ber-optic window of
the output of the "rst stage is thin so as to transmit
as many photons as possible, the second II has to
be in #oating operation, that is, its cathode voltageof 15 kV relative to the anode corresponds to 35 kV
relative to the ground. The third II and the EBCCD Hamamatsu Co. Ltd., Toyooka Factory, Shimokanzo 3145, Toyooka, Iwata, Shizuoka 438-0193, Japan.
 DHT Co. Ltd., 1-636 Maruko-doori, Nakahara, Kawasaki,
Kanagawa 211-0006, Japan.
 GEOSPHAERA Inc., Vinitskaya 3/136, Moscow 117192,
Russia.
 HOYA}SCHOTT Co. Ltd., Shinjuku 3-23-7 , Tokyo 1600022, Japan.

141

are operated in the gate mode by independent external triggers, which are created by fast detectorsof the spectrometer. The third II is set at about
850 V on MCP, 7 kV on the anode and the gate
voltage on the photocathode is !200 V. The applied voltage to the EBCCD is tuned at around
7 kV to get sharp images. The voltage on the
EBCCD's gate is about 700 V. The external triggers
consist of two di!erent levels, which is described in
Section 5.2.
The photoelectrons produced by the intensi"ed
photons are accelerated and directly bombard theback side of a thinned-CCD chip in the EBCCD.
Since no phosphor screens or optic devices before
a CCD chip are required, there is no further loss ofbrightness information and a better spatial resolution of images can be achieved. The CCD chip
consists of 532;580 pixels of 17 lm;23 lm size.
The e!ective area of the CCD chip is9.04 mm;13.34 mm. The 10 cm diameter area of
the input window of the "rst II is demagni"ed by
0.25 in the "rst II itself and by 0.32 in the "ber-optic
taper, then "nally it is reduced to 8 mm diameter,
which "ts into the e!ective size of the CCD. The
EBCCD is set so that the direction of a row of
580 pixels (13.34 mm) coincides with the beam axis
in the SCIFI block. The frame transfer to read thecharge from the CCD is started by the external
trigger. The driving clock of the CCD is 10 MHz
and the readout time is 45 ms.
The raw CCD image su!ers a distortion causedby both the lens system of the IIs and the stray "eldof the spectrometer magnet in use. In order tocompensate for this distortion, a 5 mm thick black
acrylic plate with holes was placed on the opposite
side of the SCIFI block and illuminated withelectro-luminescent panels from the outside. Eachhole on the plate is 300 lm in diameter and they
were positioned at 5 and 10 mm intervals along the
U-(V-) and Z-direction, respectively. The compensation was performed with a two-dimensional spline"tting, as shown in Fig. 5. After a compensation,
scales were converted to the real dimension at the"rst II input surface. In the Z-direction, 10 mm
intervals of holes on the plate shrink to 5 mm since"ber sheets are bundled from 200 to 100 mm at the
II surface. The hole centers are aligned within an
accuracy of 50 lm.

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J.K. Ahn et al. / Nuclear Instruments and Methods in Physics Research A 457 (2001) 137}150

Fig. 4. Schematic drawing of the chain structure of the four IIs.

Table 1
Characteristics of the chain of four Image Intensi"ers
II stage

First

SecondThird

Tapered "berFourthInput diameter (mm)Output diameter (mm)Magni"cation
Gain
Phosphor
Decay time (1/e) (ns)

100
25
0.25
+10
PS-5
575

25
25
1
6}10
P-46
(130)

25
25
1
10}10
P-20(8690)

25
8
0.32(0.32)
**25*
1
10}10**

The magni"cation of 0.32 of a tapered "ber optic reduces photon gain by a factor of (0.32).

Fig. 5. Position compensation is performed with a two-dimensional spline "tting. (a) Before compensation, the raw CCD image su!ers
a distortion caused by both the lens system of the IIs and the stray "eld of the spectrometer magnet. (b) After compensation, scales areconverted to the real dimension at the "rst II input surface.

J.K. Ahn et al. / Nuclear Instruments and Methods in Physics Research A 457 (2001) 137}150

143

5. Hyperon-proton scattering and triggers for the
SCIFI detector

lengths are moderately long and suitable for the
resolution and size of the SCIFI block.

The Yp scattering experiment was performed at
the K2 beam line of the 12-GeV Proton Synchrotron at KEK. The SCIFI block and the II chain
was assembled as shown in Fig. 6. A separatedp> (1.64 GeV/c) or p\ (1.32 GeV/c) beam was used
to produce &>'s, "'s, or &\'s through the (p!, K>)
reaction in a SCIFI target as follows:

5.1. Experimental layout

p>pPK>&>, p>nPK>", p\pPK>&\.

Fig. 7 shows a top view of the experimental
setup, which consisted of the beam line, a spectrometer and the SCIFI target system. The spectrometer
was set at a "nite angle in order to reduce overlapping of tracks of an outgoing K> and a hyperon
produced in the image data. A p! beam with anintensity of 1&2;10/spill (one spill was about 2 s

Subsequent scattering of hyperons was observed in
the SCIFI target. In the &>p scattering case, forexample, &>'s are "rst produced by the above reaction in the active scintillating "ber block target.
The production includes the &>'s both from thefree protons and bound protons in carbon (quasifree production) in the target. The produced &>'s
are identi"ed by tagging the outgoing K>'s in theforward direction and observing the decay vertices
from &>Ppp or np> inside the "ber block. Someof the &>'s interact with another proton or carbon
in the same production target, and elastic scatteringwith protons can be identi"ed by kinematics obtained from tracking information of the event images. In this case, the mean #ight paths of related
charged particles are about 8, 5, 18, 19, and 44 mm
for incident &>, scattered &>, recoil proton, decay
proton, and decay p>, respectively. These #ight

Fig. 6. SCIFI block and the II chain assembled. The p beam
comes from the left in this photo and the K's from the production reaction go to the right (the spectrometer side).

Fig. 7. Top view of the experimental setup.

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J.K. Ahn et al. / Nuclear Instruments and Methods in Physics Research A 457 (2001) 137}150

length with 0.25 Hz repetition) was focused on the
SCIFI target. The beam particle identi"cation was
performed by measuring the time-of-#ight between
the T1 and T2 scintillation counters, supplemented
by a silica aerogel Cherenkov counter (BAC) anda gas Cherenkov counter (GC). The particle trajectory was measured by sets of drift chambers(BDC1&3) and multi-wire proportional chambers(BPC1&5). The outgoing charged particles weremomentum analyzed and detected with a spectrometer consisting of a dipole magnet (SP), multiwire proportional chambers (PC1 and PC2), drift
chambers (DC1, DC2, and DC3), an aerogel
Cherenkov counter (FAC), a lucite Cherenkov
counter (FLC) and a TOF hodoscope (FTOF).

5.2. Triggers on the II chain
The "rst-level trigger was formed by incident
p!'s, vetoed by scattered pions, to apply the gate of
the third stage II. A coincidence matrix between thehodoscope (CH) at the entrance of the SP magnet
and the FTOF counters was also incorporated intothe "rst-level trigger to provide charge and approximate momentum information on the outgoing particle. Since the decision time of the "rst-level trigger
was 190 ns and the power supply took 170 ns formaking the gate voltage, it took 360 ns in total
to open the gate after the passage of the beam.
The "rst-level trigger opened the gate of the third II
for 2 ls. The rate of the "rst-level trigger was150}170 events/spill
for
(p>,
K>)
and
200}250 events/spill for (p\, K>) at 1.5;10p!/
spill. The fourth stage EBCCD was gated by a second-level trigger, which we called a `mass triggera.
The trigger was formed by the signal from theregion around the mass of K>'s in the `mass triggera logic. The mass-trigger selection was determined by momentum information given by the
combination of hit positions on CH and FTOF andthe time-of-#ight information between T2 andFTOF counters obtained from the fast-encoding
TDC modules. The decision time of this trigger
took 14 ls after the occurrence of an event. The
gate of the EBCCD was opened for 1 ms. The
trigger rate at this level was 10}14 events/spill for(p>, K>) and 5}8 events/spill for (p\, K>).

5.3. Photons in the II chain
Photoelectrons from the photocathode are accelerated and hit the phosphor at the output window
to make another image. Phosphors at each II's
output work as an image holder during its decay
time until the triggers "re. The decay time of thephosphor of the "rst II will determine the acceptable beam rate of the system, so as to reduce theoverlap of images at the phosphor caused byshort-beam intervals. Since it took 360 ns to open
the gate of the third II, we decided to use thephosphor PS-5 for the "rst II, which has beenreported to have 700 ns decay time [11], whereas
we employed P-24 (s(10%)&2.4 ls) in the prototype [10]. The gain and decay time were measured
for a test II with PS-5 with single-photon level lightby using a blue LED. The number of photons
emitted per electron generated in the photocathodeof the test II with PS-5, c/e, increases linearly withthe applied voltage as shown in Fig. 8. It was alsoveri"ed that there is no di!erence in gain between
intense and single-photon level light sources. Although the c/e value was lower than P-24 by about
30% at 20 kV (see footnote 16), the decay time of
PS-5 was measured to be about 575 ns, as shown in
Fig. 9, which is a better timing for the "rst-level
trigger than that of P-24.
The phosphor P-46 (s (10%)"300 ns ) is
used for the second II, then the light output of thesecond stage as a function of time is described asfollows :G G
f (t)"   (e\RO !e\RO )
(5.1)

q !q


where s denotes the decay time of PS-5 (575 ns)
and q is for P-46 (130 ns). G is the gain of the nthL
II. The peak of the light output is at about 250 ns,
as shown in Fig. 10, in which gains of the "rst,second and third II are assumed to be 10, 10 and
10, respectively. Since it takes about 360 ns toopen the gate of the third II after the passage ofa particle, 65% of the total light output of thesecond II is passed to the third II within the 2 ls
gate width. Then the light output from the According to the measurement by Hamamatsu Co. Ltd.

J.K. Ahn et al. / Nuclear Instruments and Methods in Physics Research A 457 (2001) 137}150

Fig. 8. Gain measurement as a function of the voltage applied toPS-5; c/e is the number of output photons per electron generated
in the photocathode of the II.

145

Fig. 10. Expected light output at the second stage and at thethird stage of II as a function of time after the passage of thebeam. The gains of the "rst, second and third II are assumed tobe 10, 10 and 10, respectively. The peak of light output at thesecond stage is at about 250 ns, and 65% of the total light outputof the second II is passed to the third II within the 2 ls gate
width of the trigger. Then the EBCCD receive 22% of the third
II's light output when the trigger for the EBCCD is invoked at
14 ls with its width of 1 ms. Horizontal lines in the "gure show
the timing and widths of each trigger.

0(04t(360 ns )G
" q  e\RO R f (t)eRYO dt (3604t(2360 ns )G e\RO f (t)eRYO dt (t52360 ns )
 
q

(5.2)

Fig. 9. Decay time of PS-5 is measured to be about 575 ns,
which suits the timing of the "rst-level trigger.

phosphor P-20 (s (10%)"20 ls (see footnote 16))of the third II is given by the following equations:RGf (t)"  e\RO f (t)eRYO dt


q



where q is the decay time for P-20. The light
output curve is shown in Fig. 10. The trigger for the
EBCCD is invoked within 14 ls and its width is
1 ms. The EBCCD receives 22% of the third II's
light output in case of a decision time of 14 ls.
Therefore, in this gate condition under the EBCCD
gain of 100, the total gain of the II chain is
1.4;10("10;10;0.65;10;0.22;10).
6. Detector performance
A typical example of the image for &\p scattering is shown in Fig. 11. The average data size is

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J.K. Ahn et al. / Nuclear Instruments and Methods in Physics Research A 457 (2001) 137}150

Fig. 11. ;}Z and <}Z view of a &\p scattering event observed with the SCIFI detector.

about 7 kbyte/image. The energy deposit of &\ andrecoil protons is di!erent from mesons in this reaction. The incident p\ in this reaction deposits anenergy close to a minimum ionizing particle (MIP).
The momentum of &\ produced by the (p\, K>)
reaction in the SCIFI target is typically about
500 MeV/c. The energy deposit of such a slow
particle is about three-times larger than that ofa MIP. Fig. 12 shows the average brightness perunit length (mm) of (a) the incident p\ and (b) &\.
So the clusters of tracks are dense and the track of
a &\ is recognized as a continuous line as shown in
Fig. 11.
6.1. Thin track (p\)
A contiguous set of bright pixels is de"ned asa cluster. The centroid of the brightness is de"ned
as the center of the cluster and is considered to be
the input position of the photo-electron. The r.m.s.
of the brightness distribution around the center isregarded as the size of the cluster. Figs. 13(a) and (b)show the distributions of cluster sizes along andperpendicular to the beam axis, respectively, for the
1.32 GeV/c p\ track. The average cluster sizes areboth 0.77 in pixel unit, which correspond to 18 lm(13 lm) on the CCD chip and 221 lm (163 lm) on
the "rst II surface along (perpendicular to) the
beam axis. The cluster density along the particle
track for p\'s is found to be 0.42 clusters/mm in theFig. 12. Average brightness per unit length (mm) of (a) incident
p\'s and (b) &\'s.

SCIFI block detector, which corresponds to0.25 clusters/sheet.
A trajectory in the projected image is "tted witha straight line. The distribution of the center of eachcluster around the straight line is shown in
Fig. 14(a) for tracks longer than 20 mm. The "t toJ.K. Ahn et al. / Nuclear Instruments and Methods in Physics Research A 457 (2001) 137}150

147

a "ber. Thus, at the design stage of a detector
system, one can adjust its spatial resolution by
selecting a size of a "ber or a demagni"cation factor
of an II system depending on the experimental
requirement. The track width, which is de"ned as
the brightness residual around the straight line, isshown in Fig 14(b), and it is found to be 299 lm.
This corresponds to each cluster having a brightness size of 200}250 lm.
6.2. Thick track (&\)

Fig. 13. Distributions of cluster sizes for p\ tracks, (a) along and(b) perpendicular to the beam axis. The cluster sizes are 0.77 in
pixel units both, which corresponds to 18 lm (13 lm) on the
CCD chip and 221 lm (163 lm) on the "rst II surface along(perpendicular to) the beam axis. The brightness distribution ofclusters is shown in (c).

a Gaussian distribution gives a spatial resolution ofp"165 lm for this SCIFI block detector. Thisvalue can be explained by the sources of uncertainties in Table 2.
p"(p #p #p #p +155 lm. (6.1)

 
!!"
''
Along the beam direction, the CCD pixel size
(23 lm) with the demagni"cation 0.08 gives similar
contribution in a spatial resolution to the size ofSince the clusters are thick and dense enough tostick pixels together, the track of slow particles like&\ is recognized as a continuous line, as shown in
Fig. 11. Fig. 15 shows the width of the &\ track,and the "t to a Gaussian distribution with
a p"436 lm. This value is considerably larger
than in the p\ case. Since the II's gain was tuned todetect pion tracks e$ciently, the blooming aroundthe cluster might result for the observed &\ track in
the CCD.
The angular deviation of the &\ track is shownin Fig. 16 as a function of the &\ #ight length. The
angular deviation is de"ned as the di!erence between the predicted direction with the spectrometersystem and the observed direction obtained by theimage data, for &\'s produced from hydrogen. The
track direction in the image data is determined by
identifying the end-points on the video pictures.
The deviation is "tted with two Gaussians; a narrow peak corresponds to &\ production from hydrogen and a broader component corresponds tothose from protons in carbon. The "tting for eachlength is displayed on the right upper side ofFig. 16. This angular deviation was "tted with thefollowing function of l:*h(r.m.s.)" (0.014)#a #(0.02;(l) (rad).
l(6.2)

The "rst term represents the missing momentum
vector resolution which was estimated to be
14 mrad by a Monte Carlo simulation. The momentum resolution of p\ and K> in the simulation
was assumed to be 0.7 and 1.0%, respectively,
which reproduced the missing mass resolution of

148

J.K. Ahn et al. / Nuclear Instruments and Methods in Physics Research A 457 (2001) 137}150

Fig. 14. (a) Distribution of the center of each cluster (p"165 lm) and (b) the residual brightness or track width (p"299 lm) around the"tted straight line.
Table 2
Sources of position uncertainties
Possiblesources

Approximate size inlm at the input surfaceof the "rst II

p
 
p
!!"
p
''
p


330/(12
17/(12/0.08
50
p
 

Size of a "ber with paintSize of a CCD pixel
Accuracy of the II system
Misalignment of the stacking

Fig. 16. Angular deviation *h(r.m.s.) of &\ tracks as a functionof the &\ #ight length (full squares). The solid line shows theresult of "tting by Eq. (6.2). The dotted line represents theresolution of the missing momentum vector (14 mrad). The
dashed line represents the calculated e!ect of multiple Coulomb
scattering (0.02;(l ). The "tting of the angular deviation foreach length with two Gaussians is also displayed on the rightupper side of the "gure.
Fig. 15. Width of the &\ track, p"436 lm.

7.4 MeV/c of the real data. The second term represents the position resolution of pointing between
the end-points on the video pictures perpendicular
to the track, whose value is `aa 0.46 mm, where `ladenotes the #ight length in mm. This includes the
position resolution of both the production anddecay vertices of the &\. The third term corresponds to the e!ect of multiple Coulomb scattering,J.K. Ahn et al. / Nuclear Instruments and Methods in Physics Research A 457 (2001) 137}150

149

50 to 1103 in the center of mass frame. The
recognition e$ciency of the scattering pattern in
the image data is decreased due to short #ight
lengths of recoil protons at forward angles, as described above, and those of hyperons at backward
angles.

7. SummaryFig. 17. Comparison of the elastic p}p di!erential cross-section
by the present method (crroses) with the published data (squares[12]). Event candidates were selected by eye-scanning of image
data on the video picture, and kinematical constraints wereapplied, such as the coplanarity condition and the momentum
balance.

and was calculated to be 0.02;(l for 500 MeV/c&\ in scintillator. The main component of thisangular deviation at short distance is caused by the
position resolution in the identifying procedure of
the end-points of the line segments on the video
picture.
6.3. A control through p}p elastic scattering
measurement
To test this detector system, proton}proton elastic scattering has been measured. A proton beam of
750 MeV/c was incident on the SCIFI target detector. The result of the elastic p}p di!erential
cross-section is shown in Fig. 17. Event candidates
were selected by eye-scanning of image data on thevideo picture and kinematical constraints were applied, such as the coplanarity condition and themomentum balance. The p}p di!erential cross-section is known to be almost #at at about 3.5 mb/sr inthis momentum region [12]. The eye-scanning e$ciency drops at angles smaller than 503, since the#ight length of recoil protons become shorter than
3.5 mm and it is getting hard to identify. However
scattering at angles larger than 503 are well recognized and the obtained values of the di!erential
cross-sections are consistent with the existing data.
In hyperon proton scattering case, the detectable
angular region of this SCIFI block detector is fromWe have constructed a new active target witha SCIFI block detector for hyperon}proton scattering measurements at hyperon momenta of several hundred MeV/c. The SCIFI block consists of
2;10 plastic scintillating "bers, KURARAYSCSF78, where each "ber is 30 cm long andhas a 300 lm;300 lm square cross-section.
600 sheets, 10 cm wide and 30 cm long with
330 "bers each, are stacked alternately so as to form
an e!ective target volume of 10;10;20 cm. This"ber block is viewed from two orthogonal directions by two sets of II chains in order to reconstruct
the particle trajectories inside the target three-dimensionally.
For Yp scattering, hyperons are produced by (p,K) reactions from nucleons in the SCIFI block.
This SCIFI detector is linked with the spectrometersystem, and the signals of incident p's and outgoing
K's from the spectrometer detectors are used toopen gates of the II chains to take data event by
event e$ciently. Hyperon production, scatteringand decay are observed in the "ducial volume of the
SCIFI detector and the kinematics of the reactions,which occur in the SCIFI block, is deduced fromthe image data together with the spectrometer information.
For p\'s of 1.32 GeV/c, a hit density of 0.42 clusters/mm and a spatial resolution of p"165 lm are
obtained for this SCIFI detector. For thick tracks,
like &! and protons, tracks are recognized as contiguous lines, and the width of the tracks becomeswider than for the p\ case. Although further improvements will be required for higher beam intensity and higher event rates of the system, the active
target with this SCIFI block detector will givea breakthrough for hyperon experiments, such as
Yp scattering and searches for such particles as the
H-dibaryon.

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J.K. Ahn et al. / Nuclear Instruments and Methods in Physics Research A 457 (2001) 137}150

Acknowledgements
We wish to thank Mr. O. Shinji of KURARAY
Co. Ltd for o!ering valuable information and many
samples of scintillating "bers. The design and assembling of the II chain was expedited by the suggestions of Dr. J.-P. Fabre and Dr. H.T. Wong at
CERN. We express our appreciation to Professor
K. Niwa and Dr. O. Sato at Nagoya University for
their help with the measurement of the decay timeof PS-5. The dedication and support of many of the
KEK sta! during this development are very muchappreciated. We would like to thank Professors H.
Sugawara, S. Yamada, K. Nakamura, T. Ohshima,
J. Chiba, and Y. Yoshimura for their continuous
encouragement throughout this work. We also
thank the KEK accelerator crew and the beam
channel group members for their continuous tuning and the stable operation of the machines. This

work was supported partially by Grants-in Aid for
Scienti"c Research no. 06640425 of Japan Ministryof the Education, Science and Culture.

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[4] B. Sechi-Zorn et al., Phys. Rev. 175 (1968) 1735.
[5] G.R. Charlton et al., Phys. Lett. 32B (1970) 720.
[6] J.A. Kadyk et al., Nucl. Phys. B 27 (1971) 13.
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[12] K. Abe et al., Phys. Rev. D 12 (1975) 1.

1

Active Target with Plastic Scintillating Fibers forHyperon-Proton Scattering ExperimentJ. K. Ahna,1, H. Akikawaa , J. Arvieuxb,2, B. Bassalleckc , M. S. Chunga,3, H. En’yoa , T. Fukudad, H. Funahashia ,S. V. Golovkine , A. M. Gorine , Y. Gotoa,4, M. Hanabataf, T. Hayakawag,5, A. Ichikawaa , M. Ieirid, *, K. Imaia ,M. Ishinoa,6, H. Kandaa,7, Y. D. Kimh,8, Y. Kondoa,9, E. N. Kozarenkoi , I. E. Kresloi , J. M. Leej,10,
A. Masaikea,11, S. Miharaa,6, K. Nakaig, K. Nakazawaf, K. Ozawaa,12, A. Satog, H. D. Satoa , K. S. Simk,
T. Tabarua , F. Takeutchil , P. Tlustym , H. Toriia , K. Yamamotoa,13, S. Yokkaichia , M. Yoshidaa,4a Department of Physics, Kyoto University, Kyoto 606-8502, Japanb Laboratoire National Saturne, F-91191, Gif-sur-Yvette, CEDEX, Francec Department of Physics, University of New Mexico, Albuquerque, N.M. 87131, U.S.A.
d IPNS, KEK, High Energy Accelerator Research Organization, Oho 1-1, Tsukuba 305-0801, Japane Institute for High Energy Physics, RU-142284, Protvino, Russia
f Physics Department, Gifu University, Gifu 501-1193, Japang Department of Physics, Science University of Tokyo, Chiba 278-8510, Japanh Physics Department, Seoul National University, Seoul 151-742, Korea
i Joint Institute for Nuclear Research, RU-141980, Dubna, Russia
j Department of Physics, Yonsei Univ., Seoul 120-749, Korea
k Department of Physics, Korea University, Seoul 136-701, Korea
l Department of Physics, Kyoto Sangyo University, Kyoto 603-8555, Japan
m Nuclear Physics Institute, 250 68 Rez, Czech Republic
Abstract-- An active target with plastic scintillating fibers
has been constructed. This active target served as a production
target of hyperons as well as a scattering target, and worked
as a 4 π -detector for charged particles. This system isdesigned for measurement of hyperon-proton scattering for thehyperon momenta of several hundred MeV/c. A spectrometersystem for incident beam and outgoing particles of hyperon
production reaction is linked with this target-detector in order* Corresponding author. E-mail address: masaharu.ieiri@kek.jp
Present Addresses:
1 Research Center for Nuclear Physics, Osaka University, Ibakaki, Osaka
567-0047, Japan.
2 Institut de Physique Nucleaire, Universite Paris-Sud, F-91406
ORSAY-Cedex, France.
3 P-25, MS H846, Los Alamos National Laboratory, Los Alamos, NM
87545, U.S.A.
4 RIKEN, Institute of Physical and Chemical Research, Wako, Saitama
351-0198, Japan.
5 Central Japan Railway Company, Meieki 1-1-4, Nakamura, Nagoya,
Aichi, 450-6101, Japan.
6 International Center for Elementary Particle Physics, University of Tokyo,
Tokyo 113-0033, Japan.
7 Physics Department, Graduate School of Science, Tohoku University,
Sendai 980-8578, Japan.
8 Physics Department, Sejong University, Seoul, 143-747, Korea.
9 KEK, High Energy Accelerator Research Organization, Oho 1-1,
Tsukuba 305-0801, Japan.
10 Korea Research Institute of Standards and Science, 1 Toryong-dong,
Yusong-gu, Taejoen 305-600, Korea.
11 Fukui University of Technology, Fukui 910-8505, Japan.
12 Center for Nuclear Study, University of Tokyo, Tokyo 113-0033, Japan.
13 Department of Physics, Osaka City University, Osaka 558-8585, Japan.

to provide triggers for the imaging device of this detector
system. As a control of this detector, the decay constant of Σhyperon has been evaluated.

I. INTRODUCTION
We have developed an active target with a block ofscintillating fibers (SCIFI) in order to measure hyperonproton (Yp) scattering [1,2], which is viewed by two chainsof image intensifiers (IIs) to reconstruct three-dimensionallyreaction kinematics of the event inside the SCIFI block. The
existing data of Yp scattering are from bubble chamber
experiments performed in the 1960’s [3]. In recent nuclear
physics experiments with strangeness, a precise tracking of
hyperons decays plays an essential role in order to get
information about the event itself or about the reaction
sequence [4].
The SCIFI block target and the imaging device are
explained in sect. 2 and 3, respectively. The process oftreating image data is described in sect. 4. In sect. 5, theanalysis of Σ- production events is mentioned, and the decay
constant of Σ- is deduced as a control of the detector system.
A summary is presented in sect. 6.
II. S CINTILLATING F IBER B LOCK TARGETThe SCIFI block consists of plastic scintillating fibers,
KURARAY SCSF-78 [5]. Each fiber is 30 cm long and has
a 300 µm × 300 µm square cross section. The core of the

2fiber is 288 µm × 288 µm square, which is coated with a 6µm thick cladding.
The SCIFI block is assembled by stacking alternate fibersheets, as shown in Fig. 1-(a) and (b). Each fiber sheet
consists of about 330 fibers, corresponding to 100 mm inwidth. The fiber-sheets are bonded with white water paintwhich eliminates any cross talk across the fiber-sheets bydiffused reflection of the untrapped photons. The overlapping
area between alternate fiber sheets is 100×100 mm2. About
600 sheets are stacked to form an effective target thickness of
200 mm along the beam direction. The fiber-sheets in eachdirection are bundled and polished at the readout arms to
make readout surfaces of 100×100 mm2. The effective
volume serving as an active target is the overlapping area of
100×100 mm2 with 200 mm thickness in the beam direction.

make a line of bright points as the projected track of acharged particle. By using two sets of these projected tracks
in the U-Z and V-Z plane of the SCIFI block, we reconstructthree-dimensional track(s) of charged particle(s) in the target
volume, and the kinematics of the reaction inside the SCIFI
target is calculated together with the information of incident
pion and outgoing kaon of the hyperon production reaction
obtained by the spectrometer (SP).
III.

IMAGE INTENSIFIER T UBES

Photons from the projected track of charged particles in the
SCIFI block are amplified by IIs. Two sets of the II chain are
attached at the readout arms for U-Z and V-Z planes of the
SCIFI block. The chain of IIs are shown in Fig. 2.

Fig. 2. The SCIFI target is viewed by two chains of IIs. The first(V4440PX : HAMAMATSU [6]) and the second (PP0030X : DEP [7])stages are of the electrostatic type. The third stage (BV2563McG :PROXITRONIC [7]) is of the micro channel plate (MCP) type. At the
fourth stage, an electron bombarded CCD (EBCCD : GEOSPHAERA [8])is employed. A tapered fiber optic (HOYA-SCHOTT [9]) is used between
the third stage and the fourth stage to match the image size to the sensitive
area of the CCD.

Fig. 1. Schematic drawing of (a) a cross section of a fiber and a fiber
sheet, (b) the structure of the SCIFI block. The SCIFI block is assembled bystacking alternate fiber sheets.

Photons created by the interaction of a charged particle in afiber reach the end of each fiber, and those end-surfaces will

The third II and the electron bombarded CCD (EBCCD)
are operated in the gate mode by independent external triggers,which are created by fast detectors of the SP. The external
triggers consist of two different levels. The 1st level trigger
made by fast signals from photomultipliers of SP opens thethird MCP gate. The 2nd level trigger, called “mass trigger”
is made from combinations of hodoscope hits and time-offlight information to open the EBCCD gate. The image datafrom CCD are digitized in real time with a 6-bit flash-ADC
and the digitized data were compressed with a compactor
module in a CAMAC crate. A typical example of the imagefor Σ-p scattering is shown in Fig. 3. The average data size isabout 7 kbyte/image. Details of the IIs chain and triggers
made by detectors of the SP are described in ref. [1,10].

3Fig. 3.

U-Z and V-Z view of a Σ-p scattering event.

IV. IMAGE DATA ANALYSIS
A. Analysis procedure
Raw image data are distorted by the pin-hole distortion of
the electrical lens system and a stray field from SP. First of
all the processes, this distortion should be corrected.
To derive observed values and their errors, the efficiencies
and accurateness of the analysis should be estimated. For thispurpose, Monte Carlo simulations were performed.
Simulated image data were generated and analyzed with thesame processes as those for the real.
Then, pictures were eye-scanned and categorized according
to the event patterns. The events including visible Σ-'s wereselected and used for further analysis. After the categorization
was completed, the reaction points were marked on a graphic
display with a pointing device.
B. Position calibration
Raw images on the CCD suffered a pin-hole distortion
caused by the electrical lens system of the IIs. A Stray fieldfrom SP also distorted the images. The actual position ofeach pixel in the SCIFI target was compensated by placing a
black lucite-plate with holes on the opposite side end of the
SCIFI block, and uniformly illuminating them with anelectro-luminescence panel from outside. Fig. 4 shows
pictures of calibration patterns before and after the correction.
The diameter of the hole was 300µmφ, and the pitches
were 10mm and 5mm longitudinal and transverse to thebeam direction, respectively. The pixel position wastransformed to the real position with a formula obtained byusing a two-dimensional spline fitting. The centers of theholes were aligned within the accuracy of 50 µm after
compensation.

Fig.4. Position compensation by using a calibration pattern illuminatedwith an electro-luminescence panel. (a) A picture of the raw pattern. (b)After compensation, the scales are converted to the real dimension in the
SCIFI block.

C. Image Data Simulation
In order to estimate analyzing efficiencies and accuratenessof pointing and to evaluate systematic errors, the Monte
Carlo simulation was performed by using GEANT 3 [11]. To
evaluate the scanning efficiency, the simulated image data
were mixed into the real data. The Σ-'s were generated by "- +(p) → K+ + Σ- reaction, and scatter or react with protons inthe simulation. The Fermi motion in carbon nuclei wasreproduced with the distribution asN0N( p) =
(1)p(GeV/ c) − 0.1
1 + exp 
0.05
The energy loss of the charged particles, effect of multiple
Coulomb scattering were calculated by GEANT. The
response of the SCIFI detector system was simulated toreproduce the characteristics of the real tracks, and simulated
image data were created. Finally, the brightness distributionsof "- and Σ- tracks and the track width of them were well
reproduced.
The generated image data were analyzed through the sameprocess as the real data, and the efficiencies were evaluatedfrom the surviving rate of the simulation events. The angularresolution and the accuracy of the track-length measurements
were also estimated using the simulation data by comparing
the generated positions to the positions obtained by the
pointing process.
V. ANALYSIS OF Σ- PRODUCTION AND ITS DECAY CONSTANT
A. Σ-p elastic scattering experiment
We have performed Σ-p elastic scattering. Σ-'s of 400 to
700 MeV/c were produced through the in-flight p("-, K+) Σreaction in the SCIFI with a 1.32 GeV/c "- beam. Subsequentelastic scattering of Σ- hyperons was observed in the SCIFI
active target. Details of the analysis of Σ-p elastic scattering
are described in ref. [1]. Here, we should like to explain
about the analysis of Σ- hyperon production reaction, which

4
is important in obtaining the absolute value of cross sectionsof Σ-p elastic scattering, since produced Σ-'s act as “beam” forΣ-p scattering.
B. Requirement for the Σ- Production
The pointing was done for 2947 event samples of the Σbeam candidates to evaluate the amount of the “Σ- beam”.
This correspond to 1.6% of all the data in the Σ-p scatteringexperiment. A typical Σ- production event is shown in Fig. 5.

Fig. 6. The Σ - flight length dependence of (a) the error of length
obtained by the pointing process, and (b) the analyzing efficiency. In figure(b), open squares represent the efficiency of the event categorization andpointing, and closed squares represent the total efficiency including thekinematical cut efficiency to select the H-target Σ - beam (The definition of
H-target is described in the next subsection.) They were obtained with theanalysis of the simulated image data. The threshold value for the Σ - track
length was set to 5mm, as indicated by the lines in the figure.

Fig. 5. An example of a Σ - production event observed with the SCIFI
detector. Open squares represent the input positions given in the pointing
process. The solid lines just on the "- and K+ tracks are the fitted lines, andthe dotted lines slightly deviated from the tracks represent the predictedtracks with the external wire chambers.

The weak decay of Σ- → "-n was easily recognized in the
pointing process, because the brightness of the Σ- and "tracks was quite different. The start point of the Σ- track was
the vertex point of the ("-, K+) reaction and the end point was
the kinked point or the point where the brightness of the
track suddenly changed.
The production vertex of the ("-, K+) reaction wasreconstructed by using the image data. The vertex position
was defined as the closest point of the Σ- and K+ tracks. The
vertex resolution along the z-axis was estimated to be 0.6
mm from the simulation data.
The measurement error of the track length of the Σ- causes
systematic errors in the cross section. Additionally, if thelength of the Σ- is too short, the event selection becomesambiguous. Figure 6-(a) shows the difference of the flight
length between the generated one by the simulation and themeasured one by the pointing process. Figure 6-(b) shows theefficiencies for the event categorization, pointing andkinematical cut to select the H-target Σ- beam as a function of
the flight length of the Σ-. The definition of H-target Σ- beamand the details of the kinematical cut are described below.
The Σ- whose flight length was longer than 5mm was used
for the Σ- beam. To avoid ambiguities in the categorizationfor events which occurred at the upstream and downstream
ends of the SCIFI, the z-position of the vertex was required
to be within -90 mm ~ 60 mm, where the origin of thecoordinate was the center of the SCIFI block.

C. Σ- Produced on Free Protons and Carbon Nuclei
Since the SCIFI target consisted of (CH)n, Σ-'s were
produced both on free protons (hydrogen target) and carbon
nuclei (carbon target). We call them the H-target and C-target
Σ- beams, respectively. For the C-target beam, themomentum of the Σ- beam is not well determined by the ("-,
K+) kinematics due to the Fermi motion.
In order to separate the H-target Σ- beam, we adopted the
H-target cut, which include the angle-difference and missingmass cuts. The angle-difference for the Σ- production
kinematics (Δθprod) was defined asΔ θ prod = cos 

p
d
−
−
 Σ prod Σ prod −1 pΣ − prod ⋅ dΣ − prod(2)

where, dΣ ¯prod is the measured direction of the Σ- obtained
from the pointing process, and pΣ ¯prod denotes the predicted
momentum vector of the Σ- calculated from the momentum
vectors of the "- and the K+. The momenta of the "- and K+
were obtained from the analysis of the spectrometer data, andthe directions were measured with the SCIFI detector and theexternal wire chambers. The energy loss in the SCIFI target
was taken into account to calculate the production kinematics.
Figure 7-(a) shows the distribution of Δθprod versus theflight length of the Σ- (flΣ ¯). The Δθprod distribute around zero
within the resolution of the SCIFI system for the H-target Σbeam. These events make a narrow peak, and which can be
distinguished from the broader distribution of the C-target Σbeam. The region inside the lines in Figure 7-(a) was theaccepted region for the Σ-'s produced on free protons. The
threshold was set to be
1.35
Δ θ prod <+ 0.03, flΣ− > 5 mm
(3)
flΣ −

5
by taking projections of Figure 7-(a) onto Δθprod in someslices of (flΣ ¯).
Figure 7-(b) shows the missing-mass spectrum of the p("-,+K )X reaction (MMprod) after the angle-difference cut wasapplied. The missing-mass resolution was 7.4 MeV/c2(r.m.s.). The criterion for the missing-mass cut was
1.174 < MMpro < 1.218 (GeV/c2).
(4)
The events which satisfy (3) and (4) were regarded as the
H-target Σ- beam, and the other events were recognized as the
C-target Σ- beam. The contamination from the C-target in the
H-target Σ- beam was estimated to be 10% from Figure 7-(b).

Fig. 8. Distribution of the Σ - flight length for the H-target Σ - beam
Fig. 7. (a) Scatter plot of Δθprod versus the flight length of the Σ - (flΣ¯ ).
(b) Missing-mass spectrum of the p("- , K+)X reaction after the angledifference cut was applied. The lines on the spectrum are obtained with a
two-Gaussian fitting. The events inside the region indicated by the arrows
were regarded as the Σ - 's produced on free protons (H-target Σ - beam).

D. Lifetime of ΣTo confirm the Σ- event selection and test the detector
system, the lifetime of the Σ- was derived as following.
The mean flight length, x, of the particle with the life-timeτ is given asp
x = c τ ⋅ β γ = cτ ⋅(5)mwhere m and p are the mass and momentum of the particle,
respectively. For charged particles in material, the energy lossof the particle along its path should be taken into account,then x•m/p is replaced by L, which is defined as
x m
L =∫
⋅ dξ
(6)
0 p(ξ )The decay rates as a function of L before and after theefficiency correction were evaluated in the region of L
between 10 mm and 100 mm, as shown in Figure 8. The
efficiency for each bin of L was obtained by using thesimulation data. The decay constant was calculated with thelogarithmic likelihood method.
The result is:cτ = 43.3 ± 3.2 mm,(7)which is consistent with known value [12]: cτ = 44.34 ±0.33 mm.

VI. S UMMARY
A new active target with plastic scintillating fibers has
been constructed for hyperon-proton scattering. The SCIFI
block consists of 2 × 105 plastic scintillating fibers,
KURARAY SCSF78, where each fiber is 30 cm long andhas a 300 µm × 300 µm square cross section. 600 sheets, 10cm wide and 30 cm long with 330 fibers each, are stacked
alternately so as to form an effective target volume of 10
× 10 × 20 cm3. This fiber block is viewed from two
orthogonal directions by two sets of II chains and data are
stored as two CCD images, in order to reconstruct theparticle trajectories inside the target three-dimensionally.
To evaluate the scanning efficiency of image data, thesimulated image data were mixed into the real data. The
decay time of Σ- hyperon is well obtained as a control of thisdetector system.
VII. ACKNOWLEDGMENT
The dedication and support of many of the KEK staff
during this development are very much appreciated. We
would like to thank Professors H. Sugawara, S. Yamada, K.
Nakamura, and Y. Yoshimura for their continuous
encouragement throughout this work. We also thank the KEK
accelerator crew and the beam channel group members fortheir continuous tuning and the stable operation of themachines. This work was supported partially by Grants-in
Aid for Scientific Research no. 06640425 of Japan Ministryof the Education, Science and Culture.

6
VIII. R EFERENCES
[1]
[2]
[3]

Y. Kondo et al., Nucl. Phys. A676 (2000) pp.371-387.
Y. Goto et al., Nucl. Phys. A648 (1999) pp. 263–279.
R. Engelmann et al., Phys. Lett. 21 (1966) 587; G. Alexander et al.,Phys. Rev. 173 (1968) 1452; B. Sechi-Zorn et al., Phys. Rev. 175(1968) 1735; G. R. Charlton et al., Phys. Lett. 32B (1970) 720; J. A.
Kadyk et al., Nucl. Phys. B27 (1971) 13; F. Eisele et al., Phys. Lett.
37B (1971) 204; J. M. Hauptman et al., Nucl. Phys. B125 (1977) 29.
[4] J. K. Ahn et al., Phys. Lett. B378 (1996) 53; Nucl. Phys. A648 (1999)
263.
[5] KURARAY Co. Ltd., Kurashiki 2-28, Nakajo, Kitakanbara, Niigata
959-2653, Japan.
[6] Hamamatsu Co. Ltd., Toyooka Factory, Shimokanzo 314-5, Toyooka,
Iwata, Shizuoka 438-0193, Japan.
[7] DHT Co. Ltd., 1-636 Maruko-doori, Nakahara, Kawasaki, Kanagawa
211-0006, Japan.
[8] GEOSPHAERA Inc., Vinitskaya 3/136, Moscow 117192, Russia.
[9] HOYA-SCHOTT Co. Ltd., Shinjuku 3-23-7 , Tokyo 160-0022, Japan.
[10] J. K. Ahn et al., Nucl. Instr. Meth., accepted for publication.
[11] GEANT – Detector Description and Simulation Tool, CERN Program
Libraly Long Writeup W5013.
[12] Particle data group, Eur. Phys. J. C15 (2000) pp. 1-878.

E307 H. C. Bhang
Lifetimes and Weak Decay Widths of Light and Medium-Heavy Λ Hypernuclei
H. Park et al.
[attached]
Lifetime Measurement of Medium-Heavy Λ Hypernuclei
Phys. Rev. C61 (2000) 054004.
● O. Hashimoto et al.
[attached]
Proton Energy Spectra in the Non-mesonic Weak Decay of 12ΛC and 28ΛSi Hypernuclei
Phys. Rev. Lett., 88 (2002) 042503.
◇ H. Outa et al.
Mesonic and Non-mesonic Decay Widths of 12ΛC
Nucl. Phys. A670 (2000) 265c-268c.
◇ O. Hashimoto et al.
Total and Partial Decay Widths of Medium-Heavy Λ Hypernuclei
8th Asia Pacific Physics Conference (APPC 2000), Taipei, Taiwan, 7-10 Aug 2000.
Published in *Taipei 2000, APPC 2000* 429-434.
◇ H. Bhang et al.
Overview of Λ-Hypernuclear Weak Decay Results Obtained with the SKS
Nucl. Phys. A691 (2001) 156c-162c.
◇ Y. Sato et al.
The π- Mesonic Decay Rates on 12ΛC and 28ΛSi and ΛFe
Nucl. Phys. A691 (2001) 189c-192c.
●________________________________________________________________________● Physics papers published in refereed journal.
○★
◇＊

Technical papers.
PhD theses.
Conference and Symposium.
Internal Report and others.

PHYSICAL REVIEW C, VOLUME 61, 054004

Lifetime measurements of medium-heavy ⌳ hypernuclei
H. Park,* H. Bhang, and M. Youn†
Department of Physics, Seoul National University, Seoul 151-742, Korea

O. Hashimoto, K. Maeda, Y. Sato, and T. Takahashi
Department of Physics, Tohoku University, Aoba-ku, Sendai, Miyagi 980-77, JapanK. Aoki, Y. D. Kim,‡ H. Noumi, K. Omata, H. Outa, M. Sekimoto, and T. Shibata
High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan

T. Hasegawa,§ H. Hotchi, and Y. Ohta
Graduate School of Science, University of Tokyo, Tokyo 113-0033, JapanS. Ajimura and T. Kishimoto
Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan
共Received 10 December 1999; published 21 April 2000兲
12
28
56
We have measured the lifetimes of the weak decay of ⌳C, ⌳Si, and ⌳Fe hypernuclei which were produced⫹⫹
via a ( ␲ ,K ) reaction at KEK 12-GeV PS. The lifetimes were obtained by direct measurement of the
production and decay time of the hypernucleus using a fast timing counter system whose time resolution was
1228
about 83 ps. Our results for the lifetimes of ⌳C, ⌳Si, and ⌳ Fe are 231⫾15, 206⫾11, and 215⫾14 ps,
11
27
respectively. The lifetimes of ⌳ B and ⌳ Al with the application of the gate to the p ⌳ state of the formation
1228
spectra of ⌳C and ⌳
Si were also determined to be 211⫾13 and 203⫾10 ps. From the present work, it was
found that the total decay width saturates according to the hypernuclear mass number even at A⬃12. Thissuggests the importance of the proper treatment of the short range nature of the nonmesonic decay of ⌳hypernuclei.

PACS number共s兲: 21.80.⫹a, 13.30.Eg, 13.75.Ev, 21.10.Tg

I. INTRODUCTION

The weak decay of ⌳ hypernuclei has been studied overmany years since its discovery in 1952 in an emulsion experiment. Hypernuclei are typically produced in excitedstates 共or ground states兲 through the strangeness production
reaction and can reach their ground state through nucleon
emission and/or electromagnetic decay. Eventually, they will
decay through weak interaction processes which involve theemission of pions or nucleons.
A free ⌳ hyperon decays mostly into a nucleon and a pion
via the weak nonleptonic decay⌳→

再p⫹ ␲ ⫺ ⫹37.8 MeV 共63.9%兲,n⫹ ␲ 0 ⫹41.1 MeV

共35.8%兲,

共1兲

with a lifetime of 263 ps. The branching ratios of the twochannels are 35.8% and 63.9% 关1兴 and are consistent with asimple calculation from the empirical ⌬I⫽1/2 rule. The energy released in the free ⌳ decay is about 40 MeV and the*Present address: High Energy Accelerator Research Organization 共KEK兲, Tsukuba, Ibaraki 305-0801, Japan.
†
Present address: University of Houston, Houston, TX 77004.
‡
Present address: Department of Physics, Sejong University,
Seoul 144-747, Korea.
§
Present address: School of Allied Health Sciences, Kitasato University, Sagamihara 228-8555, Japan.
0556-2813/2000/61共5兲/054004共11兲/$15.00

corresponding momentum of a nucleon and a pion in theircenter-of-mass frame is about 100 MeV/c.
When a ⌳ hyperon is bound in the nucleus, the mesonicdecay channel is highly suppressed except for the very lightnuclei because of Pauli blocking of the final-state nucleon.
Nucleon-induced decay, the so-called nonmesonic decay, becomes dominant. This channel does not occur in the free ⌳关2兴,⌳⫹ p→n⫹ p⫹176 MeV,

共2兲⌳⫹n→n⫹n⫹176 MeV.

共3兲The energy release of this decay mode is about 176 MeV andthe momentum of each final-state nucleon is about 400
MeV/c, which is much higher than the Fermi momentum. As
a result, this decay mode becomes dominant in medium andheavy hypernuclei.
Thus, the total decay width (⌫) and the lifetime ( ␶ ) of ⌳hypernuclear weak decay can be written1⌫⫽ ⫽⌫ ␲ ⫹⌫ nm ,
␶共4兲

⌫ ␲ ⫽⌫ ␲ ⫺ ⫹⌫ ␲ 0 ,共5兲⌫ nm ⫽⌫ p ⫹⌫ n ⫹ 共 ⌫ 2p2h 兲 ,共6兲

where ⌫ ␲ ⫺ ( ␲ 0 ) is the partial decay width of ␲ ⫺ ( ␲ 0 ) mesonicdecay, ⌫ p(n) is that of proton-共neutron-兲 induced nonmesonic

61 054004-1©2000 The American Physical Society

H. PARK et al.

PHYSICAL REVIEW C 61 054004

decay, and ⌫ 2p2h is that of the two-nucleon-induced nonmesonic decay which has been suggested 关3兴 but not yet confirmed.
The ⌳N→NN interaction is the strangeness changing⌬S⫽1 analogy of the weak NN→NN interaction responsible for nuclear parity violation 关4兴. In the case of the NN→NN weak interaction, the parity-conserving part is not observable because it is overwhelmed by the strong interaction.
At present, a direct study of the hyperon-nucleon weakinteraction is difficult due to the lack of either a hyperon
beam or target and the low cross section, even if the timereversed process pn→⌳ p were to be made possible by new
facilities 关5兴. So nonmesonic decay provides the most practical means to investigate the four-fermion weak vertex and⌳N→NN weak interaction which occurs only in the nuclear
medium.
In addition, one of the most important characteristics ofthe nonmesonic decay process is the large momentum transfer of about 400 MeV/c. Such a large momentum transfer
implies that it probes short distances of the order of 0.5 fm
and might, therefore, expose the role of explicit quark degrees of freedom in the nucleus. Furthermore, in investigating the nonmesonic decay of hypernuclei, one can explore
the question of whether the ⌬I⫽1/2 rule that governs ␲mesonic decay applies to nonmesonic decays.
The total decay width 共or lifetime兲 is the observable
which can be measured most accurately and interpreted freefrom the nuclear final-state interaction. The lifetime measurement can be the starting point for obtaining the various
observables, e.g., partial decay widths of mesonic or nonmesonic decay, the ratio of proton- to neutron-induced nonmesonic decay rates, etc. In addition, the variation of lifetime
with hypernuclear mass number can provide a qualitative
understanding of the nuclear medium effects on the baryonbaryon weak interaction.
Table I shows the status of the previous lifetime measurements. For light hypernuclei of A⭐5, there are old emulsion
data for the lifetimes with large uncertainties. Recently,
some counter experiments were performed at BNL 关17,16兴
4
4
5
H, ⌳
He, and ⌳He hypernuclei. In theand KEK 关12,13兴 for ⌳medium mass region, there are only two sets of data. Graceet al. 关17兴 obtained the lifetimes by applying various gates12
on excitation energies of the ⌳C hypernucleus. The lifetimeof the hypernucleus of A⬃16 was measured at LBL 关18兴using a 16O beam, but the production of a hypernucleus was
not identified explicitly in the experiment. For heavy hypernuclei with A⬎200, there exists some lifetime data using
antiproton or proton beams to measure the delayed fission
fragments. Recently, the lifetime of the hypernuclear fission
fragments from the p⫹ 238U interaction was measured withinreasonable uncertainties using a proton beam at COSY 关22兴.
In these measurements for heavy hypernuclei with A⬎200,
the hypernuclear production was not explicitly identified.
Before the present experiment, there had existed only one
measurement in the mass region from A⫽12 to 200, which
has large errors and could not identify the hypernuclear formation clearly. The present experiment will provide the lifetimes in this region with clear hypernuclear formation spectra.

TABLE I. The status of the lifetime mesurements.
A␶ 共ps兲

Ref.

Expt.

3
⌳H

⫹220
90⫺40
⫹127
285⫺105⫹35
128⫺26⫹84264⫺52⫹62
246⫺41⫹250
180⫺70⫹490
360⫺130⫹166
268⫺107⫹24
194⫺26⫹233
228⫺129245⫾24256⫾27⫹190
140⫺50⫹150180⫺60⫹190
251⫺73⫹60
274⫺50256⫾21
201⫾30192⫾22
211⫾31
⫹33
86⫺26
⫹250
250⫺100

关6兴
关7兴
关8兴
关9兴
关10兴
关6兴
关11兴
关7兴
关12,13兴
关7兴
关14兴
关13兴
关6兴
关11兴
关7兴
关15兴
关16兴
关16兴
关17,16兴
关17,16兴关18兴
关19兴

Emulsion
Emulsion
Emulsion
Bubble chamber
Bubble chamber
Emulsion
Emulsion
Emulsion⫺⫺(K stop ,␲ )
Emulsion(K ⫺ , ␲ ⫺ )⫺⫺(K stop ,␲ )
Emulsion
Emulsion
Emulsion
Emulsion(K ⫺ , ␲ ⫺ )(K ⫺ , ␲ ⫺ )(K ⫺ , ␲ ⫺ )(K ⫺ , ␲ ⫺ )16
O beam, K ⫹ tagging
Delayed fissionp̄⫹ 238U

180⫾40⫾602700⫾500⫹80
100⫺40

关20兴
关21兴
关19兴

Delayed fission
Delayed fission
Delayed fissionp⫹ 238Up⫹ 209Bi

130⫾30⫾30240⫾60145⫾7⫾23

关20兴
关22兴
关23兴

Delayed fission
Delayed fission
Delayed fission

4
⌳H

4
⌳ He

5
⌳ He

9
⌳ Be
11
⌳B
12
⌳C
16
⌳Z
209

p̄⫹

Bi

e⫹ 209Bi

The goals of this experiment were the precise measurement of 共1兲 the lifetimes of hypernuclei, 共2兲 the branchingratios of ␲ ⫺ mesonic decay and proton-induced nonmesonicdecay, and 共3兲 to obtain the energy spectra of the protons
from nonmesonic decay. In this paper, the results for lifetimes will be presented, which may show the mass dependence of the hypernuclear lifetime. The lifetimes of the
12
28
56
C, ⌳
Si, and the bound region of ⌳Fe haveground state of ⌳been reported 关24兴.
In Sec. II, the details of the experimental apparatus will bepresented. Section III will show the analysis procedures andthe lifetime results. In Sec. III, the systematic uncertainties ofthe present results also will be considered. We will compare
the present measurements with previous results and the theoretical calculations in Sec. IV.
II. EXPERIMENT⌳ hypernuclei were produced via the ( ␲ ⫹ ,K ⫹ ) reaction.
The lifetimes of the ⌳ hypernuclear weak decay were extracted from the distribution of the time differences between
beam injection and the emission of protons from the hypernuclear weak decay.
The ( ␲ ⫹ ,K ⫹ ) reaction is suitable for measuring the weak

054004-2

LIFETIME MEASUREMENTS OF MEDIUM-HEAVY ⌳ . . .

PHYSICAL REVIEW C 61 054004

FIG. 2. Schematic view of the coincidence detector system usedin the lifetime measurement of hypernuclei. The superconducting
kaon spectrometer 共SKS兲 is not shown and is located farther to theright.

FIG. 1. Schematic view of the K6 beam line of KEK-12 GeV PS
and the superconducting kaon spectrometer 共SKS兲.

decay of medium and heavy ⌳ hypernuclei because it can
produce the deeply bound state of hypernuclei due to thehigh momentum transfer which is as much as 0.4 GeV/c,
while the (K ⫺ , ␲ ⫺ ) reaction populates preferentially the substitutional states due to the reaction selectivity. In addition,the ( ␲ ⫹ ,K ⫹ ) reaction gives a very clean inclusive spectrum
because it is free from the decay of beam particles. Thispermits us to estimate the yield of hypernuclear formation in
the inclusive spectrum rather precisely.
A. General description

The ⌳ hypernuclei were produced via the ( ␲ ⫹ ,K ⫹ ) reaction using the 1.05 GeV/c ␲ ⫹ beam at the K6 beam line 关25兴of the KEK 12-GeV PS. The scattered kaons were taggedusing a superconducting kaon spectrometer 共SKS兲 关26兴 toidentify the production of hypernuclei and reconstruct thehypernuclear mass spectrum.
Figure 1 shows a schematic view of the beam line spectrometer and the scattered particle spectrometer 共SKS兲. The
acceptance of the SKS is about 100 mSr and a good momentum resolution of 0.1% full width at half maximum 共FWHM兲
was achieved at the previous experiment 共KEK-PS E140a兲
for the hypernuclear spectroscopy 关26,27兴.
We used natural carbon, silicon, and iron as a target. We
placed targets at finite angles to the beam as shown in Fig. 2.
The reason for this asymmetric configuration is as follows:
共a兲 in order to increase the effective target thickness for the
beam to generate a larger hypernuclear production rate and共b兲 in order to decrease the path length of the decay productsin the target material. This will reduce the detection thresh-

old and the deterioration of the energy distribution. The tilt
angle for the target was about 10o, giving a target about 5
times thicker for the beam than for the decay products. In
Table II, the specifications for each target are summarized.
Each run typically lasted 2 h and the beam spill was about
1.7 s out of every 4 s. The typical beam intensity was about
3.7⫻106 pions/spill. The ( ␲ ⫹ ,K ⫹ ) event rate at the trigger
level was about 400/spill and most of these events were related to the background ( ␲ ⫹ , ␲ ⫹ ) or ( ␲ ⫹ ,p) events.
B. Coincidence part: For the detection of the weak decay

The charged decay products of the hypernucleus were detected by the coincidence detector system which consists offast timing counters, range counters, and drift chambers. The
acceptance was estimated to be about 30% by GEANT-关29兴
based Monte Carlo simulation and the detection threshold
was about 30 MeV for protons and 12 MeV for negative
pions. Figure 2 shows a schematic view of the coincidence
detector system which is located symmetrically above andbelow the target. Table III shows the specifications of eachdetector.
The first element 共T2兲 is the fast timing counter which
determines the timing of charged decay products. T2 is divided into six segments of plastic scintillator with
4关 W兴 ⫻ 18关 L兴 ⫻ 0.6关 T兴 cm3 with double-sided readout.
The intrinsic time resolution of each segment is about 37 ps
for a 100 MeV proton and about 70 ps for minimum ionizing
particles obtained from a separate test experiment 关30兴 after
time walk correction. The dE/dx information given by theT2 counter was used for particle identification.
TABLE II. The specifications of experimental targets.
Target
12

C
Si
56
Fe
28

054004-3

Thickness(g/cm2 )Abundance
共%兲

N beam(⫻109 )

6.45
10.35
7.00

98.89 共natural兲
92.23 共natural兲
91.80 共natural兲

272
1470
959

H. PARK et al.

PHYSICAL REVIEW C 61 054004

TABLE III. The specifications of the coincidence detectors.

T1
T2
RG1
RG2
RG3
RG4
VETO
PDC

Active area
(cm2 )

Thickness
共cm兲

8⫻9.6
24⫻18
34⫻34
40⫻40
46⫻46
52⫻52
54⫻54
26⫻26

1.2
0.6
2.4
2.4
2.4
2.4
0.4
4 mma

␦ t 共FWHM兲
E threshold
Acceptancea

Two layers of four segments
Six segments 共4 cm wide兲Six layers 共0.4 cm thick兲Six layers 共0.4 cm thick兲
Four layers 共0.6 cm thick兲
Four layers 共0.6 cm thick兲—Six planes (XX ⬘ Y Y ⬘ XX ⬘ )200 ps30 MeV for proton, 12 MeV for ␲ ⫺30%

FIG. 3. The scatter plot of two PID functions. The x axis is the
PID 1 using dE/dx and RANGE and the y axis the PID 2 using
dE/dx and E. The events inside the box were accepted as protons.

Drift distance.

The next element 关multiwire drift chamber 共PDC兲兴 is themultiwire drift chamber which has six planes(XX ⬘ Y Y ⬘ XX ⬘ ), where X is along the beam direction and Y isperpendicular to the beam direction. Each plane has a 4 mm
drift distance and the anode wires of the primed planes are
shifted by half a cell size 共4 mm兲 to resolve left/right ambiguity. The PDC gives the tracks of the decay particles. The
reaction point of hypernuclear production was determined by
the beam line spectrometer and SKS. Since the recoil velocity of a hypernucleus is very small, we can assume that the
decay of the produced hypernucleus occurs at its production
vertex. However, the spatial resolution of the production vertex along the beam direction was as low as a few cm and the
decay vertex was estimated by the intersection of the tracks
of a decay particle and beam.
The third element is the range counter. The range counteris divided into four sets. The first two sets (RG1,2) consistof six layers each 4 mm thick and the other two sets (RG3,4)
consist of four layers each of 6-mm-thick plastic scintillator.
The total thickness of the range counter block is 9.6 cm. The
range of the decay particle (RANGE) was determined by thepath length in the range counter after the incident angle correction. The energy of the incident particle was calculated
from the range after the particle was identified.
The last element 共VETO兲 is to veto the charged particles
which pass through the whole set of the range counter.
The particle identification has been done using
dE/dx-RANGE and dE/dx-E relations. dE/dx was definedas the energy deposit in the T2 counter in unit length.
RANGE is the path length in the coincidence detectors andE was defined as the total energy deposit digitized by analogto-digital converters 共ADC’s兲 measured in T2 and the range
counter:
PID 1 ⫽lndE
⫹ f 1 共 ln RANGE 兲 ,dx

PID 2 ⫽dE
⫹ f 2共 E 兲 ,dx

where f 1 and f 2 are the first order polynomials. Figure 3
shows the scatter plot of PID 1 and PID 2 for the hypernuclear bound state events. The events inside the box were
accepted as protons for the lifetime analysis. Figure 4 shows
the projection of PID 1 . The shaded section represents theevents remaining after applying the PID 2 cut and the events
between the two arrows were accepted as protons. From thefigure, the pion fraction in the proton gate is estimated to beless than 2%.
C. Fast timing measurement system

The hypernuclear lifetime is the mean value of the differences between the production time (t p ) and the decay time
(t d ) of the hypernuclei (⌬t⫽t d ⫺t p ). The production time
(t p ) was extracted from the beam timing (t 1 ) and the flighttime (t f b ) of the beam from the beam timing counter 共T1兲 tothe production vertex (t p ⫽t 1 ⫹t f b ). The decay time (t d ) was
determined by subtracting the timing (t 2 ) of decay products
at the T2 counter and the flight time (t f d ) from the decay
vertex to the T2 counter (t d ⫽t 2 ⫺t f d ). The time resolution is

共7兲

共8兲

FIG. 4. Particle identification plot ( PID 1 ) using dE/dx andRANGE shows proton and pion peaks. The shaded region represents the events after PID 2 (dE/dx vs E) cut. The events of theshaded region between the two arrows are accepted as protons.

054004-4

LIFETIME MEASUREMENTS OF MEDIUM-HEAVY ⌳ . . .

due to four terms: ␴ ⫽PHYSICAL REVIEW C 61 054004

冑␴ t2 ⫹ ␴ 2t ⫹ ␴ t2 ⫹ ␴ 2t . The two terms
2

fd

1

fp

related to the flight time are predominantly determined by
the geometry of the timing counter system 共flight length兲 andthe others are the intrinsic resolutions of the timing counters( ␴ t 1,2). The contribution from each term is estimated to beabout 70–94 ps in FWHM.
Since we used the leading-edge discriminator, a correction due to the time walk effect was essential to achieve ahigh time resolution. We corrected for the time walk effectusing the relation ␦ t⬃(1/冑q⫺1/冑q 0 ), where ␦ t is theamount of correction, and q and q 0 the integrated charge in
the ADC and its mean value 关31兴.
The beam timing was determined by T1 which consists of
two layers 共T11 and T12兲 of plastic scintillators 共see Fig. 2兲.
Each layer of T1 is segmented into 4 with a size of8关 L兴 ⫻ 1.3(3.5) 关 W兴 ⫻1.2关 T兴 cm3 for the center 共side兲 twosegments. The center segments have a smaller width becauseof the beam profile thus making the beam intensity about
0.4⫻106 ␲ ⫹ ’s/s for the side segments and about 0.65
⫻106 ␲ ⫹ ’s/s for the center ones. Each segment has photomultiplier tubes 共PMTs兲 on both ends and a three-stage
booster in a high voltage chain is used to avoid the nonlinearity of PMTs at high beam rates. We constructed T1 fromtwo layers of 1.2 cm thickness because the improvement in
the time resolution due to the number of scintillating photons(N p.e. ) for the minimum ionizing particle seems to saturate
like ␴ ⬀ 冑1/N p.e. at a scintillator of 1.2 cm thickness.
The intrinsic time resolution 共FWHM兲 of T1 counter isabout 108 ps for the center segments and about 127 ps forthe side segments at a beam intensity of 2 ⫻106 ␲ ⫹ ’s/s after
time walk correction and it is about 50% worse than the low
rate incidence 关30兴. For beam timing, we used the average ofthe timings of two layers and the beam time resolution ( ␴ t 1 )is about 82 ps in FWHM.
The timing of decay products was determined by T2 andwas described in Sec. II B.

FIG. 5. The time response function of the system which is obtained using the prompt ( ␲ ⫹ ,pp) events. The total resolution is 200
ps FWHM 共85 ps in ␴ ).

corrected using the prompt time spectrum of the ( ␲ ⫹ ,pp)
events. The relative timings of T1 共8 segments兲 and T2 共12segments兲 was matched mostly every 4 h. Figure 5 shows the
typical prompt time spectrum, the resolution of which is 200
ps FWHM.
III. ANALYSIS AND RESULTS
A. Singles spectra

Figure 6 shows the reconstructed hypernuclear mass spec12
C: 共a兲 inclusive, 共b兲 in coincidence with energetic
tra of ⌳protons, and 共c兲 with the pions from the hypernuclear decay.
The inclusive spectrum shows a constant background levelof about 43 events per 1 MeV but the coincidence spectra are

D. Time response function of the system

In order to obtain the time response function of the system, we used prompt events from the ( ␲ ⫹ , pp) reaction
which results from the strong interaction.
The ⌬t distribution of the ( ␲ ⫹ , pp) events — the prompt
time spectrum — was used for the time response function of
the system. Since most events identified as kaons in the trigger level were protons or pions, we could collect plenty of( ␲ ⫹ ,pp) events simultaneously with hypernuclear weak decay events. The recoiled proton was tagged by SKS and theother proton was detected and analyzed by coincidence detectors. Since the ( ␲ ⫹ , pp) reaction is a strong interaction,the ( ␲ ⫹ ,pp) events could give a prompt response.
Since the timing counters are segmented into 8 共T1兲 or 12
共T2兲, it is essential to match the relative timing of each segment. In addition, as the experiment was performed over a
period of 1 yr, the time drift due to the temperature or otherenvironmental changes during the experiment should be considered. The long-term time drift was typically a few tens ofpicoseconds and at most about 200 ps. All these effects were

12
FIG. 6. The hypernuclear mass spectra for ⌳C, 共a兲 inclusive, 共b兲
in coincidence with protons, and 共c兲 in coincidence with pions.
M HY ⫺M A means the mass difference between a ⌳ hypernucleus
and a target nucleus. B ⌳ is the binding energy of a ⌳ hyperon.

054004-5

H. PARK et al.

PHYSICAL REVIEW C 61 054004

28⌳ Si, 共a兲 inclusive,

FIG. 7. The hypernuclear mass spectra for共b兲 in coincidence with protons, and 共c兲 in coincidence with pions.
M HY ⫺M A means the mass difference between a ⌳ hypernucleus
and a target nucleus. B ⌳ is the binding energy of a ⌳ hyperon.

almost background free. The x axis indicates the mass difference between a hypernucleus (M HY ) and a target nucleus(M A ). The ⌳ binding energy (B ⌳ ) is also shown. The absolute scale of the spectra was adjusted so that the ⌳ binding
12
C was 10.8 MeV 关27兴. Theenergy of the ground state of ⌳proton coincidence spectrum shows two clear peaks at
M HY ⫺M A ⫽184 MeV (B ⌳ ⫽11 MeV) and 194 MeV 共0
MeV兲 which are well-known states with a neutron-hole⫺1⫺112s ⌳ 兴 and 关 0p 3/2
p ⌳ 兴 of ⌳C,⌳-particle configuration of 关 0p 3/2
11
respectively. The core-excited states of C with the ⌳ hyperon in the s ⌳ orbit 关28兴 could not be resolved due to thelimited resolution. The p ⌳ state was a particle unbound stateand it is known to decay through proton emission to the11
B hypernucleus 关32兴.
lighter ⌳28
Si hypernucleus, the measurements were done
For the ⌳in two separated beam cycles; one was run in February 1995
and the other in July 1995. Both cycles showed consistent
hypernuclear mass spectra. Figure 7 shows the mass spectracollected in February 1995. The inclusive spectrum 共a兲shows a constant background level of about 160 events per 1
MeV and the peak structure is not clean. The mass spectrum
in coincidence with the energetic protons 共b兲 shows three(B ⌳ ⯝17 MeV),
peaks
at
M HY ⫺M A ⯝178 MeV⯝188 MeV (⯝5 MeV), and ⯝195 MeV (⯝⫺12 MeV)
with a much smaller background level. Again, as a result of
the limited resolution of the spectrum, the whole structure of28⌳ Si could not be resolved 关27,33兴. Figure 7 in Ref. 关27兴
shows four clear peaks in the ⌳ bound region and the whole28
Si was fitted with five Gaussians. In thismass spectrum of ⌳analysis, as shown in Fig. 7共b兲, the fitting was done withthree Gaussian functions, considering the first two peaks of
Fig. 7 in Ref. 关27兴 as one peak and the next two peaks as
another peak. However, the configuration of each peak could

56
FIG. 8. The hypernuclear mass spectra for ⌳Fe, 共a兲 inclusive,
共b兲 in coincidence with protons, and 共c兲 in coincidence with pions.
M HY ⫺M A means the mass difference between a ⌳ hypernucleus
and a target nucleus. B ⌳ is the binding energy of a ⌳ hyperon.

be assigned as s ⌳ , p ⌳ , and d ⌳ states, following Ref. 关27兴.
28
Si and the p ⌳ state
The s ⌳ state is the ground state of ⌳decays through proton emission to the lighter hypernucleus
27
Al because the proton emission threshold is at B ⌳of ⌳⯝9 MeV. Neutron emission is forbidden because of thenegative reaction Q value. The d ⌳ state is above the ⌳ emission threshold (B ⌳ ⫽ 0 MeV兲.
56
Fe: 共a兲 inclusive, 共b兲
Figure 8 shows the mass spectra of ⌳in coincidence with the energetic protons, and 共c兲 with thepions from the hypernuclear decay. The inclusive spectrum
shows a constant background level of about 50 events per 1
MeV but the background level of the coincidence spectra is
negligible. The spectra in coincidence with protons and pions
show that the region of M HY ⫺M A ⬍187 MeV corresponds
to the ⌳ bound state. In this case, the hypernuclear mass
spectrum in coincidence with energetic protons corresponds
56
Fe states but the coincident protons come not only from
to ⌳
56
Fe
states but also from the lighter hypernuclei which are⌳produced through particle emission. From a simple estimate
using the reaction Q value and Coulomb barrier, the residual
55
55
hypernuclei after particle emission can be ⌳Fe or ⌳
Mn.
From now on, for convenience, ⌳ Fe will be used to represent
56
all those hypernuclei including ⌳Fe.
B. Lifetime measurements

The hypernuclear decay rate should follow the probability
distribution
1
P 共 t 兲 ⫽ e ⫺t/ ␶ ,␶

共9兲

where ␶ is the mean life of a hypernucleus and P(t) is the
decay rate at time t. Since the measurement has a finite reso-

054004-6

LIFETIME MEASUREMENTS OF MEDIUM-HEAVY ⌳ . . .

PHYSICAL REVIEW C 61 054004

lution, the actual time distribution S(t) is the convolution ofP(t) with the response function R(t) of the detector system,
S共 t 兲⫽冕⬁0

P 共 t ⬘ 兲 R 共 t⫺t ⬘ 兲 dt ⬘ .

共10兲To zeroth order, the lifetime of the hypernucleus can be
determined by the mean difference between the prompt time
spectrum and the time spectrum of hypernuclear weak decay
which would be delayed due to the hypernuclear lifetime 共thedelayed time spectrum兲. Figure 9 shows the comparison ofprompt and delayed time spectra for 共a兲 the ground state of
1228⌳ C, 共b兲 the ground state of ⌳ Si, and 共c兲 the whole ⌳ bound56
28
Si contains dataregion of ⌳ Fe, where the spectrum for ⌳taken at the February run 共only half of the whole data兲. The
differences of the mean values of prompt and delayed time
spectra are 237, 205, and 219 ps, respectively. The time
spectrum for each target contains 324, 281, and 428 events
for the delayed spectrum and 1.7⫻104 , 7.8⫻104 , and 1.9⫻104 events for the prompt spectrum.
The lifetime was extracted by fitting the delayed time
spectra of hypernuclear decay events with the function S(t),considering the lifetime as a free fitting parameter. The fitting was performed with the maximum likelihood fitting
method adopting Poisson statistics for each measurement
which has been shown to produce fewer fitting errors than
the normal least squares method 关34兴. As a response function, we used the prompt time spectra themselves from

FIG. 9. The prompt and the delayed time spectra are compared12
for 共a兲 the ground-state region of ⌳C, 共b兲 the ground-state region of
28
56
Si,
and
共c兲
the
whole
bound
regionof ⌳Fe.
⌳( ␲ ⫹ ,p p) events which were obtained simultaneously withthose of the hypernuclear production 共see Sec. II C兲. The
solid curves in Fig. 9 show the fitting results and the reduced
1228
C, 0.90 for ⌳Si,chi squares ( ␹ 2 / ␯ ) of the fits are 0.67 for ⌳and 0.79 for ⌳ Fe.
The results of maximum likelihood fitting ( ␶ FIT ) are⫹15
12
␶ FIT ⫽230⫺14
ps 共 with 324 events for the ground state region of ⌳
C兲 ,
⫹12
12
ps 共 with 263 events for the p ⌳ state region of ⌳C兲 ,⫽211⫺12⫹1128
ps 共 with 528 events for the ground state region of ⌳Si兲,⫽206⫺10⫹1028
ps 共 with 837 events for the p ⌳ state region of ⌳Si兲,⫽204⫺10⫹1456⫽215⫺13
ps 共 with 428 events for the ⌳ bound region of ⌳Fe兲,

where the quoted errors correspond to one standard deviation
共68% confidence level兲 and are only statistical. For the life28
Si, the lifetime was obtained indepentime analysis of ⌳28
dently for two separate data sets and the final lifetime of ⌳Si
was obtained by averaging over the two fitting values. The
28
values for ⌳Si were the averaged 共or summed兲 values over56
Fe, the value is the
all the silicon data. In the case of ⌳55
55
56
average of those of ⌳ Mn, ⌳ Fe, and ⌳ Fe. The event sample
for each hypernuclear state was selected with the gate condition shown in Table IV. The asymmetric gate condition isapplied in order to reduce the fraction of other states of ⌳ asmuch as possible. The simplified fitting mentioned in Sec. III
tells us that the fraction of other states is at most 13%. However, the estimation of the fraction depends on the fitting

method for the hypernuclear mass spectra. When the fitting
was done using all the peaks known from the other experiment 关27兴, the fraction of other states reaches as high as
20%.
TABLE IV. The gate condition for each hypernuclear singleparticle state and the fraction of the various states.

12
⌳C
28
⌳ Si
⌳ Fe

054004-7

Gate window
M HY ⫺M A (MeV)

Numberof events

1s
state

1p
state

Higherstates175–186 (1s gate兲
188–197 (1p gate兲
171–181 (1s gate兲
183–188 (1p gate兲
160–185 共bound states兲

323
338
527
661
309

97%
1%
87%
1%
—

3%
93%
13%
94%
—

0%
6%
0%
5%
—

H. PARK et al.

PHYSICAL REVIEW C 61 054004

C. Systematic uncertainties of lifetime measurements

TABLE V. The summary of lifetime measurements. The value
55
55
56
for ⌳ Fe represents the average lifetime of ⌳Mn, ⌳Fe, and ⌳Fe.

1. Contribution from the response function

Since the prompt time spectrum of the ( ␲ ⫹ , p p) eventswas used as the time response function of the system, thelifetime analysis of hypernuclear weak decay mainly depends on the analysis of ( ␲ ⫹ , pp) events.
The prompt time spectrum is expected to follow a Gaussian distribution, whose mean is at t⫽0 and whose standard
deviation corresponds to the time resolution of the detector
system. However, as shown in Fig. 5, the prompt time spectrum has a non-Gaussian component, predominantly in the
delayed time region.
In order to investigate the source of the non-Gaussiancomponent, we checked the time of flight of the beam. The
time of flight of the beam particles (TOF beam ) is determinedby TOF beam ⫽t 1 ⫺t 0 , where t 1 is the timing at the T1
counter located at the target area and t 0 is the timing at theBH1 counter in the most upstream section of the beam line
共see Fig. 1兲. The TOF beam distribution also shows a nonGaussian shape and the non-Gaussian component of theprompt time spectrum was strongly correlated with that of
TOF beam . Most of the non-Gaussian component in theprompt spectrum came from the events of TOF beam ⬍0 andthe prompt time spectrum becomes a Gaussian shape for the
events of TOF beam ⬎0. The TOF beam spectrum of ( ␲ ⫹ ,p p)
events and that of the hypernuclear decay events showedalmost the same distribution. We compared the fitting results
for the events of TOF beam ⬎0 and those of TOF beam ⬍0.
The results of the fitting to the two separate data sets agree
well with each other to within statistical fluctuations.
The timing offset should also be the same for both events
because the difference in timing offsets directly affects the
final lifetimes. In order to confirm this point, we set thedifference in timing offsets as a fitting parameter and performed the maximum likelihood fitting. The fitting functionwasS ⬘共 t 兲 ⫽

冕␶

⬁1

0

e ⫺t ⬘ / ␶ R 共 t⫺t ⬘ ⫹t 0 兲 dt ⬘ ,

12
⌳C
11
⌳B
28
⌳ Si
27
⌳ Al
⌳ Fe

Numberof events

Lifetime
共ps兲⌫(⌫ ⌳ )

323
338
527
661
309

231⫾15
211⫾13
206⫾12
203⫾10215⫾14

1.14⫾0.08
1.25⫾0.08
1.28⫾0.08
1.30⫾0.07
1.22⫾0.08

shown in these analyses are within statistical ranges. In addition, since we used the time difference of two distributions
共the prompt and the delayed time spectra兲 which were measured at the same time, most of the instrumental errors suchas errors in the relative positions of timing counters would becanceled out.
Since the resolution of the hypernuclear mass spectra islimited due to the use of a thick target, each state of ⌳ could
not be well separated and the fraction of other states in thegate for the various ⌳ single-particle states should be considered. From the chi-square fitting of hypernuclear mass
1228
spectra for ⌳C and ⌳
Si, the fraction of the various ⌳ states
was estimated as shown in Table IV. The effect of otherstates should be small, not only because the fraction of otherstates was small, being at most ⬃ 10%, but also because thefitting results of each gate are not so different from eachother 共less than 10%兲.
The correction was made by solving following simultaneous equations, with the assumption that the fitting value ofhigher states 共h.s.兲 共see Table IV兲 is affected only by thelower states:␶ 1 ⫽˜␶ 1s w 11⫹˜␶ 1p w 12⫹˜␶ h.s. w 13 ,
␶ 2 ⫽˜␶ 1s w 21⫹˜␶ 1p w 22⫹˜␶ h.s. w 23 ,共11兲

␶ 3 ⫽˜␶ 1s w 31⫹˜␶ 1p w 32⫹˜␶ h.s. w 33 ,where t 0 as well as ␶ was a fitting parameter. In order toavoid the histogram binning effect, the step size in the variation of t 0 was fixed to 25 ps. The fitting shows that ␹ 2 wasminimized for all sets of data at t 0 ⫽0. We conclude that thetiming offset of the prompt spectrum and the delayed spectrum should be same.
From the above investigations, we conclude that no systematic differences in the time response of the detector system for ( ␲ ⫹ ,pp) and hypernuclear decay events were observable and that the systematic uncertainties of the lifetime
due to the ( ␲ ⫹ ,pp) events are negligible compared to thestatistical uncertainties.

where ␶ i is the fitting result for each ith gate, ˜␶ j , where j is
1s, 1 p, or h.s., is the real lifetime of each state, and w i j is
the weighting of the jth state (1s, 1 p, h.s.兲 to the ith gatedregion listed in Table IV. Table V shows the results of thecorrections. As expected, the correction is much smaller than
the statistical uncertainty.
In conclusion, we cannot find any systematics comparable
to the statistical uncertainties. The experimental error level
was predominantly determined by the statistical errors.

2. Other considerations

In Table VI, the present lifetime results are compared12
11
C and ⌳
B
with the earlier measurements. The lifetimes of ⌳are consistent with the previous measurements of Grace
12C
et al. 关17兴 with much lower uncertainties. The previous ⌳⫺lifetime measurement was done with the K beam at BNL-

We investigated the other systematics, e.g., the independent lifetime analysis for up- and down-coincidence detector
systems, the dependence of decay proton energy, and thebeam momentum dependence of lifetimes. The fluctuations

IV. DISCUSSION
A. Comparison with the previous experimental results

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PHYSICAL REVIEW C 61 054004

TABLE VI. The comparison with the previous measurements.
55
The value for ⌳ Fe represents for the average lifetime of ⌳
Mn,
55
56
Fe,
and
Fe.
⌳
⌳

11
⌳B
12
⌳C
⬃16
⌳ Z
27
⌳ Al
28
⌳ Si
⌳ Fe

Present
results 共ps兲Previousmeasurement 共ps兲211⫾13231⫾15—203⫾10206⫾12
215⫾14

192⫾22 关17,16兴
211⫾31 关17,16兴
⫹33
86⫺26
关18兴
—
—
—

AGS. It produced a hypernucleus using the (K ⫺ , ␲ ⫺ ) reaction and tagged pions to identify the hypernuclear production. The hypernuclear mass spectrum was reconstructed.
The only earlier measurement in the medium mass region
12
C is the lifetime of A⬃16 hyperfor nuclei heavier than ⌳⬃16nuclei ( ⌳ Z) measured by Nield et al. 关18兴. They used a 2.1
GeV/nucleon 16O beam at the Lowrence Berkeley Laboratory 共LBL兲 Bevalac and a polyethylene target. The production of low momentum K ⫹ mesons and their delayed decay
after stopping were used as the trigger for the hypernuclear
production. They could not reconstruct the hypernuclear
mass spectrum and identify the hypernuclear species. They
extracted the lifetime from the recoil distance distribution
which was measured by spark chambers. They had a lot ofbackground events which came from the interaction with theAl window of the spark chamber and also the trigger counterat downstream of the target. After the subtraction of the large
background, the remaining events constituted only 22 eventsand the lifetime from the maximum likelihood fitting of the⫹33
ps, which is muchrecoil distance distribution was ␶ ⫽86⫺26
12
28
smaller than the lifetime of ⌳ C and ⌳ Si.
B. Comparison with the calculations

Up to now, three groups have presented the calculations
for lifetimes of various ⌳ hypernuclei. First, in 1985, Osetand Salcedo 关35兴 made lifetime calculations using ⌳ selfenergies for nuclear matter. They included only one-pion exchange for the nonmesonic decay which governs the hypernuclear weak decay for the medium and heavy mass regions.
In order to apply their calculation to the finite hypernuclei,
they used the local-density approximation and obtained the5
208
He to ⌳
Pb. Their calculations give much
lifetimes of ⌳shorter lifetimes than the measurements with values about
60–70 % of the measurements.
Ramos et al. 关36兴 calculated the lifetimes in 1994 basedon Oset and Salcedo’s calculation. They used a one-pionexchange model including the two-nucleon-induced decay
mode (⌳NN→NNN). In ⌳NN→NNN, the strongly correlated two nucleons 共mainly proton-neutron pair兲 and ⌳ exchange a virtual pion. They gave the lifetimes for medium
12208
C to ⌳
Pb). Their calculationand heavy hypernuclei 共from ⌳also gave similar values to Oset and Salcedo’s, which wereabout 60–70% of the measurement values. However, the cal-FIG. 10. The mass dependence of hypernuclear lifetimes. The
open circles represent the data from the previous counter experiments and the solid circles the present results. The dot-dashed line
shows the calculations by Itonaga et al. and the dashed line shows
the calculations by Ramos et al.

culations of Ramos et al. yield slightly shorter values thanOset and Salcedo’s.
Recently, Itonaga et al. calculated the hypernuclear lifetime by adopting a one-pion and 2 ␲ / ␳ , 2 ␲ / ␴ exchange potential for nonmesonic decay, including p-shell nucleons aswell as s-shell nucleons 关37兴. They gave the lifetimes of
5
12
28
56
⌳ He, ⌳ C, ⌳ Si, and ⌳ Fe. Their calculation reproduced the⌳ hypernuclear lifetimes quite well even though they gave
28
56
Si and ⌳Fe.
slightly shorter values for ⌳C. Lifetimes for the overall hypernuclear mass region

Figure 10 shows the hypernuclear lifetimes for the overall
hypernuclear mass region. The solid circles are the present
results and the open circles are the results from previous
counter experiments with explicit identifications of the generated ⌳ hypernuclei. The horizontal error bars for the ⌳ Fe
data represents the mass range of the hypernuclei. The hypernuclear lifetimes are shorter than the free ⌳ lifetime except for the very light hypernuclei and are almost constant at
about 80% of the free ⌳ value in the hypernuclear mass⬎28
12. The lifetimes of ⌳Si and ⌳ Fe are slightly
region of A ⬃12shorter than that of ⌳ C. However, the differences are within
experimental uncertainties.
The theoretical calculations are overlaid indicated by thedash-dotted line 共the calculation of Itonaga et al.兲 关37兴 andthe dashed line 共Ramos et al.兲 关36兴 in Fig. 10. The calculations of Ramos et al. show that the lifetime does not changevery much according to the hypernuclear mass number.
However, their mass number dependence reflects only theoverlap area of the wave functions of the ⌳ and nucleon
because they used the local-density approximation. The calculations of Itonaga et al. also show a saturation of lifetimes
but the effect seems to be a little weaker than for the measurements. This may indicate that their 2 ␲ exchange potential has a longer range than the actual one.
V. CONCLUSION
11
12
27
28
We have measured the lifetimes of ⌳
B, ⌳
C, ⌳
Al, ⌳
Si,
and ⌳ Fe hypernuclei. For the hypernuclei with A⬎12 which

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H. PARK et al.

PHYSICAL REVIEW C 61 054004

by counting the number of ⌳N bonds responsible for thenonmesonic weak decay 关38兴.
The present lifetime data apparently require us to take
into account the short range nature of the nonmesonic weak
decay, which does not seem to be properly incorporated in
the current meson exchange model.

are in the mass range beyond the p-shell hypernuclei this is
the first measurement which can explicitly identify the ⌳hypernuclear bound states.
The present results together with the previous measurements show that the lifetimes are almost constant at about
80% of the free ⌳ value in the hypernuclear mass region
above A⫽12. This weak mass dependence should be related
to the mechanism of nonmesonic decay, which is dominant
in the present mass region, and to the baryon-baryon shortrange correlation inside the hypernucleus.
Assuming the short range nature of the nonmesonic weak
decay process, one can employ a zero range approximation
for a qualitative understanding. The nonmesonic weak decay
widths will then be proportional to the overlap integral of the
wave functions of a nucleon and a ⌳, and the hypernuclear
lifetimes will be almost constant in the mass region beyond
carbon where the overlap integrals almost saturate. A simple
discussion of the weak mass dependence was also made recently by assuming a sufficiently short range interaction and

We are grateful for the generous support from the staff
members of KEK and the old INS. We also deeply appreciate the continuous encouragement by Professor K. Nakai andProfessor T. Yamazaki throughout this experiment from the
beginning. We very much benefited from discussions withProfessor Itonaga, Professor T. Motoba, Professor T.
Fukuda, and Professor T. Nagae. H.B. acknowledges partial
support from the Korea-Japan collaborative research program of KOSEF and from the Ministry of Education throughthe Research Institute of Basic Science of Seoul National
University.

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ACKNOWLEDGMENTS

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PHYSICAL REVIEW C 61 054004

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VOLUME 88, NUMBER 4

PHYSICAL REVIEW LETTERS

28 JANUARY 2002

28
Proton Energy Spectra in the Nonmesonic Weak Decay of 12
L C and L Si Hypernuclei

O. Hashimoto,1 S. Ajimura,5 K. Aoki,3 H. Bhang,2 T. Hasegawa,4, * H. Hotchi,4, † Y. D. Kim,2,3, ‡ T. Kishimoto,5K. Maeda,1 H. Noumi,3 Y. Ohta,4 K. Omata,3 H. Outa,3 H. Park,2 Y. Sato,1,3 M. Sekimoto,3 T. Shibata,3T. Takahashi,1 and M. Youn2,3
1
Department of Physics, Tohoku University, Sendai 980-8578, Japan
Department of Physics, Seoul National University, Seoul 151-742, Korea
3
High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan
4
Graduate School of Science, University of Tokyo, Tokyo 113-0033, Japan
5
Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan(Received 28 September 2001; published 14 January 2002)
2

Numbers of protons per G hypernuclear weak decay were measured as a function of proton energy
above 40 MeV, explicitly identifying production of G hypernuclei by the 共p 1 , K 1 兲 reaction. The ratios
between the neutron-stimulated to proton-stimulated nonmesonic decay widths, G共Ln ! nn兲兾G共Lp !
np兲 共苷 Gn 兾Gp 兲 were extracted by fitting the proton energy spectra. The present result claims that the
28
proton yields are suppressed and the Gn 兾Gp ratios are close to 1 both for 12
L C and L Si in contradiction
to theoretical expectations based on meson exchange models.
DOI: 10.1103/PhysRevLett.88.042503

PACS numbers: 21.80. +a, 13.30.Eg, 13.75.Ev, 23.40. –sNonmesonic weak decay of L hypernuclei plays aunique role for the investigation of the baryon-baryon
weak interaction, which is not easily studied experimentally. Although a L hyperon decays by the mesonic weak
decay process, L ! pN, in free space, the nonmesonicweak decay becomes dominant over the mesonic weak
decay for L hypernuclei heavier than A 艐 10, in which
the nucleon emission is blocked by the Pauli exclusion
principle. The total decay width of a L hypernucleus isexpressed as a sum of the mesonic 共Gp 兲 and nonmesonic
共Gnm 兲 decay widths as Gtot 共苷 1兾t兲 苷 Gp 1 Gnm . The
nonmesonic decay process is mostly the one-nucleon
共1N兲 process, LN ! NN , which is further divided into a
proton-stimulated 共Gp :Lp ! pn兲 and neutron-stimulated
共Gn :Ln ! nn兲 processes [1]. Recently, the two-nucleon
共2N 兲 process, LNN ! NNN has been also discussed[2,3], although its experimental evidence has yet to beshown. Thus, the nonmesonic weak decay width canbe decomposed as Gnm 苷 Gn 1 Gp 共1 G2N 兲. The ratio,
Gn 兾Gp , between the neutron- and proton-stimulated 1N
decay widths is known to be sensitive to the structure of
the LN weak interaction and has been intensively investigated both experimentally and theoretically. However,it has been a puzzle that experimental Gn 兾Gp ratios areclose to 1 or greater than 1 for 5L He and 12
L C [4,5], while
theoretical calculations based on meson exchange models
imposing the DI 苷 1兾2 rule predict small Gn 兾Gp 共ø 1兲
and cannot account for the experimental ratios. There
have been efforts to solve the puzzle, such as employing
exchange processes of mesons heavier than pions (r’s,kaons, etc.) and/or 2p exchange process [6–9] and direct
quark exchange process [10,11] in order to take into account of the short range nature of the LN weak interaction
and/or examine the role of the DI 苷 1兾2 rule. Since thepion exchange process which has a large tensor component

plays an important role, isospin of final two nucleons ispreferred to be 0, resulting in small Gn 兾Gp ratios in themeson exchange models in most of L hypernuclei. Thereis also an attempt to construct a hybrid model that takes
into account a direct quark exchange process for the shortrange part and a meson exchange process for the longer
range [12]. Although the direct quark exchange processoffers larger Gn 兾Gp , its contribution to the total decay
width seems to be much smaller than that of the mesonexchange process. It was also pointed out that the protonenergy spectra would be shifted toward lower energy
due to the 2N process, suggesting that apparently greater
Gn 兾Gp would be obtained from the experimental spectra
if the 2N process were not considered [3]. However, noneof the theoretical models can consistently describe thetotal decay width and Gn 兾Gp so far.
At the same time, the reliability of the experimental data
has been often questioned since large errors are quoted
for Gn 兾Gp , partly due to poor statistics of the coincidence
spectra and also experimental difficulty of detecting neutron spectra. In addition, for light L hypernuclei, it isnecessary to determine pionic decay widths since theircontribution is large. In medium-heavy L hypernuclei,
however, the nonmesonic decay width becomes dominantand contribution of the mesonic decay width is less thana few percent of the total decay width. Therefore, measurement of proton energy spectra readily provides a great
deal of information on the nonmesonic weak decay process even without measuring that of neutrons, once thetotal decay widths are known. In the present paper, we report on high-quality energy spectra of protons emitted in28
the nonmesonic weak decay of 12
L C and L Si, particularly
paying attention to the derivation of absolute number ofprotons per weak decay, and discuss their nonmesonic decay widths.

042503-1© 2002 The American Physical Society

0031-9007兾02兾 88(4)兾042503(4)$20.00

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VOLUME 88, NUMBER 4

PHYSICAL REVIEW LETTERS

The 共p 1 , K 1 兲 reaction was used to populate L hypernuclei since it gives us considerably cleaner excitation
spectra in the hypernuclear bound region compared withthose by the 共K 2 , p 2 兲 reaction. A positive pion beam of
1.06 GeV兾c was delivered at the K6 beam line of KEK
12 GeV PS, irradiating natural carbon 共12 C:99%兲 and silicon 共28 Si:92%兲. The targets were tilted so as to maximize
the thicknesses in the beam direction to be 6.4 (C) and10.3 共Si兲 g兾cm2 , while keeping the thicknesses seen by thecoincidence detector thin enough to minimize the energy
threshold. Kaons emitted from the target were detected
and momentum-analyzed by the SKS spectrometer, fully
utilizing the advantage of its large solid angle of 100 msr[13]. The low energy charged particles emitted in the weak
decay of L hypernuclei were detected by two sets of coincidence counter systems installed above and below thetargets. Each system consists of timing counters, tracking chambers, and 11 layers of range counters. Energiesof the charged particles were derived from the range information. The measured energy region was 30 –140 MeV
for protons and 12–70 MeV for pions, respectively. Theenergy thresholds were governed by the thicknesses of thetargets and the timing scintillators. Protons and pions wereclearly identified by the DE 2 E information, and the pion
contamination in the proton spectra was kept less than 1%.
The experimental setup is the same as that for the precision
measurement of hypernuclear lifetimes [14]. Details of the
SKS spectrometer and the coincidence counter system aredescribed in Ref. [13,15].
An inclusive hypernuclear mass spectrum and a proton
coincidence spectrum for Si are shown as an example inFig. 1(a), where bumps corresponding to major-shell orbitals are indicated by sL , pL , and dL .
A proton energy spectrum per weak decay for 28
L Si wasobtained as follows. First, the inclusive energy spectrum[Fig. 1(a)] was fitted by five Gaussians and a polynomial
function representing a quasifree component as shown by
the solid lines. Overall normalization and spectral energy
resolution were allowed to vary, while peak positions andrelative intensities among the peaks were fixed taking thevalues from the previous high-resolution spectrum [16].
The proton-coincidence hypernuclear-mass spectrum[Fig. 1(b)] was fitted by the same function, with theintensities of the major peaks allowed to vary but theirpeak positions and resolution fixed as predetermined in
the fitting of the inclusive spectrum (6.3 MeV FWHM).
The first two peaks of the spectrum represented nucleon
bound states which eventually weak decayed from theground state [16]. A similar procedure was taken for
12
L C, in which case the energy resolution was 4.8 MeV(FWHM). Numbers of measured protons in the boundregion “per weak decay” are plotted as shown in Fig. 2 asa function of “experimentally measured” proton energies,
exp
exp exp
Ep . The proton spectrum is obtained as Rp 共Ep 兲 苷
expYcoin 共Ep 兲兾Yinclusive, in which Yinclusive is the numberof counts for the nucleon-bound peaks in the inclusive
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28 JANUARY 2002

FIG. 1. Hypernuclear mass spectrum of the natural Si共p 1 , K 1 兲reaction. (a) inclusive, (b) proton coincidence spectra.
exp

hypernuclear-mass spectrum and Ycoin 共Ep 兲 that of thecorresponding proton-coincidence spectrum. Only proton
spectra above 40 MeV are discussed to avoid experimental
ambiguity near the detector threshold of 30 MeV.
exp
exp
The measured Rp 共Ep 兲 is related to the number ofprotons “per nonmesonic weak decay” coming out of the
weak decay nucleus, Np 共Ep 兲, as
Rpexp共Epexp 兲 苷 具´coin VcoinNp 共Ep 兲典 共1 2 bm 兲 .
The Np 共Ep 兲 spectrum can be calculated theoretically. The´coin and Vcoin are efficiency and solid angle of the coincidence counters, and bm the sum of branching ratios of thecharged and neutral mesonic weak decay. The solid angle,
Vcoin兾4p, was evaluated to be 27 6 1% by a Monte Carlosimulation and the average detection efficiency, ´coin, was28
found to be 共84 6 2兲% for 12
L C and 共86 6 2兲% for L Si.
Thus, about 23% of the weak decay protons were detectedby the coincidence counters.
The branching ratios for negative pions were measured in the present experiment to be 9.8 6 1.0共stat兲 6
0.4共sys兲% for 12
L C and 3.6 6 0.8共stat兲 6 0.2共sys兲% for
28
Si,
while
that
of the neutral pionic decay for 12
LLC
was taken from the value reported in Ref. [17] 关17.4 6
5.7共stat兲 6 0.8共sys兲%兴. However, since the branchingratio of the neutral pionic decay is not known for 28
L Si,
it was calculated from the theoretical ratio, 2.3, of
neutral to charged mesonic weak decay widths [18].
Thus, the branching ratios of mesonic weak decay were
given to be 27.2 6 1.0共stat兲 6 5.8共sys兲% for 12
L C and28
11.9 6 0.8共stat兲 6 8.3共sys兲% for L Si.
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VOLUME 88, NUMBER 4

FIG. 2. Proton energy spectra measured by the coincidence
counter system identifying hypernuclear bound states producedby the 共p 1 , K 1 兲 reaction. The vertical axes are normalized
to the number of protons per hypernuclear weak decay. Calculated spectra that took into account final state interaction, energy
losses in the target and detectors, and the acceptance of the proton detectors are shown for comparison. The solid histogram
represents the best fit ones.

The spectra shown in Fig. 2 fall slowly above the threshold and do not show a bump structure centered around
80 MeV, which is expected for the two-body 1N process.
Energy loss and multiple scattering of protons in the target
and the coincidence detectors are partly responsible, butthey can be interpreted as needing consideration of otherprocesses such as LNN ! NNN and/or the stronger finalstate interaction.
The calculated proton energy spectra were comparedwith the measured ones in the following way and theGn 兾Gp ratios were derived. Protons and neutrons, which
are originated from the weak interaction between a L
hyperon and a nucleon with Fermi motion, subsequently
undergo final state interactions, where initial numbers of
weak decay protons and neutrons at the weak vertex are
dependent on Gn 兾Gp . Some of the protons can be also
converted to neutrons (and vice versa) during the cascade.
28
Thus, proton energy spectra emerging from 12
L C and L Si
were modified and they were calculated with an intranuclear cascade code [19,20]. The calculations based on
the essentially same code well accounted for proton spectra after pion absorption, in which protons are emitted in
the similar energy region as that in the nonmesonic decay [21]. The protons to be observed by the coincidence
counters are then followed by a GEANT Monte Carlo codetaking into account of energy loss and multiple scattering both in the target and detectors. Those proton spectra
042503-3

28 JANUARY 2002

PHYSICAL REVIEW LETTERS

both from the proton- and neutron-stimulated weak decay
were summed incoherently according to a given Gn 兾Gp ratio. The calculated proton spectra “per nonmesonic weak
decay” were also renormalized to “per weak decay” taking into account the branching ratios of mesonic weak decay and are shown overlaid in Fig. 2 for Gn 兾Gp 苷 0.1,
1.0, and 2.0. It indicates that the proton yields in theenergy region above 40 MeV are considerably less thanthose calculated for small Gn 兾Gp 苷 0.1. By fitting theenergy spectra with Gn 兾Gp as a free parameter assumingthat the nonmesonic weak decay occurs through the 1N
28
process only, Gn 兾Gp was obtained for 12
L C and L Si to be
10.09
10.20
10.13
10.27
1.1720.08 共stat兲20.18 共sys兲 and 1.3820.11 共stat兲20.21 共sys兲 withstatistical and systematic errors, respectively, as shown bya solid histogram in Fig. 2. The statistical errors camefrom the errors of spectral fitting while systematic errors
were due to the errors of the mesonic decay widths and detection efficiencies 共´V兲 of the coincidence detectors. In
addition, the spectra were also fitted considering contributions from the 2N process, which was calculated following
the prescription of Ref. [19]. In Table I, the Gn 兾Gp ratios
derived assuming “1N only” and “1N and 2N” processes
are listed. Even when the contribution of the 2N process
is taken to be 32% and 29% of the nonmesonic weak de28
cay width for 12
L C and L Si, respectively, the lower boundof Gn 兾Gp still stays as large as 0.66 and 0.77 for 12
L C and28
L Si. It is because the number of protons per weak decay
is more strongly dependent on Gn 兾Gp in the region 0 ,Gn 兾Gp , 1 and the lower bound of the Gn 兾Gp is not very
sensitive to the contribution of the 2N process in the region around Gn 兾Gp 苷 1 [19]. It is noted that stronger
final-state interaction possibly lowers the Gn 兾Gp ratios
28
TABLE I. Gn 兾Gp ratios measured for 12
L C and L Si are compared with previous data and recent calculations. The extracted
Gn 兾Gp ratios in the column “1N only” assumed no 2N process in fitting the proton energy spectra. Those in the column“1N and 2N” were obtained taking the ratio between the decay
width of the 2N process 共G2 兲 and that of nonmesonic weak decay 共Gnm 兲 as given in parentheses. See text for details.

Gn 兾Gp“1N and 2N”“1N only”

Refs.

Experiment12LC

10.0910.20
1.1720.0820.18

28
L Si

10.1310.27
1.3820.1120.25

12
LC

12
LC

28
L Si

10.1010.22
0.9620.0920.21
共G2 兾Gnm 苷 0.32兲
10.1410.30
1.1820.1320.28
共G2 兾Gnm 苷 0.29兲

Present
Present

11.12
1.3320.81
10.32
1.87 6 0.5921.00

[4]
[5]

Theory0.31
0.53
0.288 – 0.341
0.34

[23]
[9]
[22]
[23]

042503-3

VOLUME 88, NUMBER 4

PHYSICAL REVIEW LETTERS

since an energy spectrum can be shifted toward the lower
energy side and beyond the proton energy threshold of thepresent measurement. The energy spectra in the lower energy region are subject to further theoretical and experimental investigations.
The obtained Gn 兾Gp ratio for 12
L C is consistent withthose reported previously [4,5], but the quality of the data
has been greatly improved. For 28
L Si, the Gn 兾Gp ratio has
been for the first time measured and was found to be near
unity as in the case of 12
L C. In Table I, the Gn 兾Gp ratios
predicted by various models are also given. Although thetotal decay widths are reasonably well reproduced by thosemodels, the experimental Gn 兾Gp ratios are not accounted
for by any theoretical calculations presently available.
In summary, energy spectra of protons emitted in the28
nonmesonic weak decay of 12
L C and L Si were measured
1
1by the 共p , K 兲 reaction with special attention to the normalization per weak decay. The measured proton spectra
above 40 MeV per weak decay were compared with those
calculated based on a meson-exchange model, applying
an intranuclear cascade code and a GEANT simulation forthe coincidence detector response. The Gn 兾Gp ratios were
derived assuming the 1N process alone, and also including the 2N process in addition. The present high-quality
proton-energy spectra show Gn 艐 Gp not only for 12
LC28
but also L Si even when a sizable contribution of the 2N
process is considered. The puzzle of large Gn 兾Gp ratios
remains, being contradictory to most of the available theoretical models which predict small ratios. Spectral distortion due to the final-state interaction and contribution of
the 2N process will be further issues to be studied experimentally and theoretically. In this regard, measurements ofneutron energy spectra as well as correlation of pair nucleons in the nonmesonic weak decay have been conducted
recently and the analyses are underway [24,25]. Those
data together with the present proton energy spectra are
expected to further reveal the nonmesonic weak decay process, providing clearer information on the contribution of
the 2N process and the degree of the final state interaction.
The authors would like to express their deep gratitude
to Professor A. Ramos and Professor E. Oset for providing
us with the calculated proton energy spectra and for intensive discussion on the proton spectra. Discussion withProfessor T. Motoba and Professor K. Itonaga was also indispensable to complete the present paper. We thank Professor K. Nakai, Professor T. Yamazaki, and Professor S.
Homma for their continuous encouragement to the present
work, Dr. R. Chrien for critical reading of the manuscript,
and Dr. G. Garbarino for theoretical comments. Excellent

042503-4

28 JANUARY 2002

operation and support of the KEK 12 GeV PS staffs arevery much appreciated.
The work was carried out partly under the JapanKorea collaborative research program by JSPS of Japanand KOSEF of Korea.

*Present address: School of Allied Health Sciences, Kitazato University, Sagamihara 228-8555, Japan.
†
Present address: Physics Department, Brookhaven National
Laboratory, Upton, NY 11973.
‡
Present address: Department of Physics, Sejong University,
Seoul 143-747, Korea.
[1] E. Oset and A. Ramos, Prog. Part. Nucl. 41, 191 (1998).
[2] W. M. Alberico, A. De Pace, M. Ericson, and A. Molinari,Phys. Lett. B 256, 134 (1991).
[3] A. Ramos, E. Oset, and L. L. Salcedo, Phys. Rev. C 50,
2314 (1994).
[4] R. Grace et al., Phys. Rev. Lett. 55, 1055 (1985); J. J.
Szymanski et al., Phys. Rev. C 43, 849 (1991).
[5] H. Noumi et al., Phys. Rev. C 52, 2936 (1995).
[6] J. F. Dubach, G. B. Feldman, B. R. Holstein, and L. de la
Torre, Ann. Phys. (N.Y.) 249, 146 (1996).
[7] A. Parreno, A. Ramos, and C. Bennhold, Phys. Rev. C 56,
339 (1997).
[8] K. Itonaga et al., Nucl. Phys. A639, 329c (1998).
[9] D. Jido, E. Oset, and J. E. Palomar, Nucl. Phys. 694, 525(2001).
[10] T. Inoue, S. Takeuchi, and M. Oka, Nucl. Phys. A597, 563(1996).
[11] K. Sasaki, T. Inoue, and M. Oka, Nucl. Phys. A669, 331(2000); A678, 455(E) (2000).
[12] T. Inoue, M. Oka, T. Motoba, and K. Itonaga, Nucl. Phys.
A633, 312 (1998).
[13] T. Fukuda et al., Nucl. Instrum. Methods Phys. Res.,
Sect. A 361, 485 (1995).
[14] H. Bhang et al., Phys. Rev. Lett. 81, 4321 (1998).
[15] H. Park et al., Phys. Rev. C 61, 054004 (2000).
[16] T. Hasegawa et al., Phys. Rev. C 53, 1210 (1996).
[17] A. Sakaguchi et al., Phys. Rev. C 43, 73 (1991).
[18] T. Motoba, K. Itonaga, and H. Bando, Nucl. Phys. A489,
683 (1988).
[19] A. Ramos, M. J. Vicente-Vacas, and E. Oset, Phys. Rev. C
55, 735 (1997).
[20] A. Ramos and E. Oset (private communication).
[21] L. L. Salcedo et al., Nucl. Phys. A484, 557 (1988).
[22] A. Parreno and A. Ramos, Phys. Rev. C (to be published).
[23] K. Itonaga, T. Ueda, and T. Motoba, Nucl. Phys. A691,
197 (2001); (private communication).
[24] H. Bhang et al., Nucl. Phys. A691, 156c (2001).
[25] KEK PS experiment E462, spokesperson H. Outa, 2000.

042503-4

E325 H. Enyo
Study of Chial Property of Dense Nuclear Matter through Measurement of Meson
Mass Modification in Medium
K. Ozawa et al.
[attached]
Observation of ρ/ω Meson Modification in Nuclear Matter
Phys. Rev. Lett. 86 (2001) 019-5022.
○ M. Ishino et al.
[attached]
Mass production of hydrophobic silica aerogel and readout optics of cerenkov light
Nucl.Instrum.Meth. A457 (2001) 581-587.
○ M. Sekimoto et al[attached]
Spectrometer for measurements of φ mesons in nuclear matter produced through 12-GeV p+Areactions
Nucl. Instr. Meth. A516 (2004) 390-405.
★ S. Yokkaichi
Study of Nuclear Dependence of Production and Decay of φ meson in 12-GeV p + A interaction.
Kyoto University, 2000.
★ M. Ishino
Study of nuclear–matter effect on K+K– invariant mass measured in 12-GeV p + A interaction.
Kyoto University, 2000.
★ K. Ozawa
Observation of ρ/ω Meson Modification in Nuclear Matter
KyotoUniversity, 2002.
◇ H. En’yo
Study of Chiral Property of Nuclear Matter through the Measurements of φ Meson Decays
Physics of Hadrons and Nuclei
14-17 December 1998,Sanjo Conference Hall, University of Tokyo, Tokyo,Japan.
Nucl.Phys.A670 (2000) 182c.
◇ K. Ozawa
Measurements of Mass Modification of Vector Mesons in Nuclear Matter
YITP Workshop On Possible Existence Of The Sigma Meson And It Implications To Hadron
Physics (Sigma Meson ) 2000.6.12-14, published in their proceedings.
◇ S. Yokkaichi
Observation of Meson Modification in Nuclear Matter at KEK-PS
Nuclear Physics and the 21st Century(INPC 2001), Berkeley, California, 30 Jul - 3 Aug 2001.
Published in *Berkeley 2001, Nuclear physics in the 21st century* 551-555.
◇ M. Naruki et al
Observation of ρ/ω meson modification in nuclear matter at KEK-PS.
Physics AIP Conf. Proc. 610(2002) 551-555.
◇ S. Yokkaichi
Observation of Meson Modification in Nuclear Matter at KEK-PS
International Nuclear Physics Conference (INPC 2001) 2001.7.30-8.05 Berkeley, CA.,
Nucl. Phys. A698 (2002) 535-538.
◇ K. Ozawa
Modification of Vector Mesons in Nuclear Matter Measured in 12-GeV P + A reactions at KEK-PS
International Conference On Ultra relativistic Nucleus-Nucleus Collisions (QM2001) 2001.1.15-20,
Stony Brook, N.Y. Published in Nucl.Phys.A698 (2002) 535-538.
◇ Naruki et al.
Measurement of Invariant Mass Spectra of Vector Meson Decaying in Nuclear Matter
Prepared for 16th International Conference on Particles and Nuclei (PANIC 02), Osaka, Japan, 30
Sep - 4 Oct 2002. Published in Nucl. Phys. A721 (2003) 297-300.
◇ H. En’yo et al.
Nuclear Media Effects on Production and Decay of Vector Meson Studied in 12-GeV P + A●________________________________________________________________________● Physics papers published in refereed journal.
○★
◇＊

Technical papers.
PhD theses.
Conference and Symposium.
Internal Report and others.

Interaction
YITP - RCNP Workshop on Chiral Restoration in Nuclear Medium, Kyoto, Japan, 7-9 Oct 2002.
Published in Prog. Theor. Phys. Suppl.149 (2003) 49-55.
＊ H. En’yo
Nuclear Matter Probed with φ Meson - exploring the lost symmetry
Lecture at 10th RIKEN Winter School on Nuclear Physics with Electromagnetic Probes at RIKEN
RI-Beam Factory - Unstable Nuclei and Hadrons -, Hotel Blanc, Higashi Naruse-mura, Akita, Japan
January 9-14, 2000.
＊ H. En’yo
Measurements of in-medium decay of vector mesons at KEK-PS
Invited talk at Hawaii 2001, First Joint Meeting of the Nuclear Physics of APS and JPS, October
17-20, 2001, Wailea, Maui, Hawaii.

________________________________________________________________________● Physics papers published in refereed journal.
○★
◇＊

Technical papers.
PhD theses.
Conference and Symposium.
Internal Report and others.

VOLUME 86, NUMBER 22

PHYSICAL REVIEW LETTERS

28 MAY 2001

Observation of r兾兾v Meson Modification in Nuclear Matter
K. Ozawa,* H. En’yo, H. Funahashi, M. Kitaguchi, M. Ishino,† H. Kanda,‡ S. Mihara,† T. Miyashita,§ T. Murakami,
R. Muto, M. Naruki, F. Sakuma, H. D. Sato, T. Tabaru, S. Yamada, S. Yokkaichi,k and Y. Yoshimura¶
Department of Physics, Kyoto University, Kitashirakawa Sakyo-Ku, Kyoto 606-8502, Japan

J. Chiba, M. Ieiri, M. Nomachi,** O. Sasaki, M. Sekimoto, and K. H. Tanaka
Institute of Particle and Nuclear Studies, KEK, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan

H. Hamagaki
Center for Nuclear Study, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan(Received 14 November 2000)
We measured the invariant mass spectra of electron-positron pairs in the target rapidity region of
12-GeV p 1 A reactions. We have observed a significant difference in the mass spectra below the v
meson between p 1 C and p 1 Cu interactions. This difference indicates that the spectral shape ofmesons is modified at normal nuclear-matter density.
DOI: 10.1103/PhysRevLett.86.5019

Recently, the chiral property of QCD in hot (T ﬁ 0) or
dense (r ﬁ 0) nuclear matter has attracted wide interestin the field of hadron physics. The dynamical breaking ofchiral symmetry in the QCD vacuum induces an effective
mass of quarks, known as constituent quark mass, whichthen determines the known mass of all the hadrons. In hot
and/or dense matter, this broken symmetry is subject tobe restored either partially or completely and, hence, theproperties of hadrons can be modified. To observe suchan effect, measurements of the in-medium decay of vector
mesons are highly desirable for the direct determination of
the meson properties in matter.
Although many heavy-ion experiments have been carried out in CERN-SPS and BNL-AGS to study hot anddense matter, only one experiment could address the massmodification of vector mesons. The CERES/NA45 Collaboration measured low-mass electron-pair productions in
Pb-Au collisions at 158 A GeV [1], and observed an enhancement of the e1 e2 pair yield in the mass range of
0.3 , mee , 0.7 GeV兾c2 over the expected yield fromthe known hadronic sources in pp collisions. This striking
effect could be understood as a consequence of the massmodification of a r and a v meson in hot matter.
On the other hand as an interpretation of K 1 -12 C scattering data [2], Brown et al. proposed a possible modification of the r meson mass at normal nuclear density.
After this suggestive but an indirect information, the
TAGX Collaboration reported the direct observation of amodified r meson in p 1 p 2 invariant mass spectra in the
3
He共g, p 1 p 2 兲X channel [3]. Further experimental data,
especially regarding the lepton channel which is free fromthe effect of final state interactions, are awaited, since
the TAGX data were based on subthreshold r meson
production and not free from the influence of the limited
phase space.
There are several theoretical approaches to the investigation of meson modification and its density dependence
0031-9007兾01兾86(22)兾5019(4)$15.00

PACS numbers: 25.40.Ve, 14.40.Aq, 21.65.+f, 24.85. +p[4,5]. For example, one model based on the QCD sum rulehas been proposed by Hatsuda and Lee [4]. According
to this model, the mass decrease at normal nuclear density is ⬃120 180 MeV兾c2 for the r and v mesons and⬃20 40 MeV兾c2 for the f meson. Brown and Rho proposed another scaling law which predicts that the massesof r and v mesons decrease 20% at normal nuclear density [5]. Thus, the measurements of such mesons, which
are produced and decayed in a nucleus, are of great interest. The present experiment is one of several experimental
efforts [6] to investigate the in-medium properties of thevector mesons at normal nuclear-matter density.
The experiment, KEK-PS E325, was designed tomeasure the decays of the vector mesons, f ! e1 e2 ,r兾v ! e1 e2 , and f ! K 1 K 2 , in the kinematicalregion where the decay probability inside a target
nucleus was enhanced (0.6 , rapidityee , 2.2, 0.0 ,PTee , 1.5 GeV兾c). Considering the observed kinematical distributions, it is expected that about 60% of r
mesons and about 10% of v mesons decay statisticallyinside a copper nucleus if no mass broadening takes placein the nuclear medium. This rate increases according to
the width broadening in the nucleus. We thus expect tosee a “free-space” peak consisting of mesons decayedoutside the nucleus. And the effect of meson modification
could be seen as an excess over the known spectra.
The spectrometer was built at the primary beam line
EP1B in the 12GeV-PS at KEK. We have been taking
data since 1998. This report describes the e1 e2 triggereddata of 5.6 3 107 events collected in 1998 using 2.2 31014 protons on the targets. The layout of the detectors
is shown in Fig. 1. The spectrometer had two electron
arms and two kaon arms, which shared the dipole magnet
and the tracking devices. The electron arms covered 612±
to 690± horizontally and 622± vertically. The kaon arms
covered 612± to 654± horizontally and 66± vertically.
Primary protons with a typical intensity of 7 3 108 Hz© 2001 The American Physical Society

5019

VOLUME 86, NUMBER 22

FTOF Counter
AerogelCherenkov
KDC
HodoscopeCounter
Start
TimingCounter

a)

b)
Start
TimingCounterbeam

1000mmtargetAerogel
FGC RGC Cherenkov
RLG KDCFTOFCountertargetchamber
BDC
CDC

3980mm

RGC
RLG
SLG
BDC
FGC
CDC

PHYSICAL REVIEW LETTERS

HodoscopeCounter

FIG. 1. Schematic view of the experimental setup of the E325
spectrometer: (a) the top view and (b) the side view. The sideview shows the cross section along the center of the kaon arm(see text).

were delivered to the targets at the center of the dipole
magnet. We used three kinds of targets (carbon, polyethylene, and copper) aligned in-line with interaction lengths
of 0.028%, 0.061%, and 0.020%, respectively. The carbon
and copper targets were glued onto target supports madeof paper (C6 H12 O6 ), whose interaction length was 0.033%.
The combination of an intense beam with a thin target wasessential to keep the g conversion rate below the Dalitz
decay rate. The typical interaction rate was as high as
1.2 MHz. The three targets were separated by 40 mm, andthe target images were reconstructed by the charged tracks
with 4.2 mm resolution in the beam direction.
Tracking was performed using a cylindrical drift chamber (CDC) and barrel-shaped drift chambers (BDC). The
dipole magnet had circular pole pieces of 880 mm in radius. The magnet provided the field of 0.78 T at the center, and the field integral of 0.81 Tm from the center to1600 mm in radius where the BDC’s were located. The
acceptance of the CDC was 612± to 6132± horizontallyand 622± vertically, while the BDC had the same vertical
coverage as the CDC with a smaller horizontal coverageof 67.5± to 694.5±. The CDC consisted of ten layers ofthe drift cells in the radial region of 445.0 to 830.0 mm
including four stereo layers. The BDC’s had four layers,
5020

28 MAY 2001

located in the radial region of 1600 to 1650 mm, including two stereo layers. All of the drift cells of the CDC
had the same horizontal angular coverage of 1.5± with respect to the target, and the drift cells of the BDC had anangular coverage of 0.75±. Both the CDC and BDC usedargon-ethane mixed gas of 50% and 50% at 1 atm. The
total thickness of the materials from the target to the frontof the BDC was 3.1% of the radiation length. A position
resolution of 350 mm was obtained in the present analysis.
For electron identification, the whole region of theelectron arm was covered by two stages of electronidentification counters. The first stage of electron identification was performed by the front gas-Čerenkov
counters (FGC), which covered from 612± to 690±
horizontally and 622± vertically. They were horizontallysegmented into 13 units in each arm so that one segment
covered 6±. The second stage consisted of three types ofelectron-identification counters. The rear gas-Čerenkov
counters (RGC) covered 612± to 654± horizontally and66± vertically with seven horizontal segments in each arm.
These regions corresponded to the kaon-arm acceptance.
The rear lead-glass electromagnetic (EM) calorimeters(RLG) covered the same horizontal angle as the RGCs
with 12 segments in each arm, but were vertically covered
outside the kaon-arm acceptance, from 66± to 622±.
In the backward region, where the horizontal angle waslarger than 57±, the second-stage electron identification
was performed by side lead-glass EM calorimeters (SLG)
which covered 657± to 690± horizontally and 622±
vertically with nine horizontal segments in each arm.
Both of the gas-Čerenkov counters used isobutane witha refractive index of 1.0019. The threshold momentum forpions was 2.3 GeV兾c. Both the RLG and the SLG consisted of SF6W lead glass. The typicalp energy resolutionof the RLG and the SLG was 15%兾 E. To suppress any
fake triggers caused by pions and protons, we set the discriminater threshold for the gas-Čerenkov counters higher
than the optimum setting for electrons; thus, the electron
efficiency was sacrificed. For electrons with a momentum
greater than 400 MeV兾c, the overall efficiencies, including the trigger threshold and the off-line cut, were 55% for
FGC, 86% for RGC, and 85% for the calorimeters, RLG
and SLG. We achieved a pion rejection of 6.7 3 1024
using a cascade operation of FGC and the EM calorimeters, and 3.9 3 1024 with FGC and RGC for 400 MeV兾cpions.
An electron trigger having three levels was adopted
to perform the data accumulation. In the first level, we
selected electron-pair candidates using the coincidence
signals of the front-stage (FGC) and the rear-stage
detectors (SLG, RLG, RGC), by requiring horizontal
position matching. To suppress electron pairs having
small opening angles, such as from Dalitz decays and gconversions, we required the two FGC hits to be morethan two segmentations apart. In the second level, we
required the pair to be oppositely charged by using thedrift-chamber hits associated with the FGC hits. The sign

VOLUME 86, NUMBER 22

PHYSICAL REVIEW LETTERS

of a track can be roughly determined by hits in the outermost layers in the CDC and in the BDC together with thetarget position. We eliminated the electron-pair candidates
which had an apparent “11” or “22” configuration.
We also required the remaining pairs to be more than 12±
apart at the r 苷 825 mm position in order to suppress
electron pairs having small opening angles. In the thirdlevel, the approximate opening angles of the pairs were
calculated. Because the radius of the BDC layers was almost twice that of the radius of the outermost CDC layers,
the opening angle of the pair at the target, Qopen , could
be approximated by Qopen ⯝ 2 3 Qcdc 2 Qbdc , where
Qcdc is the opening angle of the pair at the outermost
CDC layer and Qbdc at the BDC layer with respect to thetarget position. We required Qopen to be in the range from50± to 150± in the trigger. The typical trigger rates were
450, 330, and 280 Hz in the first, second, and third level
triggers, respectively. We maintained the live time of thedata acquisition at around 60%.
To evaluate the performance of the spectrometer, themass resolution was examined for the observed peaksof the L ! p 1 p 2 and Ks ! p 1 1 p 2 decays, as
shown in Figs. 2a and 2b. For the L peak we obtained thecentroid at 1115.5 MeV兾c2 (known to be 1115.7 MeV兾c2 )with a Gaussian resolution of 1.8 6 0.1 MeV兾c2 , andfor Ks 493.9 MeV兾c2 (known to be 497.7 MeV兾c2 ) and3.6 6 0.6 MeV兾c2 , respectively. The observed peak positions and widths provide the systematic uncertainty of themass scale and the mass resolution of the present analysis.
The results were compared to Monte Carlo simulations inwhich we took the chamber resolution and the multiple
scattering into account. The observed widths were well
reproduced by the simulation (1.9 6 0.6 MeV兾c2 for L
and 3.5 6 0.6 MeV兾c2 for Ks ), and the energy scale
uncertainties for the v and f mesons were estimated to

be 4 and 7 MeV兾c2 , and the mass resolution to be 9.6 and12.0 MeV兾c2 , respectively.
Figure 3 shows the e1 e2 invariant mass spectra: (a) forthe carbon and polyethylene targets (light nuclear targets) and (b) for the copper target (heavy nuclear target).
These histograms depict the events when the electron andthe positron are detected in different arms, so that thelow-mass part of the spectra is largely suppressed. Although the clear peaks of the v meson decaying into e1 e2
are visible in the spectra, a significant shape difference isobserved between the light nuclear targets and the copper
target, and an excess at the low-mass side of the v peakcan be seen.
We tried to reproduce the mass shape of the obtained
histograms using the combinatorial background and the[events / 50MeV/c 2]
100

500

a)

[events / 5MeV/c 2]
m=493.9, σ = 3.6
25[MeV/c2]
20

400

b)

from lighttargets
fit result
ω,ρ,φ e+e−ω π 0 e+e−
combinatorial

60
40
20
0

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2[GeV/c2]e+ e- invariant mass[events / 50MeV/c 2]

25
20
15
10

15

a)

80

30
[events / 1MeV/c 2]
m=1115.5, σ = 1.8[MeV/c2]600

28 MAY 2001b)from heavy
targetfit result
ω,ρ,φ e+e−
ω π0 e+e−
combinatorial

5

300
10
200

0
5

100
0
1.09

1.11

1.13
[GeV/c2]p π- invariant mass

0
0.45

0.49

0.53
[GeV/c 2]π + π- invariant mass

FIG. 2. Invariant mass spectrum of pp 2 (a) and p 1 p 2(b). The lines are the best-fit results by applying a Gaussianwith a linear background.

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2[GeV/c2]e+ e- invariant mass

FIG. 3. Invariant mass spectrum of the e1 e2 pair: (a) for the
carbon and polyethylene targets and (b) for the copper target.
The solid lines show the best-fit results of the known hadronicsources with the combinatorial background. The dotted lines
indicate the contributions from r, v, and f decays. The dashedlines indicate v ! p 0 e1 e2 decays and the dot-dashed lines
indicate the combinatorial background.

5021

VOLUME 86, NUMBER 22

PHYSICAL REVIEW LETTERS

known hadronic sources. The origins of the combinatorial
background were pairs which were picked up from two
independent Dalitz decays or g conversions, and pairs likee2 p 1 or e1 p 2 due to misidentification. The remaining
e2 p 1 and e1 p 2 background was estimated to be about
13% in the spectra and contaminations such as p 1 p 2 tobe negligibly small. The distribution of the combinatorial
background was obtained using the event-mixing method.
As known hadronic sources, r ! e1 e2 , v ! e1 e2 ,f ! e1 e2 , h ! e1 e2 g, and v ! e1 e2 p 0 wereconsidered. Contributions from particles with higher
masses are known to be insignificant [7]. The Dalitz
decay, p 0 ! e1 e2 g, is negligible in the mass acceptanceof the present data. The shapes of the e1 e2 invariant
mass spectra from the Dalitz decays, h ! e1 e2 g andv ! e1 e2 p 0 , were taken from Ref. [8]. The mass
shape of the r, v, and f mesons was given as theBreit-Wigner function with the natural widths 150, 8.41,
and 4.43 MeV兾c2 , respectively. The Breit-Wigner functions were smeared with the estimated mass resolutionsof 9.6 MeV兾c2 for the v and 12.0 MeV兾c2 for the fmeson. Because the present experiment used targets witha radiation length of less than 0.3%, the radiative tail andthe multiple scattering due to the target thickness were
negligible. To obtain the mass shape of the known sourcesin the observed spectra, we evaluated the experimental
mass acceptance using the particle distributions obtainedby the nuclear cascade code, JAM [9], which reproduced
the rapidity and Pt spectra of the observed e1 e2 pairs.
The relative abundances of the known sources and the
combinatorial background were obtained through fittingwith four parameters, the amplitudes of r兾v ! e1 e2 ,f ! e1 e2 , h ! ge1 e2 , and the combinatorial background. We assumed that the production cross sectionof r is equal to that of v following the data obtained at
the same energy [10]. The amplitude of the other source,
v ! p 0 e1 e2 , was given by the branching ratio. The
best fits are overplotted in Fig. 3. As a result of the fits,
the contribution of h ! ge1 e2 turned out to be negligible, and we found 75.5 6 9.0 v mesons and 7.4 6 5.8 f
mesons from the light target and 20.0 6 4.8 v mesons and5.2 6 2.7 f mesons from the copper target. The invariant
mass shapes were well reproduced, except for the mass region below the omega peak.
To evaluate the excess, we would like to avoid introducing “modified” spectra which are not reliably predictable.
We thus repeated the same fit procedure excluding the mass
region from 550 to 750 MeV兾c2 , where the excess is visible. The excess was estimated by subtracting the amplitudeof the fit function from the data. The amount of the excessof the light target was 19.6 6 11.7 and that of the copper
target was 29.5 6 8.7. The excess is statistically significant for the copper target data. The ratios to the amplitude of the v peak are 0.26 6 0.16 for the light target and1.48 6 0.56 for the copper target. The difference between
the two cases should have originated from the difference
5022

28 MAY 2001

in the nuclear size. The relative yields of the r兾v mesonfor both targets including the excess follow A0.6760.12 , being consistent with A2兾3 [11], where A is the nuclear mass
number.
A natural explanation for the shape change is that the
spectral modification of r兾v mesons takes place inside a
nucleus. Although the spectra of the modified meson isdifficult to predict, it should be noted that the excess inthe copper target data is visible in the mass range of about
200 MeV below the v peak.
In summary, we have observed a significant differencein the e1 e2 invariant mass spectra around the r兾v region
between carbon and copper targets, which indicates that the
spectral shapes of mesons are modified at normal nuclearmatter density.
We would like to thank all of the staff members of
KEK-PS, especially the beam channel group for their helpful support. This work was partly funded by the Japan Society for the Promotion of Science and a Grant-in-Aid for
Scientific Research from the Ministry of Education, Science, and Culture of Japan (Monbusho).

*Present address: Center for Nuclear Study, Graduate
School of Science, University of Tokyo, 7-3-1 Hongo,
Tokyo 113-0033, Japan.
Email address: ozawa@cns.s.u-tokyo.ac.jp†
Present address: ICEPP, University of Tokyo, 7-3-1 Hongo,
Tokyo 113-0033, Japan.
‡
Present address: Physics Department, Graduate School of
Science, Tohoku University, Sendai 980-8578, Japan.
§
Present address: Fujitsu Corporation, 4-1-1, Kamikodanaka, Nakahara, Kawasaki, Kanagawa 211-8588, Japan.
kPresent address: RIKEN, 2-1 Hirosawa, Wako, Saitama
351-0198, Japan.
¶
Present address: Xaxon Corporation, 1-3-19, Tanimachi,
Chu-ou, Osaka, Japan.
**Present address: Department of Physics, Osaka University,
1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan.
[1] G. Agakichiev et al., Phys. Lett. B 422, 405 (1998).
[2] G. E. Brown et al., Phys. Rev. Lett. 60, 2723 (1988).
[3] G. J. Lolos et al., Phys. Rev. Lett. 80, 241 (1998); M. A.
Kagarlis et al., Phys. Rev. C 60, 025203 (1999).
[4] T. Hatsuda and S. H. Lee, Phys. Rev. C 46, R24 (1992).
[5] G. E. Brown and M. Rho, Phys. Rev. Lett. 66, 2720 (1991).
[6] HADES Collaboration, W. Schoen et al., Acta Phys. Pol. B
27, 2959 –2963 (1996). GSI/SIS proposal S214, Search forbound h- and v-nuclear states using the recoilless (d, 3 He)
reaction. T. Nakano et al., Nucl. Phys. A629, 559c (1998).
KEK-PS E325 proposal (http://www.pn.scphys.kyotou.ac.jp/phi/E325_project.html).
[7] T. Akesson et al., Z. Phys. C 68, 47 (1995).
[8] A. Faessler, C. Fuchs, and M. I. Krivoruchenko, Phys. Rev.
C 61, 035206 (2000).
[9] Y. Nara et al., Phys. Rev. C 61, 024901 (1999).
[10] V. Blobel et al., Phys. Lett. 48B, 73 (1974).
[11] M. Binkley et al., Phys. Rev. Lett. 37, 571 (1976).

Nuclear Instruments and Methods in Physics Research A 457 (2001) 581}587

Mass production of hydrophobic silica aerogel and readout
optics of Cherenkov light
M. Ishino *, J. Chiba, H. En'yo , H. Funahashi , A. Ichikawa ,M. Ieiri, H. Kanda , A. Masaike , S. Mihara , T. Miyashita , T. Murakami ,A. Nakamura, M. Naruki , R. Muto , K. Ozawa , H.D. Sato , M. Sekimoto,T. Tabaru , K.H. Tanaka, Y. Yoshimura , S. Yokkaichi ,M. Yokoyama, H. Yokogawa
Department of Physics, Faculty of Science, Kyoto University, Kyoto 606-8502, Japan
IPNS KEK, High Energy Accelerator Research Organization, 1}1 Oho, Tsukuba 305-0801, Japan
Mohri Oil Mill Co., Ltd., 1819-4 Okuchi, Matsuzaka, Mi-e 515, Japan
Central Research Laboratory, Matsushita Electric Works, Ltd.,1048 Kadoma, Osaka 571, Japan
Received 15 May 2000; accepted 27 June 2000

Abstract
We have produced 500 l of hydrophobic silica aerogel for the threshold-type Cherenkov detectors to trigger kaons in
the momentum region of 0.5}2.0 GeV/c. The refractive index of the products is 1.034 with piece by piece variation of$0.004. The transmission length is found to be 25$4 mm at 400 nm. The details of the production scheme and thecharacteristics of the produced silica aerogel are reported. We also describe our studies of the light collection schemes of
the aerogel Cherenkov counters.  2001 Elsevier Science B.V. All rights reserved.
PACS: 29.40.Ka; 42.15.Eq; 24.85.#p; 11.30.R
Keywords: Hydrophobic silica aerogel; Kaon trigger; Particle identi"cation (p/K); Threshold-type Cherenkov counter

1. Introduction
The experiment KEK-PS E325 measures decayof the mesons produced in 12-GeV p # A reac* Corresponding author. Tel.: #81-75-753-3861; fax: #8175-753-3795.
 Present address, Tohoku University, Sendai, 980-8578,
Japan.
 Present address, Fukui University of Technology, Fukui,910-8505, Japan.
 Present address, ICEPP, University of Tokyo, Tokyo, 1130033, Japan.
E-mail address: ishino@nh.scphys.kyoto-u.ac.jp (M. Ishino).

tions both in the PK>K\ and Pe>e\ channels to investigate the experimental signatures ofmass shift of mesons in nuclear matter at normal
density. To trigger the PK>K\ decays, the experiment requires threshold-type Cherenkov
counters which can separate kaons from pions in
the momentum region of 0.5}2.0 GeV/c together
with the fast encoding time-of-#ight measurements[1]. The details of the experiment can be foundelsewhere [2,3].
Silica aerogel has been widely used for the kaon
trigger [4,5]. The aerogel used in the former experiments was hydrophilic, so it had to be kept dry in

0168-9002/01/$ - see front matter  2001 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 7 8 6 - 5

582

M. Ishino et al. / Nuclear Instruments and Methods in Physics Research A 457 (2001) 581}587

order to avoid any deterioration in the transparency. The recent innovative work by the KEKBELLE PID group and Matsushita Electric Worksestablished a new method to produce hydrophobic
silica aerogel [6]. The products were found to havetypically 2.5 times longer transmission length than
the traditional ones. Based on this new technology
the mass production of hydrophobic silica aerogel
whose refractive index is 1.034 was carried out by
the authors using the autoclave facility at Mohri
Oil Mill Co., Ltd.
We have built two sets of aerogel Cherenkov
counters using this product. Each of them has an
e!ective area of 706 mm in height and 2640 mm inwidth segmented into 16 cells. In the design of theCherenkov counter, the longer transmission lengthof the silica aerogel enables us to optimize the lightcollection scheme by the ray-tracing calculation.
We have achieved the uniform detection e$ciency
over one segment which is 706 mm in heightand 160 mm in width viewed by a pair of 5 in.
photomultipliers.
The detailed procedure of mass production of
aerogel is described and the characteristics of the
products are reported in Section 2. The light collection scheme adopted into the aerogel Cherenkov
counters described here and the performancecompared with other light collection schemes are
in Section 3. Finally, the conclusion is given in
Section 4.

the laboratory, descriptions can be found in
Refs. [6,7].
Gelation takes place by mixing two solutions,oligomer MS51 (methyl silicate 51) and distilledwater with ammonia, both of which are dissolved inmethanol. The solutions are poured into a plastic
container whose bottom is covered with a stainless-steel plate coated with Te#on. The coating
ensures smooth removal of the gel from the container later. The mixture is gelated for 5 min andstored in a hermetic box to prevent the solvent from
evaporation. The following polymerization continues and alcogel +SiO , is formed in 3 days with Lammonia as catalyst:nSi(OCH ) #4nH OPnSi(OH) #4nCH OH,nSi(OH) P+SiO , #2nH O.
 LPrior to the mass production, the mixture was
optimized through the test runs to obtain the index
of 1.030 using ethanol as solvent. The results are
summarized in Table 1. The mixture sample no. S5
in Table 1 was selected for the mass production. It
was pointed out after the test runs, the silica aerogel
using methanol as a solvent has 1.2 times longer
transmission length at 500 nm than using ethanol
at this refractive index. We changed the mixture ofsolvent from ethanol to methanol keeping the volume to be constant since density of methanol is
0.2% larger than ethanol. The mean value of therefractive index in the mass production has been

2. Mass production of hydrophobic silica aerogel
We have produced 400 silica-aerogel pieces of283;200;21 mm with refractive index of 1.034.
All of them were produced in two and a halfmonths from early spring to summer in 1996.
The production was divided into 4 batches taking
into account the size of the 500 l autoclave forthe supercritical drying. Each batch was organized
as follows. For the small-scale production in The silica aerogel is produced under the mutual agreement
between Kyoto University, Mohri Oil Mill Co., Ltd. and Matsushita Electric Works, Ltd. Note that the present production
scheme of silica aerogel is a patent of Matsushita ElectricWorks, Ltd. [7], presently to whom one can order silica aerogel.

Table 1The list of sample mixtures and refractive indices obtained. All
samples include 1.0 mol of methyl-silicate 51 and 20.0 mol of
H OSamplename

Ethanol
(mol)

NH

(mol)

Density(g/cm)Refractive
index

S1
S2
S3
S4
S5
S6
S7
S8
S9

33.0
33.0
33.0
38.0
38.0
38.0
43.0
43.0
43.0

0.14
0.17
0.20
0.14
0.17
0.20
0.14
0.17
0.20

0.11400.11950.11150.10520.11070.10800.09890.0993
0.09851.03241.03231.03231.02991.03011.0292
1.02801.0274
1.0275

M. Ishino et al. / Nuclear Instruments and Methods in Physics Research A 457 (2001) 581}587

increased by 0.004 which is still quite acceptable forour application.
Aging period is needed to remove water from thealcogel and to proceed further polymerizations.
The alcogel is moved from the plastic container toa stainless-steel mesh-cage and kept at 303 for 14
days in ethanol which is replaced once in 7 days. It
has been experienced that a longer aging period athigher temperature gave better transmission lengthof the "nal product. It should be noted that thetemperature must be lower than 403 to avoid deforming of the products and shortening of thetransmission length.
Hydrophobic reaction is to replace hydrophilic-OH group in alcogel with hydrophobic -OSi(CH ) group.
The alcogel is moved into ethanol solvent containing 12.5 vol% of (CH ) -Si-NH-Si-(CH ) .
The hydrophobic reaction is completed in twodays. This process is the key point to make the "nalproducts hydrophobic. After that, an aging periodof 14 days is needed to remove the ammonia fromthe alcogel produced in the hydrophobic reaction.
Ethanol is replaced once in 7 days.
Drying at supercritical point takes place to transform alcogel into silica aerogel by removing
the ethanol from the alcogel without creatingcracks and shrinking the volume. The alcogel
is moved to the autoclave "lled with ethanol.
The autoclave is set at the supercritical point of
CO (160 atm, 803C). By circulating CO for
one and a half days, ethanol in alcogel is completely
removed. Through the period of this mass production, aerogel which had signi"cant cracks wereabout 30%. Typical transmission length ofthe product as a function of wavelength is shownin Fig. 1. The present hydrophobic silica
aerogel has about 2.5 times longer transmission
length than the conventional ones made by AirglassInc.
To check the batch by batch and piece by piece
quality of the products, we measured the transmission lengths and the refractive indices of randomly
sampled pieces. The results are shown in Fig. 2. It
was found that transmission lengths were within
25$4 mm at 400 nm and the indices were within
1.034$0.004. The variations were well within ourrequirement. The index 1.034 corresponds to the583

Fig. 1. Transmission length of the produced silica aerogel.
Circles are for the present silica aerogel (n"1.034) and triangles
are for the one produced by the traditional method (made by
Airglass Inc. n"1.041).

Cherenkov light threshold for pion of 0.532 GeV/cand kaon of 1.881 GeV/c.
We have examined the uniformity of the transmission length at 400 nm and the refractive index
within one piece. The results are $2.0 mm and$0.0005, respectively, which are comparable withthe resolution of the measurement system and negligibly smaller than the variations over the production period.
The major source of the variations is attributed
to the temperature during the gelation and aging
period. We observed signi"cant changes in gelating time, which is slower in the lower room
temperature. In addition, density of the solvent
changes with temperature. For the case of methanol which is a solvent in our case, the expansion coe$cient of volume is 1.190;10\/K(273}303 K) [8]. The production period coveredfrom early spring to summer and the temperaturechanged by more than 203, which changes the density by 2.4%. A temperature control during themixing and the gelating phase is recommended tokeep the refractive index stabler than in the present
work.

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M. Ishino et al. / Nuclear Instruments and Methods in Physics Research A 457 (2001) 581}587

Fig. 2. Distributions of the refractive index and the transmission length of the produced silica aerogel.

3. Design of the light collection system of theCherenkov counterWe compared various light collection schemes
by a half-cell model. These are schematically shownin Fig. 3. Type A is a conventional di!usion box
covered with white paper Millipore whose re#ection e$ciency is better than 93%. Type B has #at
mirrors made of MgF -coated aluminized mylar
whose re#ection e$ciency is better than 88% for
the wavelength longer than 200 nm. Types A and
B are simple solutions whereas they need two 5 in.
photomultipliers (PMTs). To reduce the number ofPMTs, we tried Types C and D having a cylindricalconcave mirror.
Type D is the "nal version applied for the present
detector. The layout is shown together with thesimulated Cherenkov photon trajectory. The
counter covers a 2640;706 mm active area which
is segmented into 16 cells with an area of

160;706 mm. Cherenkov photons are readout
with a pair of 5 in. PMTs, a cylindrical concave
mirror and a Winston funnels [9] attached on theboth PMTs.
The mirrors are made of acryl and the funnels are
made of ABS resin, onto which aluminum andMgF are successively evaporated. The cells areseparated with double coated aluminized mylar
sheets (Al#MgF ). We have adopted 125 mmthick silica aerogel as a Cherenkov radiator which
consists of six layers of the silica aerogel plates.
These schemes were tested at the T1 beam line at
KEK 12GeV-PS using 0.7 GeV/c positive pions(b"0.981). The thickness of the silica aerogel was
90 mm. The number of photo electrons observed
are plotted in Fig. 4 as a function of the incident
beam positions. For 2-PMTs scheme (Types A andB), the number of photoelectrons from both PMTs
were added. The PMTs were located at
X"500 mm and >"0 mm. During the scanningM. Ishino et al. / Nuclear Instruments and Methods in Physics Research A 457 (2001) 581}587

585

Fig. 4. The number of observed photoelectrons for 0.7 GeV/cpositive pions as functions of particle position. PMT is locatedat X"500 mm and >"0 mm. Triangles are for the lightcollection Type A, rectangles for Type B, and circles for Type C.

Fig. 3. The light collection schemes studied with a test beam.
Type A is a di!usion box covered with white paper Millipore,
Type B consists of #at mirrors, and Type C is similar to TypeD but without a Winston funnel. Type D is the "nal version
applied for the present detector.

along the X direction (X: 0}500 mm), the > position was kept at #5 mm, and while scanningalong the > direction (>: !80 to 80 mm), theX position was kept at 250 mm. It can be seen thatthe di!usion box solution (Type A) is not adequate
for the present detector geometry whose segmented
structure requires the long drift distance for
Cherenkov light from X"0 mm to PMTs. Type
B is applicable but an expensive solution (read with
2-PMTs). Type C has a strong X dependence characterized by the dip at X"300 mm. An optics
calculation shows that at this point the center ofa Cherenkov ring of 140 mm diameter falls outside
the PMT surface of 125 mm in diameter. The"nal version (Type D) is the re"ned design of TypeC, where the geometry of optics is optimized by

ray-tracing calculations and Winston funnels areused. The number of observed photoelectrons for
1.0 GeV/c positive pions is compared in Fig. 5 asa function of the beam incident position for two
di!erent thicknesses of aerogel, 90 mm and
150 mm. Compared to the di!usion box solution
the gain of the photon yield is obvious even though
the required number of PMTs is just a half. An
almost linear increase of photoelectron yield wasobserved according to the change of the thicknessof aerogel, which is another important bene"t ofthe high transmission length.
The present results di!ers from the previous
works [4,5], where less transparent silica aerogel
was used and the di!usion box solution waschosen. The present silica aerogel with 2.5
times longer transmission length than the previous
one allows us to design the e$cient light collection optics based on the ray-tracing calculation.
It should be noted that recent successful measurements of a Cherenkov light ring [10,11] were
performed by using silica aerogel whose refractive
index is about 1.03 and which was produced by
the same procedure described in the previous
section.

586

M. Ishino et al. / Nuclear Instruments and Methods in Physics Research A 457 (2001) 581}587

In Fig. 6 we show the pulse height distribution of
the aerogel Cherenkov counter constructed for theexperiment KEK-PS E325. A secondary beam of
1.4 GeV/c positive pions and protons were delivered at the center of the segment. We obtained
5.4 photoelectrons for pions. The photoelectronsobserved for protons are attributed to d rays or
scintillation lights. Setting the threshold level at 1.1
photoelectron we have achieved the pion rejection
e$ciency of 99.0% and an over-killing rate for
proton of 0.3%.

4. Conclusion

Fig. 5. Position dependence of the number of photoelectronsfor 1.0 GeV/c positive pions observed with Type D. The solidline is the expected photoelectron yield with a di!usion box
solution (scaled from Type A in Fig. 4). Dashed lines are to guidethe eyes.

We have carried out the mass production of
500 l of hydrophobic silica aerogel. These products
are used as a Cherenkov radiator of the thresholdtype counter to veto pions in the momentum regionof 0.5}2.0 GeV/c. Through the present work wehave successfully industrialized the mass production scheme of hydrophobic silica aerogel. We haveobtained the optical properties of the products
whose refractive index is 1.034$0.004 and thetransmission length is 25$4 mm at 400 nm.
The transmittance is typically 2.5 times larger thanthe previous one, which is the essential di!erencein choosing the light collection scheme. The undiffused Cherenkov photons can be collected e!ectively with the optics optimized by the ray-tracing
method. The aerogel Cherenkov counter constructed for the KEK-PS E325 spectrometer has
achieved 99.0% pion rejection e$ciency and 0.3%
proton over-killing rate at 1.4 GeV/c.

Acknowledgements

Fig. 6. Pulse height distribution of the E325 aerogel Cherenkov
counter for the secondary beam of 1.4 GeV/c positive pions andproton. When we set the threshold at ADC count 40 (equivalent
to 1.1 photoelectrons) the e$ciency for pion is 99.0% andproton over-killing rate is 0.3% in the veto operation.

The authors are grateful to Prof. T. Sumiyoshi,
Dr. R. Suda and the members of the KEK-BELLE
PID group for their advice to complete the present
work. They are also thankful to Prof. H. Kawai,
Mr. T. Matsumoto and the KEK-PS E248 group.
This work was supported by a Grant-in-Aid for
Scienti"c Research from Japanese Ministry of Education, Science and Culture. Some of the authors
were supported by the Japan Society for the Promotion of Science.

M. Ishino et al. / Nuclear Instruments and Methods in Physics Research A 457 (2001) 581}587

References[1] Y. Yoshimura et al., Nucl. Instr. and Meth., in preparation.
[2] KEK-PS E325 proposal, 1994. http://www.pn.scphys.kyotou.ac.jp/phi/e325}project.html.
[3] S. Yokkaichi et al., Proceedings of Quark Matter 97; Nucl.
Phys. A 368 (1998) 435c.
[4] J.K. Ahn et al., Phys. Lett. B 378 (1996) 53.
[5] T. Hasegawa et al., Nucl. Instr. and Meth. A 342 (1994)
383.

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[6] I. Adachi et al., Nucl. Instr. and Meth. A 255 (1995) 390.
[7] Matsushita Electric Works, Ltd., Japanese Patent Early
Publication (KOKAI) No. 1992-198238.
[8] Landolt-BoK rnstein Zahlenwerte und Funktionen aus
Physik, Chemie, und Technik, 6 Au#., II, Band, 1 Teil,
Springer, Berlin, 1971, pp. S. 378}718.
[9] H. Hinterberger, R. Winston, Rev. Sci. Instr. 37 (1996)
1004.
[10] R. De Leo et al., Nucl. Instr. and Meth. A 401 (1997) 187.
[11] D. Ryckbosch, Nucl. Instr. and Meth. A 433 (1999) 98.

ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research A 516 (2004) 390–405

Spectrometer for measurements of f mesons in nuclear matterproduced through 12-GeV p+A reactions
M. Sekimotoa, J. Chibaa, H. Funahashic, H. Hamagakid, M. Ieiria, M. Ishinoe,
M. Kitaguchic, S. Miharae, T. Miyashitac, T. Murakamic, R. Mutoc, M. Narukic,
M. Nomachia,1, K. Ozawad, F. Sakumac, O. Sasakia, T. Tabarub, K.H. Tanakaa,
S. Yamadac, S. Yokkaichib, Y. Yoshimurac, H. En’yob,*aInstitute of Physical and Nuclear Studies, High Energy Accelerator Research Organization (KEK), Tsukuba-shi,
Ibaraki 305-0801, Japanb
RIKEN(The Institute of Physical and Chemical Research), Wako-shi, Saitama 351-0198, Japanc
Department of Physics, Kyoto University, Kyoto 606-8502, JapandCenter for Nuclear Study, Graduate School of Science, University of Tokyo, Tokyo 113-0033, Japane
ICEPP, University of Tokyo, Tokyo 113-0033, Japan
Received 18 February 2003; received in revised form 15 August 2003; accepted 16 August 2003

Abstract
A spectrometer was constructed to measure the invariant mass spectra of r; o and f mesons through their eþ e or
K K decays. The KEK experiment E325 was successfully carried out using the new spectrometer, and observed thesignature of an in-medium mass modiﬁcation of the r and/or o mesons for the ﬁrst time. The details of the spectrometerare described.
r 2003 Elsevier B.V. All rights reserved.
þ

PACS: 24.85.+p; 25.40.Ve; 29.30.Aj; 29.40.Ka
Keywords: Spectrometer; f meson; o meson

1. Introduction
A new spectrometer for studying light vector
mesons produced in nuclei through high-energy
proton–nucleus reactions was designed and constructed at the 12 GeV proton synchrotron of the*Corresponding author.
E-mail address: enyo@riken.go.jp (H. En’yo).
1
Present address: Department of Physics, Osaka University,
Toyonaka-shi, Osaka 560-0043, Japan.

High Energy Accelerator Research Organization(KEK-PS). An experiment, E325 [1], was carriedout using this spectrometer to investigate the chiralproperty in dense matter. It is believed that chiralsymmetry, which is spontaneously broken in ourworld, should be restored in a system with a ﬁnite
temperature and/or a ﬁnite density. Theoretically,it has been suggested that a signature of the chiralsymmetry restoration could be seen as a possible
mass shift of vector mesons in nuclear matter [2].
Experiment E325 was proposed to measure the0168-9002/$ - see front matter r 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.nima.2003.08.168

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M. Sekimoto et al. / Nuclear Instruments and Methods in Physics Research A 516 (2004) 390–405

mass of r; o and f mesons produced, and decays
in nuclei, through a determination of the invariant
mass from the eþ -e pairs, or the Kþ -K pairs
following their decays. The experiment was
successful to observe o and f mesons in theeþ -e invariant mass spectra, and reported thesignature of an in-medium mass modiﬁcation of
the mesons [3]. This was the ﬁrst observation in the
leptonic in-medium decay of vector mesons at a
normal nuclear matter density.
The spectrometer was built at primary beam line
EP1-B [4]. This beam line was designed totransport 12-GeV protons with a maximum
intensity of 4  109 per 2-s spill with a 4-s
repetition cycle, and with a small beam halo.
There are two key points concerning this spectrometer. The ﬁrst is to detect slowly moving vector
mesons which have a larger probability to decay
inside a nucleus. The typical acceptance is
0:5oyee o1:5 and 1obgee o3: The second is touse a high-quality primary beam on thin targets.
The combination of an intense beam with thintargets is essential to minimize the background bykeeping the g conversion rate below the Dalitz
decay rate. In the typical data-taking condition theinteraction rate is as high as 1 MHz using 1  109
beam protons per 2-s spill on the three targets witha total interaction length of 0.2%.
The layout of the detectors is shown in Figs. 1
and 2. The spectrometer has two electron arms andtwo kaon arms, which share a dipole magnet andtracking devices. The electron arms cover from712 to 790 horizontally and 723 vertically.

Fig. 1. Schematic view of the E325 spectrometer (top view).

391

Fig. 2. Schematic view of the E325 spectrometer; a crosssection along the center of the kaon arm is shown.

The kaon arms cover from 712 to 754horizontally and 76 vertically.
Tracking has been performed with a cylindricaldrift chamber (CDC) and barrel-shaped drift
chambers (BDC), but lately a vertex drift chamber(VTC) was added.
For electron identiﬁcation, the whole region of
the electron arm is covered by two stages ofelectron-identiﬁcation counters. The ﬁrst stage ofthe electron identiﬁcation is performed by front
gas-Cherenkov counters (FGC), which cover from712 to 790 horizontally and 723 vertically.
The second stage is either gas Cherenkov counters
or EM calorimeters. The rear gas-Cherenkov
counters (RGC) cover 712 to 754 horizontallyand 76 vertically. These regions correspond to
the kaon-arm acceptance. The rear lead-glass EM
calorimeters (RLG) cover the same horizontal
angle as RGCs, but vertically cover outside thekaon-arm acceptance, from 76 to 723 : Another set of lead glass calorimeters (forward leadglass EM calorimeters: FLG) were recently added
to improve the electron identiﬁcation in the kaonarm acceptance. In the backward region, where thehorizontal angle is larger than 57 ; second-stage
electron identiﬁcation is performed by side leadglass EM calorimeters (SLG), which cover 757
to 790 horizontally and 723 vertically.
For kaon identiﬁcation, segmented aerogel
Cherenkov counters and time-of-ﬂight counters
are used. The aerogel Cherenkov counter (AC) is a
threshold-type Cherenkov detector using aerogel,ARTICLE IN PRESS
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M. Sekimoto et al. / Nuclear Instruments and Methods in Physics Research A 516 (2004) 390–405

φ2590
φ2120

φ1760

Beam

130

830

130

844

911

3980

Beam

2320

907

1522
1720

2. Spectrometer magnet and tracking devices

coil

2120

whose refractive index is 1.034, to separate kaons
from pions in the momentum region of 0.53 to
1:88 GeV=c [5,6]. Since we cannot count beam
protons particle by particle, due to the highintensity, the event time-zero is given by starttiming counters (STC) placed at 380 mm from thetargets. The forward time-of-ﬂight counters(FTOF) are used to measure the time of ﬂight ofparticles in the kaon arm acceptance. They are also
used in combination with hodoscope counters(HC) to determine the charge and the crude
momentum of the particles, and to make kaon
selection possible by b ðv=cÞ measurements in thetrigger.
In the following section, details concerning theexperimental components and their performanceare described.

2.1. Magnet
1950

2.2. Target holder
We use three or ﬁve targets aligned in-line. The
typical conﬁguration is a set of carbon, polyethylene and copper, with each thickness being about
0.05%–0.1% of an interaction length and 0.2%–
0.5% of a radiation length. These targets areplaced at the center of the magnet, and typically
44 mm apart from each other. The target holder isrotated by 180 about the vertical axis every 12 h
to minimize the position dependence of theacceptance for each target position.
To decrease the interaction of the beam protons
with air, the entire beam line is kept as vacuum

1950
830

The spectrometer magnet is a dipole type withan overall weight of 300 tons, which was recycledfrom an old cyclotron magnet. The dimensions are
5655 mm in width, 3980 mm in height and
2120 mm in depth. The gap between the poles is
907 mm: Other dimensions can be seen in Fig. 3.
The magnet provides a ﬁeld integral of 0:81 Tmfrom the center to 1600 mm in radius, where BDCs
are located. At the center of the magnet, the ﬁeld
strength is 0:71 T:

2731

2664
5395
5655

Fig. 3. Schematic view of the spectrometer magnet.

from the extraction point to the entrance of thevertex drift chamber, where the region of vacuum
is terminated with a 200-mm-thick Mylar sheet.
Downstream of the vertex chamber, a helium-ﬁlled
beam pipe (5:5 m long) follows.
2.3. Cylindrical drift chamber
A schematic view of the CDC is shown in Fig. 4.
The outer diameter of the CDC is 1760 mm; theinner diameter is 800 mm and the height is
840 mm: The CDC is located on the pole pieceof the spectrometer magnet. The acceptance of
CDC is from 712 to 7132 horizontally and723 degrees vertically.
The CDC consists of 10 radial layers of drift
cells, which are grouped into three super-layers.
The ﬁrst super-layer is in the radial region from

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TOP View

Ceramic
Parts

Guard Wirewire
regionSense Wire

393
Potential Wire

10.47 mmX’-layer
X-layer
R = 800
U-layertarget7.89 mmX’-layer
X-layerbeam
SIDE View

to Amp Discri.

V’-layerR = 602.5

V-layer
5.8 mmgas

840 mmU-layer
R = 445

X’-layer
X-layerHigh Voltage
1760 mmFig. 4. Schematic view of the CDC. Ceramic plates (60 in total)
are supported by an aluminum structure (see Section 2.6).

445.0 to 475:0 mm; having three layers of drift cells
ðX ; X 0 ; UÞ; the next is from 602.5 to 642:5 mm;
having four layers ðV ; V 0 ; X ; X 0 Þ; and the last isfrom 800.0 to 830:0 mm having three layers
ðU; X ; X 0 Þ: In the X and X 0 layers, the directionof the wires is vertical, and in the U and V layers
the wires are tilted by about 70:1 radian: An
argon(50%)–ethane(50%) mixed gas is used at
1 atm:The structure of the drift cells is shown in Fig. 5.
All of the drift cells of the CDC have the same
horizontal angular coverage of 1:5 with respect tothe targets. For the inner-most super-layer, a highvoltage of 2:1 kV is applied to the potential wires
and 1:37 kV to the guard wires. For the next
super-layer and the outer-most super-layer, highvoltages of 2:2 and 1:43 kV are applied to thepotential and guard wires, respectively. The sense
wires are kept at the ground level. A typicalresolution of 350 mm is obtained.

Fig. 5. Cell structure of CDC.

The structure of the CDC was designed torestrict its active region to only the spectrometer
acceptance. The top and bottom end plates have a
step-like structure with three stages, as shown in
Fig. 4. This structure is effective to gain mechanical strength, and thus to minimize the support
structure within the spectrometer acceptance.
2.4. Barrel-shaped drift chambers
The structure of the BDC consists of 90 arcshaped endplates at the top and bottom, which aresupported by three aluminum bars, as shown in
Fig. 6. The inner radius is 1570 mm; the outerradius is 1680 mm and the height is 1420 mm: The
acceptance of the BDCs are the same as the CDC
for the vertical coverage, but with a smaller
horizontal coverage from 77:5 to 794:5 : The
BDC have four layers of drift cells with an XX 0 UV
conﬁguration, located at r ¼ 1600 to 1650 mm: In
the X and X 0 layers the direction of the wires isvertical, and in the U and V layers the wires are

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394

wire region
110
Top
View

applied to the potential wires, þ0:1 kV to theguard wires and þ1:5 kV to the sense wires, inorder to reduce the HV leakage current from thepotential wires due to the tight assembly. A typical
position resolution of 350 mm has been obtained.

90 degree1570

2.5. Vertex drift chamber

6 degreeBeam axis

Side
View

1420

Fig. 6. Schematic view of the BDC.

Guard Wire

Sense Wire

Potential Wire

10.41 mm

A schematic view of the VTC is shown in Fig. 8.
The VTC was installed to give tracking information in the vicinity of the beam and the targets.
A signiﬁcant beam halo of about 100 kHz with a
120 mm long and 1 mm wide scintillator has been
observed at 20 mm away from the beam. To makethe chamber operational under such a circumstance, special cell arrangements as shown in
Fig. 8, have been chosen. The forward three layers
cover the horizontal region from 76 to 724 at
r ¼ 200 mm and the backward three layers coverfrom 718 to 7141 at r ¼ 100 mm: All of thelayers have a hexagonal cell structure, as shown in
Fig. 9, in order to minimize the cell size. In the
backward layers, the drift length is 2:62 mm; andin the forward layer it is 1:75 mm: In the typical
data-taking condition, the maximum hit rates perwire are 100 kHz in the backward cells and 300
kHz in the forward cells. As the chamber gas, anargon(50%)–ethane(50%) mixture is used throughan ethanol bubbler at room temperature. It was

V-layerBe

U-layerX’-layer
X-layerFig. 7. Cell structure of BDC.

tilted by about 70:1 radian: The same gas mixture
as that for the CDC is used.
The structure of the drift cells is shown in Fig. 7.
All of the drift cells of the BDC have the same
horizontal angular coverage of 0:75 with respect
to the target position. The size of the drift cells of
the BDC is the same as that of the outer superlayer of the CDC. A high voltage of 0:7 kV was

200mm

R = 1600

am

44

5m

m

m

0m

32

Fig. 8. Schematic view of the VTC. The typical target
arrangement is also shown.

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Guard wires
R = 209.09 mm

Field wires
Sense wires

2.02 mm

3.49 mm

R = 200 mm
R = 196.97 mm

0.5 degrees

Fig. 9. Cell structure of the forward layer of the VTC. The
backward layer has a similar structure, but with a 1.5-times
larger scale.

found that without ethanol the chamber could notwork at such high rates. The typical position
resolution is 280 mm: The frame of the VTC is
made of aluminum with dimension of 445 mm
long, 320 mm wide and 200 mm high. To minimize
the material in the spectrometer acceptance, thetop and bottom end-plates are supported only at
the upstream side.

395

made of gold-plated brass of a 1:6 mm diameter
pipe. To secure electrical connection between thepipes and the circuit pattern, cut-ring springs wereinserted between the feedthroughs and the throughholes. The ceramic board on the top provides
signal connections to the sense wires, and those on
the bottom provide high-voltage connections. The
ceramic boards are 100 mm in width, 200 mm inlength and 25 mm in thickness, and are locatedprecisely on the end plates by knock pins with amachining precision of about 10 mm: In between
the ceramic boards there are the ribs of thealuminum end plate. Feedthroughs made of Delrin(acetal resin), as shown in Fig. 10(b), are used forwires on the ribs. They are also used as the
feedthroughs in the BDC.
In the VTC, to minimize the drift-cell size,
thinner feedthroughs are used. In the backward
cells, 2:9 mmf NORYL (degeneration PPE resin)
feedthroughs are used with a pitch of 3:02 mm(Fig. 10(c)). In the forward cells, all of the cathodewires are grounded through metallic feedthroughs
which are directly mounted on the end-plate(Fig. 10(d)). For the anodes, NORYL feedthroughs as backward cells are used. A positivehigh voltage is applied, and the signals are read out
through AC coupling.

2.7. Readout electronics
2.6. Wire support structure
All of the chambers use gold-plated tungsten of
30 mmf for the sense wires and Be–Cu of 100 mmf
for the potential wires. These wires are supported
by feedthroughs with a bush inserted at the end.
We use bushes with an 80 mmf hole for the sense
wires, so that the precision of the wire positioning
is about 25 mm: For potential wires bushes with a
200 mmf hole are used.
Different types of feedthroughs have been
introduced for each chamber. The end plates andthe support structure of the CDC are made ofaluminum. Thirty ceramic plates are mounted oneach aluminum end plates, as shown in Fig. 4. The
ceramic plates have a print-circuit pattern, which
provides electrical connections to the feedthroughpins, as shown in Fig. 10(a). The feedthroughs are

The read-out electronics of the CDC consists ofa pre-ampliﬁer, a post-ampliﬁer, a discriminatorand a time-to-digital converter (TDC). The preampliﬁers are mounted on the CDC. Radeka typepre-ampliﬁers, which are charge-sensitive bipolar
ampliﬁers with a gain of 200 mV=pC; are used [7].
The output signals of the pre-ampliﬁer are sent toa post-ampliﬁer via 10 m-long twisted-pair cables.
The gain of the post-ampliﬁer is 10, and the output
is discriminated to the ECL logic signal andtransfered to the counting house via 50 m-longtwisted-pair cables.
Those pre-ampliﬁers were lately replaced byampliﬁer–shaper–discriminator ASICs developed
for the ATLAS experiment [8], in order togain a threshold margin for the noise andoscillation.

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M. Sekimoto et al. / Nuclear Instruments and Methods in Physics Research A 516 (2004) 390–405

(a)

(b)

(c)

(d)

Fig. 10. Feedthrough structure: (a) for ceramic endplates (CDC), (b) for aluminum endplates (BDC), (c) and (d) for the backward andthe forward VTC cells, respectively.

3. Electron identiﬁcation counters3.1. Start timing counter
The time zero of an event is deﬁned by the starttiming counters (STC). We located STC 380 mmfrom the center of the spectrometer, covering from
712 to 760 in the horizontal angle and 728 inthe vertical angle. In each arm STC was segmented
into eight units horizontally.
The scintillator of STC is made of Bicron
BC404. The dimensions of the scintillator segment
are 400 mm in height, 40 mm in width and 5 mmin thickness. The scintillation light is transferedthrough light guides to a pair of photomultipliers
attached to the top and bottom ends. The
photomultipliers are H6154 of Hamamatsu Photonics; the diameter of the photocathode is 2 inches

and the structure of the dynodes is a ﬁne-meshtype. They can be operated under a magnetic ﬁeldof 0:71 T with a typical gain of 1  106 : They aremounted with an angle of 30 degrees with respect
to the ﬁeld direction to obtain the maximum gain.
Since the coincidence rate of the top and bottom
photomultipliers has reached 1 M counts per spillin the forward segments, the high-voltage breeder
was modiﬁed to supply a sufﬁcient current to thelast three dynodes. The typical breeder current is
0:4 mA; which has been increased to 4 mA at thelast three dynodes. Discriminated signals fromthe top and bottom photomultipliers are fed into
the mean timer and provide the timing of thesegment. The timings of all the STC segments areadjusted to within 1 ns: These signals are OR-ed
and used as the event time-zero signal. In an ofﬂine
analysis, a time resolution of about 300 ps has

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been achieved. The obtained time resolution islimited due to the high rate of operation and thelower gain of the PMT in the magnetic ﬁeld.
3.2. Front and rear gas-Cherenkov counter
The front gas-Cherenkov counters (FGC) coverfrom 712 to 790 horizontally and 723
vertically. They are horizontally segmented into
13 units in each arm so that one segment covers 6 :The rear gas-Cherenkov counters (RGC) cover712 to 754 horizontally and 76 verticallywith seven horizontal segments in each arm. The
cross-sections of FGC and RGC are schematicallyshown in Fig. 11, together with the light-collection
scheme.
Iso-butane gas with a refractive index of 1.0019(at STP) is used as a Cherenkov photon radiator.
The threshold momentum for pions is 2:3 GeV=c:Since the path length of particles in the counters israther short, we use a gas having a high refractive
index. Another merit of iso-butane was its
transparency for ultra-violet light down to awavelength of 220 nm: It is important for thedetection of Cherenkov light because the intensityof Cherenkov light is higher at a short wavelengthwith a 1=l2 dependence. In FGC, radiated
RLG
FGC

3" FineMesh PMT
Winston funnel

Winstonfunnel

2nd reflection
mirrorreflection
mirror

1st reflection
mirror

700 mm397

Cherenkov photons are reﬂected twice on ﬂatand arc-shaped mirrors, while in RGC, photons
are reﬂected once on the arc-shaped mirror.
To suppress multiple scattering, it is required touse thin mirrors. For the ﬂat mirror we use
0:05 mm aluminized Mylar supported by a honeycomb backplane made of paper. Arc-shaped
mirrors were made with a 3 mm-thick acrylic
plastic, on which aluminum and SiO were successively evaporated. Light is also reﬂected at thewalls for segmentation. The walls are MgF2 coated double-sided aluminized Mylar. The reﬂection efﬁciency is more than 88% for wavelengths
down to 200 nm:
The Cherenkov photons are focused with a
Winston funnel [9] attached to the photomultipliers. Since the photomultipliers of the FGCs areoperated under a magnetic ﬁeld of about 0:2 T; we
use R5542 of Hamamatsu Photonics, which has 19
stages of ﬁne-mesh dynodes. For the detection of
Cherenkov photons, its window is made of UVtransparent glass with a diameter of 3 inches. For
RGC we use R1652 of Hamamatsu Photonics witha diameter of 3 in; together with a m-metal shield.
The typical number of observed Cherenkov
photo-electrons is 3.5 in the FGC and 3.9 in the
RGC, under the typical data taking condition. The
efﬁciency uniformity of the Cherenkov counters
are plotted in Fig. 12. We had to increase thethreshold of FGC to reduce the trigger rate causedby pions, so that the electron efﬁciency of FGC
was sacriﬁced especially at larger horizontal
angles. Average electron efﬁciencies are 83% and
93% for FGC and RGC, respectively, while thepion rejections are 1%(FGC) and 2%(RGC).
Electron efﬁciencies are obtained by using puriﬁedeþ or e samples which do not participate in thetrigger decision but belong to the zero mass peakof eþ e pairs from Dalitz decays or gamma
conversions.
3.3. Lead–glass EM calorimeters

RGC

Fig. 11. Schematic view of FGC, RGC and RLG. Typicalparticles trajectories and Cherenkov light paths are shown.

The rear lead–glass EM calorimeters (RLG)
cover the same horizontal angle as the RGCs with
12 segments in each arm, but vertically coveroutside the kaon-arm acceptance, from 75 to723 ; as shown in Fig. 11. Inside the kaon arm

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M. Sekimoto et al. / Nuclear Instruments and Methods in Physics Research A 516 (2004) 390–405

Fig. 12. Uniformity of electron efﬁciencies of Front and Rear Gas Cherenkov counters, the left ﬁgure for the vertical angles and theright for the horizontal angles of particles incident to the detectors.

acceptance, the forward lead–glass EM calorimeters (FLG) are located behind the kaon
identiﬁcation counters. The side lead–glass EM
calorimeters (SLG) cover 757 to 790 horizontally and 723 vertically with nine horizontal
segments in each arm. The cross-sections of thecalorimeters are schematically shown in Fig. 13.
The calorimeters are made of SF6W lead–glass
blocks, which have a size of 330 mm height,
120 mm width and 110 mm thickness. These
blocks were recycled from the EM calorimeter of
TOPAZ of TRISTAN at KEK [10]. To cover alarge region with a small number of segments,
unusual arrangements are adapted, as shown in
Figs. 11 and 13. Cherenkov photons from an EM
shower are transported vertically by the total
reﬂection to the photomultipliers attached at theend of the block. The PMTs with a diameter of
3 in are used, Hamamatsu R1652 for RLG and
FLG and Hamamatsu H1911 for SLG.
For SLG and FLG, we reshaped and repolished the blocks in Tochigi Nikon Corporation
into a rectangular shape from the original tapered
shape. Five(three) units of the lead–glass blocks
for SLG(FLG) were stacked vertically and glued
with epoxy resin to form one segment of
SLG(FLG). The one segment of SLG(FLG) hada size of 1650ð950Þ mm in height, 120 mm in widthand 110 mm in thickness.
The light attenuation length through the total
reﬂections is about 100 cm; as can be seen in
Fig. 14. The sum of both ends, however, wasalmost position-independent. The loss of light at

Winston 3" Fine Meshfunnel PMT
FGC

EM shower

2nd reflection
mirror

1st reflection
mirror

700 mmSLG
Fig. 13. Schematic view of FGC and SLG. Typical particle
trajectories and Cherenkov light paths are shown.

the glued boundaries was small enough comparedto the typical energy resolution.
Although the effective thickness for the EM
shower-development is only 6.5 radiationpﬃﬃﬃﬃ lengths,a typical energy resolution of 15%= E has beenobtained for electrons. Fig. 15 shows the resolution of RLG as a function of electron momentum.

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4.1. Time-of-flight counters(arbitrary)

2

1

0

0

20

40

60

80

100

120

140

(cm: from TOP PMT)

Fig. 14. Position dependence of the SLG pulse height: closed
circles for the top, open squares for the bottom, and closed
squares for the sum of the photomultipliers.

20

dE/E (%)

15399

E (GeV)
2

10
1
5

0

P (GeV/c)

1

2

0.6

0.7

0
0.5

0.8

Momentum (GeV/c)

Fig. 15. The energy resolution of RLG
pﬃﬃﬃasﬃ a function of
measured momentum. The curve is 15%= E dependence. The
correlation between RLG energy and track momentum isinserted.

The inserted scatter plot shows the correlation of
the track momentum and the calorimeter energy,
which is obtained by the puriﬁed electron samples
as explained in the previous section.

4. Kaon identiﬁcation counters
Kaon identiﬁcation is performed by the combination of time-of-ﬂight measurements and threshold-type silica-aerogel Cherenkov counters.

The start timing is given by STC, as iscommonly used for electron detection. Hodoscopecounters (HC) are used to determine the sign of acharged particle and its rough momentum in
combination with the forward time-of-ﬂight counters (FTOF). Signals from HC are used in the online ﬁrst-level and second-level kaon-triggers, as
described in Section 5.
The dimensions of the HC scintillator are
660 mm in height, 70 mm in width, and 5 mm inthickness. Two segments located most backward
have a different width of 200 mm: They arearrayed with an overlap of 5 mm: They coverfrom 712 to 754 in the horizontal angle,
and 76 in the vertical angle, and are placed
2632 mm from the center of the spectrometer.
Each segment is read by a pair of photomultipliers
from the top and bottom ends through lightguides. The photomultipliers are H1161 of Hamamatsu Photonics with a photocathode diameterof 2 in:
The FTOF is used to measure the ﬂight time ofcharged particles together with STC. Particle
identiﬁcation is performed with this information
together with the ﬂight length and the momentum
given by the track ﬁtting. The FTOFs are locatedat the most-downstream part of the kaon arm.
They cover from 712 to 754 in the horizontal
angle, and 76 in the vertical angle. The distancefrom the center of the spectrometer to the center of
the FTOF array is 3951 mm: The FTOF consistsof 16 segments in each arm. Two types ofscintillator slabs are used: one is 800 mm in height,
200 mm in width and 30 mm in thickness; theother is 800 mm in height, 170 mm in width and
20 mm in thickness. They are arrayed alternatively
with an overlap of 2 mm: The scintillation light isread out with a pair of photomultipliers throughlight guides mounted on both ends of the slabs.
The photomultipliers are H1949 of Hamamatsu
Photonics with a photocathode diameter of 2 in:This photomultiplier is specialized for time-ofﬂight measurements with a high gain of 2:0  107
and a typical signal rise-time of 1:3 ns: The
resolution of the time-of-ﬂight measurements is
360 ps together with STC.

ARTICLE IN PRESS400

M. Sekimoto et al. / Nuclear Instruments and Methods in Physics Research A 516 (2004) 390–405
2

M kaonpositive particles

squared mass

2

[ GeV ]

2

M kaonnegative particles

squared mass

2of the transparency. Recent innovative work by
the KEK-BELLE PID group and Matsushita
Electric Works has established a new method to
produce hydrophobic silica aerogel [5]. The
products have been found to typically have a
2.5-times longer transmission length than the
traditional ones.
The ACs are located just in front of the FTOF
counters; both of them have geometricallymatched 16 segments in each arm. When a hit in
an FTOF segment is associated with a hit in eitherof three AC segments in the front, it is considered
to be a pion having a momentum above
0:53 GeV=c:
The ACs cover 712 to 754 in the horizontal
angle and 76 in the vertical angle. Each of ACs
has an effective area of 2640 mm in width and706 mm in height. The dimensions of one segment
are 160 mm in width and 706 mm in height.
Radiated Cherenkov photons are read out asshown in Fig. 17 by a pair of photomultipliers[ GeV ]
Winston funnel

Fig. 16. Distribution of the mass square for particles whose
momenta are less than 2:0 GeV=c without an associated hit in
the aerogel Cherenkov counters. The hatched areas represent
contaminations from protons and pions.

The squared-mass distributions are shown in
Fig. 16 for particles whose momenta are less than
2:0 GeV=c: The over-plotted hatched histograms
represent the contaminations of protons andpions. The shape of the kaon peaks is given by
convoluting the time resolution over the momentum distributions. The cut region of the kaon
identiﬁcation was from 0.04 to 0:49 GeV2 ; asshown in Fig. 16 by the arrows. With thisselection, the kaon identiﬁcation efﬁciency was
96%, which contained 6% of the miss-identiﬁed
particles. The purity of Kþ K pairs was estimatedto be 90%.

particle

Reflection Mirror

Aerogeln = 1.034

4.2. Aerogel counters
To trigger kaons, we use threshold-type aerogel
Cherenkov counters (AC). Silica aerogel has been
widely used for kaon triggers. The ones used informer experiments were hydrophilic, so they had
to be kept dry in order to avoid any deterioration

1000 mmFig. 17. Schematic view of the aerogel Cherenkov counters.
Typical particle trajectories and Cherenkov light paths areshown.

ARTICLE IN PRESS

through an optical system made with cylindricalconcave mirrors and Winston funnels [9] attachedto each photomultiplier.
The equipped photomultipliers are single-photoelectron sensitive H6527 of Hamamatsu Photonics
with a photocathode diameter of 5 in: They have
14 dynodes of the linear-focusing type. They areoperated with a typical gain of 1:4  107 :We examined the performance of ACs using
data triggered by requiring any two charged
particles. Since the Cherenkov angle for b ¼ 1
particles is 0:257 radian; those tracks whose
vertical position was within 7120 mm from thecenter made hits in both the top and bottom
photomultipliers, and other tracks made hits onlyat either side. The threshold level was set at 0.5
photoelectrons for the sum of the two photomultipliers.
The AC efﬁciency as a function of the pion
momentum was obtained as shown in Fig. 18. For
those pions with a momentum greater than
1:0 GeV=c; the vertical position dependence of
the AC efﬁciency is plotted in Fig. 19 which
demonstrates a uniform efﬁciency over the kaonarm acceptance.
The details of the mass production of theCherenkov radiators, the construction and theperformance of the ACs are described in ourearlier publication [6].

100efficiency [%]

80

60

40

20

0
0

0.5

1

1.5

2

pion momentum [GeV/c]
Fig. 18. Pion momentum dependence of the AC efﬁciencies.

efficiency [ %]M. Sekimoto et al. / Nuclear Instruments and Methods in Physics Research A 516 (2004) 390–405

401

100

90
for negative pions
80-400 -300 -200 -100 0 100 200
track vertical position [mm]

300

400

Fig. 19. Vertical position dependences of the AC efﬁciencies forpions whose momentum was larger than 1:0 GeV=c: The arrows
indicate the kaon-arm acceptance.

5. Trigger and data acquisition
We apply the three-level trigger logic for eþ eevents and two-level logic for Kþ K events. Whenthe higher trigger conditions are satisﬁed, thetypical dead time of the data taking is 600 ms torecord an event. Otherwise, the dead time is
reduced to 50 ms by a fast-clear signal.
5.1. Electron trigger
For the ﬁrst level of the electron trigger, weselect electron-pair candidates using the coincidence signal of the front-stage (FGC) and the rearstage detectors (SLG, RLG, RGC), by requiringhorizontal position matching. The hit signals ofeach segment are deﬁned as an ORed signal of the
top and bottom PMTs. The size of the matchingwindows corresponds to a momentum range
greater than 0:4 GeV=c: To suppress electron pairs
with a small opening angle, such as from Dalitz
decays and g conversions, we require the two FGC
hits to be more than two segmentations apart. The
typical trigger rate of the electron ﬁrst level is
640=spill:In the second level, we require the pair to be
oppositely charged by using drift-chamber hits
associated with the FGC hits. The hit positions in
the chambers are obtained from the OR-ed signalof the X and X 0 layers of the BDC and of the outersuper-layer of the CDC. The sign of a track can beroughly determined by these segmentations together with the target position. We eliminate any
electron pair candidates having an apparent ‘‘þþ’’

ARTICLE IN PRESS
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M. Sekimoto et al. / Nuclear Instruments and Methods in Physics Research A 516 (2004) 390–405

or ‘‘ ’’ conﬁguration. We also require theremained pairs to be more than 12 degrees apartat the outer super-layer of the CDC ðr ¼ 825 mmÞ
to suppress electron pairs with a small opening
angle. This logic is implemented with hand-mademodules using Field Programmable Gate Arrays(FPGA, Lattice ispLSI 3256). The typical trigger
rates of the electron second level is 550/spill.
In the third level, the approximate opening
angles of the pairs are calculated. The radius of the
BDC layers is almost twice larger than the radiusof the outer super-layer of the CDC, so that the
opening angle of the pair at the target, Yopen ; canbe approximated by Yopen ¼ 2  Ycdc  Ybdc ;where Ycdc ðYbdc Þ is the difference in the horizontal angle coordinate of the pair at the outersuper-layer of the CDC (at the BDC layer). We
require the Yopen to be in the range of 50 to 150in the trigger. This logic is implemented with thesame modules as the second-level trigger. The
typical trigger rate of the electron third level is
430/spill.

Typical kaon ﬁrst-level trigger rate is 40 K=spill
and it is down-scaled by about 20 to keep thetrigger live time at a level of 70–80%. Afterthe second level trigger, the kaon trigger rate is
400–500/spill. The trigger rejection at the ﬁrst
level is limited by low energy pions below
the silica aerogel threshold. This determined
our statistical limitation in the kaon pair channel.

6. Performance of the spectrometer
Overall performance of the electron identiﬁcation of the present spectrometer is demonstrated inFig. 20, as the energy spectrum of the lead glass
calorimeter (RLG) after requiring the electron
candidate in the front gas Cherenkov counter(FGC). By requiring the energy–momentum
matching in the calorimeter the cross-hatchedregion are selected as electron candidates. The
remained part of the energy spectrum is well
reproduced by the calorimeter response to pions

5.2. Kaon trigger
250

200

countsThe kaon ﬁrst-level trigger requires at least two
FTOF hits which are not associated with geometrically matched AC hits. To select tracks whose
momentum is larger than 0:4 GeV=c; certain
combinations of these AC-vetoed FTOF signals
and the HC signal are selected using a matrix
coincidence logic. To invoke an event trigger, atleast one STC hit both in the left and right arms is
also required.
The kaon second-level trigger uses ﬂight-time
information from the FTOF counters and the hit
information of the HC counters. A timing window
for kaons is determined for each FTOF–HC pair.
We require at least two FTOF–HC hit pairs whose
ﬂight time is inside the kaon window and thecharges are at least one positive and one negative.
The kaon second-level logic is implemented byusing FPGA (XILINX XC3190A) modules communicating with the TDC modules. The logic
reduces the trigger rate to about 32% and thedecision time to less than 100 ms; including theconversion time of the fast TDCs (LeCroy FERAFERET).

150

π data sample
RLG e-ID100

FGC e-ID

50

0

0

0.5

1

1.5

2

2.5

Energy (GeV)
Fig. 20. The lead–glass energy spectrum for the tracks
identiﬁed as electron in FGC. The cross-hatched region
corresponds to the tracks which pass energy–momentum
matching in the calorimeter. The shaded area is the energy
spectrum for pions and protons which is over plotted after
rescaling.

ARTICLE IN PRESS
M. Sekimoto et al. / Nuclear Instruments and Methods in Physics Research A 516 (2004) 390–405

and protons as the over plotted shaded area in theﬁgure.
The electron efﬁciencies and pion rejection
efﬁciencies are obtained and summarized inTable 1. By applying the two-stage cascade
Table 1
The typical electron identiﬁcation efﬁciencies and pion rejection
capabilities of the each detector components. The listed values
are after applying typical analysis cuts. Average values for thetwo-stage coincidence are also shown
Countere7

p7

Gas Cherenkov

FGC
RGC

0:82570:010
0:9370:011

0:014270:00006
0:021270:0008

Calorimeter

SLG
RLG

0:8770:03
0:9770:01

0:00970:001
0:03170:001

0:7870:01

0:0002770:00001

Two stage coincidence

Fig. 21. Momentum resolution for kaons and electrons,
obtained with a Monte Carlo calculation.

403

operation of the electron identiﬁcation counters,
the overall electron efﬁciency is 78% with a pion
rejection power of 3  104 : The pion contamination in the ﬁnal electron-pair sample for a physics
analysis is another key issue. The remaining e pþand eþ p background is estimated to be about
19% in the ﬁnal electron-pair sample with anegligible contamination of pþ p of about 1%.
The momentum resolution of the spectrometer
was estimated by a Monte Carlo simulation bytaking into account the tracking chamber resolutions together with energy losses and scatterings in
the detector materials. The results are shown in
Fig. 21. In the track ﬁtting, we impose theconstraint that two tracks from a pair should beoriginated from the same vertex. The effect of sucha constranit is also included in the ﬁgure.
The acceptance of the spectrometer for electron
pairs are shown in Fig. 22 as the functions ofinvariant mass, rapidity and pT : Those values are
obtained with use of a nuclear cascade code,
JAM[12], by also taking all the trigger effects into
account. We found that JAM describes observed
rapidity and pT distributions fairly well.
As another direct check of the performance ofthe spectrometer, the mass resolution was examined for the observed peaks of L-p þ p decay,
as shown in Fig. 23. For the L peak we obtained acentroid at 1115:570:03 MeV=c2 (known to be
1115:7 MeV=c2 ) with a Gaussian resolution of
1:870:1 MeV=c2 : The results were compared toMonte Carlo simulations in which we took the

Fig. 22. The mass (left), rapidity (center) and pT (right) acceptance of the present spectrometer after the analysis cuts.

ARTICLE IN PRESS
M. Sekimoto et al. / Nuclear Instruments and Methods in Physics Research A 516 (2004) 390–405

404

chamber resolution and the multiple scattering
into account. The observed widths were well
reproduced by the simulation (1:9 MeV=c2 for L).
According to the simulation, the mass resolution
in f-Kþ K was estimated to be 2:4 MeV=c2 ;o-eþ e to be 9:6 MeV=c2 and f-eþ e to be
12:0 MeV=c2 : Fig. 24 shows the eþ e and Kþ K
spectrum for a carbon target, where clear peaks ofo and f are observed. The estimated mass
resolutions were included in the ﬁtting of thepeaks. Discrepancies between the ﬁt result and theobserved data, as shown in Fig. 24, are closely
m = 1115.5MeV/c4600

σ = 1.8MeV/c2

400
300

7. Summary
A spectrometer was newly constructed at thehigh-intensity primary beam-line at KEK-PS todetect the f-eþ e ; r=o-eþ e and f-Kþ K
channels simultaneously. The construction ofexperiment E325 started in 1995. Data taking
was performed for 3200 h from 1997 to 2002. The
spectrometer has been successfully operated, resulting in a clear observation of f-eþ e innuclear reactions with the lowest beam energy inhistory. A data analysis for the earlier data (1998)
has been completed, and the deformation of themass spectra was observed, as reported in Ref. [3].
Further physics outcomes will be reported in near
future.

200

Acknowledgements

100
0
1.09

1.1

1.11

1.12

1.13

1.14

[GeV/c2][counts/bin]

Fig. 23. Invariant mass spectrum of L-pp : The curve is thebest-ﬁt result with a Gaussian and a linear background. The
vertex position are required to be more than 20 mm apart fromthe target.

We are grateful to the staff members of KEKPS. This work was partly supported by Japan
Society for the Promotion of Science and a Grantin-Aid for Scientiﬁc Research of the Japan
Ministry of Education, Culture, Sports, Scienceand Technology (MEXT).

[counts/bin][events/1MeV/c2]

500

related to our physics goal, and they should be
discussed elsewhere [11].

Fig. 24. Invariant mass spectra of the eþ e and Kþ K channels, obtained for a carbon target. The lines are the contributions of the
backgrounds and the known pair sources [11].

ARTICLE IN PRESS
M. Sekimoto et al. / Nuclear Instruments and Methods in Physics Research A 516 (2004) 390–405

References[1] KEK-PS E325 proposal, http://www.pn.scphys.kyoto-u.
ac.jp/phi/E325 project.html.
[2] T. Hatsuda, S.H. Lee, Phys. Rev. C46 (1992) R34.
[3] K. Ozawa, et al. (PS-E325 collaboration), Phys. Rev. Lett.
86 (2001) 5019.
[4] M. Takasaki, et al., KEK Internal 95-1.
[5] I. Adachi, et al., Nucl. Instr. and Meth. A 355 (1995)
390.
[6] M. Ishino, et al., Nucl. Instr. and Meth. A 457 (2001)
581.
[7] M.M. Morii, et al., KEK Preprint 88-80 November 1988.

405[8] O. Sasaki, KEK, Japan, ATLAS TGC Collaboration,
http://online.kek.jp/~sosamu/.
[9] H. Hinterberger, R. Winston, Rev. Sci. Instr. 37 (1966)1094.
[10] S. Kawabata, et al., Nucl. Instr. and Meth. A 270 (1988)11.
[11] M. Naruki, et al., Proceedings of the XVI Particle andNuclei International Conference (PANIC02), Nucl. Phys.
A721 (2003)297c;
M. Ishino, Memoirs of the Faculty of Science, Kyoto
Univ., Series of Physics, Astrophysics, Geophysics andChemistry, Vol. 42, No. 1 Article 1, 2001, to be published.
[12] Y. Nara, et al., Phys. Rev. C61 (1999) 024901.

E352 R. J. Petersen
Quasielasic π Meson Scattering
Y. Fujii et al.
Quasielastic "--nucleus scattering at 950 MeV/c
Phys. Rev. C64 (2001) 034608.
● R. J. Peterson et al.
Mesonic y scaling
Phys. Rev. C65(2002) 054601.
●________________________________________________________________________● Physics papers published in refereed journal.
○★
◇＊

Technical papers.
PhD theses.
Conference and Symposium.
Internal Report and others.

[attached][attached]PHYSICAL REVIEW C, VOLUME 64, 034608

Quasielastic ␲ À -nucleus scattering at 950 MeVÕc
Y. Fujii,1,* O. Hashimoto,1 T. Nakagawa,1 Y. Sato,1 T. Takahashi,1 J. T. Brack,2 C. J. Gelderloos,2 M. V. Keilman,2R. J. Peterson,2 M. Itoh,3 H. Sakaguchi,3 H. Takeda,3 K. Aoki,4 H. Hotchi,4 H. Noumi,4 Y. Ohta,4 H. Outa,4 M. Sekimoto,4
M. Youn,4 S. Ajimura,5 T. Kishimoto,5 H. Bhang,6 H. Park,6 and R. Sawafta7
1

Department of Physics, Tohoku University, Sendai 980-8578, Japan
Nuclear Physics Laboratory, University of Colorado, Boulder, Colorado 80309-0446
3
Department of Physics, Kyoto University, Kyoto 606-8502, Japan
4
High Energy Accelerator Research Organization (KEK), Ibaraki 305-0801, Japan
5
Department of Physics, Osaka University, Osaka 560-0043, Japan
6
Department of Physics, Seoul National University, Seoul 151-742, Korea
7
Physics Department, North Carolina A & T State University, Greensboro, North Carolina 27411
共Received 14 May 1999; revised manuscript received 31 May 2000; published 22 August 2001兲
2

Quasielastic scattering cross sections have been measured with a 950 MeV/c ␲ ⫺ beam on targets of 2 H,
Li, C, Ca, Zr, and 208Pb, over a range of three-momentum transfers from 350 through 650 MeV/c. Results forcarbon are compared to a finite-nucleus continuum random-phase approximation calculation including distortions. The pion spectra at our lowest range of momentum transfers show less scalar/isoscalar correlation than
predicted.
6

DOI: 10.1103/PhysRevC.64.034608

PACS number共s兲: 25.80.Ls

I. INTRODUCTION

Quasielastic scattering is a process in which an incident
particle elastically and incoherently interacts with only one
nucleon inside a nucleus, with all the other nucleons being
spectators. At intermediate energies this process dominates
the nuclear response, showing two characteristic features:
one is that the position of the peak corresponds to that ofelastic scattering by a free nucleon, and the other is that thewidth of the peak reflects internal motion of individual
nucleons in the target nucleus. Nuclear correlations can bestudied with this process, by measuring the distribution of
the quasielastic strength 关1兴. Quasielastic scattering of electrons has been extensively studied using their short wavelengths, deep penetration, and well-known couplings tonucleons. However, the coupling of electrons to nucleons isalmost entirely electromagnetic, and hadronic probes areneeded to explore the full set of spin and isospin couplings.
A recent example is the study of isovector spin-transverseand spin-longitudinal quasielastic scattering with the polarized (p,n) reaction 关2,3兴.
Mesonic probes can be used to study other couplings thanreached by lepton and baryon beams. Quasielastic noncharge
exchange 共NCX兲 scattering by pion and K ⫹ beams occurs
largely through scalar-isoscalar couplings, while pion singlecharge exchange 共SCX兲 acts through an isovector, largely
scalar, coupling. Both ( ␲ , ␲ ⬘ ) 共NCX兲 关4兴 and ( ␲ ⫾ , ␲ 0 )共SCX兲 关5兴 reactions were measured using pions of
624 MeV/c at LAMPF. The quasielastic peak positions forthe ( ␲ ⫾ , ␲ 0 ) reaction showed shifts toward higher energy
loss than for free ␲ ⫺ p scattering below about q⫽400 MeV/c, while no shifts were observed for the ( ␲ , ␲ ⬘ )reaction. This difference was tentatively explained as arisingfrom the difference in effective particle-hole interactions for*Electronic address: fujii@lambda.phys.tohoku.ac.jp0556-2813/2001/64共3兲/034608共8兲/$20.00

different channels. These studies demonstrated that pionquasielastic scattering can be used to study nuclear responses
in the scalar-isoscalar and scalar-isovector channels. At large
momentum transfers, however, the outgoing pions fromthose studies have such low energies as to be likely to interact again to form the prominent ⌬ resonance. A higher pion
beam energy is needed to avoid this complication and tomaintain a long mean free path within the nuclear medium.
The (K ⫹ ,K ⫹ ⬘ ) quasielastic reaction at 705 MeV/c 关6兴 was
measured at the Alternating Gradient Synchrotron 共AGS兲 at
Brookhaven National Laboratory, taking advantage of thesmall K ⫹ N cross sections to give a long mean free pathwithin nuclei to reach high densities of nucleons. Theoretical
calculations compared to these data were performed in a
finite-nucleus continuum random-phase approximation
共RPA兲 framework 关7兴. The calculation described the experimental results well and provided a constraint on the strengthof the effective particle-hole interaction in the scalarisoscalar channel. The model used in that calculation is also
applied below to the present data.
The present experiment is a study of quasielastic
␲ ⫺ -nucleus scattering at 950 MeV/c, principally to investigate the nuclear response using a scalar-isoscalar dominant
probe. An advantage of the present experiment at
950 MeV/c compared to the previous one at 624 MeV/c isthe greater predominance of the scalar-isoscalar channel. The
momentum transfer dependence of the spin/isospin content
of pion-nucleon scattering cross section at the beam momentum of 950 MeV/c is shown in Fig. 1. The ␲ ⫺ N cross sections in the four allowed spin/isospin channels are takenfrom the SM95 solution to SAID 关8兴, assuming charge symmetry. The fraction of the scalar-isoscalar channel is morethan 55% and up to 75% over a momentum transfer range
from 350 to 550 MeV/c, which our spectra will emphasize.
II. EXPERIMENT AND DATA ANALYSISThe experiment was performed at the 12 GeV proton synchrotron of the High Energy Accelerator Research Organiza64 034608-1©2001 The American Physical Society

Y. FUJII et al.

PHYSICAL REVIEW C 64 034608

FIG. 1. The q dependence of the spin/isospin content of thepion-nucleon scattering cross section at a beam momentum of
950 MeV/c. The present experiment covers the range from 350
through 650 MeV/c. As elsewhere in this work, q is measured in
the laboratory frame.

tion 共KEK兲 关9兴. A negative-pion beam was delivered to thetarget by the K6 beam line, which was equipped with a DC
separator. The beam momentum was analyzed by a beam line
spectrometer, which comprises a QQDQQ magnet system,
three arrays of scintillator hodoscopes, and four sets of drift
chambers capable of tracking high-rate beam particles. The
momentum spread of the beam was ⫾1.2%. The typicalbeam intensity at the experimental target was 1.1⫻106
particles/spill, where the beam duration was 1.8 s and thespill repetition cycle was 4 s. The beam size 共rms兲 was typically 6.3 mm horizontal and 13.0 mm vertical.
Scattered pions were analyzed with the Superconducting
Kaon Spectrometer 共SKS兲 关9兴. It consists of a 11 MJ superconducting dipole magnet, four sets of drift chambers, arraysof time-of-flight 共TOF兲 scintillators, Aerogel Čerenkov
counters, and Lucite Čerenkov counters. The SKS was positioned at a fixed laboratory scattering angle of 30°. Since the
SKS has a large solid angle of 100 msr, it could cover anangular range of ⫾15°. The momentum acceptance of thespectrometer was ⫾20%. Four magnetic field settings 共860,
780, 720, and 630 MeV/c central momentum兲 were used tocover the entire quasielastic region, allowing good overlap
regions. The present setup covers momentum transfers from
350 to 650 MeV/c and energy loss up to 350 MeV, as shownin Fig. 2.
A veto plastic scintillator 共24 cm wide兲 was placed along
the beam direction 40 cm downstream from the scatteringtargets, subtending ⫾17°. This was used to reject the large
background from the pion beam not interacting with the scattering target but directly hitting a detector frame downstream. The veto counter cut into the most forward events
accepted by the SKS, so only pion scattering angles beyond
20° were analyzed. Comparing continuum spectra with andwithout the veto at the same low beam rate, the difference
was less than 3% across the quasielastic region. We conclude that use of this veto system did not affect the shape ormagnitude of our quasielastic spectra beyond 20°.

FIG. 2. Kinematics of the present experiment. The thin solid line
shows the region where measurements were made. The q-fixed lociare shown in the dashed lines. The thick solid line shows kinematics
of a quasielastic peak and the dotted lines show full width of aquasielastic peak calculated with a Fermi-gas model (k F
⫽221 MeV/c).

Natural isotopic targets of CH2 (3.5 g/cm2 ), C
(4.6 g/cm2 ), Ca (3.1 g/cm2 ), and Zr (3.4 g/cm2 ) and highly
isotopically enriched targets of C2 H2 (1.4 g/cm2 ),6 2
Li H (2.0 g/cm2 ), and 208Pb (4.6 g/cm2 ) were irradiated in
the present experiment. The size of the targets was 4.6 cm
wide and 4.1 cm high for the lead target and 10 cm wide and10 cm high for the other targets, more than 3 times as large
as the beam size. The targets were chosen to study the A
dependence of the quasielastic scattering over a wide rangeof nuclear sizes. The deuteron target was used to ensure that
we correctly account for scattering on both neutrons and protons.
Free ␲ ⫺ p scattering cross sections were measured with
the CH2 and C targets at several beam momenta. The differential cross sections were derived using an SKS acceptancefunction determined by a Monte Carlo simulation, in whichmeasured field maps, geometry and efficiencies of detectors,
and the beam profile measured during the experiment wereused. Figure 3 shows an example of the mapped acceptanceand the loci of elastic proton events at several beam momenta. These cross sections were compared to those fromSAID 关8兴 as shown in Fig. 4 and normalized to the SAID crosssections. The normalization factor was 1.05. The overall accuracy of this normalization was 5% for the quasielastic region, as determined by the comparison between the shapes ofthe measured and the SAID cross sections. In addition, theelastic scattering cross sections for carbon at 900 MeV/c arein agreement with those of Takahashi et al. at 895 MeV/c关10兴 within 15%. The systematic error of the cross sections
was estimated to be 11%, determined by the above agreement with SAID, the accuracy with which overlapping spectra
agreed 共never worse than 7%), and the effects of the vetocounter. We include no uncertainty for the ␲ ⫺ p cross sections of SAID, since any theoretical comparisons to our data
are likely to use these same cross sections.

034608-2

QUASIELASTIC ␲⫺-NUCLEUS SCATTERING AT 950 MeV/c

PHYSICAL REVIEW C 64 034608

angular resolution of 0.8°, to get enough statistical accuracy
events at a range of angles and outgoing energies.
Doubly differential cross sections for 2 H, 6 Li, C, Ca, Zr,
and 208Pb are presented as a function of energy loss ␻ for agiven momentum transfer bin, where ␻ was defined as thedifference between the initial and final laboratory kinetic energies of the scattered pions. Deuteron spectra were extracted
by subtracting the C spectra from C2 H2 spectra after normalization. The 6 Li spectra were similarly obtained from 6 Li2 H
and deuteron spectra.

III. RESULTS
A. Doubly differential cross sections
FIG. 3. Effective solid angle of the SKS for the highest SKS
field setting (860 MeV/c central momentum兲, covering most of thequasielastic spectra as determined by ray tracing through the measured field profile. The loci of elastic proton events at several beam
momenta used for checks of the normalization are shown as dottedlines.

The energy resolution was 3.0 MeV 关full width at half
maximum 共FWHM兲兴, as determined by the carbon elastic
peak. This is good enough for quasielastic scattering studies.
Events were collected into momentum transfer bins of full
width 25 MeV/c or about 1.8°, which was wider than the

Figure 5 shows the doubly differential cross sections for
H, C, and 208Pb at fixed q of 350, 500, and 650 MeV/c.
The measured spectra clearly show the characteristic shapeof a quasielastic peak, centered near the energy loss corresponding to that of free ␲ ⫺ N elastic scattering 共125 MeV atq⫽500 MeV/c) and broadened due to the internal motion ofindividual nucleons inside the nucleus. These features arefamiliar from electron scattering continuum spectra, which,however, require an awkward radiative unfolding procedure
not required in the pion scattering.
The observed spectra were fitted by a sum of three components: a quasielastic peak, a background, and additional
Gaussian peak共s兲 for elastic and inelastic scattering. For the
nuclei other than the deuteron, an asymmetric Gaussian withexponential cutoff factor C( ␻ ) was used to describe eachquasielastic peak as follows:2

f qe共 ␻ 兲 ⫽再冋 冉 冊册
冋 冉 冊册

c 1 C 共 ␻ 兲 exp ⫺1 ␻ ⫺c 2 2
2
c3

if

␻ ⭐c 2 ,c 1 C 共 ␻ 兲 exp ⫺1 ␻ ⫺c 2 2
2
c4

if

␻ ⬎c 2 ,再 冉
0C共 ␻ 兲⫽1⫺exp ⫺␻ ⫺ ␻ elc5

冊

if ␻ ⭐ ␻ el ,if ␻ ⬎ ␻ el ,2
␻ el⫽ 冑q 2 ⫹M tgt⫺M tgt ,FIG. 4. Differential cross sections for ␲ ⫺ p elastic scattering
measured with the highest SKS field setting. The open circles show
the cross sections before normalization to the SAID cross sections.
The solid lines show the results of SAID.

共1兲

共2兲

共3兲

where ␻ el is the energy loss due to target recoil and M tgt is atarget mass. This formalism was introduced by Erell et al.
关11兴. The cutoff factor, C( ␻ ), describes the suppression due
to Pauli blocking. In addition, in the case of fitting calcium
spectra, an additional Gaussian was included to describe acontribution from a small hydrogen contamination. The
width and central energy of the additional Gaussian were
fixed to those of the ␲ ⫺ p scattering data.
For the deuteron spectra, a shifted Lorentzian distribution
was used to describe each quasielastic peak:

034608-3

Y. FUJII et al.

PHYSICAL REVIEW C 64 034608

FIG. 5. Doubly differential cross sections for
H, C, and 208Pb at fixed q of 350, 500, and
650 MeV/c. The error bars show statistical uncertainties only. The lines show the fitted quasielastic peak 共solid兲, contribution from nonquasielastic process 共dashed兲, and peak共s兲 forelastic and inelastic scattering 共dotted兲.
2

f qe共 ␻ 兲 ⫽冦

0if ␻ ⭐ ␻ el ,

c1

冋

1
共 ␻ ⫺ ␻ el⫺c 2 兲 ⫹c 3
2⫺

1
共 ␻ ⫺ ␻ el⫹c 2 兲 2 ⫹c 3

if ␻ ⬎ ␻ el .

册共4兲The original form 共without ␻ el) was introduced by Esbensen
and Bertsch 关12兴 to parametrize a free response of a semiinfinite slab model.
There will also be contributions to the strength under thequasielastic peak from nonquasielastic processes: multiple
scattering 关7兴, two-particle–two-hole (2p-2h) excitations
关13兴, and pion production 关14兴, for example. In the present
analysis, these contributions were subtracted as ‘‘background,’’ assuming the shape as a linear function of ␻ ,f bg共 ␻ 兲 ⫽

再

0

if

␻ ⭐ ␻ el ,c 0 共 ␻ ⫺ ␻ el兲if␻ ⬎ ␻ el .

共5兲Each of the spectra in Fig. 5 shows examples of the fitting. The spectra were well fitted by the present fitting functions. Then, cross sections and the centroids and widths of
the quasielastic peaks were deduced. The quasielastic crosssections for calcium were deduced from the extracted crosssections, which include a small correction from an oxygen
contaminant, assuming from the observed H peak some contaminant of Ca(OH) 2 .
The model uncertainty of the fits due to the ‘‘background’’assumption was estimated by also ascribing the ‘‘background’’ only to ⌬ production. A Breit-Wigner peak at theenergy loss corresponding to ⌬ production was assumed andfolded with the quasielastic peak shape in the fitting. For the
carbon data at q⫽500 MeV/c, this procedure gives changesof ⫹15% to the quasielastic cross section, ⫹6% to thewidth, and ⫹2 MeV to the centroid. These changes can betaken to represent a systematic uncertainty in the effects ofthe continuous background under the quasielastic peak. At
the largest q of 650 MeV/c, these changes increase to⫹40%, ⫹10%, and ⫹5 MeV, respectively. Error bars from
Figs. 6 to 9 include all the systematic errors.
B. Quasielastic cross sections

Figure 6 shows the extracted quasielastic cross sectionsfor ␲ ⫺ -nucleus scattering as a function of momentum transfer. The solid lines show fitted results to a noninteracting
Fermi-gas model,

冉 冊

d␴
d␴
,
⫽A effB 共 q,k F 兲d⍀d⍀ ␲ N

共6兲

with a parameter A eff . B(q,k F ) is the Pauli blocking factor

034608-4

QUASIELASTIC ␲⫺-NUCLEUS SCATTERING AT 950 MeV/c

PHYSICAL REVIEW C 64 034608
TABLE I. Extracted values for the effective nucleon numbers
A eff , Fermi momenta k F , and peak shifts relative to free scattering⌬ ␻ 0 共at q⫽500 MeV/c). A eff and k F were obtained from fits tocross sections and widths at all q’s.
Target
2

H
Li
C
Ca
Zr
208
Pb
6

FIG. 6. Integrated quasielastic cross sections for 2 H, 6 Li, C, Ca,Zr, and 208Pb as a function of momentum transfer. The solid lines
show fits to the cross sections, using average free scattering crosssections and a scale factor A eff , and the dotted lines show the same
fits without the Pauli blocking factor.

关15兴, and k F is the Fermi momentum of the target nucleus.
The dotted lines show the fits without the Pauli blocking
factor. The factor A eff can be interpreted as the effective
number of nucleons participating in the quasielastic scattering. Since effects of Pauli blocking are small for the measured kinematical region, k F is fixed to the experimental values of the (e,e ⬘ ) data of Ref. 关16兴 for this figure. Better
results will follow below. (d ␴ /d⍀) ␲ N is the elementary␲ ⫺ N elastic scattering cross section at 950 MeV/c, averaged
over all nucleons in the nucleus. We use the free ␲ ⫺ N crosssections, even for scattering within nuclei. A recent calculation of meson properties in nuclei suggests little change ofthe ␲ mass and little change of a ␴ meson mass that carries
the scalar-isoscalar interaction favored by our experiment关17兴. It therefore seems appropriate to use free ␲ ⫺ N scattering amplitudes that were obtained with the phase shift code
SAID 关8兴 in the present analysis. The present analysis reproduces the q dependence of d ␴ /d⍀ with the single parameter
A eff as shown in Fig. 6, showing the validity of the factorization of the quasielastic cross sections as given in Eq. 共6兲.

A

A eff

kF
(MeV/c)⌬ ␻ 0 (q⫽500 MeV/c)
共MeV兲

2.0
6.0
12.0
40.1
91.2
208.0

⫹0.08
1.76⫺0.03⫹0.10
2.77⫺0.10⫹0.19
3.57⫺0.08⫹0.40
6.75⫺0.20⫹0.57
9.21⫺0.31⫹0.72
11.70⫺0.29⫹1.1
53.4⫺1.1⫹6.2
146.6⫺5.4⫹9.9
183.4⫺3.8⫹8.0
198.7⫺5.3⫹12.5
214.3⫺10.8⫹14.3
192.5⫺8.4

2.5⫾2.5
3.3⫾3.0
11.7⫾2.7
7.5⫾2.7
4.6⫾3.2
1.6⫾3.0

␣ is 0.42⫾0.01, smaller than the exponent of 0.56⫾0.03
found for ␲ ⫺ NCX scattering at 624 MeV/c 关4兴. The exponent ␣ ⬍1 indicates a shadowing of nucleons in the nucleus
by neighboring nucleons. For electron scattering, ␣ is near
unity because of the weakness of electron-nucleon interaction. The present value of 0.42 indicates that pions arelargely absorbed at the nuclear surface and thus scatteredonly from the surface region of a nucleus.
The value of A eff can be estimated by using an eikonal
approximation based on Glauber theory 关18兴:
calc
⫽
A eff

冕
冕

T共 b 兲⫽d 2 b T 共 b 兲 e ⫺ ␴ T T(b) ,⫹⬁⫺⬁dz ␳ 共 冑b 2 ⫹z 2 兲 ,

共8兲
共9兲

where ␳ (r) is the proton density taken from the ground statecharge distributions of Ref. 关19兴. The neutron density distribution is assumed to be the same as that of protons. ␴ T is thetotal ␲ ⫺ N cross section, averaged over all nucleons in the
nucleus using values for ␲ ⫺ p 共53.77 mb兲 and ␲ ⫺ n 共23.75
mb兲 scattering at 950 MeV/c 关8兴. The open circles in Fig. 7
show the results of the calculation. The slope of the A dependence is well reproduced, though the calculated A eff aresmaller than the experimental observations. The larger ex-

C. Effective number of nucleons

The fitted values of A eff from Fig. 6 are listed in Table I.
A eff is expected to follow a power law of atomic mass A,
exp⫽N 0 A ␣ ,
A eff

共7兲

as shown in Fig. 7 and as is also known for other quasielastic
meson scattering 关4,6,18兴. The best fit value for the exponent

FIG. 7. Fit of the effective number of nucleons to a power law.
The open circles are the results of an eikonal calculation.

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Y. FUJII et al.

PHYSICAL REVIEW C 64 034608

FIG. 8. Fermi momenta extracted from measured widths of thequasielastic peaks are compared. The solid circles are the present
results and the open squares are the values from the (K ⫹ ,K ⫹ ⬘ )reaction 关6兴. The open circle for carbon is at a beam momentum of
624 MeV/c 关4兴.

perimental A eff may be due to a change of the ␲ ⫺ N total
cross section in the nuclear medium. It is noted that thisdifference is present regardless of the assumed ‘‘background’’ shape, since the systematic uncertainties from thealternative treatment of this subtraction are all positive.
D. Peak width

In the simple relativistic Fermi-gas model the width
共FWHM兲 of the quasielastic peak is determined by the Fermi
momentum k F of the struck nucleons:

⌫ FG⫽

1

冑2

关 冑M * 2 ⫹ 共 q⫹k F 兲 2 ⫺ 冑M * 2 ⫹ 共 q⫺k F 兲 2 兴 ,共10兲

where M * is the effective mass of the struck nucleon, here
taken to be the free mass of a nucleon. The Fermi momentum
for each target nucleus was thus derived using Eq. 共10兲 fromthe experimental widths obtained by fitting the quasielastic
peak. For the present spectra the reduced ␹ 2 values ranged
from 0.4 to 1.7. These Fermi momenta are summarized inTable I and shown as the solid circles in Fig. 8. They arecompared to results obtained from K ⫹ scattering 关6兴 共the
open squares兲. The K ⫹ penetrates more deeply into nuclei
where the nuclear density and thus local Fermi momentum ishigher, resulting in a wider quasielastic peak. The Fermi momentum previously extracted from the ␲ ⫺ -C quasielastic
scattering at P ␲ ⫽624 MeV/c 共the open circle兲 is very similar with the present observation 关4兴. Equation 共10兲 was applied to the deuteron data so that the data can be included in
the figure as a reference, although the Fermi-gas model is not
appropriate.
E. Peak position

The peak positions of the quasielastic scattering process
in a nuclear target would be shifted from that of the free

FIG. 9. Quasielastic peak shifts relative to scattering from hydrogen for 共a兲 the present result, and 共b兲 ␲ ⫺ 共solid squares兲 and ␲ ⫹共open squares兲 SCX at 624 MeV/c 关5兴.

space scattering due to the interaction of the struck nucleon
with its mean field or with its neighbor nucleons. In Fig. 9共a兲
we show the energy loss shift of the peak positions of thequasielastic scattering on the nuclear target relative to that ofhydrogen. The shift is positive for a quasielastic peak withgreater energy loss relative to the free kinematics. In Fig.
9共b兲 we also show shifts for the quasielastic SCX ofnegative-pion 共the solid squares兲 and positive-pion 共the open
squares兲 beams at 624 MeV/c 关5兴 for comparison with thepresent mostly scalar-isoscalar quasielastic scattering. These
are to be averaged to cancel the Coulomb effect. The striking
difference of the q dependence between mostly scalarisoscalar and scalar-isovector probes can be explained fromthe interactions presented in Ref. 关20兴.
In the previous ( ␲ , ␲ ⬘ ) NCX experiment at 624 MeV/c
关4兴 no significant peak shift was observed for momentum
transfers up to about 600 MeV/c. However, those results are
not inconsistent with the present data due to large uncertainties of the previous data.

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QUASIELASTIC ␲⫺-NUCLEUS SCATTERING AT 950 MeV/c

PHYSICAL REVIEW C 64 034608

FIG. 10. Computed cross sections for ␲ ⫺ - 12C quasielastic scattering without interactions 共dashed-dot lines兲 and in the RPA in thescalar-isoscalar channel only 共dashed lines兲.
F. Comparison with a theoretical calculation

Doubly differential cross sections for carbon are compared with a theoretical calculation based on a finite-nucleus
continuum RPA framework using a density-dependent
particle-hole interaction 关21兴. This model does not use thefactorized form of Eq. 共6兲, but takes into account distortionsof pions by means of a Glauber-type calculation. The same
framework was successfully applied to other reactions withstrongly interacting beams, (p,n) 关22兴 and (K,K ⬘ ) 关7兴.
Figure 10 shows a comparison between a free 共dashed-dot
lines兲 response and RPA 共dashed lines兲 response in thescalar-isoscalar channel for ␲ ⫺ - 12C quasielastic scattering.
As seen in the figure, there is little difference between thesecalculations at q⫽650 MeV/c, but the nuclear scalarisoscalar interactions have an important effect at
350 MeV/c. Figure 11 shows ␲ ⫺ - 12C quasielastic cross sections compared with the RPA calculations. The open circles
show the quasielastic portion of the experimental data; thelinear background determined by the fitting described in Sec.
III A has been subtracted from the experimental data. The
solid lines show the cross section with full interaction, while
the dashed 共dotted兲 lines represent the cross sections throughthe scalar-isoscalar 共non-scalar-isoscalar兲 channel. The RPA
responses shift toward higher energy loss with higher q,
which is consistent with the present data. The origin of thepositive shift of the RPA responses depends on q. For q⫽350 and 450 MeV/c, the shift is due to a q dependence ofthe scalar-isoscalar response. However, at q⫽550 and
650 MeV/c it is due to an increasing contribution from nonscalar-isoscalar responses. The q dependence of the observed
peak shift on the carbon target is qualitatively reproduced by
the RPA calculation, except for the lowest q of 350 MeV/c.
At the larger q, the agreement between the RPA spectra andthe experimental spectra would imply that interactions inscalar-isoscalar and the other channels are appropriately
taken into account in the calculation. At low-energy losses
for q⫽350 MeV/c, the interacting RPA overestimates thepresent data even though the interaction strength has been
already set to half of its expected value, as needed for similar

FIG. 11. ␲ ⫺ - 12C quasielastic cross sections compared with the
RPA calculation. The open circles show the quasielastic portion of
the experimental data. The solid lines show the RPA cross section
with all channels. The dashed lines show the RPA cross section onlywith the scalar-isoscalar channel, and the dotted lines show thecontributions from non-scalar-isoscalar channels.

RPA calculations in Ref. 关7兴 to reproduce the K ⫹ quasielastic
data. The present data would agree best with an RPA prediction with very small or no scalar-isoscalar nuclear interaction.
IV. CONCLUSIONS

We have measured quasielastic and continuum scatteringof 950 MeV/c ␲ ⫺ on nuclei ranging from 2 H through 208Pb.
The measurement was done with single fixed spectrometer
angle taking advantage of the large solid angle of the SKS
spectrometer, with which good angular and energy resolution
were also realized. The overall normalization of the measured spectra was confirmed by comparing the cross sectionof ␲ ⫺ p elastic scattering with those of the SAID calculation.
The present experiment offered high-quality quasielastic
spectra for laboratory three-momentum transfers from 350
through 650 MeV/c. Since radiative corrections are notneeded for pion beams in contrast to electron beams, these
spectra are simple and clean even for 208Pb; those data arevery similar in quality and shape to those for carbon.
We have summarized our observations by fitting the spectra above a background and obtained singly differential crosssections, and centroids and widths of the quasielastic peaks.

034608-7

Y. FUJII et al.

PHYSICAL REVIEW C 64 034608

The integrated cross sections follow a power law in A, as
observed for other meson quasielastic reactions, and arenearly as predicted by a simple Glauber model. Peak widths
were used to extract Fermi momenta, which approach saturation as nuclear densities saturate for heavy targets. Our
Fermi momenta are lower than those found for K ⫹ scattering, as expected since pions interact at lower nuclear densities. The energy shifts of the quasielastic maxima follow thegeneral trend expected from the momentum dependence of
NN interactions, in a direction opposite to that found for
SCX quasielastic spectra sensing only isovector NN interactions.
At the lowest q of 350 MeV/c, the present data are sensitive to scalar-isoscalar residual interactions, as included in
the recent RPA calculations 关7兴 to which we compare thedata. Calculations of quasielastic K ⫹ scattering based on thismodel found a need to make those interactions weaker than
expected. The present higher-quality ␲ ⫺ spectra show a further need to reduce collectivity than is provided by the RPA
calculation. At larger q, the other spin/isospin channels become as important as the scalar-isoscalar channel. The good
agreement with the present spectra would indicate that no
relative change in those interactions is needed within the
carbon nucleus.

关1兴 W.M. Alberico, M. Ericson, and A. Molinari, Nucl. Phys.
A379, 429 共1982兲.
关2兴 L.B. Rees, J.M. Moss, T.A. Carey, K.W. Jones, J.B. McClelland, N. Tanaka, A.D. Bacher, and H. Esbensen, Phys. Rev. C
34, 627 共1986兲.
关3兴 T.N. Taddeucci et al., Phys. Rev. Lett. 73, 3516 共1994兲.
关4兴 J.E. Wise et al., Phys. Rev. C 48, 1840 共1993兲.
关5兴 R.J. Peterson, S. Ho” ibråten, J. Ouyang, M.R. Braunstein, X.Y.
Chen, M.D. Kohler, B.J. Kriss, D.J. Mercer, D.S. Oakley, and
D.L. Prout, Phys. Lett. B 297, 238 共1992兲.
关6兴 C.M. Kormanyos et al., Phys. Rev. C 51, 669 共1995兲.
关7兴 A. De Pace, C. Garcı́a-Recio, and E. Oset, Phys. Rev. C 55,
1394 共1997兲.
关8兴 R.A. Arndt, I.I. Strakovsky, R.L. Workman, and M.M. Pavan,
Phys. Rev. C 52, 2120 共1995兲.
关9兴 T. Fukuda et al., Nucl. Instrum. Methods Phys. Res. A 361,
485 共1995兲.
关10兴 T. Takahashi et al., Phys. Rev. C 51, 2542 共1995兲.
关11兴 A. Erell, J. Alster, J. Lichtenstadt, M.A. Moinester, J.D.
Bowman, M.D. Cooper, F. Irom, H.S. Matis, E. Piasetzky, and

U. Sennhauser, Phys. Rev. C 34, 1822 共1986兲.
关12兴 H. Esbensen and G.F. Bertsch, Ann. Phys. 共N.Y.兲 157, 255
共1984兲.
关13兴 W.M. Alberico, A. Molinari, A. De Pace, M. Ericson, andMikkel B. Johnson, Phys. Rev. C 34, 977 共1986兲.
关14兴 J.D. Zumbro et al., Phys. Rev. Lett. 71, 1796 共1993兲.
关15兴 G.F. Bertsch and O. Scholten, Phys. Rev. C 25, 804
共1982兲.
关16兴 R.R. Whitney, I. Sick, J.R. Ficenec, R.D. Kephart, and W.P.
Trower, Phys. Rev. C 9, 2230 共1974兲.
关17兴 A.M. Rakhimov, F.C. Khanna, U.T. Yakhshiev, and M.M.
Musakhanov, Nucl. Phys. A643, 383 共1998兲.
关18兴 J. Ouyang, S. Ho” ibråten, and R.J. Peterson, Phys. Rev. C 47,
2809 共1993兲.
关19兴 H. de Vries, C.W. de Jager, and C. de Vries, At. Data Nucl.
Data Tables 36, 495 共1987兲.
关20兴 T. Shigehara, K. Shimizu, and A. Arima, Nucl. Phys. A492,
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ACKNOWLEDGMENTS

The authors would like to thank Dr. A. De Pace for allowing us to use the spectra of the RPA calculation. The support
by the KEK-PS staff for the running of this experiment isgreatly appreciated. Dr. R. Chrien kindly loaned us the
6 2
Li H sample.

034608-8

PHYSICAL REVIEW C, VOLUME 65, 054601

Mesonic y scaling
R. J. Peterson,1 J. T. Brack,1 Y. Fujii,2 O. Hashimoto,2 T. Takahashi,2 M. Itoh,3 and H. Sakaguchi2
1

Nuclear Physics Laboratory, University of Colorado, Boulder, Colorado 80309-0446
2
Department of Physics, Tohoku University, Sendai 980-8578, Japan3
Department of Physics, Kyoto University, Kyoto 606-8502, Japan
共Received 3 October 2001; published 17 April 2002兲

The concept of y scaling has been demonstrated in the scattering of high-energy electrons to the nuclear
continuum, demonstrating the incoherent elastic scattering of the projectile from individual bound nucleons.
Scattering of intermediate energy ␲ ⫺ and K ⫹ from complex nuclei also shows scaling for cases of low density
or small cross sections, but not for all nuclei. These results are used to evaluate the use of such mesonic yscaling as a means to measure meson-nucleon elastic differential cross sections within the nuclear medium.
DOI: 10.1103/PhysRevC.65.054601

PACS number共s兲: 25.80.Ls

I. INTRODUCTION

Inclusive inelastic electron scattering at high beam momenta has been shown to follow a y-scaling response. Underassumptions of incoherent, single nucleon elastic scattering,
kinematic relations have been shown to transform spectra fora range of beam momenta and scattering angles into thesame y-scaled responses 关1,2兴. The transformations of doubly
differential cross sections and energy loss ␻ 共all in the laboratory frame兲 at each momentum transfer q are defined to be
关3兴

冋

册

F共 y 兲⫽q eff
d 2 ␴ /d⍀d ␻ 1,2
d ␴ /d⍀ 共 free兲 A eff 冑m N ⫹ 共 q eff⫹y 兲 2

共2兲

˜ 共␻
˜ ⫹2m N 兲 ⫺q eff ,y ⬁ ⫽ 冑␻共3兲

冑m N2 ⫹ 共 q eff⫹y ⬁ 兲 2y⬁y⫽y ⬁ 1⫺2 共 A⫺1 兲 m N
y ⬁ ⫹q eff

共1兲

with

with

˜ ⫽ ␻ ⫺SE,␻冋q eff⫽q 1⫾

4Z ␣ បc
3Er 0 A 1/3册,

r 0 ⫽1.12 fm.

共4兲Separation energies 共SE兲 are listed in Table I, as used in Ref.
关3兴 or interpolated from values reported there. Free nucleon
masses m N are used throughout. At sufficiently large q, theobservable y may be thought of as the component of thenucleon’s internal momentum along the direction of q at the
moment it was struck.
Thus we know that the nucleus offers us this y-scaling
response F(y). We here address the question of whether thisscaling can be seen in the scattering of high-energy mesons,differing from the electrons in their strong interactions with
nucleons and nuclei, and whether such meson data can beused to extract d ␴ /d⍀, the in-medium elastic mesonnucleon differential cross section, from continuum spectra. If
0556-2813/2002/65共5兲/054601共8兲/$20.00

so, these y-scaling spectra would offer a direct measure ofthis important ‘‘medium effect,’’ searching for differences in
the interactions of mesons within complex nuclei from thosemeasured in free space.
Moreover, electron scattering has been demonstrated toshow a ‘‘superscaling’’ response, whereby the continuum
spectra of all nuclei have been shown to be the same under acertain transformation 关3兴. If similar methods can be shown
to be valid for meson scattering, we could even measure thenuclear target, or density, dependence of these medium effects.

II. METHODS

Most pion data for this analysis come from Refs. 关4,5兴,
where ␲ ⫺ at beam momenta from 780 to 1050 MeV/c were
scattered from targets of D, 6 Li, C, Ca, Zr, and 208Pb. Momentum transfers ranged from 350 to 650 MeV/c, binned
into steps 25 MeV/c wide. Some ␲ ⫹ data from Ref. 关6兴 arealso used, at a beam momentum of 624 MeV/c. K ⫹ continuum scattering data at 715 MeV/c from Ref. 关7兴 are used,at the maximum momentum transfer of 480 MeV/c fromthat work. In each case, the absolute normalization of thecross sections was very secure, being calibrated on scattering
TABLE I. Numerical values used to evaluate the scaling transformations in the text. Separation energies 共SE兲 are from Ref. 关3兴,or interpolated from values there. Fermi momenta for electron scattering are from Ref. 关3兴; those for ␲ ⫺ are from Refs. 关4,5兴, andthose for K ⫹ from Ref. 关7兴. All are obtained by fits to the shape ofthe quasielastic peak to the form of the relativistic Fermi gas, withk f in MeV/c. Interpolated values are in parantheses. Effective numbers of nucleons are from an eikonal calculation.
Target SE 共MeV兲 k f (ee ⬘ ) A eff( ␲ ⫺ ) k f ( ␲ ⫺ ) A eff(K ⫹ ) k f (K ⫹ )
D
6
Li
C
Ca
Zr
Pb

65 054601-1

2.0
10.0
15.0
17.0
22.0
25.0

220
共232兲
共236兲
240

1.27
2.66
2.66
4.95
6.68
8.68

53
147
183
199
214
193

2.0

73

6.1
14.9

190
220

33.3

230

©2002 The American Physical Society

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PHYSICAL REVIEW C 65 054601

FIG. 2. The world’s supply of ␲ -carbon total cross sections isshown, with circles for ␲ ⫺ and crosses for ␲ ⫹ 关11,12兴. Above thedelta resonance the solid line shows results from the eikonal method
used here to compute A eff in Eq. 共2兲. Squares show K ⫹ -carbon total
cross sections 关15兴, with a corresponding eikonal theory curve.

FIG. 1. Doubly differential cross sections measured for scattering of 950 MeV/c ␲ ⫺ from C and 208Pb are shown at three valuesof the laboratory frame momentum transfer q 关4,5兴. Coherent scattering is noted at low energy losses ␻ , and a broad incoherent
quasielastic peak dominates the continuum.

from free protons and using the known elastic cross sections
关8兴.
Figure 1 shows representative ␲ ⫺ spectra for C and 208Pb
at three momentum transfers. The prominent quasifree peakis noted at all values of q. At 350 MeV/c we still see tracesof coherent elastic and inelastic scattering at low energy
losses. The energy resolution of these spectra was 2.5 MeV
关full width at half maximum 共FWHM兲兴.
Since we desire to use the incoherent scattering from individual nucleons for this study, we need care with the kinematic conditions. These are spelled out in Chap. 11 of Ref.
关9兴. The momentum transfer q must satisfy bothqⰇ 冑8mT/3⬵255 MeV/c
andq 2 /2mⰇ 兩 U 兩 ⫽40 MeV,

with T the nucleon Fermi level in the nucleus and U theaverage nucleon-nucleus potential. At the nominal momentum transfer of q⫽500 MeV/c, the first is probably, but notdecisively, satisfied.
The beam momentum k, both incoming and outgoing,must satisfy 1/kⰆL, the mean spacing between nucleons,with about 1.9 fm between the centers of objects each about
0.6 fm in radius. This scale requires L⫽0.7 fm, or k
Ⰷ280 MeV/c. If nucleons were larger in the nucleus than infree space, with smaller spacings between their surfaces, thisstandard becomes stricter. We study the low density examples of D and 6 Li to vary these densities or spacings.
The effective number of nucleons A eff sensed by the mesonic beams is less than the total number in the nuclear targets because of absorptions and distortions. This quantity ishere computed by a Glauber or eikonal method 关10兴, compared to the present pion data in Ref. 关4兴 and to K ⫹ data in
Ref. 关7兴. A different set of geometrical parameters for thetarget nuclei is used here, compared to Ref. 关4兴, so values for
A eff differ slightly. Meson-nucleon total cross sections used
for these calculations are from Ref. 关8兴, evaluated for eachnucleus including the nuclear Coulomb effect on the beamenergy and averaged over neutrons and protons. Values ofA eff are listed in Table I.
Resonances in the fundamental ␲ -nucleon cross sections
are a prominent feature in the beam energy range we cover.
Within a complex nucleus, however, these resonances otherthan the delta are not visible in measurements of total crosssections 关14兴, compared to an eikonal calculation. Figure 2
shows the world data set for carbon 关11,12兴 compared to a
curve computed for energies above 300 MeV by the eikonal
method. Bumps expected from free space interactions are notfound for the carbon target, but the overall agreement withthe magnitude of the data confirms the reliability of the eikonal method we use at our beam energies near 800 MeV. A
similar conclusion was reached in an isobar model of pionnucleus total cross sections between 400 and 1000 MeV 关13兴,

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MESONIC y SCALINGPHYSICAL REVIEW C 65 054601

with theoretical cross sections somewhat lower than thoseshown here. Also shown in Fig. 2 are K ⫹ -nucleus total crosssections 关14兴, and the eikonal calculation.
Meson-nucleon differential cross sections appearing in
Eq. 共2兲 were computed from a standard phase shift analysis,
kept up-to-date with the latest data 关8兴. Coulomb effects fromthe nuclear targets were included to select the beam energies
for these evaluations. Except at y⫽0, the meson is colliding
with a moving nucleon, and the scattering is off the mass
shell. The method of evaluating such off-shell cross sectionsin the optimum frame 关15兴 minimizes the difference between
on and off-shell scattering. For the extreme cases in the dataof Fig. 1, we find differential cross sections for the q⫽650 MeV/c case to vary by no more than 4% across thefull range of the spectrum for the range of beam energies
appropriate to the optimum reference frame for each energy
loss ␻ . For q⫽500 MeV/c differential cross sections are
12% higher at the low energy loss side for ␻ ⫽30 MeV, andlarger by 29% for the large energy loss limit of ␻⫽260 MeV. Since the resonances that provide these variations are damped within the nuclear targets, and since theeffects are not large, we use only free space differential elastic ␲ -nucleon cross sections in Eq. 共2兲, averaged over neutrons and protons in the complex nucleus.
Electron scattering y-scaling responses can sense either
the proton charges incoherently or the 共largely兲 isovector
magnetic moments. Our meson scattering is sensitive primarily to an isoscalar, nonspin, coupling to the nucleons in the
nuclear medium, with about 60% of the cross section at q⫽500 MeV/c 关8兴. The smaller pieces do not include any
term from pion exchange; the mesons scatter from nucleons
only with a short range interaction. If a single meson were
responsible for the largest part of the ␲ -N interaction, it
would be the putative sigma.
III. RESULTS

The transformations of Eqs. 共1兲–共4兲 have been applied to␲ ⫺ spectra such as shown in Fig. 1 to form the y-scalingplots of Figs. 3 and 4. Only spectra at selected values of q
have been used here. The incoherent quasifree peak is found,
centered very near y⫽0. In contrast to some electron scattering spectra, these pion data also decrease and scale atpositive y. This is because the small scattering angle has
precluded spin transfers, and thus quenched pion production
which destroys the y scaling for transverse electron scattering
for y⬎0.
The very light nuclei D and 6 Li show very good scaling
responses. The sharp peaks near y⫽⫺130, ⫺180, and⫺210 MeV/c in the D spectra are from elastic deuteronscattering. The 6 Li sample shows good scaling down to y⫽⫺200 MeV/c.
For the carbon sample, however, scaling is not observedfor q less than about 500 MeV/c. The heavier targets show
decreasing agreement with the scaling hypothesis, even atlarge q.
Another way to sense y scaling is by plotting spectra at
fixed q, but varying beam energy and scattering angle. Figure
5 shows ␲ ⫺ data from the same experiment as Refs. 关4,5兴, as

FIG. 3. Scaling functions for 950 MeV/c ␲ ⫺ scattered fromlight nuclei are shown at selected values of the momentum transfer
q. The sharp peaks near y⫽⫺130, ⫺180, and ⫺210 MeV/c in the
deuteron spectra are from elastic deuterium scattering. The D and6
Li spectra show good y-scaling responses, but the carbon casescales only for larger values of q. Data are from Ref. 关4兴.

well as ␲ ⫹ data from Ref. 关6兴. Scaling is only approached
for the higher beam momenta, as we might expect from thekinematic conditions. Note that the free ␲ -nucleon cross sections are changing strongly in this resonance range of beam
momenta. Although optimum frame considerations 关15兴 will

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PHYSICAL REVIEW C 65 054601

FIG. 5. Scaling spectra are shown for q⫽500 MeV/c at fivecombinations of beam momentum and scattering angle. The four
highest ␲ ⫺ curves are from Refs. 关4,5兴, while the 624 MeV/c caseis for ␲ ⫹ 关6兴. Scaling is only apparent for the higher beam
momenta.

⫽⫺100 MeV/c. In a similar plot of electron scatteringy-scaling responses, the charge response is found to be nearF(y⫽0)⫽0.0027 MeV⫺1 for carbon 关1兴.
Longitudinal electron scattering responses for carbon
were treated with the same scaling transformations used here
for pions. Specifics used here are described in the Appendix.
Data from Ref. 关16兴 at q⫽400 and 550 MeV/c result in thescaling responses shown in Fig. 7, with magnitudes much as
shown in other analyses of these data with not exactly thescaling transformation used here 关17,18兴. The ␲ ⫺ scaling
responses for carbon scale much as these electron data, butwith a magnitude larger by a factor of 3.5. This will be

FIG. 4. Scaling functions for the heavier nuclei of our study are
shown. The spike atop the Ca spectrum at q⫽350 MeV/c is theresult of some hydrogen contamination in the sample. These cases
do not show y scaling except near the largest values of q obtained in
our measurements. Data are from Ref. 关4兴.

be more important at the lower beam energies, they have noeffect at y⫽0.
Scaling responses should be approached from below as
the momentum transfers increase 关1兴. Figure 6 shows theseapproaches for the light targets at y⫽0 and at y

FIG. 6. Values of the scaling data at y⫽0 and y⫽⫺100 MeV/c are shown for the light nuclei for each of the bins of
momentum transfer measured in our experiment 关4兴. Results for D
and 6 Li approach an equilibrium value, while y⫽0 values continue
to increase with q for C. No Ca data are shown for y⫽0 because ofsome hydrogen contamination in the target.

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MESONIC y SCALINGPHYSICAL REVIEW C 65 054601

FIG. 7. Separated longitudinal electron scattering response datafor carbon are transformed by the methods of this work to they-scaling format. Crosses are for 400 MeV/c and diamonds for
550 MeV/c from Ref. 关16兴.

FIG. 8. Continuum K ⫹ spectra at q⫽480 MeV/c are showntransformed as in the text to the superscaling format. Although statistical uncertainties are large, these data do superscale. Data arefrom Ref. 关7兴.

discussed in Sec. V. A difference in the location of the scaling maximum is also noted in comparing Figs. 7 and 3共c兲.

tude of these plots by more than 10%. Cross sections for D
and 6 Li track remarkably closely.
In Fig. 10 the values of f (Y ) are plotted for each target
mass for K ⫹ and ␲ ⫺ at Y ⫽0 and for ␲ ⫺ at Y ⫽⫺0.5. Longitudinal electron scattering responses for C 关16兴, Ca 关19兴,and Fe 关17兴 were transformed by the present usages to thesuperscaling responses shown in Fig. 11. Pion, K ⫹ , and longitudinal electron superscaling responses are very similar,
but the pion data are larger by a factor of 3 than the electron
data, and the K ⫹ data are larger by a factor of 1.8.
For both y-scaling and superscaling responses, we find themeson responses to exceed those measured with electrons.
Might this be evidence for a ‘‘medium dependence’’?

IV. SUPERSCALING

A means to compare a larger body of meson and electron
scattering results is by way of the concept of ‘‘superscaling’’关3兴. The y-scaling resonses are further transformed by
Y ⫽y/k f ,共5兲

f 共 Y 兲 ⫽k f F 共 y 兲 .

共6兲In Ref. 关3兴 the required Fermi momenta k f were obtained
from fits of the shape for a relativistic Fermi gas to thewidths of the electron scattering quasielastic peaks; values
are listed in Table I. The ␲ ⫺ data were also so fitted 关4兴, aswere the K ⫹ data 关7兴. This form was also used for D, although the Fermi gas model is not appropriate. Results arecompared in Table I. The ␲ ⫺ interact at larger radii andlower density and lower Fermi momenta than the K ⫹ and theelectrons.
Figure 8 shows the K ⫹ superscaling responses at q
⫽480 MeV/c, the largest momentum transfer reached inthat experiment 关7兴. Although statistical error bars are large,
these data scale well near y⫽0, for y⬎0, and 共except for
Pb兲, for y⬍0. Although several data points for D seem to lie
below the trend for the other targets, these are fluctuations
because of poor statistical accuracy. Many of the D points
are in the same band as the many other points, and hard to
distinguish. In the simplest nonrelativistic Fermi gas model,
with all scaling conditions met, the universal superscaling
response would be a parabola reaching between Y ⫽⫺1 and⫹1, with a magnitude of 0.75.
In Fig. 9 we show ␲ ⫺ superscaling responses at q⫽500,
600, and 650 MeV/c. These data seem to approach a uniform response for heavier targets. Uncertainties in the Fermi
momenta used for this analysis will not change the magni-

V. DISCUSSION

It was observed in the electron scaling and superscaling
responses that longitudinal 共charge兲 scattering data scaled
well, but transverse (S⫽1, T⯝1) data increase with q 关1,3兴.
Pion scattering also shows a sensitivity to S⫽1 scattering,roughly proportional to sin2(␪) where ␪ is the ␲ ⫺ -N centerof-mass scattering angle for quasifree scattering 关8兴. The approaches to scaling at a fixed beam momentum of
950 MeV/c shown in Fig. 6 could also be thought to indicate this, since the increase in q is provided by larger scattering angles, but the increase is far below proportionality tosin2␪. The scaling data at fixed q with varying beam energy
shown in Fig. 5, however, indicate otherwise; F(y) at its
maximum decreases with scattering angle for the lowest
beam momentum. The present data set at lower energies islimited in its kinematic suitability to the scaling hypothesis,
but we conclude that we can provide no evidence for S⫽1enhancements with pions.
It is striking that our pion y scaling and superscaling responses lie above similar results from electron scattering, by
about a factor of 3. A major difference in the analyses is ourneed to use both computed values of d ␴ /d⍀ and A eff in Eq.
共2兲. If in-medium ␲ -N total cross sections increase, A eff de-

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PHYSICAL REVIEW C 65 054601

FIG. 10. At our largest effective momentum transfer q eff⫽650 MeV/c, the superscaling responses are plotted for eachnuclear target for Y ⫽0 for K ⫹ 共diamonds兲 and ␲ ⫺ 共circles兲, andfor Y ⫽⫺0.5 for ␲ ⫺ 共squares兲. A rise to a saturating value is seen.
Longitudinal electron scattering superscaling responses are near 0.5
at Y ⫽0.

as to incur a larger probability of a second scattering than isprovided by the eikonal method; this would lead to a smaller
value of A eff and would raise the diamond points in Fig. 5.
If differential cross sections increase in proportion to total
cross sections, the product in the denominator of Eq. 共2兲
changes little. This is demonstrated in Fig. 12, showing freed ␴ /d⍀ for q⫽500 and 650 MeV/c and A eff computed for␲ ⫺ in the eikonal method using free-space total cross sections for a range of beam energies. Resonances are seen inboth terms of the denominator of Eq. 共2兲, but with a productthat varies too slowly to be the cause of our excess in F(y)and f (Y ) near 820 MeV. No resonances are found for theK ⫹ -N system near our beam momentum. Both differential
elastic and total K ⫹ -nucleus cross sections are above expectations based on free K ⫹ -N scattering, and again the mutual

FIG. 9. Superscaling data for ␲ ⫺ for all of our samples areshown at three values of the effective momentum transfer. All show
the same trend, shifting somewhat to lower values of Y and increasing to saturation for heavier nuclei.

crease. The eikonal model used to compute A eff assumes only
small angle collisions with negligible energy losses. Thismodel is used here for angles as large as 45°, and may not be
reliable for heavy nuclei. This could explain the observed
failure to scale for larger momentum transfers. In particular,
the outgoing pion energy for the case of 624 MeV/c is such

FIG. 11. Separated longitudinal electron scattering response data
are superscaled by the methods of this work. Circles are for carbon
at q⫽550 MeV/c 关16兴, squares for Ca at 400 MeV/c 关19兴, anddiamonds for Fe at 570 MeV/c 关17兴.

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MESONIC y SCALINGPHYSICAL REVIEW C 65 054601

means the resulting F(y) and f (Y ) would increase in ourrange of energies and angles, as noted in the data.
We have definitely seen some ‘‘medium effect’’ in mesonic y scaling in differential cross sections. This result isapparently more sensitive to medium effects than shown by
the 31– 42 % increase in ␲ -N interaction strength in all channels estimated from total cross sections 关13兴. The failure toscale for heavy nuclei may indicate an inadequacy in themethods used to generate scaling functions, but enhancements are also seen for light nuclei exhibiting good scaling.
Data at higher beam energies may remove this kinematic
impediment in the future and provide a surer reaction
mechanism.
ACKNOWLEDGMENTS
FIG. 12. The solid curve shows A eff computed in our eikonalmodel, reflecting the large free ␲ -N total cross sections near two
prominent resonances. Pi-N isospin averaged differential cross sections at q⫽500 MeV/c and 650 MeV/c are also shown, as dashedand dot-dashed curves, the latter multiplied by 2. Their product, as
appears in the denominator of Eq. 共2兲, changes little. If medium
effects altered both ␲ -N total and differential cross sections
equally, our scaling responses would show little sensitivity tochanges from free-space values.

changes will have little effect on the magnitude of the K ⫹scaling.
The present analysis finds that continuum ␲ meson scattering does exhibit y scaling for light nuclei, but fails forlarger systems. K ⫹ data do seem to superscale. The magnitudes of f (Y ⫽0) for pions are larger than observed withelectrons by a factor of 3, while for K ⫹ the maximum islarger than found for electrons by a factor of 1.8. The longer
mean free path of K ⫹ within nuclei gives them a penetrationintermediate between pions and electrons. Pion responses atlarge q do show a larger continuum for y⬎0 than do electron
charge responses.
If the radial extent of nucleons in the interior were toincrease, an effect hypothesized to account for a large bodyof K ⫹ data 关20兴, but their ingredients remain the same, wemight suppose that ␲ -N total cross sections in the nuclear
medium do not increase, nor do values of A eff . Diffraction of
the pions around a larger radius R would concentrate thein-medium differential cross sections to smaller angles andincrease their magnitude at fixed values of qR. By this关1兴 D. B. Day, J. S. McCarthy, T. W. Donnelly, and I. Sick, Annu.
Rev. Nucl. Part. Sci. 40, 357 共1990兲.
关2兴 J. Arrington et al., Phys. Rev. Lett. 82, 2056 共1999兲.
关3兴 T. W. Donnelly and I. Sick, Phys. Rev. C 60, 065502
共1999兲.
关4兴 Y. Fujii et al., Phys. Rev. C 64, 034608 共2001兲.
关5兴 Y. Fujii, Ph.D. thesis, Tohoku University, 1998.
关6兴 J. E. Wise et al., Phys. Rev. C 48, 1840 共1993兲.
关7兴 C. M. Kormanyos et al., Phys. Rev. C 51, 669 共1995兲.

We wish to thank all collaborators in KEK E352 for assistance in obtaining the data used here. R.J.P. thanks T. W.
Donnelly and D. B. Day for useful comments. This work was
supported in part by the United States Department of Energy.
APPENDIX

Separated longitudinal electron scattering responses areoften shown in the literature, and several methods have demonstrated how these scale and superscale 关1兴. In the present
work, the transformations given in the body of the papertransform cross sections. Here we specify how we scale published electron responses to generate the data shown in Figs.
7 and 11, for a clear comparison of the data using several
projectiles.
Responses R L are defined from separated electron scattering longitudinal cross sections using the Mott electronnucleon cross section. Scaling responses use the actual
electron-nucleon elastic diferential cross sections, including
the proton form factor for longitudinal or charge scattering.
Here we use the dipole charge form factor for the proton,with a p ⫽840 MeV/c. With Q 2 ⫽q 2 ⫺ ␻ 2 we create longitudinal scaling functions by

冉 冊冉

K Q 4
Q2
1⫹
F 共 y 兲 ⫽R L 共 q, ␻ 兲
Z q
4m N2

冊冉 冊
1⫹

Q2
a 2p

4

共A1兲

with R L (q, ␻ ) from the literature, Z as the nuclear charge,
and K as in Eq. 共2兲. Superscaling functions are created in thesame fashion as used for mesons, with k f from published fits
to the widths of electron scattering quasifree cross sections.

关8兴 R. A. Arndt and L. D. Roper, program SAID 共unpublished兲; R.
A. Arndt, J. M. Ford, and L. D. Roper, Phys. Rev. D 32, 1085
共1985兲.
关9兴 M. L. Goldberger and K. M. Watson, Collision Theory 共Wiley,
New York, 1964兲.
关10兴 J. Ouyang, S. Hoibraten, and R. J. Peterson, Phys. Rev. C 47,
2809 共1993兲.
关11兴 R. J. Peterson, Few-Body Syst., Suppl. 9, 17 共1995兲.
关12兴 C. J. Gelderloos et al., Phys. Rev. C 62, 024612 共2000兲.

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PHYSICAL REVIEW C 65 054601

关13兴 C. M. Chen, D. J. Ernst, M. F. Jiang, and M. B. Johnson, Phys.
Rev. C 52, 485 共1995兲.
关14兴 E. Friedman, A. Gal, and J. Mares, Nucl. Phys. A625, 272
共1997兲.
关15兴 S. A. Gurvitz, Phys. Rev. C 33, 422 共1986兲.
关16兴 P. Barreau et al., Nucl. Phys. A402, 515 共1983兲.

关17兴 J. Jourdan, Nucl. Phys. A603, 117 共1996兲.
关18兴 J. M. Finn, R. W. Lourie, and B. H. Cottman, Phys. Rev. C 29,
2230 共1984兲.
关19兴 C. F. Williamson et al., Phys. Rev. C 56, 3152 共1997兲.
关20兴 R. J. Peterson, Phys. Rev. C 60, 022201共R兲 共1999兲.

054601-8

E360/E546 A. Shinohara
Measurement of Electronic X rays correlated with Pionic X rays
A. Shinohara[attached]
Formation Process of Pionic Atoms and their Behavior in Material
J. Nucl. Radiochem. Sci., 1(1) (2000) 33-37.
● A. Shinohara, et al.
[attached]
Pion Transfer Process of Pionic Atoms in the Gas Mixtures Containing Hydrogen at Various
PressuresRadiochim. Acta, 89 (2001) 849-852.
◇ A. Shinohara, et al.
Pion Transfer Processes of Pionic Atoms in the Gas and Liquid Phases
5th Int. Conference on Nuclear and Radiochemistry, Pontresina, Sep. 3-8, 2000.
◇ K. Goto, et al.
Development of the Measuring System for Electronic X Rays Following Atomic Capture of
Negative Pions
Asia-Pacific Symposium on Radiochemistry 2001, Fukuoka, Oct. 30 – Nov. 1, 2001.
●________________________________________________________________________● Physics papers published in refereed journal.
○★
◇＊

Technical papers.
PhD theses.
Conference and Symposium.
Internal Report and others.

Journal of Nuclear and Radiochemical Sciences, Vol. 1, No. 1, pp. 33−37, 2000

33

Formation Process of Pionic Atoms and Their Behavior in Material
Atsushi Shinohara*
Department of Chemistry, Graduate School of Science, Osaka University, Machikaneyama-cho, Toyonaka, Osaka
560-0043, Japan
Received: December 9, 1999
Exotic atoms are the interesting object as a new probe in material science and a new chemical species in futurechemistry. Our group (πAX) has recognized the mesoatomic and mesomolecular phenomena as a subject ofchemistry, and continued the experimental study for the capture process of negative pions since 1982. In the
recent study, we have investigated the pion capture process mainly in liquid and gas phases, focusing to the behavior of pionic hydrogen atoms. A novel method to measure selectively the pion capture for a particular hydrogen ina molecule revealed the effect of the chemical state of the hydrogen on the transfer process. A proposed model,
which combines a LMM model with a transfer process, was useful to wholly understand the capture process
observed. The outline of the formation process of pionic atoms and its behavior in material is described briefly,
and the main results of our studies are reviewed.

1. Background of the Present Studies
Recent progress of an accelerator has allowed us to apply
various exotic atoms to material science, analytical chemistry,
medical science, etc, other than particle physics. Especially,
the use of muons has been increasing, such as µSR, muonium
chemistry, µCF, and so on, in addition to the use of positrons.
Muonic or pionic atoms, which consist of a negative muon orpion and an ordinary atom (nucleus), have been studied since
the discovery of the atoms, and the outline of the formationand decay process of the atoms has been established. 1 - 5
However, more detailed and fundamental studies for formation
mechanism and behavior of such atoms are required to develop
a new chemistry of the 2nd generation substance (muonic or
pionic atoms and molecules) and new application of the phenomenon.
It is accepted that the formation and decay of a pionic atomtake place according to the following step. A negative pion in
the first stage of the slowing down in matter loses the energy
by ionization, and it is slowed down by collision with electronof comparable velocities until finally it is captured by the
Coulomb field of an atom. The pion captured in an outer Bohrorbit cascades down accompanied by the emission of Auger
electrons and by the pionic X rays. In the capture process,“chemical effects” are observed because electrons are considered to affect the Coulomb field in the capturing stage. In thelower levels, the strong interaction affects the cascade schemeand finally causes the nuclear absorption of the pion.
Complete understanding has not yet been attained for themechanism of molecular and atomic capture of pions.
A large mesomolecular (LMM) model is more useful todescribe the capture process including the chemical effects
among various capture models proposed. This model was proposed by Ponomarev et al.1 to explain the obvious chemical
effect observed in hydrides, and extended to the capture phenomenon in general molecules by Scheuwly et al.3 In the firststep (probability P) of the model, it follows that the pion is
captured on the LMM orbital when the valence electron emission occurs, and on the atomic orbital when the core electronof the corresponding atom is emitted as Auger electron. The
pion on the LMM orbital moves to an atomic state of any atomin the molecule and then the pionic atom forms in the second*Corresponding author. E-mail: shino@chem.sci.osaka-u.ac.jp.
FAX: +81-6-6850-5418.

step (probability Q). If the pion is captured by hydrogen, thecapture probability of hydrogen (WH) is expressed by
WH = P Q R,

(1)where R means the probability that the pion in the pionichydrogen is finally captured by the proton without transfer toother atoms.
Ponomarev well explained the hydrogen effect by assuming
the second step to be radiative process. Schneuwly could takethe chemical effects into account in the LMM model by treating the probability P as a localization factor defined by the ionicity parameter σ introduced by Paurling. In our previous
works,6-9 we proposed a modified LMM model, in which thesecond step proceeds via Auger transition, based on the large
chemical effects observed for the pion capture ratio in a seriesof beryllium borides (BexBy) and in MBH4, MOH, and MF(M=Li, Na, K).
As for the pion transfer phenomenon, many gas phase
experiments performed mainly by Russian group showed thatthe pion transfer from pionic hydrogen atoms to other heavier
atoms occurs in gas phase in spite of its short life time. The
semi-empirical formulae for the transfer rate were also derived
from the systematic measurements. 2,4,10,11 The influence ofchemical environment on the transfer process, however, wasscarcely observed and was not involved in such phenomenological treatment.
2. Theme and Goal of the πAX Project
The goal of the πAX project lies in comprehensively understanding the formation process and behavior of pionic atoms
and molecules in order to apply the phenomenon to materialscience, analytical chemistry, bio and medical science and todevelop a new chemistry of 2nd generation substance (muonicor pionic atoms). Especially, the quantitative treatment for
pionic hydrogen atoms will provide a unique probe for hydrogen in material. Figure 1 shows the capture and decay
processes of negative pions in hydrogen-containing molecule(Z-H ). In the present studies (Visiting Researcher’s Programof KEK-PS: E202, E262, E360), we focused the formation andbehavior of pionic hydrogen atoms, as shown with the boldlines in the figure.
The observable quantities are the pionic X rays and two γrays from π0 decay (indication of the final capture by hydro-© 2000 The Japan Society of Nuclear and Radiochemical Sciences

Shinohara

34 J. Nucl. Radiochem. Sci., Vol. 1, No. 1, 2000

Figure 1. Pion capture process in hydrogen-containing molecules (Z-H).

gen). The former includes the information on the internaltransfer, and the change of the latter against the concentrationof Z'-atom reflects the external transfer rate. The selection of
the bond nature and structure of the Z-H molecule is also additional degree of freedom in the research technique from theviewpoint of chemistry.
Many experiments were carried out with a newly developed
apparatus, which allows us to measure simultaneously two γrays and low-energy pionic X rays, to achieve the goal mentioned above. These experiments are classified into the following theme.
(1) Selective measurement for the capture probability on thehydrogen at a particular site in the molecule.14,15
Systems: R(alkanes), ROH/ROD, RCOOH/RCOOD.
(2) Measurement of the external transfer rates by two-component systems in liquid and gas phases.16,17
Systems: C6H12/C6H6+CCl4/CS2, H2/CH4+Z(rare gas).
(3) Measurement of pressure dependence of transfer rate ingas phase (in progress).
Systems: H2/CH4+Ar(chaining pressure).
(4) Transfer and structure effects appearing in the intensity
patterns of pionic X rays.18,19
Systems: Fullerenes, H2/D2/CH4+SF6, & All samples studied.
(5) Measurement of the fundamental data (Pionic X-ray
yield, correction data,...).20
Systems: Various elements, H2O+D2O,...
(6) Application of the LMM model.21
Systems: Mx(acac)y, MHx.
(7) Proposal for the combined LMM model (LMM + transfer).14
Based on (1) and (2).
(8) Proposal for a new model including the dynamical
behavior of pionic hydrogen (in progress).
3. Experimental Method
The experimental setup was installed at the πµ-channel22 of
the 12-GeV proton synchrotron of the High Energy
Accelerator Research Organization (KEK). The channel comprises a D4Q2D2Q lens system and has a length of 11.0 m
from the production target to the sample position. The apparatus comprised an energy degrader, a collimator, a radiation
shield, a defining counter system, and a measuring chamber, as

Figure 2. Schematic view of the experimental setup.

shown in Figure 2. Negative pions were slowed down by thegraphite degrader so as to stop in the target. Incident and stopping pions in the sample were counted using a conventionalcounter telescope comprising four plastic scintillation counters(PS1, PS2, PS3, and PS4 (veto)).
The pionic X rays were measured with two or three Ge−
detectors in coincidence with the stop events (1*2*3*4 ). Twocoincident γ rays from a charge-exchange reaction, p + π− -> n
+ π 0 , π 0 -> 2γ (70 MeV each, 60% branching ratio), weredetected using a pair of Cherenkov-detector arrays. The beam
size at the sample position was defined as 4(H) × 3(V) cm2 by
the collimator and the defining counters (PS2 and PS3 in
Figure 2).
The solid or liquid sample was set in the chamber filled withhelium gas in order to avoid any disturbance from air.14 A rectangular container [5.5(H) × 7.0(V) cm2 in area and 2.5, 5.0, or
7.5 mm in thickness] made of thin beryllium or aluminum foil
was used for liquid and powder samples. The self-supporting
sample was also prepared for some compounds as a pressed
method. A high-pressure gas chamber was specially designed
for measurement of low-energy pionic X rays and π0 decays
from a gas sample.18 The chamber had cylindrical form to bearhigh pressure (50kgf/cm 2 ) and had the sample volume of
125cm3. The 2.5mm thick chamber wall was made of berylli-Formation Process of Pionic Atoms and Their Behavior

J. Nucl. Radiochem. Sci., Vol. 1, No. 1, 2000 35

um to measure low-energy pionic X rays (ex. 18keV for carbon). PS-3 and PS-4 counters were mounted inside the chamber and used as the phoswich type counter system.
4. Main Results of the Present Project
Among the above theme, the results of (1) , (3), and (7)were the noticeable ones in our work. Especially, the selective
measurement for hydrogen was successfully done by using thedeuterated compounds.15 The dependence of the transfer rateon gas pressure was also observed at the first time in the
CH4+Ar system. The former will provide a useful technique toinvestigate individually the behavior of the pionic hydrogen
atom originating from a hydrogen atom at different chemical
states in a molecule or material. These results require a newmodel for the capture and transfer processes, which include thelife time of the pionic hydrogen atom and the chemical effects
in the transfer process. Main results of the project are briefly
described below.
4.1. Measurements for External Transfer in Alkanes anda Combined LMM Model.14 We attempted to reveal the contribution of the external transfer process in the pion capture forhydrogen-containing compounds in the condensed phase. We
then measured precisely the capture process in a series ofhydrocarbon (liquid alkanes), which had been analyzed withno contribution of the external transfer so far.23 To explain the
result obtained, we proposed a new model involving a modified LMM model and an external transfer process as mentioned below.
In the modified LMM model,7,9 we introduced a parameteraγ to express the capture probability on hydrogen, where aγ isdefined as a ratio of the sum of the radiative and the nuclear
absorption rates to the Auger transition rate. The probabilityof pion capture by hydrogen WH is obtained from eq 2:ν(1+σ) a γ
WH =
,
(2)
(N+ν)(1-σ)Zeff
where N is the sum of the core electrons relevant to the capture
process (N=kn2+ln1, n1=0), ν presents the total valence electrons (ν=kν2+l).9 The factor a γ /Zeff is thought to correspond to
the the product of the parameter Q and R from a comparison
with eq 1. In this model, therefore, we approximate the parameter a γ as the product of two factors, a γ' and R', where R' is
the non-transfer probability associated only with the externaltransfer. The capture probability WH is then rewritten asν(1+σ) a γ'
WH =
R'.
(3)
(N+ν)(1-σ)Zeff

given as an average over the chemically different states of theelement in the molecule. Observation for the pion capture
probability on a particular atom in the molecule, if possible,
will give significant information for understanding the pion
capture mechanism. We then tried to extract the capture rate
for a particular hydrogen atom using deuterated compounds.
The capture events of pionic hydrogen undergoing no transfer
can be measured by two-γ-rays of π0 produced by the charge
exchange reaction. On the contrary, the reaction of a pionic
deuterium (π−d) is strongly suppressed (about 10−4).24 The
measurement for deuterated molecules therefore provides thecapture probability for hydrogen except for hydrogen at thedeuterated site. Thus we can obtain the capture rate for a particular hydrogen atom with a comparison between the ordinary
compound and the deuterated one.
We selected a series of alcohols, carboxylic acids, and thecorresponding compounds with a deuterated hydroxyl (-OD) or
carboxyl (-COOD) group to study the behavior of the pionic
atoms originating from hydrogen in different chemical states.
As a result, we found that pionic hydrogen atoms from hydroxyl and carboxyl groups show different behavior as demonstrated
in Figures 3a and 3b. The pionic hydrogen from hydroxyl
group strongly favors the transfer to oxygen and that from carboxyl group shows almost no transfer phenomenon. The transfer rate parameters ΛO of individual pionic hydrogen for oxygen
were determined based on the combined LMM model usingΛC=1.7 mentioned above. The result indicates that the chemical state of the hydrogen in the molecule affects the life or

Figure 3a. Capture ratios per atom of the hydrogen bound to carbon
relative to the carbon, RCH/RC, as a function of the number (n) of carbon atoms for alcohols ( ) and carboxylic acids ( ). The dashedline represents the capture ratio for polyethylene. Solid curves are themodel calculations.

The probability R' can be expressed approximately by
R' = 1/(1 + ΛZCZ),(4)

based on a phenomenological model, where ΛZ is the reduced
pion-transfer rate relative to the charge-exchange rate in thecollision with Z-atoms, and CZ is the ratio of the number of theZ-atom to the hydrogen atom. The a γ' value should be recognized as being the probability of isolation of a π−p from the ZH bond or of absorption by the proton without experiencing aninternal transfer (the latter probability is assumed to be negligibly small in eq 2) and hence is determined as a particular parameter for the kind of Z-H bond.
In conclusion, we found the contribution of the externaltransfer in liquid alkanes and obtained the transfer rate parameter for carbon as ΛC=1.7+0.2 and the parameter for isolation
process from the molecule as a γ'=0.140+0.008 based on the
proposed model. The obtained transfer parameters are summarized in Table 1 together with the semi-empirical estimates.4
4.2. Selective Measurement for the Capture on Hydrogen.15
In the analysis of the pion capture process, the main problem isthe fact that the capture probability for a particular Z atom is
2

Figure 3b. Capture ratios per atom of the hydrogen bound to oxygen
relative to the oxygen, ROH/RO, as a function of the number (n) of carbon atoms for alcohols ( ) and carboxylic acids ( ). Solid curve isthe model calculations with ΛC=1.7 and ΛO=4.1, and the dotted curve
represents those with a γ'=0.14 for alcohols. No external transfer was
found for carboxylic acids.

Shinohara

36 J. Nucl. Radiochem. Sci., Vol. 1, No. 1, 2000
velocity of the isolated pionic hydrogen atom and causes thechemical effect in the transfer process.
4.3. Measurement of the External Transfer by TwoComponent Systems in Liquid and Gas Phases.16,18 The
external transfer process is explicitly observed in two-component system consisting of the hydrogen-containing moleculeand the hydrogen-non-containing one. For the liquid phase,
we selected C6H12 or C6H6 + CCl4 or CS2 systems which don’treact with each other and mix with each other in any concentration. The external transfer rate was derived from the dependence of two-γ-events on the capture fraction of C6H12 or C6H6
molecule, which was obtained from the pionic X-ray measurement. In this study, we revealed the structure effect for thetransfer process by comparing between the transfer rate forcarbon of alkaline (sp3 structure) and that for carbon of benzene (sp2 structure) as shown in Table 1. The transfer rates at
51kgf/cm2 were also measured for the H 2 + Z and CH4 + Z(Z=He, Ne, Ar, Kr, Xe) systems to compare with the systematics for the atomic number Z. The obtained external transfer
rate parameters are listed in Table 1 and plotted as a functionof Z1/3 in Figure 4 together with the predictions by the semiempirical formulae.4,10,11
TABLE 1: Transfer Rate Parameters Obtained in the Present
Study
ΛZ
System (liq.) Z=COCl(CCl4)R(alkanes)
1.7±0.2
−
−
ROH
1.7*
4.1±0.5
−
RCOOH
1.7*
4.5±0.5−
C6H12+CCl4
1.7*−
5.9±0.4
C6H6+CCl4
3.4±0.6−
5.9*
Systematics**
2.5
3.0
4.7
System (gas) Z=
He
Ne
Ar
Kr
Xe
H2+Z
1.39±0.10 4.83±0.22 7.21±0.43 8.35±0.49 9.89±0.95
CH4+Z
1.2 ±0.1 2.64±0.22 3.59±0.17 4.23±0.42−* The parameter was fixed in the analysis procedure.
** Estimated from the semi-empirical formula for CmHn+Z (Ref. 4).

4.4. Pressure-Dependence of the Transfer Rate in the Gas
Phase (in Progress). The dependence of the transfer rate on
the gas pressure has not been observed so far in the muon orpion capture process for gas mixtures. The pressure-dependence may be observed for the pionic atoms from the boundhydrogen, which may have a short life as mentioned above, if
the running path of the pionic hydrogen is short and comparable to the mean free path in the material. We then measured
the change of the transfer rates at the pressure 50kgf/cm 2 ,13.5kgf/cm2, and 6.0kgf/cm2 for the H2 + Ar and CH4 + Ar systems.

Figure 4. Z-dependence of the transfer rate parameters (ΛZ). Solidline is the semi-empirical estimates for H2+Z system,4,10 and dashedline for CnHm+Z system.4,11: H2+Z, : CH4+Z, : CmHn+Z(liq.),: C6H6+Z(liq.), Z=alkanes.

Figure 5 shows the pion capture ratios of hydrogen to carbon against the fraction of Ar for different pressures for the
CH 4 + Ar system. One can see small change but apparentchange of the transfer rate with pressure in the figure. On theother hand, we can observe no change of the transfer rate for
the H2 + Ar system.
We can roughly estimate for the mean free path in Ar to beabout 10−8m in gas phase at the pressure of 50kgf/cm2, assuming that the effective interaction radius is an atomic dimension,and for the traveling length (tv) of the pionic hydrogen atom tobe 6×10−10 to 2×10−8 m, providing that the velocity (v) of it isestimated from the kinetic energy of 1−2 eV.25 Then overlapping between the ranges of the mean free path and the travelinglength suggests the possibility that the transfer rate changes
depending on the gas pressure and between the gas and condensed phases. If the observed result is attributed to the mechanism mentioned above, the fact that the pressure-dependence
was observed only for the CH4 system is consistent with thedifference in the life time (t) between the excited states of thepionic hydrogen atom from CH4 and that from H2.
4.5. Transfer and Structure Effects Appearing in the
Intensity Patterns of Pionic X Rays.13,18 Change of the intensity pattern and yields of pionic X rays will be caused by thecontribution of the pion transferred from a pionic hydrogen as
well as the different angular momentum distribution on the initial stage of the capture process. The relation of the change ofintensity ratio was investigated from the viewpoint of the molecular structure and the number of hydrogen for the pionic X
rays of carbon and oxygen for most samples studied. It was
found that the pion brought by the internal transfer mainly contribute the atomic cascade for the pionic X rays of the atombound to hydrogen. The pure structure effect was observed in
the measurements for carbon allotropes including fullerenes
and for various single substances. We discussed about theatomic levels concerned with the transfer based on the cascadecalculations including the contribution of the transferred pion.
5. Summary
Figure 4 shows the dependence of the transfer rate parameters obtained on the atomic number and a comparison between
the transfer rates in the liquid phase and those in gas phase.
One can see that the present results slightly deviate from theZ1/3-dependence which is assumed in the empirical formulae,
and that the transfer rates in liquid phase are rather faster thanthose in gas phase for alkane systems. The pressure-dependence of the transfer rate was also found for alkane systems. It
is generally accepted that the difference between the H2 system

Figure 5. Pion capture rations of hydrogen to carbon atoms against: 13.5the fraction of Ar at different gas pressure.
: 51kgf/cm2,
kgf/cm2, : 6.0kgf/cm2.

Formation Process of Pionic Atoms and Their Behavior
and CH4 system in gas phase is attributed to the fact that thepionic hydrogen atom from the bound hydrogen (CH4) lies in alower excited state compared with that from H2 molecule.4 If
this means that the life of the pionic hydrogen from alkanes isshorter than that from H2, the results of the pressure dependence is qualitatively consistent with the difference between
the liquid and gas phases. The present result also indicates thata dynamical model, in which the life, velocity, molecular
structure, and so on are taken into consideration, is required forthe pionic hydrogen (with short life) originating from boundhydrogen.
In conclusion, the combined LMM model and the cascadecalculation including transfer process successfully explained
the results of the capture rates, transfer rates, and pionic X-ray
intensity patterns for most sample systems studied. Here wecan conclude that these model calculations are very useful tounderstand the formation and decay process of most pionic
atoms in various materials. After this, quantitative discussion
about the discrepancy between the results of liquid and gas
phases and further accumulating the data of a γ' and ΛZ for various chemical systems are needed to achieve the goal of ourstudy.
Acknowledgements. The πAX project is a collaboration by
Drs. T. Saito, T. Miura, A. Yokoyama, T. Muroyama, K.
Takamiya, Y. Hamajima, H. Muramatsu, S. Kojima, Y.
Yoshimura, N. Imanishi, I. Fujiwara, M. Furukawa, H. Baba,
and A. Shinohara, and many graduate students of Nagoya,
Osaka, Kyoto, and Niigata Universities. The experiments were
performed under the Visiting Researcher’s Program of KEK.
We are grateful to Professors K. Nakai, K. Nakamura, K.
Takamatsu, T. Oshima, J. Chiba for their interest and supports.
We also wish to thank the staff of the KEK-PS and the beam
channel for steady provision of the pion beam.
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Radiochim. Acta 89, 849–852 (2001)
 by Oldenbourg Wissenschaftsverlag, München

Pion transfer processes of pionic atoms in the gas mixturescontaining hydrogen at various pressures
By Atsushi Shinohara1 , ∗, Taichi Miura2 , Akihiko Yokoyama1 , † , Koichi Takamiya3 , Tetsuya Kaneko4 ,Toshiharu Muroyama5 , Tadashi Saito6 , Yasunori Hamajima7 , Hisakazu Muramatsu8 , Sadao Kojima9 , Hiroshi Baba6 andMichiaki Furukawa10
1

Department of Chemistry, Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan
Radiation Science Center, High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japan
3
Research Reactor Institute, Kyoto University, Osaka 590-0491, Japan
4
Faculty of Science, Niigata University, Niigata 950-2181, Japan
5
Low Level Radioactivity Laboratory, Kanazawa University, Nobi-gun, Ishikawa 923-1224, Japan
6
Radioisotope Center, Osaka University, Toyonaka 560-0043, Japan
7
Faculty of Science, Kanazawa University, Kanazawa 920-1192, Japan
8
Faculty of Education, Shinshu University, Nagano 390-8621, Japan
9
Radioisotope Research Center, Aichi Medical University, Nagakute-cho, Aichi 480-1195, Japan
10
Faculty of Environmental and Information Science, Yokkaichi University, Yokkaichi 512-8045, Japan
2(Received March 5, 2001; accepted in revised form August 31, 2001)

Negative pion capture / Pionic X rays /Pion transfer process / Pionic atoms / Pionic hydrogen /Gas phase

Summary. The pion capture process in gas mixtures containing hydrogen was studied by measuring pionic X raysand π 0 decays. A little difference in pion transfer process
was found between the H2 + D2 and H2 O + D2 O systems.
Pressure-dependence of the transfer rates was also revealedin the CH4 + Ar system by the measurements at several gas
pressures. The mechanism of the pion transfer process was
discussed based on a comparison between transfer parametersin the gas and liquid phases.

1. Introduction
Muonic or pionic atoms, which consist of a negative muonor pion and an ordinary atom (nucleus), have been studied
since the discovery, and the outline of the formation anddecay process has been established [1–6]. However, moredetailed and fundamental studies of the formation mechanism and behavior of the exotic atoms are required to develop
new applications of the phenomenon.
A negative pion is slowed down in matter through ionization or collisions with electrons at the first stage and captured by the Coulomb field of an atom. The pion captured
in an outer atomic orbit cascades down accompanied by theemission of Auger electrons and by the pionic X rays. In thecapture process of molecules, “chemical effects” have been
observed so that the electron distribution and chemical structure are considered to play an important role in the captur*Author for correspondence†(E-mail: shino@chem.sci.osaka-u.ac.jp).
Present address: Faculty of Science, Kanazawa University,
Kanazawa 920-1192, Japan.

ing stage. To explain the obvious chemical effect observedin some hydrides, a large mesomolecular (LMM) model was
proposed by Ponomarev et al. [1]. The capture model was extended to the capture phenomenon in general molecules byScheuwly et al. [4], and revised to better reproduce the results for light-element compounds in our previous work [6].
As for the pion transfer phenomenon, many gas phase experiments performed mainly by a Russian group showed thatthe pion transfer from pionic hydrogen atoms to other heavier atoms occurs in gas phase in spite of its short life-time.
Semi-empirical formulae for the transfer rate were also derived from the systematic measurements [2, 3, 5]. Recently,
we successfully performed the selective measurement forhydrogen by using deuterated compounds [7] and proposeda new model involving a modified LMM model and an external transfer process [8]. This method will provide useful
information on the individual behavior of pionic hydrogenatoms from different sites in a molecule or material. The
influence of chemical environment on the transfer process,however, has been scarcely observed and is not involved ina phenomenological treatment [5].
We have investigated the pion capture process for various
molecules in liquid and gas phases, focusing on the behavior of pionic hydrogen atoms [7–10]. Here, the recent resultsof the gas phase experiments at various gas pressures arereported.

2. Experimental
The experimental setup was installed at the πµ-channel [11]of the 12-GeV proton synchrotron of the High Energy Accelerator Research Organization (KEK). Fig. 1 shows theschematic view of the experimental setup. Negative pions
were slowed down with a graphite degrader so as to stopin the target. Incident and stopping pions in the sample

850

A. Shinohara et al.

3. Results and discussion
The external transfer rate can be derived from the intensity change of 2γ events on the capture fraction of eachcomponent, which was obtained from the pionic X-ray
measurement. The true 2γ events, from which the chargeexchange probability (WH ) was derived, were obtained fromthe trajectory analysis of individual events recorded in the
Cherenkov-detector arrays and the measurements without
target.
The WH value in the H-D mixture system is expressed by
the following formula as a function of CD (the ratio of thenumbers of deuterium atoms to hydrogen atoms),
WH =Fig. 1. Schematic view of the experimental setup. The apparatus comprised an energy degrader, a collimator, a radiation shield, a defining
counter system, and a measuring chamber. The measuring chamber was replaced by a high-pressure gas chamber in the gas-sample
measurement.

were counted using a conventional counter telescope (PS1,2,3,4). A typical intensity of the incident pion beam was
1 × 105 π − /4 sec. The pionic X rays were measured with twoor three Ge detectors in coincidence with the stop events, andtwo coincident γ rays (2γ ) from a charge-exchange reaction,p + π − → n + π 0 (60% branching ratio), π 0 → 2γ(70 MeV),
were detected using a pair of Cherenkov-detector arrays. The
capture rates for all elements other than hydrogen and helium
can be obtained from the pionic X-ray measurements and thecapture on hydrogen is derived from the 2γ events.
A high-pressure gas chamber was specially designed for themeasurement of low-energy pionic X rays and π 0 decays ina gas sample [10]. The chamber had a cylindrical form withthe sample volume of 125 cm3 and its 2.5 mm thick wall
was made of beryllium to bear high pressure (50 kg/cm2 )and to measure low-energy photons (e.g., 18 keV for a 3–2
transition in pionic carbon). PS-3 and PS-4 counters were
mounted inside the chamber and used as phoswich type
counter system.
The external transfer process is explicitly observed in
the two-component system consisting of the hydrogencontaining molecule and the hydrogen-non-containing one.
We selected the H2 + D2 system and the H2 or CH4 + Ar
system with the mixing fraction ranging from 5% to 80%.
The concentration was analyzed with a gas chromatograph.
The measurements of the H2 or CH4 + Ar system weremade at the gas pressure of 50 kgf/cm2 , 13.5 kgf/cm2 , and6.0 kgf/cm2 to investigate the behavior of a pionic hydrogenin various atomic densities.
Table 1. Transfer parameters obtained for
the H2 + D2 system.
ΛκΛ(1 + κ)1 + κCD1,
1 + CD 1 + κCD + ΛCD(1)where κ is the ratio of nuclear capture probability in theπ − p + d to π − p + p collision, and Λ is the ratio of thetransfer probability in the π − p + d collision to the nuclear capture probability in the π − p + p collision. The results for the H2 O + D2 O system have been already reported
elsewhere [9]. Fig. 2 shows the obtained transfer probability, 1 − (1 + C)WH , for the H2 + D2 system as a functionof C together with the fitted values (doted curve). There
was no difference between the results at 45 kgf/cm2 and at
15 kgf/cm2 within the attached errors. The present results
are compared with the previous ones in Table 1. The obtained parameters are close to the values of “World fit” [12].
In a comparison between the H2 + D2 and H2 O + D2 O systems, we can conclude that the transfer rate from H to D
does not change in these two systems from the transfer pa-

Fig. 2. Transfer probabilities [1 − (1 + C)WH ] as a function of C. The
filled circles are the results for 45 kgf/cm2 , and the open triangles for
15 kgf/cm2 . The doted curve presents the fit for the present results.

Present work

Weber et al. [12]“World fit” [12]cf. H2 O + D2 O [9]0.50 ± 0.05
0.92 ± 0.15
0.35 ± 0.04

0.65 ± 0.07
1.40 ± 0.22
0.32 ± 0.03

0.45 ± 0.04
0.93 ± 0.14
0.33 ± 0.03

0.52 ± 0.04
0.73 ± 0.11
0.42 ± 0.04

851

Pion transfer processes of pionic atoms in the gas mixtures

ical model [2, 4].
IH
1
,
=C
IH (pure)
1 + ΛZ CZIZC ΛZ CZ,= (1 − C) +IZ (pure)
k 1 + ΛZ CZ

Fig. 3. Pion capture ratios of hydrogen to carbon atoms against thefraction of Ar in the CH4 + Ar system at different gas pressures.

rameter Λ observed, while we can see a small differencein the transfer parameter κ. This finding may lead to further understanding of the mechanism of the pion transfer
process.
Fig. 3 shows the pion capture ratios of hydrogen to carbon versus the fraction of Ar for different pressures for
the CH4 + Ar system. One can see a small change of thetransfer rate with pressure in the figure, although the dataobtained are rather scattered. On the other hand, we canobserve no change of the transfer rate for the H2 + Ar
system.
Here, we will discuss briefly the observed pressuredependence of the transfer rate in the gas phase. Such a dependence has not been observed so far in the muon or pion
capture process for gas mixtures. The pressure-dependence
may be, however, observed for the pionic atoms from thebound hydrogen, if the traveling length of the pionic hydrogen is shorter than or comparable to the mean free pathof the pionic hydrogen in the material. The new finding
about the kinetic energy of pionic hydrogen by Daum’s
group [13] may provide an important information on thetraveling length. However, the obtained data is too poor to
discuss the relation between the Coulomb de-excitation processes of pionic hydrogen and the transfer parameters κ.
We roughly estimated the mean free path in Ar to be about
10−8 m in gas phase at the pressure of 50 kg/cm2 , assuming that the effective interaction radius is an atomic dimension, and the traveling length (tv) of the pionic hydrogen atom is estimated to be 6 × 10−10 to 2 × 10−8 m. Here
we deduced the velocity (v) from the average kinetic energy of 1–2 eV [14], neglecting the high-energy component of the kinetic energy [13]. The overlap between theranges of the mean free path and the traveling length suggests the possibility that the transfer rate changes depending on the gas pressure and between the gas and liquid
phases.
In the H2 + Z system, the transfer rate was determinedfrom the 2γ events (IH ) and X-ray intensity (IZ ) per incident
pion by the following formulae based on the phenomenolog-

(2)
(3)

where ΛZ is the transfer rate parameter for a Z-atom, CZ
is the ratio of the number of Z-atoms to that of hydrogenatoms, k is the ratio of the capture rate for a Z-atom tothat for a hydrogen atom, and C is the concentration ofhydrogen. The second term in Eq. (3) corresponds to theincrease of the transfer from hydrogen to the Z-atom. This
term is generally small because k
1 except for helium.
The analysis of the pionic X-ray spectrum showed that thedependence of IZ on the concentration C was approximately
linear. This fact means that the above formulae hold for thiscase. We can also use the similar formulae for the CH4 (oralkanes) +Z system if there is no pressure-dependence. The
transfer parameters obtained are summarized in Table 2 together with the result for the liquid sample reported elsewhere [10].
Fig. 4 shows the values of log(ΛZ ) versus the mean freepath of a neutral atom in the corresponding gas pressure. We
can deduce from the figure that the life-time of the pionic hydrogen atom in the CH4 + Ar system is shorter than the time,
which the pionic hydrogen atom takes to collide with theneighboring atom, while in the H2 + Ar system the life-time
is much longer than that time. The fact that the pressuredependence was observed only for the CH4 + Ar system isFig. 4. Dependence of log(ΛZ ) versus the mean free path of a neutral
atom in the gas phase at the pressure studied. Open circles: CH4 + Ar
system, open triangles: H2 + Ar system, filled circle: Liquid alkane +CCl4 system.

Table 2. Transfer parameter ΛZ (Z = Ar) for various pressures.
Pressure (kgf/cm2 )H2 system

CH4 system(Liquid)
51.0
13.5
6.0

−
7.21 ± 0.43
7.13 ± 0.45
6.55 ± 0.71

5.9 ± 0.4 a
3.59 ± 0.17
2.52 ± 0.20
1.61 ± 0.38

a: Taken from [10], Z = Cl.

852

also consistent with the difference in the excited state between the pionic hydrogen atom from CH4 and that from H2 .
The contribution from the Coulomb de-excitation processesof pionic hydrogen will also provide qualitatively the similar relationship for the traveling length and the excited stateof the pionic hydrogen atom. If the pressure dependence ofthe transfer rate were confirmed, the result would provide animportant insight in the dynamics of the pionic hydrogen.

4. Conclusions
In conclusion, we found a small difference in the transfer
parameters between the H2 + D2 and H2 O + D2 O systems,
and the dependence of transfer rate on the density for thepionic hydrogen originating from the bound hydrogen. The
mechanism of the pion transfer from pionic hydrogen toother atoms was briefly discussed based on a comparison between transfer parameters in the liquid and gas phases. The
pressure-dependence observed only for the CH4 system isconsistent with the previous result that the life-time of thepionic hydrogen atom from the bound hydrogen would beshorter than that from H2 . Further improved measurements
are needed to confirm the pressure dependence of the piontransfer process and to quantitatively discuss the dynamical
behavior of the pionic hydrogen.
Acknowledgment. The experiments were performed under the Visiting
Researcher’s Program of KEK. We are grateful to Professors K. Nakamura and Y. Yoshimura for their interest and supports. We also wish to
thank the staff of the KEK-PS and the beam channel group for steadyprovision of the pion beam.

A. Shinohara et al.

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PhD theses.
Conference and Symposium.
Internal Report and others.

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27 Jul - 2 Aug 2000.
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XXII Physics in Collision Conference Stanford, California, June 20-22, 2002,
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10th Vienna Conference on Instrumentation, Feb. 16-21 2004, Vienna, Austria.
KK JooResults from the K2K Experiment
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Physics Letters B 511 (2001) 178–184
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Detection of accelerator-produced neutrinos at
a distance of 250 km
S.H. Ahn h , S. An h , S. Aoki g , H.G. Berns t , H.C. Bhang o , S. Boyd t , D. Casper b ,T. Chikamatsu f,1 , J.H. Choi c , S. Echigo g , M. Etoh q,2 , K. Fujii g , S. Fukuda r ,Y. Fukuda r , W. Gajewski b , U. Golebiewska s , T. Hara g , T. Hasegawa p , Y. Hayato f ,J. Hill j , S.J. Hong h , M. Ieiri f , T. Inada n , T. Inagaki i , T. Ishida f , H. Ishii f , T. Ishii f ,H. Ishino f,3 , M. Ishitsuka r , Y. Itow r , T. Iwashita g , H.I. Jang c,4 , J.S. Jang c , E.J. Jeon f ,E.M. Jeong c , C.K. Jung j , T. Kadowaki n , T. Kajita r , J. Kameda r , K. Kaneyuki r ,I. Kato i , Y. Kato f , E. Kearns a , S. Kenmochi k , B.H. Khang o , A. Kibayashi e ,D. Kielczewska s , B.J. Kim o , C.O. Kim h , H.I. Kim o , J.H. Kim o , J.Y. Kim c , S.B. Kim o ,S. Kishi n , M. Kitamura g , K. Kobayashi r , T. Kobayashi f , Y. Kobayashi r , M. Kohama g ,D.G. Koo h , Y. Koshio r , W. Kropp b , G. Kume g , E. Kusano f , J.G. Learned e , H.K. Lee c ,J.W. Lee h , S.B. Lee f , I.T. Lim c , S.H. Lim c , H. Maesaka i , K. Martens j,5 ,T. Maruyama p,6 , S. Matsuno e , C. Mauger j , C. McGrew j , M. Minakawa f , S. Mine b ,M. Miura r , S. Miyamoto f , K. Miyano k , S. Moriyama r , S. Mukai i , M. Nakahata r ,K. Nakamura f , M. Nakamura k , I. Nakano l , T. Nakaya i , S. Nakayama r , K. Nakayoshi f ,K. Nishijima q , K. Nishikawa i , S. Nishiyama g , S. Noda g , H. Noumi f , Y. Obayashi r ,J.K. Oh h , A. Okada r , M. Onchi g , T. Otaki g , Y. Oyama f , M.Y. Pac d , H. Park f ,S.H. Park h , S.K. Park h , A. Sakai f , M. Sakuda f , N. Sakurai r , N. Sasao i , K. Sato g ,K. Scholberg a,7 , E. Seo o , E. Sharkey j , K. Shiino f , A. Shima i , M. Shiozawa r , H. So o ,H. Sobel b , A. Stachyra t , J.L. Stone a , L.R. Sulak a , A. Suzuki g , Y. Suzuki f , Y. Suzuki r ,M. Takasaki f , M. Takatsuki g , K. Takenaka g , H. Takeuchi r , Y. Takeuchi r , N. Tamura k ,K.H. Tanaka f , Y. Tanaka g , K. Tashiro g , K. Tauchi f , T. Toshito r , Y. Totsuka r ,V. Tumakov f , T. Umeda l , M. Vagins b , C.W. Walter a , R.J. Wilkes t , S. Yamada r ,T. Yamaguchi l , Y. Yamanoi f , C. Yanagisawa j , H. Yokoyama i , H. Yokoyama n , J. Yoo o ,M. Yoshida m , S.Y. You ca Department of Physics, Boston University, Boston, MA 02215, USAb Department of Physics and Astronomy, University of California, Irvine, CA 92697-4575, USAc Department of Physics, Chonnam National University, Kwangju 500-757, South Korea
d Department of Physics, Dongshin University, Naju 520-714, South Korea
e Department of Physics and Astronomy, University of Hawaii, Honolulu, HI 96822, USA
f Institute of Particle and Nuclear Studies, KEK, Tsukuba, Ibaraki 305-0801, Japang Kobe University, Kobe, Hyogo 657-8501, Japanh Department of Physics, Korea University, Seoul 136-701, South Korea
i Department of Physics, Kyoto University, Kyoto 606-8502, Japan

0370-2693/01/$ – see front matter  2001 Published by Elsevier Science B.V.
PII: S 0 3 7 0 - 2 6 9 3 ( 0 1 ) 0 0 6 4 7 - 5

K2K Collaboration / Physics Letters B 511 (2001) 178–184

179

j Department of Physics and Astronomy, State University of New York, Stony Brook, NY 11794-3800, USA
k Department of Physics, Niigata University, Niigata, Niigata 950-2181, Japanl Department of Physics, Okayama University, Okayama, Okayama 700-8530, Japanm Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japann Department of Physics, Science University of Tokyo, Noda, Chiba 278-0022, Japano Department of Physics, Seoul National University, Seoul 151-742, South Korea
p Research Center for Neutrino Science, Tohoku University, Sendai, Miyagi 980-8578, Japanq Department of Physics, Tokai University, Hiratsuka, Kanagawa 259-1292, Japanr Institute for Cosmic Ray Research, University of Tokyo, Kashiwa, Chiba 277-8582, Japans Institute of Experimental Physics, Warsaw University, 00-681 Warsaw, Polandt Department of Physics, University of Washington, Seattle, WA 98195-1560, USA

Received 22 March 2001; accepted 15 May 2001
Editor: W. Haxton

Abstract
The KEK to Kamioka long-baseline neutrino experiment (K2K) has begun its investigation of neutrino oscillations suggested
by atmospheric neutrino observations. Twenty-eight neutrino events have been detected in coincidence with the expected arrival
time of the beam in the 22.5 kt fiducial volume of Super-Kamiokande, the far detector at 250 km distance. The expectation is37.8+3.5−3.8 , derived using measurements of neutrino interactions in a near detector and extrapolation using a beam simulation
validated by a measurement of pion kinematics after production and focusing. The background is of order 10−3 events.  2001
Published by Elsevier Science B.V.
PACS: 14.60.Pq; 13.15.+g; 23.40.Bw; 95.55.Vj

The Standard Model of electroweak interactions has
been tested with exceptional precision. The model,
however, does not address the question of the originof generations and their mixing. The existence ofneutrino oscillations implies that neutrinos are massive
and that lepton flavors are not conserved quantum
numbers. It enables us to study relations between massand flavor eigenstates in the lepton sector. Experiments
on atmospheric neutrinos have found a significant
1 Present address: Miyagi Gakuin Women’s College, Sendai,
Miyagi 981-8557, Japan.
2 Present address: Research Center for Neutrino Science, Tohoku
University, Sendai, Miyagi 980-8578, Japan.
3 Present address: Tokyo Institute of Technology, Tokyo 1528550, Japan.
4 Present address: Department of Civil Engineering, SeokangCollege, Kwangju, 500-742, South Korea.
5 Present address: Department of Physics, University of Utah,
Salt Lake City, UT 84112, USA.
6 Present address: Institute of Particle and Nuclear Studies,
KEK, Tsukuba, Ibaraki 305-0801, Japan.
7 Present address: Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.

deficit in the flux of νµ which have travelled anearth scale distance[1–3]. The interpretation of theseresults provides not only strong evidence of νµ → ντ(or νµ → νsterile ) oscillations [4,5] but also evidence
for different mixing properties in the lepton andquark sectors. For a neutrino energy Eν (GeV) anda distance from the source L (km), the oscillation
probability can be written in terms of the mixing
angle θ and the difference of the squared masses
m2 (eV2 ) in two flavor approximation as P (νµ →νx ) = sin2 2θ sin2 (1.27 m2L/Eν ).
The KEK to Kamioka long-baseline neutrino experiment (K2K) is the first accelerator-based experimentwith hundreds of km neutrino path length. The intense,nearly pure neutrino beam (98.2% νµ , 1.3% νe , and
0.5% ν̄µ ) has an average L/Eν ≈ 200 (L = 250 km,
Eν  ≈ 1.3 GeV). The neutrino beam properties aremeasured just after production, and the kinematics
of parent pions are measured in situ to extrapolate
the measurements at the near detector to the expectation at the far detector. K2K focuses on the study ofthe existence of neutrino oscillations in νµ disappear-

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K2K Collaboration / Physics Letters B 511 (2001) 178–184

ance that is observed in atmospheric neutrinos, andon the search for νµ → νe oscillation with well understood flux and neutrino composition in the m2 
2 × 10−3 eV2 region. In this Letter, the first resultson the event rates with 100 days of data taken from
June 1999 to June 2000 corresponding to 2.29 × 1019
protons on target, and the performance of the criticalcomponents of the long-baseline neutrino experimentare described.
The K2K neutrino beam [6,7] is a horn-focusedwide band νµ beam. The primary beam for K2K
is 12 GeV kinetic energy protons from the KEK
proton–synchrotron (KEK–PS) [8]. Every 2.2 s, approximately 6 × 1012 protons in nine bunches are fastextracted in a single turn, making a 1.1 µs beam spill.
These protons are focused onto a 30 mm diameter,
66 cm long aluminum target which is a current carrying element in the first of a pair of horn magnets
operating at 250 kA [9]. This design maximizes theefficiency of the toroidal magnetic field to focus positive pions produced in proton–aluminum interactions,while sweeping out negative secondary particles.
Downstream of the horn system, before the 200 m
long decay volume where the pions decay to νµ andmuons, a gas-Cherenkov detector (PIMON) [10] is occasionally put in the beam to measure the kinematics of the pions after their production and subsequentfocusing. After the decay volume, an iron and concrete beam dump stops essentially all charged particles
except muons with an energy greater than 5.5 GeV.
Downstream of the dump there is a “muon monitor” (MUMON) [10] consisting of a segmented ionization chamber and an array of silicon pad detectors. This monitors the residual muons spill-by-spill
to check beam centering and muon yield. The ionization chamber consists of 5 cm wide strips coveringroughly 2 × 2 m2 area with separate planes for horizontal and vertical read-out. The 26 silicon pads, eachof ∼ 10 cm2 transverse area, are distributed through a
3 × 3 m2 area.
Neutrino interactions near the production site aremeasured by a set of detectors with complementary abilities. The detectors are located approximately
300 m from the pion production target with approximately 70 m of earth eliminating virtually all prompt
beam products other than neutrinos. A one kiloton
water Cherenkov detector (1 kt) uses the same technology and analysis algorithms as the far detector,

Super-Kamiokande (SK). It has 680 20 photomultiplier tubes (PMTs) on a 70 cm grid lining a 8.6 m
diameter, 8.6 m high cylinder. The PMTs themselves
and their arrangement are the same as in SK, giving the same fractional coverage by photo-cathode(40%). A scintillating fiber detector (SciFi) [11] witha 6 ton water target has tracking capability, and allows discrimination between different types of interactions such as quasi-elastic or inelastic. Downstreamof the SciFi there is a lead glass array for tagging electromagnetic showers. A muon range detector (MRD)[12], measures the energy, angle, and production pointof muons from charged current (CC) νµ interactions.
It covers 7.6 × 7.6 m2 transverse area with four 10 cm
iron plates, followed by eight 20 cm iron plates, all interleaved with drift tubes. The total mass is 915 tons.
The 6632 drift tubes each has 5 × 7 cm2 cross section, and are arranged in horizontal and vertical readout planes.
The far detector for K2K is the SK detector located in the Kamioka Observatory, Institute for Cosmic Ray Research (ICRR), University of Tokyo, which
has been taking data since 1996. Its performance andresults have been documented in the literature [4].
Event selection for this detector uses timing synchronization with the KEK–PS via the Global Positioning
System (GPS) [13].
The beam-line was aligned by GPS position survey [6]. The precision of this survey for the line fromthe target to the far detector is better than 0.01 mr andthe construction precision for the near site alignmentis better than 0.1 mr. The predicted neutrino spectrumat 250 km is approximately the same over nearly 1 km,
giving a required accuracy of 3 mr for the pointing of
the beam.
The steering and the monitoring of the νµ beam
are carried out based on the MUMON measurement.
The νµ and muons in the decay volume originate fromthe same pion decays, so the measured center positionof the muon profile is correlated with the νµ beam
direction. The r.m.s. muon profile width is ∼ 90 cm
for both the horizontal and vertical directions. These
measurements are used in tuning the beam direction at
the start of every beam period, usually once per month.
Since there is a magnetic field in and around the target,
the direction of the secondary pions, which determines
the direction of decay νµ and muons, is sensitive
to the position of the primary proton beam at theK2K Collaboration / Physics Letters B 511 (2001) 178–184

target. A 1 mm displacement of protons at the target
face causes roughly 0.5 mr deflection to the opposite
direction for muons observed by the MUMON [14].
During beam tuning, the primary proton position at thetarget is adjusted so that the muon profile is centeredon the SK direction within 0.1 mr. The center of themuon profile is measured to be stable spill-by-spill
within ±1 mr in each dimension throughout the wholeexperimental period. The stability of the muon yieldis directly related to the stability of the horn-focusedνµ beam. The yield normalized to proton intensity
measured by a current transformer is stable spill-byspill within a measurement uncertainty of 2%.
The characteristics and stability of the νµ beamitself are directly monitored at the near site by the
MRD using νµ interactions with iron. The largetransverse area of MRD makes it possible to measure
the beam direction and width (profile), and the large
mass makes it possible to measure the time stability ofthe νµ event rate, profile and spectrum. The locationof the center of the profile gives the beam direction.
For the profile measurement, the starting point of areconstructed muon track is regarded as the vertexof the νµ -iron interaction. The data reduction for the
profile measurement requires: (a) Only tracks aroundthe time of the beam spill are accepted. (b) Tracks
with a common vertex are taken as a single event withthe longest track assumed to be a muon. (c) Muons
entering or exiting the detector are rejected. (d) Any
muons with reconstructed energy lower than 0.5 GeVor higher than 2.5 GeV are rejected in order to avoid
regions of small acceptance. The vertex distribution
is corrected for geometrical acceptance to obtain the
beam profile. As shown in Fig. 1(a), the horizontal
profile is well centered on the SK direction. The profile
width is well reproduced by the beam simulation
which is described below. The profile center is plotted
as a function of time in Fig. 1(b). The νµ direction has
been stable within ±1 mr throughout all data-taking
periods. The vertical profile is also well centered on
the SK direction and stable. For the event rate andkinematic distribution analyses no energy cut is made,but only events with vertices inside the fiducial volume
defined by a cylinder of 3 m radius in the upstream
9 iron plates are accepted. The measured rate ofνµ -iron events is typically 0.05 events/spill and canbe monitored on a daily basis with good statistics.
The event rate normalized to proton intensity is stable

181

Fig. 1. (a) Horizontal profile of the νµ beam measured by MRD ina one month period (points with error bars), and simulated (boxes;
size corresponds to error). Errors include statistics and uncertaintyof the acceptance correction. The normalization is by the area under
the histograms. (b) Stability of the νµ beam direction. The fitted
center of the horizontal profile is plotted for five day periods, witherror bars from the fits. The dashed lines show ±1 mr directions
to SK.

within the statistical error of the MRD measurement.
The muon energy and angular distributions are also
continuously monitored. They show no change as afunction of time, implying the νµ energy spectrum isstable throughout this period.
Comparison between expected and observed numbers of νµ events at the far site is done with this knowledge of measured beam stability. To predict the νµbeam characteristics at the far site, a normalization
measurement at the near site and the extrapolation of
the information from near to far are necessary. For therate normalization, the 1 kt is used so that any detector or analysis bias is suppressed. For the extrapolation, the beam simulation is used, which is validated by the PIMON measurement as described below. This simulation is based on GEANT [15] withdetailed description of materials and magnetic fields
in the target region and decay volume. It uses as input a measurement of the primary beam profile at thetarget. Primary proton interactions on aluminum are

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K2K Collaboration / Physics Letters B 511 (2001) 178–184

modeled with a parameterization of hadron production
data [16]. Other hadronic interactions are treated by
GEANT–CALOR [17].
Once the kinematic distribution of pions after production and focusing is known, it is possible to predict the νµ spectrum at any distance from the source.
The pion momentum and angular distribution is measured by the PIMON. Cherenkov photons generated
by pions in the detector are focused by a pie-shaped
spherical mirror to an array of PMTs of 8 mm effective diameter in its focal plane. Photons emitted
by particles with the same velocity and incident angle with respect to the mirror arrive at the same position in the focal plane. This is independent of photon
incident position with respect to the mirror. The photon distribution on the PMT array is a superpositionof slices of the Cherenkov rings from particles of various velocities and angles. Measurements are made atseven indices of refraction, controlled by gas pressure,to give additional information on the velocity distribution of the particles. The indices of refraction arechosen so that the corresponding pion momentum interval is 400 MeV/c. The pion two-dimensional distribution of momentum versus angle is derived by unfolding the photon distribution data with various indices of refraction. The binning of extracted pion momentum and angular distributions are 1 GeV/c and
10 mr, respectively. To avoid background from 12 GeV
primary protons, the index of refraction is adjusted
below the Cherenkov threshold of 12 GeV protons(1 − β ≈ 2.6 × 10−3 ). As a consequence, analysis isdone for Pπ  2 GeV/c, giving the νµ energy spectral
shape above 1 GeV. Fig. 2 shows the inferred νµ energy spectral shape at the near site and the far to nearνµ flux ratio along with the beam simulation result.
The beam simulation is well validated by the PIMON
measurement without any tuning.
The νµ interaction rate at the near site is measured
in the 1 kt by detecting Cherenkov light emitted fromproduced charged particles. The vertices and directions of Cherenkov rings are reconstructed with thesame methods as in SK [2]. In the 1 kt, however, ananalog sum of signals from all PMTs is recorded by an
FADC to select beam spills with only one event. The
definition of events per spill is the number of peaksof the FADC signal with > 1000 collected photoelectrons (p.e.) (≈ 100 MeV deposited energy). The
average number of events observed in the full detec-

Fig. 2. The top figure shows the νµ energy spectral shape at thenear site and the bottom figure shows the far to near νµ flux ratio.
The histograms are from the beam simulation results. The data
points are derived from the PIMON measurement. For the top figure,normalization is done by area above 1 GeV. The vertical error bars
for the data points reflect the total uncertainty.

tor is about 0.2/spill including entering background
events due to cosmic rays or νµ induced muons fromupstream (about 1.5% of events in the fiducial volume
defined below). Thus  2 events occur in about 10%of those spills with  1 event. Neutrino interactions
are selected by requiring: (a) There is no detector activity in the 1.2 µs preceding the beam spill. (b) Onlya single event is observed in the FADC peak search
for that spill. (c) The reconstructed vertex is inside the
25 t fiducial volume defined by a 2 m radius, 2 m long
cylinder along the beam axis, in the upstream side ofthe detector. The detection efficiency of the 1 kt detector for detecting CC interactions is 87% and for neutral
current inelastic (NCinel) interactions it is 55%. The
expected ratio of CC to NCinel gives an overall efficiency of 72%. The main source of inefficiency is the1000 p.e. threshold of the peak search in the FADC
signal. The neutrino interaction simulator used for efficiency calculations is the same as that used in all SK
analyses. Finally the number of νµ events in 1 kt is corrected for spills with multiple events. The average νµevent rate per proton hitting the target is 3.2 × 10−15 .
Correcting for efficiencies, relative target masses, anddetector live times, the expected νµ signal at the far

K2K Collaboration / Physics Letters B 511 (2001) 178–184

site is estimated by applying the extrapolation fromthe experimentally validated beam simulation.
Accelerator-produced neutrino interactions at thefar site, SK, are selected by comparing two Universal Time Coordinated (UTC) time stamps from the
GPS system, TKEK for the KEK–PS beam spill start
time and TSK for the SK trigger time. The time difference between two UTC time stamps, (T ) ≡ TSK −TKEK − TOF, where TOF is the time of flight of a neutrino, should be distributed around the interval from 0
to 1.1 µs to match the width of the beam spill of the
KEK–PS. Since the measured uncertainty of the synchronization accuracy for the two sites is < 200 ns,
beam-induced events are selected in a 1.5 µs window.
Data reduction similar to that used in atmospheric neutrino analyses at SK [2,3] is applied to select fully
contained (FC) neutrino interactions. The criteria are:(a) There is no detector activity within 30 µs before
the event. (b) The total collected p.e. in a 300 ns
time window is > 200 (≈ 20 MeV deposited energy).
(c) The number of PMTs in the largest hit cluster inthe outer-detector is < 10. (d) The deposited energy is> 30 MeV. Finally, a fiducial cut is applied accepting
only events with fitted vertices inside the same 22.5 kt
volume used for SK atmospheric neutrino analysis.
The detection efficiency of SK is 93% for CC interactions and 68% for NCinel interactions, for a totalof 79%. Similarly to the 1 kt, the inefficiency is mainly
due to the energy cut. Fig. 3 shows the (T ) distribu-

183tion at various stages of the reduction. A clear peak intime with the neutrino beam from the KEK–PS is observed in the analysis time window. Twenty-eight FC
events are observed in the fiducial volume. The arrival
rate of neutrinos observed by the far detector is consistent with constant within statistical fluctuation. The
1.5 µs selection window gives an expected backgroundfrom atmospheric neutrino interactions of order 10−3
events.
The systematic uncertainty of the 1 kt measurement
is 5%, for which the leading terms are due to vertex fitting and its effect on the fiducial cut (4%), andthe treatment of multiple events in a spill (3%). Other
sources such as energy scale uncertainty are relatively
small. The statistical uncertainty of the 1 kt measurement is < 1%. The systematic uncertainty of the extrapolation from the near site measurement is estimated to be +6−7 % based on the PIMON measurement
uncertainty and beam simulation uncertainty for low
energy neutrinos. The systematic uncertainty in the SK
measurement is 3%, mainly due to the fiducial cut. The
systematic uncertainty term coming from uncertainty
in the neutrino energy spectrum and cross section issmall due to cancellations. The quadrature sum of all
known uncertainties for the far site event rate predic+3.5
tion is +9−10 %. The resulting expectation is 37.8−3.8
events in the absence of neutrino oscillations. The observed and expected numbers for various categories
are summarized in Table 1. The number of Cherenkov
rings and particle identification are reconstructed by
the same algorithms as those used at SK [2], and theevent category definitions are also the same. The expected number of events for analyses of CC interactions in the other near detectors are 41.0+6.0−6.6 for MRD(iron target) and 37.2+4.6
for
SciFi
(watertarget) [18],−5.0Table 1
Summary of the observed and expected numbers of FC events in thefiducial volume at the far site

Fig. 3. Far site timing analysis: (top) (T ) distribution over a±500 µs window for events remaining after cuts (a) (unfilled
histogram) and (b) (hatched), and after all cuts except timing(shaded). The cuts are described in the text (bottom) (T ) for a± 5 µs window for FC events with vertices in the fiducial volume.

Event category

Observed

ExpectedSingle ring µ-like

14

20.9

Single ring e-like

1

2.0

Multi ring

13

14.9

Total

28

37.8+3.5
−3.8

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K2K Collaboration / Physics Letters B 511 (2001) 178–184

each of which is consistent with the value based on
1 kt analysis.
The crucial principles of K2K have been provento work: the beam direction has been continuously
monitored and controlled within ±1 mr for several
months duration by controlling the proton position
on target with millimeter accuracy and operating thehorn magnet stably. Pion kinematics have successfully
been measured in situ allowing prediction of theneutrino beam at the distance of 250 km with six toseven percent accuracy. Events at the far site havebeen identified by means of GPS timing, reducing
backgrounds to a negligible level. Twenty-eight FC
neutrino events have been observed where 37.8+3.5−3.8
are expected. The experiment expects to accumulate
1020 protons on target, providing sufficient statistics
to study neutrino oscillations by spectral analysis forνµ disappearance.

Acknowledgement
We thank the KEK and ICRR Directorates for theirstrong support and encouragement. K2K is made possible by the inventiveness and the diligent efforts of the
KEK–PS machine group. We gratefully acknowledge
the cooperation of the Kamioka Mining and Smelting Company. This work has been supported by the
Ministry of Education, Culture, Sports, Science andTechnology, Government of Japan and its grants for
Scientific Research, the Japan Society for Promotionof Science, the U.S. Department of Energy, the Korea
Research Foundation, and the Korea Science and Engineering Foundation.

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PHYSICA L R EVIEW LET T ERSVOLUME 90, N UMBER 4

week ending
31 JANUARY 2003

Indications of Neutrino Oscillation in a 250 km Long-Baseline Experiment
M. H. Ahn,1 S. Aoki,2 H. Bhang,1 S. Boyd,3,* D. Casper, 4 J. H. Choi,5 S. Fukuda,6 Y. Fukuda,6,* W. Gajewski,4 T. Hara,2
M. Hasegawa,7 T. Hasegawa,8 Y. Hayato,9 J. Hill,10,* A. K. Ichikawa,9 A. Ikeda,11 T. Inagaki,7 T. Ishida,9 T. Ishii,9M. Ishitsuka,6 Y. Itow,6 T. Iwashita,2 H. I. Jang,5,* J. S. Jang,5 E. J. Jeon,9 C. K. Jung,10 T. Kajita,6 J. Kameda,6K. Kaneyuki,6 I. Kato,7 E. Kearns,12 A. Kibayashi,13 D. Kielczewska,14,15 K. Kobayashi,6 B. J. Kim,1 C. O. Kim,16
J. Y. Kim,5 S. B. Kim,1 T. Kobayashi,9 M. Kohama,2 Y. Koshio,6 W. R. Kropp, 4 J. G. Learned,13 S. H. Lim,5 I. T. Lim,5
H. Maesaka,7 K. Martens,10,* T. Maruyama,9,* S. Matsuno,13 C. Mauger,10,* C. Mcgrew,10 S. Mine, 4 M. Miura,6
K. Miyano,17 S. Moriyama,6 M. Nakahata,6 K. Nakamura,9 I. Nakano,11 F. Nakata,2 T. Nakaya,7 S. Nakayama,6
T. Namba,6 K. Nishikawa,7 S. Nishiyama,2 S. Noda,2 A. Obayashi,6 A. Okada,6 T. Ooyabu,6 Y. Oyama,9 M. Y. Pac,18
H. Park,1 M. Sakuda,9 N. Sakurai,6 N. Sasao,7 K. Scholberg,19 E. Sharkey,10 M. Shiozawa,6 H. So,1 H.W. Sobel,4A. Stachyra,3 J. L. Stone,12 Y. Suga,2 L. R. Sulak,12 A. Suzuki,2 Y. Suzuki,6 Y. Takeuchi,6 N. Tamura,17 T. Toshito,6,*
Y. Totsuka,6,9 M. R. Vagins,4 C.W. Walter,12 R. J. Wilkes,3 S. Yamada,6 S. Yamamoto,7 C. Yanagisawa,10 H. Yokoyama,20
J. Yoo,1 M. Yoshida,9 and J. Zalipska15
1

Department of Physics, Seoul National University, Seoul 151-742, Korea
2
Kobe University, Kobe, Hyogo 657-8501, Japan3
Department of Physics, University of Washington, Seattle, Washington 98195-1560
4Department of Physics and Astronomy, University of California–Irvine, Irvine, California 92697-4575
5Department of Physics, Chonnam National University, Kwangju 500-757, Korea
6
Institute for Cosmic Ray Research, University of Tokyo, Kashiwa, Chiba 277-8582, Japan
7
Department of Physics, Kyoto University, Kyoto 606-8502, Japan
8
Research Center for Neutrino Science, Tohoku University, Sendai, Miyagi 980-8578, Japan
9
Institute of Particle and Nuclear Studies, KEK, Tsukuba, Ibaraki 305-0801, Japan10
Department of Physics and Astronomy, State University of New York, Stony Brook, New York 11794-3800
11
Department of Physics, Okayama University, Okayama, Okayama 700-8530, Japan
12
Department of Physics, Boston University, Boston, Massachusetts 02215
13
Department of Physics and Astronomy, University of Hawaii, Honolulu, Hawaii 96822
14
Institute of Experimental Physics, Warsaw University, 00-681 Warsaw, Poland
15
A. Soltan Institute for Nuclear Studies, 00-681 Warsaw, Poland
16
Department of Physics, Korea University, Seoul 136-701, Korea
17
Department of Physics, Niigata University, Niigata, Niigata 950-2181, Japan18
Department of Physics, Dongshin University, Naju 520-714, Korea
19
Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
20
Department of Physics, Tokyo University of Science, Noda, Chiba 278-0022, Japan(Received 4 December 2002; published 30 January 2003)
The K2K experiment observes indications of neutrino oscillation: a reduction of  flux together
with a distortion of the energy spectrum. Fifty-six beam neutrino events are observed in SuperKamiokande (SK), 250 km from the neutrino production point, with an expectation of 80:16:2
5:4 .
Twenty-nine one ring -like events are used to reconstruct the neutrino energy spectrum, which is
better matched to the expected spectrum with neutrino oscillation than without. The probability thatthe observed flux at SK is explained by statistical fluctuation without neutrino oscillation is lessthan 1%.
DOI: 10.1103/PhysRevLett.90.041801

PACS numbers: 14.60.Pq, 13.15.+g, 23.40.Bw, 95.55.Vj

Recent atmospheric and solar neutrino data indicate
the existence of neutrino oscillation and therefore theexistence of neutrino mass [1–3]. The zenith angle distribution of atmospheric neutrinos observed by SuperKamiokande (SK) shows a clear deficit of upward-going
muon neutrinos, which is well explained by two-flavor - oscillations with m2 around 3103 eV2 , andsin 2 2 close to or equal to unity.
The KEK (High Energy Accelerator Research Organization) to Kamioka long-baseline neutrino oscillation

experiment (K2K) [4] uses an accelerator-produced neutrino beam with a neutrino flight distance of 250 km toprobe the same m2 region as that explored with atmospheric neutrinos. The neutrino beam is produced by a
12 GeV proton beam from the KEK proton synchrotron.
After the protons hit an aluminum target, the produced
positively charged particles, mainly pions, are focused bya pair of pulsed magnetic horns [5]. The neutrinos produced from the decays of these particles are 98% puremuon neutrinos with a mean energy of 1.3 GeV.

041801-1

 2003 The American Physical Society

0031-9007=03=90(4)=041801(5)$20.00

041801-1

VOLUME 90, N UMBER 4

PHYSICA L R EVIEW LET T ERSThis analysis is based on data taken from June 1999 toJuly 2001, corresponding to 4:81019 protons on target(POT). The pion momentum and angular distributions
downstream of the second horn are occasionally measured with a gas-Cherenkov detector (PIMON) [6] inorder to verify the beam Monte Carlo (MC) simulationand to estimate the errors on the flux prediction at SK. The
direction of the beam is monitored on a spill-by-spillbasis by observing the profile of the muons from thepion decays with a set of ionization chambers and silicon
pad detectors located just after the beam dump. The
neutrino beam itself is measured in a set of near neutrino
detectors (ND) located 300 m from the proton target. The
measurements made at the ND are used to verify thestability and the direction of the beam and to determine
the flux normalization and the energy spectrum before theneutrinos travel the 250 km to SK. The flux at SK isestimated from the flux of the ND by multiplying the
far=near (F=N) ratio, the ratio of fluxes between the far
detector (SK) and ND to that of the ND.
Since both a suppression in the number of events and a
distortion of the spectrum are expected for neutrinos
which travel a fixed path length in the presence of oscillations, both the number of observed events and thespectral shape information at SK are compared withexpectation. All of the beam-induced neutrino events
observed within the fiducial volume of SK are used tomeasure the overall suppression of flux. In order to study
the spectral distortion, one ring -like events (1R) areselected to enhance the fraction of charged-current (CC)quasielastic (QE) interactions (  n !   p). Only
the muon is visible in these reactions since the proton
momentum is typically below Cherenkov threshold. Theenergy of the parent neutrino can be calculated by using
the observed momentum of the muon, assuming QEinteractions, and neglecting Fermi momentum:
Erec
 

mN E  m2 =2;mN  E  P cos (1)where mN , E , m , P , and  are the nucleon mass,
muon energy, the muon mass, the muon momentum, andthe scattering angle relative to the neutrino beam direction, respectively.
The ND is comprised of two detector systems: a 1 kt
water Cherenkov detector (1KT) and a fine-grained detector (FGD) system. The flux normalization is measured
by the 1KT to estimate the expected number of events at
SK. Since the 1KT has the same detector technology as
SK, most of systematic uncertainties on the measurement
are canceled. The energy spectrum is measured by analyzing the muon momentum and angular distributions inboth detector systems. The 1KT has a high efficiency forreconstructing the momentum of muons below 1 GeV=c,
and full 4 coverage in solid angle. However, the 1KT has
little efficiency for reconstructing muons with momen041801-2

week ending
31 JANUARY 2003

tum above 1:5 GeV=c since they exit the detector. The
FGD, on the other hand, has high efficiency for measuringmuons above 1 GeV=c, and the two complementary detectors are thus able to completely cover the relevantenergy range.
In the 1KT analysis, a cylindrical fiducial volume of
25 ton oriented along the beam direction is used. Event
selection criteria for the flux normalization are the sameas those in Ref. [4]. Events which deposit more than100 MeV of energy are used for the measurement of
the integrated flux. The measurement has a 5% systematic
uncertainty, of which the largest contribution comes fromthe vertex reconstruction [4]. For the spectrum measurement, further cuts are imposed in order to select 1R
events which stop in the detector. Among the events
selected for the flux normalization measurement, 53%of the events have one ring. The events with a muon
exiting from the detector are effectively eliminated byrequiring the maximum charge of any photomultipliertube to be less than 200 photoelectrons; 68% of the one
ring events remain after this requirement. The largest
systematic uncertainty for the spectrum measurement is
an uncertainty on the energy scale. The energy scale isunderstood to within 2
3 %, which is confirmed with bothcosmic-ray muons and beam-induced 0 s.
The FGD is comprised of a scintillating fiber and water
detector (SciFi) [7], a lead-glass calorimeter (LG), and amuon range detector (MRD) [8]. In the FGD analysis,
events containing one or two tracks with vertex within
the 5.9 ton fiducial volume of the SciFi are used. The trackfinding efficiency is 70% for a track passing through threelayers of scintillating fiber and close to 100% for morethan five layers [9]. Three layers is the minimum track
length required in this analysis. Events which have atleast one track passing into the MRD are chosen in order
to select  -induced CC interactions. The momentum of
the track is measured by its range through the SciFi, LG,
and MRD, with accuracy of 3%.
If the proton produced in the QE interaction has amomentum greater than 600 MeV=c, its track may also
be reconstructed. In the case where a second track isvisible, the kinematic information is used to enhance
the fraction of QE events in the sample. Assuming QEinteraction, the direction of the proton can be predictedfrom the muon momentum. The QE enhanced sample isselected by requiring that the direction of the second
track agrees with the prediction to within 25 . Events
where the direction of the second track differs from theprediction by more than 30 are put into a non-QE
enhanced sample. In the QE enhanced sample, 62% of
the events are estimated to be QE events from the MC
simulation. In the non-QE enhanced sample, 82% of
events are estimated to come from interactions otherthan QE. The SciFi events are divided into three event
categories: one-track, two-track QE enhanced, and twotrack non-QE enhanced samples.
041801-2

both the q2 dependence of the cross section and the total
cross section. For coherent pion production, two different
models are compared: one is the Rein and Sehgal model[11], and the other is a model by Marteau [12]. For deep
inelastic scattering, GRV94 [13] and the corrected structure function by Bodek and Yang [14] are studied. For theoscillation analysis the Marteau model and Bodek andYang structure functions are employed. Varying the
choice of models causes the fitted value of Rnqe ( 0:93)
to change by 20%. In order to account for this, anadditional systematic error of 20% on Rnqe is added tothe error matrix. The choice of models and axial massdoes not affect the spectrum measurement itself beyond
the size of the fitted errors.
The F=N ratio from the beam simulation is used to
extrapolate the measurements at the ND to those at SK.
The errors including correlations above 1 GeV, where the
PIMON is sensitive, are estimated based on the PIMON
measurements. The errors on the ratio for E below 1 GeV
are estimated based on the uncertainties in the hadronproduction models used in the K2K beam MC [4]. The
diagonal elements in the error matrix for the F=N ratio
are summarized in Table I.
The events in SK are selected using the timing information provided by the global positioning system. Events
detected in SK must occur within an expected beam
arrival time window of 1:5 sec. In addition, the detected events must have no activity in the outer detector,
and they must have an energy deposit greater than 30 MeV
with a vertex reconstructed within the 22.5 kt fiducial
volume [4]. This sample of events is referred to as the
fully contained (FC) sample. The efficiency of this selection is 93% for CC interactions. Fifty-six events satisfy(a)0 – 0.5
0.5– 0.75
0.75–1.0
1.0 –1.5
1.5–2.0
2.0 –2.5
2.5–3.0
3.0 –041801-3

MC

1.31
1.02
1.01
1:00
0.95
0.96
1.18
1.07

 ND
49
12
9.1
7.1
8.4
19
20

 F=N
2.6
4.3
4.3
6.5
10
11
12
12

 SK
8.7
4.3
4.3
8.9
10
9.8
9.9
9.9

∆θp < 25o

150

MC(QE)

2000

(c)

200

100

1000

events / 10o

5000

0.5

1

1.5

50
0
0

2

Pµ [GeV/c]

events / 0.25 GeV/c

ND

Data

3000

0
0

TABLE I. The central values of the flux re-weighting parameters for the spectrum fit at ND (ND ) and the percentage
size of the energy dependent systematic errors on ND , F=N
ratio, and SK . The re-weighting parameters are given relative
to the 1.0  1.5 GeV energy bin.

events / 0.25 GeV/c

250

events / 0.1 GeV/c

The two-dimensional distributions of the muon momentum versus angle with respect to the beam directionof four event categories (the 1KT event sample and thethree SciFi event samples) are used to measure the neutrino spectrum. A 2 -fitting method is used to compare
these data against the MC expectation. The neutrino
spectrum is divided into eight energy bins as defined inTable I. During the fit, the flux in each energy bin isreweighted relative to the values in the beam MC. These
weights are normalized relative to 1.00 for the E 
1:0–1:5 GeV bin. An overall normalization is included
as a free parameter in the fit. The parameter Rnqe is used
to reweight the ratio between the QE and non-QE crosssection relative to the MC simulation. The systematic
uncertainties of the ND are incorporated into the fitting
parameters. They are the energy scales, the track finding
efficiencies, and the detector thresholds. In addition, thespectrum measurement by PIMON is used as a constraint
on the reweighting factors.
The value of 2 is 227:2=197 d:o:f: at the best-fit point.
All the parameters including the detector systematics arefound to lie within their expected errors. The best-fit
values of the flux reweighting factors are shown in
Table I. The muon momentum and angular distributionsof 1R events in the 1KT, and the muon momentum
distributions of the two-track QE enhanced and non-QE
enhanced events in SciFi are overlaid with the reweighted
MC in Fig. 1. The fit result agrees well with the data. The
errors of the measurement are provided in the form of anerror matrix. Correlations between the parameters of the
fit are taken into account in the oscillation analysis using
this matrix. The diagonal elements in the error matrix are
shown in Table I.
The uncertainty due to neutrino interaction models is
separately studied. In QE scattering, the axial vector
mass in the dipole formula is set to a central value of
1:1 GeV=c2 and is varied by 10%. The axial mass for
single pion production is set to a central value of
1:2 GeV=c2 and is varied by 20% [10]. This affects

E (GeV)

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PHYSICA L R EVIEW LET T ERSVOLUME 90, N UMBER 4

(b)

400

4000

1

2

(d)

3

Pµ [GeV/c]∆θp > 30o

300

3000

200

2000

100

1000
0
0

50

100

150

θµ [deg.]

0
0

1

2

3

Pµ [GeV/c]

FIG. 1. (a) The muon momentum distribution of the 1KT
1R sample, (b) the angular distribution of the 1KT 1R
sample, (c) the muon momentum distribution of the SciFi
QE enhanced sample, and (d) that of the SciFi non-QE enhanced sample. The crosses are data and the boxes are MC
simulation with the best-fit parameters. The hatched histogramshows the QE events estimated by MC simulation.

041801-3

the criteria. With the timing cut, the expected numberof atmospheric neutrino background is approximately
103 events.
The expected number of FC events at SK without
oscillation is estimated to be 80:16:2
5:4 . The correlations
between energy bins from the spectrum measurement at
the ND and the F=N ratio are taken into account in theestimation of the systematic errors. The major contributions to the errors come from the uncertainties in the F=N
ratio ( 4:9%
5:0% ) and the normalization (5.0%), dominated byuncertainties of the fiducial volumes due to vertex reconstruction both at the 1KT and SK.
A two-flavor neutrino oscillation analysis, with disappearance, is performed by the maximum-likelihood
method. In the analysis, both the number of FC events andthe energy spectrum shape for 1R events are used. The
likelihood is defined as L  Lnorm  Lshape . The normalization term Lnorm Nobs ; Nexp is the Poisson probability to observe Nobs events when the expected number of
events is Nexp m2 ; sin2 2 ; f . The symbol f represents a
set of parameters constrained by the systematic errors.
These parametersdescribed in detail later. The shape
QNare
1 R
term, Lshape  i1
P Ei ; m2 ; sin2 2 ; f , is the product of the probability for each 1R event to be observed atrecErec  Ei , where P is the normalized E distribution
estimated by MC simulation and N1R is the number of
1R events.
In the oscillation analysis, the whole data sample isused for Lnorm , i.e., Nobs  56. The spectrum shape inJune 1999 was different from that for the rest of therunning period because the target radius and horn current
were different. The estimation of energy correlations inthe spectrum at the ND and in the far=near ratio has notbeen completed for this period. Thus, data taken in
June 1999 are discarded for Lshape . The discarded data
correspond to 6.5% of total POT. The number of 1R
events observed excluding the data of June 1999 is 29, andthe corresponding number of 1R events expected from
MC simulation in the case of no oscillation is 44.
The parameters f consist of the reweighted neutrino
spectrum measured at the ND (ND ), the F=N ratio, thereconstruction efficiency (SK ) of SK for 1R events, thereweighting factor for the QE=non-QE ratio Rnqe , the SKenergy scale, and the overall normalization. The errors on
the first three items depend on the energy and havecorrelations between each energy bin. The diagonal partsof their error matrices are summarized in Table I as
described earlier. The error on the SK energy scale is3% [15]. Two different approaches are taken for the treatment of systematic errors in the likelihood. The first is totreat the parameters f as fitting parameters with an additional constraint term in the likelihood (method 1) [1].
The other approach is to average the L f sampled overmany random trials weighted according to the probability density distribution of the systematic parameters f(method 2) [16].
041801-4

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PHYSICA L R EVIEW LET T ERS

The likelihood is calculated at each point in the m2
and sin2 2 space to search for the point where the likelihood is maximized. The best-fit point in the physicalregion of oscillation parameter space is found to be atsin2 2 ; m2  1:0; 2:8  103 eV2 in method 1 and at
1:0; 2:7  103 eV2 in method 2. If the whole space
including the unphysical region is considered the values
are 1:03; 2:8  103 eV2 in method 1 and 1:05; 2:7 
103 eV2 in method 2. The results from two methods are
consistent with each other. At the best-fit point in thephysical region the total number of predicted events is54.2, which agrees with the observation of 56 within
statistical error. The observed Erec distribution of the
1R sample is shown in Fig. 2 together with the expecteddistributions for the best-fit oscillation parameters, andthe expectation without oscillations. Consistency between
the observed and the best-fit Erec spectrum is checked byusing the Kolgomorov-Smirnov (KS) test. A KS probability of 79% is obtained. The best-fit spectrum shapeagrees with the observations.
The probability that the observations are due to astatistical fluctuation instead of neutrino oscillation isestimated by computing the likelihood ratio of the nooscillation case to the best-fit point. The no-oscillation
probabilities are calculated to be 0.7% and 0.4% formethod 1 and method 2, respectively. When only normalization (shape) information is used, the probabilities are

EventsVOLUME 90, N UMBER 4

12
10
8
6
4
2
0

0

1

2

3

4

5
Eνrec

FIG. 2 (color online). The reconstructed E distribution for
the 1R sample (from method 1). Points with error bars aredata. The box histogram is the expected spectrum without
oscillations, where the height of the box is the systematic error.
The solid line is the best-fit spectrum. These histograms arenormalized by the number of events observed (29). In addition,
the dashed line shows the expectation with no oscillations
normalized to the expected number of events (44).

041801-4

PHYSICA L R EVIEW LET T ERS∆m (eV )VOLUME 90, N UMBER 4

week ending
31 JANUARY 2003

2

2

has collected approximately one-half of its planned 1020
protons on target.
We thank the KEK and ICRR Directorates for theirstrong support and encouragement. K2K is made possibleby the inventiveness and the diligent efforts of the KEKPS machine and beam channel groups. We gratefully
acknowledge the cooperation of the Kamioka Mining
and Smelting Company. This work has been supportedby the Ministry of Education, Culture, Sports, Scienceand Technology, Government of Japan and its grants for
Scientific Research, by the Japan Society for Promotionof Science, by the U.S. Department of Energy, by the
Korea Research Foundation, and by the Korea Scienceand Engineering Foundation.

-2

10

-3

10

-4

10

0

0.2

0.4

0.6

0.8
1
2
sin 2θ

FIG. 3 (color online). Allowed regions of oscillation parameters. Dashed, solid, and dot-dashed lines are 68.4%, 90%, and
99% C.L. contours, respectively. The best-fit point is indicatedby the star.

1.3% (16%) and 0.7% (14.3%) for the two methods.
Allowed regions of oscillation parameters are evaluatedby calculating the likelihood ratio of each point to thebest-fit point and are drawn in Fig. 3. Both methods give
essentially the same results. In order to be conservative,
the result from method 1 is shown in the figure as it givesa slightly larger allowed region at the 99% C.L. The
90% C.L. contour crosses the sin2 2  1 axis at 1.5 and3:9  103 eV2 for m2 . The oscillation parameters preferred by the total flux suppression and the energy distortions alone also agree well. Finally, the uncertainties ofneutrino interactions are studied using the same procedure as the spectrum measurement at the ND. It is foundthat the effects of the interaction model difference on all
the results are negligible due to the cancellation caused byusing the same models in both the ND and SK.
In conclusion, both the number of observed neutrino
events and the observed energy spectrum at SK are consistent with neutrino oscillation. The probability that themeasurements at SK are explained by statistical fluctuation is less than 1%. The measured oscillation parameters
are consistent with the ones suggested by atmospheric
neutrinos. At the time of this Letter the K2K experiment

041801-5*For current affiliations, see http://neutino.kek.jp/presentaddresses0211.ps[1] Super-Kamiokande Collaboration, Y. Fukuda et al., Phys.
Rev. Lett. 81, 1562 (1998).
[2] Super-Kamiokande Collaboration, S. Fukuda et al., Phys.
Lett. B 539, 179 (2002).
[3] SNO Collaboration, Q. Ahmad et al., Phys. Rev. Lett. 89,
011301 (2002).
[4] K2K Collaboration, S. H. Ahn et al., Phys. Lett. B 511,
178 (2001).
[5] M. Ieiri et al., in Proceedings of the 11th Symposium on
Accelerator Science and Technology, SPring-8, Harima
Science Garden City, Hyogo, Japan, 1997 (SPring-8,
Hyogo, Japan, 1997), pp. 377–379.
[6] T. Maruyama, Ph.D. thesis, Tohoku Unversity, 2000.
[7] K2K Collaboration, A. Suzuki et al., Nucl. Instrum.
Methods Phys. Res., Sect. A 453, 165 (2000).
[8] K2K MRD Group Collaboration, T. Ishii et al., Nucl.
Instrum. Methods Phys. Res., Sect. A 482, 244 (2002).
[9] B. J. Kim et al., hep-ex/0206041.
[10] V. Bernard, L. Elouadrhiri, and U. G. Meissner, J. Phys. G
G28, R1 (2002).
[11] D. Rein and L. M. Sehgal, Nucl. Phys. B223, 29 (1983).
[12] J. Marteau et al., Nucl. Instrum. Methods Phys. Res.,
Sect. A 451, 76 (2000).
[13] M. Gluck, E. Reya, and A. Vogt, Z. Phys. C 67, 433(1995).
[14] A. Bodek and U.-K. Yang, hep-ex/0203009.
[15] Super-Kamiokande Collaboration, Y. Fukuda et al., Phys.
Lett. B 433, 9 (1998).
[16] J. Swain and L. Taylor, Nucl. Instrum. Methods Phys.
Res., Sect. A A411, 153 (1998).

041801-5

Search for Electron Neutrino Appearance in a 250 km Long-baseline Experiment
M.H.Ahn,1 S.Aoki,2 Y.Ashie,3 H.Bhang,1 S.Boyd,4, ∗ D.Casper,5 J.H.Choi,6 S.Fukuda,3 Y.Fukuda,7 R.Gran,4T.Hara,2 M.Hasegawa,8 T.Hasegawa,9 K.Hayashi,8 Y.Hayato,10 J.Hill,11, ∗ A.K.Ichikawa,10 A.Ikeda,12 T.Inagaki,8, ∗T.Ishida,10 T.Ishii,10 M.Ishitsuka,3 Y.Itow,3 T.Iwashita,10 H.I.Jang,6, ∗ J.S.Jang,6 E.J.Jeon,1 K.K.Joo,1 C.K.Jung,11T.Kajita,3 J.Kameda,10 K.Kaneyuki,3 I.Kato,8 E.Kearns,13 A.Kibayashi,14 D.Kielczewska,15, 16 B.J.Kim,1
C.O.Kim,17 J.Y.Kim,6 S.B.Kim,1 K.Kobayashi,11 T.Kobayashi,10 Y.Koshio,3 W.R.Kropp,5 J.G.Learned,14 S.H.Lim,6
I.T.Lim,6 H.Maesaka,8 T.Maruyama,10, ∗ S.Matsuno,14 C.Mauger,11, ∗ C.Mcgrew,11 A.Minamino,3 S.Mine,5
M.Miura,3 K.Miyano,18 T.Morita,8 S.Moriyama,3 M.Nakahata,3 K.Nakamura,10 I.Nakano,12 F.Nakata,2 T.Nakaya,8
S.Nakayama,3 T.Namba,3 R.Nambu,3 K.Nishikawa,8 S.Nishiyama,2 S.Noda,2 Y.Obayashi,3 A.Okada,3 Y.Oyama,10
M.Y.Pac,19 H.Park,10, ∗ C.Saji,3 M.Sakuda,10 A.Sarrat,11 T.Sasaki,8 N.Sasao,8 K.Scholberg,20 M.Sekiguchi,2E.Sharkey,11 M.Shiozawa,3 K.K.Shiraishi,4 M.Smy,5 H.W.Sobel,5 J.L.Stone,13 Y.Suga,2 L.R.Sulak,13 A.Suzuki,2
Y.Suzuki,3 Y.Takeuchi,3 N.Tamura,18 M.Tanaka,10 Y.Totsuka,10 S.Ueda,8 M.R.Vagins,5 C.W.Walter,13 W.Wang,13
R.J.Wilkes,4 S.Yamada,3, ∗ S.Yamamoto,8 C.Yanagisawa,11 H.Yokoyama,21 J.Yoo,1 M.Yoshida,22 and J.Zalipska16(The K2K Collaboration)

arXiv:hep-ex/0402017 v1 10 Feb 2004

1

Department of Physics, Seoul National University, Seoul 151-742, KOREA
2
Kobe University, Kobe, Hyogo 657-8501, JAPAN3
Institute for Cosmic Ray Research, University of Tokyo, Kashiwa, Chiba 277-8582, JAPAN
4
Department of Physics, University of Washington, Seattle, WA 98195-1560, USA
5
Department of Physics and Astronomy, University of California, Irvine, Irvine, CA 92697-4575, USA
6
Department of Physics, Chonnam National University, Kwangju 500-757, KOREA
7
Department of Physics, Miyagi University of Education, Sendai 980-0845, JAPAN
8
Department of Physics, Kyoto University, Kyoto 606-8502, JAPAN
9
Research Center for Neutrino Science, Tohoku University, Sendai, Miyagi 980-8578, JAPAN10High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, JAPAN
11
Department of Physics and Astronomy, State University of New York, Stony Brook, NY 11794-3800, USA
12
Department of Physics, Okayama University, Okayama, Okayama 700-8530, JAPAN
13
Department of Physics, Boston University, Boston, MA 02215, USA
14
Department of Physics and Astronomy, University of Hawaii, Honolulu, HI 96822, USA
15
Institute of Experimental Physics, Warsaw University, 00-681 Warsaw, POLAND
16
A. Soltan Institute for Nuclear Studies, 00-681 Warsaw, POLAND
17
Department of Physics, Korea University, Seoul 136-701, KOREA
18
Department of Physics, Niigata University, Niigata, Niigata 950-2181, JAPAN
19
Department of Physics, Dongshin University, Naju 520-714, KOREA
20
Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
21
Department of Physics, Tokyo University of Science, Noda, Chiba 278-0022, JAPAN
22
Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, JAPAN(Dated: May 1, 2004)
We present a search for electron neutrino appearance from accelerator produced muon neutrinos in
the K2K long baseline neutrino experiment. One candidate event is found in the data corresponding
to an exposure of 4.8 × 1019 protons on target. The expected background in the absence of neutrino
oscillations is estimated to be 2.4 ± 0.6 events and is dominated by mis-identification of events fromneutral current π 0 production. We exclude the νµ to νe oscillations at 90% C.L. for sin2 2θµe > 0.15
at ∆m2µe = 2.8 × 10−3 eV2 , the best fit value of the νµ disappearance analysis in K2K. The moststringent limit of sin2 2θµe < 0.09 is obtained at ∆m2µe = 6 × 10−3 eV2 .
PACS numbers: PACS numbers: 14.60.Pq, 13.15.+g, 23.40.Bw, 95.55.Vj

In 1998, the Super-Kamiokande (SK) collaboration reported evidence of neutrino oscillation based on atmospheric neutrino observations favoring large mixing between νµ and ντ and a ∆m2 near 2.2 × 10−3eV2 [1]. Subsequently, solar neutrino data from various experiments
have indicated νe disappearance as a result of neutrino oscillations to other active neutrino flavors (νµ or ντ ) withlarge mixing and a ∆m2 near 5 × 10−5 eV2 [2, 3]. The
KamLAND experiment also observes a deficit of reactor

ν e consistent with the same parameter values [4] as thosein the solar neutrinos. Recently, the KEK to Kamioka
long-baseline neutrino oscillation experiment (K2K) [5]reported indications of νµ → νx oscillation using an accelerator produced νµ beam. The measurement of νµdisappearance in K2K results in neutrino oscillation parameters which are consistent with the values derived
from the atmospheric neutrino oscillations.
Measurements of atmospheric and solar neutrinos sug-2gest mixing between all neutrino flavors. The νe appearance is predicted at the same ∆m2 as the one measured
using the atmospheric neutrinos (∆m2atm ) in the framework of 3-flavor neutrino oscillations with certain parameter values. In this letter, we report results from the firstsearch for νe appearance using νµ beam in the K2K experiment sensitive to the ∆m2atm region.
In K2K, an almost pure (98%) νµ beam with a meanenergy of 1.3 GeV is produced with the KEK proton synchrotron (KEK-PS) [6]. The fraction of νe is approximately 1% and the remainder are ν µ . Twelve GeV protons from the KEK-PS hit an aluminum target embedded
inside a pulsed magnetic horn system which focuses positively charged secondary particles, mainly pions, towarda far detector located 250 km far from KEK. The secondary particles decay to produce a neutrino beam. The
stability of the pulse-by-pulse beam direction is checkedby monitoring muons from pion decay with a set of ionization chambers and silicon pad detectors following thebeam dump. The measurements from these monitors
show that the beam is directed to within 1 mrad of the
far detector, SK [7], which is a 50 kt Water Cherenkov
detector located in Kamioka, Gifu Prefecture in Japan.
To reject cosmic-ray and atmospheric neutrino background, the global positioning system is employed atboth the KEK and SK sites to synchronize between beam
spills and events observed in SK [8]. The neutrino flux at
KEK is measured by a near detector complex consistingof a 1 kt water Cherenkov detector (1KT) and a finegrained detector (FGD) system. The FGD consists ofa scintillating fiber detector with segmented water targets (SciFi) [9], a plastic scintillator hodoscope (PSH),
a lead-glass calorimeter (LG), and a muon range detector (MRD) [10]. The νµ flux at SK is estimated by extrapolating the measured flux at KEK using predicted
flux ratio between SK and KEK (far/near ratio). The
far/near ratio is evaluated by a beam Monte Carlo (MC)simulation as a function of neutrino energy and is validated using secondary pion kinematic distributions measured with a gas Cherenkov detector [11] downstream of
the horn system.
Electron neutrino events in SK are selected assuming
νe charged current quasi-elastic (CC-QE) interactions inan oxygen nucleus, i.e. νe + n → e + p. Since the proton
momentum in the reaction is typically below Cherenkov
threshold in water, only the electron is visible. Thus, asingle electron-like Cherenkov ring is the signature of νeappearance. The νe contamination in the beam is estimated by the beam MC simulation which predicts theratio of the number of νe interactions to that of νµ to
be 0.9% in SK. This estimate is checked by a measurement of the νe fraction at KEK using the FGD system.
A more serious background comes from neutral current(NC) interactions where a single π 0 is produced and one
gamma-ray from its decay is not reconstructed. The NC
π 0 production rate was measured using the 1KT, con-

15

(a)

10
5
0

-10

15

0
PID (pattern)

10

(b)

10
5
0

-10

-5
0
5
PID (opening angle)

10

FIG. 1:
Distributions of PID parameters for 32 singlering events based on (a) Cherenkov ring pattern, and (b)Cherenkov opening angle. The distributions for the data(closed circles), oscillated νµ MC with (sin2 2θ, ∆m2 ) =(1.0, 2.8 × 10−3 eV2 ) (solid histograms) and expected νe signal with full mixing (dashed histograms) are shown. Shaded
histograms are the NC component of νµ MC.

straining the cross section.
This analysis is based on the data taken between June
1999 and July 2001, corresponding to 4.8 × 1019 protons
on target (POT). A total of 56 fully-contained events are
obtained in the 22.5 kt fiducial volume of SK. Singlering events are selected to enhance CC-QE interactions
against the π 0 production background. The details of
the selection criteria for fully-contained single-ring eventsare found in Ref. [5]. Particle identification (PID) is applied to reduce νµ -induced backgrounds. A Cherenkov
ring produced by an electron is diffused by its electromagnetic shower and multiple scattering, while that produced by a muon has a clear edge, and a low energy
muon has a smaller opening angle than an electron. Both
the Cherenkov ring pattern and opening angle are required to be consistent with an electron event. PID parameters are calculated from a log-likelihood difference
for the electron and the muon hypothesis. Distributionsof these PID parameters are shown in Fig.1. Negativevalues of the parameters indicate an electron-like event.
Distributions of data are consistent with the oscillatedνµ MC with (sin2 2θ, ∆m2 ) = (1.0, 2.8 × 10−3 eV2 ) , thebest-fit parameters of the νµ disappearance analysis in
K2K [5]. The visible energy is required to be larger than
100 MeV to reject low momentum charged pions andelectrons from muon decays in which muons are below
Cherenkov threshold. Finally, events which are followed
by a decay electron signal within a 30 µsec time win-

3
TABLE I: Summary of the event reduction for νe appearance search at SK. The first column lists each reduction step, and the second gives the number of observed
events after each selection. The numbers of expected background from νµ and beam νe without neutrino oscillations
are shown in the third and fourth column, respectively.
The last column corresponds to the expected number of
CC interaction events induced by νe oscillated from νµ with(sin2 2θ, ∆m2 ) = (1.0, 2.8 × 10−3 eV2 ).
FCFVSingle Ring
PID (e-like)
Evis >100MeV
w/o decay-e

DATA νµ w/o osc beam νe νe from νµ osc
56
80
0.8
28
32
50
0.5
20
1
2.9
0.4
18
1
2.6
0.4
18
1
2.0
0.4
16

dow are rejected. A small fraction of νe interactions arealso rejected by this cut when they are accompanied withdecay electrons originating from pions in inelastic interactions. The overall efficiency to select CC interactions
from the oscillated νe is 57 % for ∆m2 = 2.8 × 10−3 eV2 .
One event is selected as an electron candidate as summarized in Table I. While ring-counting algorithms evaluate this event as a single-ring event, under a carefulmanual examination it reveals that the remaining PMT
hits out of the reconstructed ring form an additional ring,and the invariant mass of these two rings is found to beconsistent with the π 0 mass. Thus, we conclude that theobserved event is consistent with a π 0 background eventfrom NC interaction.
In our simulation to predict the number of background
events, NC and CC pion production are modeled following Rein and Sehgal [12] for the resonance region, andGRV94 [13] with the correction of Bodek and Yang [14]for the deep inelastic scattering region. The axial vector mass for QE and resonance pion production in thesimulation are 1.1 GeV/c2 and 1.2 GeV/c2 , respectively.
The interaction models used in this analysis are the sameas in the νµ disappearance analysis in Ref. [5] except for
the normalization of NC with respect to CC-QE crosssection. The reduction of the background events is summarized in Table I.
The expected background from νµ interactions in thecase of no oscillation is estimated to be 2.0 events where
the normalization is determined by extrapolating the observed number of events in the 1KT [5]. It is estimatedto be 1.9 events in the case of νµ → ντ oscillation with(sin2 2θ, ∆m2 ) = (1.0, 2.8 × 10−3 eV2 ). Since the background is dominated by NC π 0 production (87%) andthe oscillated ντ has the same NC interactions, it is insensitive to the νµ disappearance oscillation parameters.
The NC π 0 production cross-section in the MC simulation is checked by a 1KT measurement of π 0 events.
In the 1KT, π 0 events are selected by requiring two e-

TABLE II: Systematic errors in the expected number of νµbackground in SK.
Jun.1999 Nov.1999∼Jul.2001+22%
NC cross-section
−27%
+15%
Ring Counting−13%
Particle ID±11%νµ Energy Spectrum
±14%
+15%
Far/Near ratio
−11%
ǫ1KT
±4%
ǫSK
±3%
POT normalization±0.9%
CC-nQE cross-section ±1%
Total
±36%

+20%
−25%
+15%
−13%

±11%
±1%
±6%
±4%
±3%
±0.6%
±0.4%
+33%
−31%

like rings whose invariant mass is between 85 MeV and215 MeV. Muon events are also collected by requiring asingle µ-like ring in the 1KT as a reference to νµ flux.
The NC/CC-QE cross section ratio is calculated fromthe ratio of the number of π 0 events to that of muon
events. The π 0 sample in the 1KT is dominated by NC
interactions (87%), while the muon sample is dominatedby CC interactions (97%). The measured NC/CC-QE
ratio is consistent with the MC simulation; the ratios ofdata to MC is 1.07+0.20−0.15 . To cover the allowed range,from 0.92 to 1.27, without changing the NC cross-section
model in our MC, an uncertainty of 30% is assigned on
the NC/CC-QE ratio. This error is used to estimate thesystematic uncertainty in the νµ background. The uncertainty in the cross-section ratio of CC interactions otherthan QE (CC-nQE) to CC-QE is estimated to be ±20%
as in Ref. [5]The uncertainty in the νµ -induced background is estimated to be ±0.6 events for the 2.0 events. Contributions
from various sources to the systematic error are summarized in Table II. Since the horn current and target diameter were different in June 1999 from the other period,
systematic errors are estimated separately for these two
periods and properly weighted to obtain the total systematic error. The uncertainty in the NC cross-section
gives the largest contribution of +20−25 %. Systematic errors from ring counting and PID are estimated by comparing the shape of the MC and data likelihood distributions for cosmic-ray muons and atmospheric neutrino
events. They are assigned to be +15−13 % and ±11%, respectively. Systematic errors from the neutrino energy
spectrum (±1.0%) and far/near ratio (±6.0%) are estimated in the same manner as in Ref. [5]. Systematic
errors from the fiducial volume definition and detection
threshold in the 1KT (ǫ1KT ) and SK (ǫSK ) are estimatedto be ±4% and ±3%, respectively.
The expected background from beam νe interactions in
SK is estimated to be 0.4 events, which is derived from4,
+ NνBG+ NνBG
Nexp = NνOSC
e
µ
e

-2

10
2

K2K 99% C.L.

2

K2K 90% C.L.

-3

10

CHOOZ 90% C.L.

-4

10

0

0.2

0.4 2 0.6
sin 2θµe0.8

1FIG. 2: The confidence interval for νµ → νe oscillations as
a function of the effective ∆m2µe at 90% C.L. (solid line) and
99 % C.L. (dashed line). The area to the right of each curve is
excluded. Dotted line shows the limit at 90% C.L. by CHOOZ
assuming sin2 2θµe = 21 sin2 2θ13 .

(1)

is the number of electron events induced
where NνOSC
eis that induced by beam νe , andby oscillated νe , NνBG
e
is
that
inducedbyboth CC and NC interactionsNνBGµof νµ and NC interactions of νe and ντ from oscillations.
, depends on the
The νµ → νe oscillation signal, NνOSC
eprobability of νe appearance, which is expressed by
P (νµ → νe ) = sin2 (2θµe ) sin2 (1.27∆m2µe L/E),-1

10∆m (eV )the νe /νµ flux ratio predicted by the beam MC simulation and the νµ flux extrapolated from the 1KT measurement. The systematic uncertainty in the number ofbeam νe events is estimated to be 0.11 events, which isdominated by the uncertainty in the νe energy spectrum.
The νe /νµ ratio has been verified by a measurement ofνe events in the FGD [15]. The νe events in the FGD are
selected by requiring 1) a vertex inside the SciFi fiducial
volume, 2) an energy deposit in the PSH of greater than
20 MeV, 2.5 times larger than expected from a muon, 3)
an energy deposited in the LG of greater than 1 GeV,
and 4) no corresponding hits in the MRD. During an
exposure of 2.9 × 1019 POT, 51 electron candidates are
selected with an estimated background of 24 νµ induced
events. The νe /νµ interaction ratio is measured to be
1.6 ± 0.4(stat.)+0.8−0.6 (sys.)% which is in agreement withthe beam MC prediction of 1.3%.
The observation of one electron event in SK is consistent with the expected background of 2.4 events in thecase of no oscillation. A constraint on neutrino oscillations from νµ to νe is obtained by comparing the observed
number of electron events with the expectation assuming
oscillations. The expected number of electron events is
calculated by(2)

where θµe and ∆m2µe are the effective mixing angle andmass squared difference between the mass eigenstates
involved, respectively, for νe appearance; L is neutrino
path length in kilometers; and E is the neutrino energy
in GeV. Descriptions of the relation between the effective parameters and the oscillation parameters in 3-flavor
framework can be found in Ref. [16]. The number of= 0.4, is treated asbeam νe induced background, NνBG
ea constant, since a contribution of νe → νx oscillation isdecreases withnegligible. The CC component of NνBGµνµ disappearance observed in K2K and atmospheric neutrino experiments , depending on the survival probability,
P (νµ → νµ ) = 1 − sin2 (2θµµ ) sin2 (1.27∆m2µµ L/E), (3)where θµµ and ∆m2µµ are the effective mixing angle andmass squared difference for νµ disappearance, respectively. In the present analysis, we assume θµµ = π4 basedon the nearly full mixing observed by atmospheric neutrino experiments, and ∆m2µµ = ∆m2µe , which is impliedin the framework of 3-flavor neutrino mixing by the small

mass difference found in solar neutrino experiments [16].
Thus, Nexp reduces to a function of two parameters, θµe
and ∆m2µe .
A probability density function (PDF) for Nexp is constructed from the Poisson distribution convoluted withthe systematic uncertainty. Given the observation of
one electron event, the systematic uncertainty has a verysmall effect on the derived confidence interval. The confidence interval of sin2 2θµe is calculated using the methodsuggested in Ref. [17]. In the calculation, the best-fit parameters are searched for in the 2-dimensional parameter
space with sin2 2θµe bounded in [0,1].
Figure 2 shows the limit on sin2 2θµe as a function of∆m2µe . The experimental limits on the neutrino mixing
for the νµ → νe oscillation hypothesis are given at 90%
C.L. for a parameter region with ∆m2µe > 6 × 10−4 eV2 .
Neutrino oscillations from νµ to νe are excluded at 90%
C.L. for sin2 2θµe > 0.15 at ∆m2µe = 2.8 × 10−3 eV2 . The
most stringent limit of sin2 2θµe < 0.09 is set for ∆m2µe =6 × 10−3 eV2 .
Assuming 3-flavor neutrino oscillations and CPT invariance, our results can be compared to reactor experiments. CHOOZ has excluded sin2 2θ13 > 0.1 at∆m213 ∼ 3 × 10−3 eV2 [18]. This corresponds to a limitof sin2 2θµe < 0.05 at ∆m2µe ∼ 3 × 10−3 eV2 assuming
θ23 = π/4, and consistent with the present analysis asshown in Fig.2. The limit on sin2 2θ13 by CHOOZ isconverted by assuming sin2 2θµe = 21 sin2 2θ13 .
The K2K experiment searched for νµ → νe oscilla-

5
tions with accelerator-produced muon neutrinos traveling 250 km. This is the first experimental search for νeappearance with sensitivity down to the ∆m2 suggested
by atmospheric neutrino oscillations. A single electron
candidate is found in SK. The observed event is consistent with the expected background event. The limit on
the νe appearance is obtained. At the best-fit parametervalues of the K2K νµ disappearance analysis, we set the
90% confidence limit of sin2 2θµe < 0.15.
We thank the KEK and ICRR Directorates for theirstrong support and encouragement. K2K is made possible by the inventiveness and the diligent efforts of the
KEK-PS machine and beam channel groups. We thankfully appreciate discussions on the statistical treatment
with Dr. Louis Lyons. We gratefully acknowledge thecooperation of the Kamioka Mining and Smelting Company. This work has been supported by the Ministryof Education, Culture, Sports, Science and Technology,
Government of Japan and its grants for Scientific Research, the Japan Society for Promotion of Science, theU.S. Department of Energy, the Korea Research Foundation, the Korea Science and Engineering Foundation, theCHEP in Korea, and Polish KBN grant 5P03B06531.

∗

For current affiliations see http://neutino.kek.jp/presentaddresses0401.ps[1] Y. Fukuda et al. (Super-Kamiokande Collaboration),
Phys. Rev. Lett. 81, 1562 (1998), hep-ex/9807003.
[2] S. Fukuda et al. (Super-Kamiokande Collaboration),
Phys. Lett. B539, 179 (2002), hep-ex/0205075.
[3] Q. Ahmad et al. (SNO Collaboration), Phys. Rev. Lett.
89, 011301 (2002), hep-ex/0204008.
[4] K. Eguchi et al. (KamLAND Collaboration), Phys. Rev.
Lett. 90, 021802 (2003), hep-ex/0212021.
[5] M.H.Ahn et al. (K2K Collaboration), Phys. Rev. Lett.
90, 041801 (2003), hep-ex/0212007.
[6] S. H. Ahn et al. (K2K Collaboration), Phys. Lett. B511,
178 (2001), hep-ex/0103001.
[7] Y. Fukuda et al. (Super-Kamiokande Collaboration),
Nucl. Instrum. Meth. A501, 418 (2003).
[8] H. G. Berns and R. J. Wilkes, IEEE Trans. Nucl. Sci. 47,
340 (2000), hep-ex/9909021.
[9] A. Suzuki et al. (K2K Collaboration), Nucl. Instrum.
Meth. A453, 165 (2000), hep-ex/0004024.
[10] T. Ishii et al. (K2K MRD Group), Nucl. Instrum. Meth.
A482, 244 (2002), hep-ex/0107041.
[11] T. Maruyama, Ph.D. thesis, Tohoku University (2000).
[12] D. Rein and L. M. Sehgal, Ann. Phys. 113, 79 (1981).
[13] M. Gluck, E. Reya, and A. Vogt, Z. Phys. C67, 433(1995).
[14] A. Bodek and U.-K. Yang, Nucl. Phys. Proc. Suppl. 112,
70 (2002), hep-ex/0203009.
[15] M. Yoshida, Ph.D. thesis, Osaka University (2001).
[16] K. Hagiwara et al., Phys. Rev. D66, 010001 (2002).
[17] G. J. Feldman and R. D. Cousins, Phys. Rev. D57, 3873
(1998), physics/9711021.
[18] M. Apollonio et al., Eur. Phys. J. C27, 331 (2003), hepex/0301017.

Nuclear Instruments and Methods in Physics Research A 453 (2000) 165}176

Design, construction, and operation of SciFi tracking detector
for K2K experiment
A. Suzuki *, H. Park, S. Aoki , S. Echigo , K. Fujii , T. Hara , T. Iwashita ,M. Kitamura , M. Kohama , G. Kume , M. Onchi , T. Otaki , K. Sato ,M. Takatsuki , K. Takenaka , Y. Tanaka , K. Tashiro , T. Inagaki, I. Kato,S. Mukai, T. Nakaya, K. Nishikawa, N. Sasao, A. Shima, H. Yokoyama,T. Chikamatsu, Y. Hayato, T. Ishida, T. Ishii, H. Ishino, E.J. Jeon,T. Kobayashi, S.B. Lee, K. Nakamura, Y. Oyama, A. Sakai, M. Sakuda,V. Tumakov, S. Fukuda, Y. Fukuda, M. Ishizuka, Y. Itow, T. Kajita,J. Kameda, K. Kaneyuki, K. Kobayashi, Y. Kobayashi, Y. Koshio, M. Miura,S. Moriyama, M. Nakahata, S. Nakayama, Y. Obayashi, A. Okada,N. Sakurai, M. Shiozawa, Y. Suzuki, H. Takeuchi, Y. Takeuchi, Y. Totsuka,T. Toshito, S. Yamada, K. Miyano, M. Nakamura, N. Tamura, I. Nakano,M. Yoshida, T. Kadowaki, S. Kishi, H. Yokoyama, T. Maruyama , M. Etoh ,K. Nishijima , H.C. Bhang , B.H. Khang , B.J. Kim , H.I. Kim , J.H. Kim ,S.B. Kim , H. So , J.H. Yoo , J.H. Choi , H.I. Jang , J.S. Jang , J.Y. Kim , I.T. Lim ,M.Y. Pac , E. Kearns, K. Scholberg, J.L. Stone, L.R. Sulak, C.W. Walter,D. Casper, W. Gajewski, W. Kropp, S. Mine, H. Sobel, M. Vagins,S. Matsuno, J. Hill, C.K. Jung, K. Martens, C. Mauger, C. McGrew,E. Sharkey, C. Yanagisawa, H. Berns, S. Boyd, J. Wilkes, D. Kielczewska,U. Golebiewska,K2K Collaboration
Kobe University, Kobe, Japan
Kyoto University, Kyoto, Japan
High Energy Accelerator Research Organization(KEK), Tsukuba, Japan
Institute for Cosmic Ray Research, University of Tokyo, Tanashi, Japan
Niigata University, Ikarashi, Niigata, Japan
Okayama University, Okayama, Japan
Osaka University, Toyonaka, Osaka, Japan
Science University of Tokyo, Noda, Chiba, Japan
Tohoku University, Sendai, Miyagi, Japan
Tokai University, Kanagawa, Japan
Seoul National University, Seoul, South Korea* Corresponding author.
 Present address: Miyagi Women's College, Sendai, Miyagi, Japan.
0168-9002/00/$ - see front matter  2000 Published by Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 6 2 4 - 0

5. TRACKING DEVICES

166

A. Suzuki et al. / Nuclear Instruments and Methods in Physics Research A 453 (2000) 165}176
Chonnam National University, Kwangju, South Korea
Dongshin University, Naju, South Korea
Boston University, Boston, MA, USA
University of California, Irvine, CA, USA
University of Hawaii, HI, USA
State University of New York, Stony Brook, NY, USA
University of Washington, Seattle, WA, USA
Warsaw University, Warsaw, Poland
Accepted 19 June 2000

Abstract
We describe the construction and performance of a scintillating "ber detector used in the near detector for the K2K(KEK to Kamioka, KEK E362) long baseline neutrino oscillation experiment. The detector uses 3.7 m long and 0.692
mm diameter scintillating "ber coupled to image-intensi"er tubes (IIT), and a CCD camera readout system. Fiber sheet
production and detector construction began in 1997, and the detector was commissioned in March 1999. Results fromthe "rst K2K runs con"rm good initial performance: position resolution is estimated to be about 0.8 mm, and track"nding e$ciency is 98$2% for long tracks (i.e., those which intersect more than 5 "ber planes). The hit e$ciency wasestimated to be 92$2% using cosmic-ray muons, after noise reduction at the o%ine stage. The possibility of using thedetector for particle identi"cation is also discussed.  2000 Published by Elsevier Science B.V. All rights reserved.

1. Introduction
1.1. K2K experiment
The Super-Kamiokande Collaboration has
shown strong evidence for neutrino oscillations in
an analysis of atmospheric neutrino data [1].
K2K [2] is a long baseline neutrino experiment
designed to allow more precise studies of neutrino
oscillations. A high-purity muon neutrino beam, generated using the KEK 12 GeV proton synchrotron, is
directed through a near detector system at KEK tothe Super-Kamiokande site 250 km away. By comparing the m beam #ux and energy spectrum betweenlthe near and far detectors, we can investigate neutrino
oscillations between m and other neutrino #avors.
l
The near detector includes a 1 kt water Cherenkov detector and a "ne-grained detector (FGD),
which in turn consists of the scintillating "ber(SciFi) detector, scintillating counters, a lead glass
counter, and a muon range detector. Data taking
for K2K began in March 1999.
1.2. Scintillating xber (SciFi) detector
The near neutrino detector is designed tomeasure the #ux and the energy spectrum of theneutrino beam as it leaves KEK. The detector must
provide good tracking capability, allowing discrimination between di!erent types of interactions suchas quasi-elastic or inelastic events. Its mass composition should be dominated by water, to allow
cancellation of common systematic uncertainties
with the Super-Kamiokande detector to the maximum extent possible. In order to study neutrino
interactions in greater detail, we decided to use
a SciFi detector constructed with "ber tracking
layers interleaving water target tanks. SciFi detectors have been used in UA2 and CHORUS
experiments [3,4]. The K2K SciFi detector is thelargest ever so far, and employs a simple design ofoptoelectronics readout and a calibration systemusing electro-luminescent (EL) plates.

2. Construction and operation of the K2K SciFi
detector
2.1. Design and structure
A schematic overview of the SciFi detector isshown in Fig. 1. It consists of 20 layers of
2.6 m;2.6 m tracking modules, spaced 9 cm apart,
each of which contains double layers of scintillating

A. Suzuki et al. / Nuclear Instruments and Methods in Physics Research A 453 (2000) 165}176

167

Fig. 2. IIT-CCD chain.

Fig. 1. Schematic overview of the SciFi tracking detector.

"ber sheets in both the horizontal and verticaldirections (i.e., XX>> layers). The "ber sheets
are coupled to image-intensi"er tubes (IITs) which inturn are read out by CCD cameras (Fig. 2). Between
the "ber modules, there are 19 layers of water target
contained in extruded aluminum tanks. Each target
layer consists of 15 tank modules, and thus there
are 285 in total. The width, height, and length of thetanks are 6, 16, and 240 cm, respectively. The thickness of the aluminum tank is 1.8 mm. The "ber is
3.7 m long and 0.692 mm in diameter.
2.2. Optimization of the scintillating xbers
2.2.1. Measurement of the light yield of various xbers
We measured the light yield and attenuation
length of various types of scintillating "bers usinga photo-multiplier tube (PMT), irradiating the "berwith a Sr b-ray source. The light yield, after
propagation of the light over a distance x in the"ber, is expressed by>(x)"> exp[!x/j](1)

where > is an initial light yield at x"0 (IITsurface) and j is an attenuation length. We estimated > by extrapolation from measurements at
x"1.5, 2.0, 2.5, and 3.0 m. The results are summarized in Table 1. Kuraray SCSF-78M is a multicladding "ber, and the others have single cladding.
From these measurements, we also estimated thedetection e$ciency for minimum ionizing particles
after light propagation over a distance x"4 m fora double-layered "ber sheet. Here, we took into
account the quantum e$ciencies of the IIT (22%)
and PMT (25%), and the re#ectivity at the far endof the "ber (70%). When we required more than
99% detection e$ciency, there were only two candidates left. In order to minimize the readout area,
we selected the smaller-diameter "ber, SCSF-78M(0.7 mm diameter).

Table 1
Measured light yield > and attenuation length j. The detection e$ciency e after 4 m light propagation for a double-layered "ber sheet
was calculated using > and j
SciFi type

Diameter (mm)> (p.e.)j (m)E$ciency (e) (%)

SCSF-78M

0.5
0.7
0.7
0.9
0.7
0.7

4.4 $ 0.2
6.5 $ 0.2
5.2 $ 0.1
7.2 $ 0.2
4.2 $ 0.1
3.6 $ 0.1

3.46 $ 0.18
3.57 $ 0.17
3.48 $ 0.13
3.86 $ 0.13
4.39 $ 0.27
3.36 $ 0.18

97.2 $ 0.7
99.6 $ 0.1
98.6 $ 0.3
99.9 $ 0.03
98.9 $ 0.3
94.2 $ 1.2

SCSF-78
SCSF-77
BCF-12

5. TRACKING DEVICES

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2.2.2. Aging
Scintillating "bers are known to have a "nitelifetime, with "ber transparency diminishingwith age. Aging is mainly caused by a chemical
reaction of the core material (polystyrene)
with oxygen in air. Fiber lifetime depends on temperature. We measured the decrease in light yieldof SCSF-78M at several elevated temperatures andestimated the "ber lifetime at lower temperaturesusing an `Arrheniusa plot, lifetime as a functionof temperature (Fig. 3). Here we de"ne a "berlifetime as the number of days at which the lightyield of the "ber drops to 90% of its initial value.
From the Arrhenius plot, the lifetimes of SCSF78M at 15 and 203C are estimated to be 21 000 and
9400 days, respectively. If we keep the temperatureof the experimental hall less than 203C, the decreasein the light yield is expected to be less than 10%
during the "ve years of the experiment. In fact, we
attempt to maintain the SciFi detector temperature
at 163C.
2.3. Fiber sheet production
We had to fabricate double-layered "ber sheets
before building the "ber tracking modules. Each"ber sheet is 370 cm long and 40 cm wide, consist-

Fig. 3. Fiber lifetime as a function of temperature. The measured "ber was Kuraray SCSF-78M version 1.1 (anti-O type).
ing of a 260 cm long sensitive area, 80 cm long lightguide, and 30 cm long IIT bundle segment (Fig. 4).
A "ber tracking module consists of six doublelayered "ber sheets laid side by side to make a total
width of 240 cm, in both horizontal and verticaldirections. For 20 such modules, we needed tofabricate 240 sheets in total, plus 8 spares. Thismethod of "ber sheet production is quite similar tothat used by the CHORUS Collaboration [5,6]. To
produce 3.7 m long "ber sheets, we prepared a
1.2 m diameter drum with a spiral groove whose
pitch was 0.7 mm. We wound "ber stock along thegroove until the sheet became 40 cm wide. Then, we
coated the sheet with white paint. After it became
dry, we wound the second layer "ber onto the "rstlayer and painted it again. Finally, we cut the sheet,removed it from the drum, and painted its innerside.
A control system for "ber sheet production
is shown in Fig. 5. Fiber stock was wound onto
the bobbin and threaded onto the winding machine. The drawing system, which consists of two
rubber-coated rollers, a rotary encoder, and a stepping motor, drew "ber from the bobbin. Because
the two layers have slightly di!erent circumferenceson the drum, we applied a constant tension to eachlayer (150 gw to the "rst and 300 gw to the second
layers, respectively) during winding. After being
removed from the drum, the prestressed sheet thenbecame #at due to the restoring force supplied by
the outer "bers. At the downstream end of thetension controller, a laser sensor monitored theFig. 4. Schematic view of a SciFi sheet. The upper "gure shows
a top view and the lower "gure shows a cross-sectional view.

A. Suzuki et al. / Nuclear Instruments and Methods in Physics Research A 453 (2000) 165}176

169

Fig. 5. Schematic view of the "ber sheet production system.

"ber diameter. When it detected a "ber section
out of tolerance (692$18 lm), the wholesystem stopped, and we cut away the segment.
Guide pullies attached to an accurate linear guide,
driven by a stepping motor, lead the "ber onto thegroove of the drum. They were synchronized withrotation of the drum and moved 0.7 mm for a turnof rotation. The drum was driven by an AC servo
motor. The rotation speed was adjustable and the
maximum speed was about 1 m/s (0.265 rps). The
whole "ber sheet production system was controlled
by a conventional PC with two custom interface
boards.
2.4. Fiber sheet modules
After all the "ber sheets were made, both endsof the sheets were polished by a milling machineand far ends of the sheets were sputtered withaluminum. We checked the quality of all the individual "bers by irradiating the sheets with Srb-source at the farthest point. The light yield of the"ber sheets was measured to be 8$1 photo-electrons.
Since the "ber sheets are very fragile, we neededa strong and light material to support the "ber sheet.
We chose a honeycomb panel with a dimension of
2.6 m;2.6 m;1.6 cm, consisting of a 1.4 cm thick
paper-honeycomb core and two 1 mm thick GFRP
skins. The area density of the honeycomb board is0.52 g/cm, and their net mass is 30 kg per board.

Fig. 6. Fiber bundle assembly. Five "ber sheets are gathered
into each bundle.

We glued six "ber sheets side by side on one
surface of the honeycomb panel in a horizontal
direction, and six more sheets on the other side ina vertical direction (referring to the "nal orientation), using epoxy adhesive CY221/HY2967.
Since we found that gas released by the glue actually accelerates "ber aging, we placed the glue
in a vacuum chamber for 5 min before gluing.
The positions and the straightness of the sheets
were controlled within 0.5 mm by a ruler bar
and alignment pins when the sheets are glued.
The straightness of each sheet was measured
by a micrometer every 10 cm with accuracy of
0.1 mm.
Since the "ber sheet modules were rather large(3.7 m;3.7 m), we made a special aluminum jig(4 m;4 m) to perform assembly, transportation,and installation of the modules safely and easily.
Throughout the installation, only about 20 "berswere broken out of 274 080.

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A. Suzuki et al. / Nuclear Instruments and Methods in Physics Research A 453 (2000) 165}176

2.5. Fiber bundles
In order to make e!ective use of the photosensitive area (10 cm diameter) of the IIT surface,"bers are bundled at the readout. All the "bers(1142;6;20;2"274 080) are grouped at thereadout end and glued into bundles, which are thenattached to the 24 IITs. Ten consecutive "ber sheets
along the beam direction are assigned to one "berbundle. Thus, there are six "ber bundles in the
upstream half of the detector, and another six "berbundles in the downstream half, in both x (horizontal) and y (vertical) directions.
One "ber bundle contains 1142;10 (or274 080/24)"11 420 "bers. Fig. 6 shows the arrangement used to accumulate "bers and makea (half) bundle from (5) "ber sheets. The remaining
half bundle is symmetrical with respect to the direction along the SciFi layer and the two halves are
glued together. In order to optically separate "bersheets from di!erent layers, acrylic "lms (200 lm
thick) are inserted between individual "ber sheets ina bundle. Epoxy glue (CY221/HY2967) was used tofabricate the bundle. For good optical contact with
the IIT surface, we trimmed and polished the surface of each bundle using a custom-made polishingmachine. The detector geometry and construction
logistics required us to perform bundle polishing
operations in the experimental hall, after installation of all the SciFi modules, with very limited
working space. A compact portable milling machine, with dual carbide and diamond bits andpre-programmed automatic feed control, wasspecially designed and built for this purpose. Following polishing, bundles were inspected for #aws
using a microscope system.
2.6. Readout system
Our opto-electronics readout comprises twostages of image-intensi"ers, an optical lens anda CCD camera (see Fig. 2). The system was designed to ensure simplicity and thus low cost. The"rst stage is an electrostatic image-intensi"er(Hamamatsu V5502UX) with 100 mm diameter PM Manufacturing, Inc., Yorkville, IL, USA.

photocathode. The image is reduced in diameter bya factor of 23% and the light yield is ampli"ed byabout 5. The bialkali photocathode has quantum
e$ciency 22% at 430 nm, which is the peakwavelength in the emission spectrum of the scintillating "ber. The second stage is a micro-channelplate (MCP) image-intensi"er (Hamamatsu MCPIIT:V1366GX). The gain in light yield is about 1000
at a typical HV value of 2600 V. We operate the
MCP-IIT with pulsed HV to reduce unwanted
events. The gate width is set to 100 ls, which corresponds to the decay time of the #uorescent phosphor screen in the "rst stage IIT. The optical lens at
the third stage reduces the image to 30% andprojects it onto the CCD camera (C3077). The
CCD camera used here has 768 pixels and 493
pixels in horizontal (x) and vertical (y) directions,
respectively, with pixel size 11 lm;13 lm. One"ber is seen by about 4;4 pixels.
The video signal from the CCD is fed to a #ashADC module housed in a NIM crate, where (x, y)
positions of the CCD and the pulse height are
digitized. The digital outputs are then sent to 12
VME FIFO modules. The readout time is about
30 ms.
2.7. EL calibration
To identify hit "bers from the CCD image, wehave to know the correspondence between "berand CCD coordinates. For this purpose, we illuminate selected "bers at regular intervals using anelectro-luminescent plate (EL). We will refer tothese position calibrations as `ELa calibrations
hereafter.
We placed ELs at the edge of the "ber sheets (seeFig. 6 in the previous section), where we coated the
sheet with black paint and then scraped the coatingo! selected "ducial "bers (Fig. 7). Thus, light fromthe EL goes only into the "ducial "bers. We illuminate only one out of every 10}20 "bers and "bersat the edges of the sheets, to save labor and data
size. Positions of the other "bers are interpolated
from these "ducial measurements. We get the "berpositions within 100 lm by this method. We measured the x, y, and z positions of the edge "bers ofeach layer with accuracy of 1 mm during installation, along with other "ducial marks on the "ber

A. Suzuki et al. / Nuclear Instruments and Methods in Physics Research A 453 (2000) 165}176

171

Fig. 7. Method for illuminating "ducial "bers with electroluminescent (EL) plates. Black paint coating the "ber sheet is
scraped o! selected "ducial "bers. Light from the EL plate
enters only these "bers (solid arrows), while the others remain
shielded (dashed arrow).

modules, to relate "ber coordinates to the experiment coordinate system. Cosmic-ray muons wereused to obtain the "nal alignment information.
3. Performance of the detector and analysis of thedata
3.1. Hit dexnition and the pulse height
The energy loss of a minimum ionizing particle isestimated to be 0.19 MeV in the "ber sheet andgenerates about 8 photoelectrons (p.e.) on the IIT
surface if the particle hits the "ber at the mid-pointof SciFi detector (2.4 m from the readout end), and6.5 p.e. if it hits at the end of the "ber (3.7 m fromthe readout).
Fig. 8 shows a typical IIT image corresponding
to a particle track, read out by CCD: the dark spots
represent hit CCD pixels. The "ber positions determined from EL calibration are also shown as small
circles. To reconstruct a hit, the hit "nding algorithm starts clustering CCD-pixels, "nds all "bers which
overlap the pixel cluster, and clusters the "bers. The
hit is de"ned as a cluster of "bers which overlap the
pixel clusters: the hit position is calculated for thecenter of gravity of the hit "bers (weighted by the
number of overlapping pixels) in the "ber cluster.
The attenuation and re#ection of photons insidethe "ber was measured during the quality test as
described in Section 2.4. The light yield is given by>(d)"> (e\BH#R e\*\BH)(2)

where > is the normalization constant, d is thedistance from the IIT surface, j is the attenuation

Fig. 8. (Top): Typical IIT image taken by CCD. The units on theaxes are CCD pixel number. The background of small circles
denotes the parameterized "ber positions determined from EL
calibration. An enlarged view of a hit is shown at bottom. The
hit is de"ned as a cluster of "bers which overlap the pixel
clusters.

length (323 cm), R is the re#ectivity of aluminumcoated side (0.74), and ¸ is the length of the "ber(370 cm).
Fig. 9 shows the mean values of the number ofpixels in a cluster (N) as a function of hit posi tion. The IIT surface is located at x"d!240 cm(the center of detector is located at x"0). The data
shown in open circles are the measured values forthe cosmic-ray muons and they agree well with theprediction by Eq. (2) (solid line). After the attenuation correction, the position dependence almost
vanished, as shown in the "gure (the solid circles).
The N
value for all track hits are normalized to that at x"0. Fig. 10 shows the typical N
distri bution of data and a pixel simulation of cosmic-ray
muons. N
is corrected for the attenuation
 
length and the incident angle.

5. TRACKING DEVICES

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A. Suzuki et al. / Nuclear Instruments and Methods in Physics Research A 453 (2000) 165}176

3.2. SciFi Monte Carlo simulation } pixel simulation

Fig. 9. The mean values of N
as a function of hit position.
 
The data (open circles) are the measured values for cosmic-ray
muons. They agree well with the prediction from Eq. (2) (solidline). After the attenuation correction, the position dependenceis negligible (solid circles).

The Monte Carlo simulation for the SciFi detector proceeds in two steps. In the "rst, charged
particles are tracked through the detector using
GEANT [7]. The energy loss of a particle (in GeV)
is computed whenever the particle traverses a "berand is then converted to the number of photoelectrons observed by the IIT/CCD imaging system, taking into account attenuation along the"ber, re#ection at the aluminium-coated end andthe quantum e$ciency of the IIT photocathodes.
The photoelectrons are then converted to an
image on the CCD surface. Each photoelectron iso!set from the center of the "ber according toa Gaussian distribution measured using single
photoelectrons during LED testing of the IITs. The
size of the pixel cluster corresponding to a single
photoelectron is then chosen from distributions
parameterized from the LED data and the pixel
cluster is built assuming circular symmetry. Finally,
the tuning of the energy scale (i.e. how many photoelectrons correspond to the energy deposition froma minimum ionizing particle) is carried out using
cosmic-ray muons.
3.3. Track and vertex reconstruction

Fig. 10. Typical distribution for the number of pixels (N
) for
 
cosmic-ray muons. Dots with error bars represent data and thehistogram shows pixel simulation results. We required
N
52.
 

The response for a minimum ionizing particle(MIP) is calibrated using cosmic-ray muons. The
N
distribution for cosmic-ray muons shows 
a peak at about 50 when it is normalized to thepulse height at x"0. This means 8 p.e. corresponds to N
"50 at the peak position, which 
agrees well with LED test data.

The track and vertex "nding algorithm proceeds
in four steps. First, the track "nder searches for
two-dimensional (2D) track candidates (in the XZor >Z planes) which hit at least 3 layers of SciFi
modules. A 3D track is reconstructed by "nding thebest combination of two X-view and >-view 2D
tracks, comparing the starting and ending points
and checking the overlap of the 2D tracks. A vertex
is de"ned as an intersection of 3D tracks, and thevertex position is determined with full 3D s minimization "t. If there is only one track in the event,the mid-point of the nearest upstream water tube
is taken as the event vertex. Finally, using
the reconstructed vertex position, the track"nder starts searching for extra tracks near thevertex, which are mainly short tracks. The last
step is very important because most proton tracks
from charge current (CC) interactions are short andmay not be found in the "rst iteration of track"nding.

A. Suzuki et al. / Nuclear Instruments and Methods in Physics Research A 453 (2000) 165}176

Fig. 11. (a) Hit e$ciency and (b) noise rate for various cut values
on N, estimated with cosmic-ray data. The arrows show the chosen analysis cut.

173

Fig. 12. Hit e$ciency for each layer estimated with cosmic-ray
muons (solid circles for data and histogram for Monte Carlosimulation).

The e$ciency of track "nding strongly depends
on hit e$ciency and the density of noise hits in theevents. Hit e$ciency (e ) was evaluated using cos
mic-ray muons which penetrate whole SciFi modules as
Number of hits on the track.
e "  Number of expected hits on the track(3)

In order to keep the hit e$ciency reasonably highand to keep the noise rate as low as 0.5%, werequired a minimum value of N
for a hit. Fig. 11 
shows the hit e$ciency and the noise rate at various cut values on N
. Based on this "gure, we decided to require N
52. As a result, the hit 
e$ciency was found to be 92$2% on average,
with almost no noise contamination ((0.5%).
Fig. 12 shows the hit e$ciency for each layer. Fig.
13 shows the track "nding e$ciency, which wasestimated using cosmic-ray muons and muontracks from m CC interactions generated in thelMonte Carlo simulation. They agree with eachother: 98$2% for long tracks (traversing morethan 7 SciFi layers) and 85$6% for short tracks.

 After upgrading the HV system of IIT in Summer 1999, we
found the hit e$ciency to be 96$2% with the same noise level.

Fig. 13. Track "nding e$ciency as a function of track length,
expressed in terms of the number of SciFi layers traversed, forcosmic-ray muons (solid circles) and for muon tracks fromm charged-current interactions generated by the Monte Carlolsimulation (histogram).

Here, the horizontal axis is the number of layers
which the muon passes through.
Vertex resolution estimated using MonteCarlo events (m CC events) was 1.5 mm inl
5. TRACKING DEVICES

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A. Suzuki et al. / Nuclear Instruments and Methods in Physics Research A 453 (2000) 165}176

Fig. 15. Typical m event candidate in the Fine-Grain detectorlincluding SciFi detector.
Fig. 14. Typical residual distribution for cosmic-ray muon track"ts for a representative layer (layer 9). To obtain this "gure, we
excluded the measured point in layer 9, and tested it against the"t results.

the beam direction and 1.5 mm perpendicular tothe beam.
The position resolution of SciFi detector was
estimated from the residual distribution for hits.
Since multiple Coulomb scattering e!ects are notnegligible, we implemented a Kalman "ltering/smoothing algorithm [8] in order to take multiple
scattering into account during the track "t. Fig. 14
shows a residual distribution of hits for cosmic-ray
muons. The "gure shows that hit position resolution is 0.8 mm. At present, performance is limited
by multiple scattering e!ects in the water tank andimperfect alignment of the sheets. If we can correct
for the detailed alignment parameters, such as thecurvature and rotation of the sheets, we expect thehit position resolution to improve to about 0.5 mm,
which is dominated by multiple scattering e!ects.

Fig. 16. Number of tracks found in m event candidates. Theltotal number of events in a Monte Carlo prediction is normalized to that in the data. The errors shown are statistical only.
The systematic error at each data point is about 5%.

3.4. Neutrino event reconstruction
Fig. 15 shows a typical m CC quasi-elastic (qe)linteraction (m #nPl\#p) candidate. The lonl
 Using a simple straight-line "t instead of Kalman "ltermethod, the rms of the corresponding residual distribution was
1.5 mm.

ger track is a muon, which hits the scintillating
counters and lead glass counters and reaches
the muon range detector, and the shorter track is
a proton.
Fig. 16 shows the distribution of the number ofthe reconstructed tracks from the m interactionlvertex in m event candidates. The total number ofl

A. Suzuki et al. / Nuclear Instruments and Methods in Physics Research A 453 (2000) 165}176

events in a Monte Carlo prediction is normalized
to that in the data. It shows that charged particles
in m CC events are successfully reconstructed andlthe data (solid circles) are well reproduced byMonte Carlo prediction (histogram). We estimatedthe fraction of tracks with misreconstructed trajectories to be less than 4%.

Fig. 17. Linearity of IIT: (a) The mean value of N
as a func tion of N(the number of photoelectrons) from the LED test.
(b) The mean value of N
as a function of p/M from the beam 
test with a proto-type detector using the proton and pion beams.
The simpli"ed Bethe}Bloch formula (solid curve) reproduces thedata points well.

175

3.5. Particle identixcation
Although tracking is the main purpose of the
SciFi detector, we may use it for particle identi"cation. Electrons can be identi"ed from SciFi hit patterns, and are useful for identifying the m events. Theenergy of an electron can be estimated from thenumber of hits in the shower, i.e. E "&10 MeV/hitfor SciFi detector, which is estimated from a Monte
Carlo study and a beam test with a prototype
detector. The energy resolution of electrons wasmeasured to be about 28% for 0.3 GeV in the beam
test and it is estimated to be 15% for 1.0 GeVelectrons.
From LED runs and the beam test, the numberof pixels (N) shows good linearity with the num 
ber of photoelectrons (N ). Fig. 17(a) shows thecorrelation of 1N
2 and N from the LED test.
 
Fig. 17(b) shows the results of the beam test of the
proton and pion beams. The horizontal axis is thep/M of the particle and the vertical axis is the meanvalue of N
. The simpli"ed Bethe}Bloch formula 
reproduces the data points well. Nof the tracks
 
may be used to separate protons from muons orFig. 18. The N
distribution for (a) l candidates and (b) 
proton candidates from m events containing two tracks. Theldata (solid circles) agree with the Monte Carlo prediction (histogram). The hatched histogram shows the proton contribution.

5. TRACKING DEVICES

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A. Suzuki et al. / Nuclear Instruments and Methods in Physics Research A 453 (2000) 165}176

pions. A feasibility study was performed using
m CCqe event candidates, which contain one muonltrack and one proton track. Fig. 18 showsN
distribution for (a) muon and (b) proton 
candidates. The "gure shows that the proton candidates have larger N
values because protons will
 lose more energy in the "ber sheet.

4. Summary
A very large scintillating "ber (SciFi) tracking
detector for the K2K long baseline neutrino oscillation experiment has been in operation since March,
1999. Track "nding e$ciency is 98$2% for longmuon tracks (those which intersect more than
5 "ber planes), and 85$6% for short tracks, whichwere estimated using cosmic-ray muons anda Monte Carlo simulation. The position resolution
per layer is about 0.8 mm. The SciFi detector has
demonstrated its capability for reconstruction ofm interactions. The pulse heights for cosmic-raylmuons have been stable within 10% after one yearof operation. In addition, the SciFi detector o!ers
the possibility of performing electron and proton
identi"cation.

Acknowledgements
We thank Mr. S. Ishikawa and Mr. T. Kawai of
Nagoya University for designing and constructing
the "ber spooling machine. We also thank Prof. K.
Niwa and his group, especially Dr. T. Nakano, ofNagoya University for their cooperation. Thanks
are also due to Mr. C. Lindenmeyer, Mr. P. Mulligan, and Mr. J. Roze, for the design, construction,and operation of the "ber bundle polishing machine.
We also thank M. Powlowski, T. Raza, and T. Kato
(SUNY) for helping the "ber sheet assembly.

References[1] Y. Fukuda et al. (Super-Kamiokande Collaboration), Phys.
Rev. Lett. 81 (1998) 1562.
[2] K. Nishikawa, Nucl. Phys. B (Proc. Suppl.) 77 (1999) 198.
[3] R.E. Ansorage et al., Nucl. Instr. and Meth. A 265 (1988) 33.
[4] P. Annis et al. (CHORUS Collaboration), Nucl. Instr. andMeth. A 412 (1998) 19.
[5] T. Nakano et al., IEEE Trans. Nucl. Sci. NS-39 (4) (1992).
[6] S. Ishikawa et al., Hoshasen 21(3) (1995) 85 (in Japanese).
[7] GEANT: Detector Description and Simulation Tool,
CERN Program Library, 1995.
[8] R. Fruhwirth, Nucl. Instr. and Meth. A 262 (1987) 444.

Nuclear Instruments and Methods in Physics Research A 482 (2002) 244–253

Near muon range detector for the K2K
experimentFconstruction and performance
T. Ishiia,*, T. Inagakib, J. Breaultc, T. Chikamatsua,1, J.H. Choid,
T. Hasegawae, Y. Hayatoa, T. Ishidaa, H.I. Jangd,2, J.S. Jangd, E.M. Jeongd,
I. Katob, A. Kibayashif, B.J. Kimg, H.I. Kimg, J.Y. Kimd, S.B. Kimg,
T. Kobayashia, W. Kroppc, H.K. Leed, S.B. Leea, I.T. Limd, K.J. Mad,
T. Maruyamae,3, S. Minec, K. Nakamuraa, M. Nakamurah, I. Nakanoi,
K. Nishikawab, Y. Oyamaa, M.Y. Pacj, A. Sakaia, M. Sakudaa, K. Shiinoa,
K. Tauchia, V. Tumakova, J. Yoog, S.Y. YoudaInstitute of Particle and Nuclear Studies, KEK, 1-1 Oho Tsukuba-shi, Ibaraki 305-0801, Japanb
Department of Physics, Kyoto University, Kyoto 606-8502, Japanc
Department of Physics and Astronomy, University of California, Irvine, Irvine, CA 92697-4575, USAd
Department of Physics, Chonnam National University, Kwangju 500-757, South Korea
e
Research Center for Neutrino Science, Tohoku University, Sendai, Miyagi 980-8578, Japanf
Department of Physics and Astronomy, University of Hawaii, Honolulu, HI 96822, USAg
Department of Physics, Seoul National University, Seoul 151-742, South KoreahDepartment of Physics, Niigata University, Niigata, Niigata 950-2181, Japani
Department of Physics, Okayama University, Okayama, Okayama 700-8530, Japanj
Department of Physics, Dongshin University, Naju 520-714, South Korea
K2K MRD Group
Received 12 April 2001; received in revised form 21 June 2001; accepted 21 June 2001

Abstract
A muon range detector (MRD) has been constructed as a near detector for the KEK-to-Kamioka long-baseline
neutrino experiment (K2K). It monitors the neutrino beam properties at the near site by measuring the energy, angle
and production point of muons produced by charged-current neutrino interaction. The detector has been working
stably since the start of the K2K experiment. r 2002 Elsevier Science B.V. All rights reserved.
PACS: 14.60.Pq; 14.60.Lm; 96.40.Tv
Keywords: Muon; Range; K2K; Drift tube; Neutrino; Long baseline*Corresponding author. Tel.: +81-298-64-5425; fax: +81-298-64-7831.
E-mail address: takanobu.ishii@kek.jp (T. Ishii).
1
Present address: Miyagi Gakuin Women’s College, Sendai, Miyagi 981-8557, Japan.
2
Present address: Department of Civil Engineering, Seokang College, Kwangju, 500-742, South Korea.
3
Present address: Institute of Particle and Nuclear Studies, KEK, Tsukuba, Ibaraki 305-0801, Japan.
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 6 8 0 - 1

T. Ishii et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 244–253

1. Introduction
The KEK-to-Kamioka long-baseline neutrino
experiment (K2K) [1] was planned to deﬁnitely
measure the neutrino oscillation, which was
originally suggested by the Kamiokande experiment [2]. An almost pure nm beam is generated
by the decay of secondary pions from the KEK
12-GeV proton synchrotron. The K2K experimentuses a near detector newly built at the KEK site
300 m from the pion production target in order toknow the beam properties just after its production,in addition to the Super-Kamiokande detector [3],the far detector 250 km away. A muon range
detector (MRD) has been constructed as a neardetector to measure the energy and angle of muons
which are produced by charged-current nm N
interaction. This determines the spectrum of theincident neutrino beam. The detector also measures the ﬂux and proﬁle of the neutrino beam.
2. Design of the muon range detector
The requirements for the near detector are:(1) that it be massive enough to measure the beam
ﬂux,(2) that it have large enough transverse dimension
to measure the beam proﬁle,245(3) and that it be capable of measuring the energyof muons in the range of the expected neutrino
spectrum, i.e. up to about 3 GeV; with reasonable resolutions.
To fulﬁll these requirements, we adopted the range
method. The MRD consists of 12 layers of ironabsorber sandwiched between vertical and horizontal drift-tube layers. The size of a layer isapproximately 7:6 m  7:6 m: In order to have agood energy resolution for the whole energy
region, the upstream four iron plates are 10 cm
thick and the downstream eight are 20 cm thick.
The total iron thickness of 2:00 m covers up to
2.8-GeV muons.
The same drift tubes were previously used by the
VENUS experiment [4] at TRISTAN. Each drifttube module consists of 8 drift tubes, which arearranged as shown in Fig. 1. The modules vary inlength from 2:45 to 7:6 m: Ideally, we make a drifttube layer by arranging 25 modules of 7:6 m lengthwith a 2-cm gap between consecutive modules.
However, for some horizontal drift-tube layers
shorter modules are combined end-to-end to makethe full 7:6 m  7:6 m dimension, since there are
not enough 7:6 m modules. We used 829 modules
in total: 377 modules of 7:6 m length, and 452
shorter ones. The available pool from VENUS was
390 and 510 modules, respectively. The setup of
the MRD is shown in Fig. 2.

Fig. 1. Structure of a drift-tube module.

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T. Ishii et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 244–253

The seventh–twelfth iron plates are also fromthe VENUS experiment; the upstream six arenew. The total weight of iron is 864 tons: Including
the aluminum drift tubes, the total mass is
915 tons:

3. Tests of the drift tubes
3.1. Gas tightness
The gas tightness of the drift tubes was tested bypressurizing them to approximately 1:25 kg=cm2
and measuring the decrease in the pressure as afunction of time. Nitrogen gas was used for thistest. About 3% of the modules showed an
excessive leak rate corresponding to more than
1 cc=min under the nominal running conditions.
Most of the leaks were found at the connection
part of the Delrin plug to the aluminum end-plate(Fig. 3). Two materials had been used for gas seals
by the VENUS group: RTV and epoxy glue. The
epoxy glue tends to form cracks after a long
period. Leaking modules were repaired using
RTV. After repair, they showed an average leakrate of less than 0:1 cc=min:
3.2. Wire tension
The wire tension was measured by searching forthe resonant frequency while supplying a pulsed
current in a magnetic ﬁeld (Fig. 4). The resonant
frequency (f (Hz)) is related to the wire tension(Mg (N)) by the following formula:rﬃﬃﬃﬃﬃﬃﬃﬃ
n Mg
f ¼
ð1Þ
2L m

Fig. 2. Setup of the MRD. The top ﬁgure shows a top view andthe bottom one shows a side view. The overall width and height
are each 7:6 m: The iron plates (empty rectangles) are 10 and
20 cm thick. Hits from a typical neutrino–iron event are alsoplotted.

where n is an odd integer, L is the wire length in m,
m is the wire line density ðkg=mÞ; and g ¼
9:8 m=s2 : For this case, the line density of a
70 mm diameter tungsten wire is 7:35  105 kg=m:
The pulsed current was made from a sine
wave generated by a variable-frequency oscillator,
which was trimmed by a diode to see a small

Fig. 3. Repair of a gas leak.

T. Ishii et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 244–253

Fig. 4. Principle of the tension measurement.

247

signal. As shown in Fig. 5, at the resonant
frequency, the wire started to oscillate in themagnetic ﬁeld inducing a current, which wasobserved as an order of 10 mV signal at the point
B of Fig. 4.
The measurement was made by a computercontrolled CAMAC system for each of
the modules. At the same time, the wire resistance was measured as well, which is anotherimportant check of the continuity of the wires. We
found 10 broken wires in total out of 7200 wires.
Most of the broken wires had been cut inside theepoxy glue which had been used for only some of
the modules to ﬁx the wire to the pin. The
measured tension distribution for the 760-cm-longmodules is shown in Fig. 6. The modules withother lengths have similar distributions. They are
distributed around 390 g with an RMS of about
10 g; which is consistent with the VENUS
measurement [5]. The broken wires were repaired
by stretching a new wire, ﬁxed to the pin only bysoldering.

4. Readout electronics
4.1. Modiﬁcation of the electronics

Fig. 5. Oscilloscope views of the tension test signals. In each
picture, channels A and B show the signal at the points A and B
of Fig. 4, respectively. The top ﬁgure shows signals at a nonresonant frequency. The middle one shows the same, but thescale of the channel B is magniﬁed. The bottom one shows
signals at the wire’s resonant frequency, where the signal due tothe resonant oscillation of the channel B is seen by thecurvature of the signal.

The VENUS electronics for the muon drift tubes[6] consists of single-hit TDCs operated in thecommon start mode. The start signal was madefrom the accelerator timing of every 5 ms in the
TRISTAN experiment. The use of a 40 MHz 6-bit
TDC in the front-end electronics boards givesa range for the TDC of 1:6 ms: However, in
the K2K experiment, the experimental condition isvery much diﬀerent from that in the TRISTAN
experiment. A beam spill every 2:2 s lasts for about
1:1 ms: If we use the accelerator timing as the startsignal, the TDC range would not cover the sum of
the beam spill time and the maximum drift time,
which is about 2:1 ms (Fig. 7). Further, the signal
from trigger counters is too late to make the startsignal.
There were two choices. One was to modify theelectronics to operate in a common stop mode, sothat the trigger counter signal could be used for thestop signal. The other was to change the TDC

248

T. Ishii et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 244–253

1000
760cm module
750

500

250

0

300

400

500

tension/g (grams)
Fig. 6. Measured tension distribution for the 760-cm-long modules. The data were corrected for t ¼ 201C:

4.2. Test of the front-end electronics
We tested the front-end electronics boards both
before and after modiﬁcation. Test pulses were
injected into the input of the preampliﬁer on theboard. The test items were linearity of the stop
timing and linearity of the start timing, as well as
the eﬃciency versus the test-pulse amplitude. We
found about 3% of the boards to be bad at eachstage; most of them were repaired.

5. Cosmic-ray test of the system
Fig. 7. Timing chart of the K2K experiment.

range to be longer than the beam spill time plus the
maximum drift time, so that we could calculate thedrift time by subtracting the trigger timing delay
later. We chose to make the TDC range longer by
replacing the 40 MHz clock chips with 20 MHz
ones. This resulted in a TDC range of 3:2 ms: This
was a simpler modiﬁcation of the electronics which
adversely aﬀects only the timing resolution
slightly, which is not critical for the range
measurement.

A cosmic-ray run was performed to test
the complete drift-tube system. P10 gas(Ar:CH4 ¼ 90:1070:5%) was made to ﬂow as in
the real experiment. The results of the cosmic-ray
test show that we have an eﬃciency plateau above
2:5 kV of high voltage (Fig. 8), and a spatial
resolution of about 2 mm: We chose the nominal
operating high voltage to be 2:7 kV:
The time-to-distance relation was also investigated. This study conﬁrmed the VENUS parameterization of the relation
x ¼ 2:11  t2 þ 2:72  t

ð2ÞT. Ishii et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 244–253

Fig. 8. Eﬃciency of the drift tubes as a function of HV.

where x is the drift length in cm and t is the drift
time in ms:
6. Gas system
In the K2K running, P10 gas is supplied
to the drift tubes using the gas recirculation
system of the VENUS experiment. As is shownin Fig. 9, it is composed of a gas-supply unit, acompressor, a puriﬁer and a gas holder. The
compressor and the puriﬁer each has a duplicate
for backup. P10 gas is pressurized by the249

compressor and then sent to the puriﬁer. The
puriﬁer includes a reaction column and anadsorption column. In the reaction column,
oxygen contamination is trapped by ﬂowing thegas through porous nickel metal.
In the adsorption column, water contamination is removed using molecular sieves. The
puriﬁed gas is ﬂowed to the drift tubes at a rate of
50 l=min: At this ﬂow rate, the total volume of thedrift tubes of 130 m3 is replaced every 2 days.
Returned gas from the drift tubes is stored in a gas
holder, which keeps the gas pressure 0:01 kg=cm2
higher than atmospheric pressure. The gas in
the gas holder is sent to the compressor andrecirculated.
Two sampling ports are located at the input
and the output of the puriﬁer. Sampled gas ismeasured by gas chromatography on a weekly
basis to check the purity. Normally, the oxygen
contamination of the return gas is less than
100 ppm; and that of the puriﬁed gas is less than
5 ppm: When the contamination level becomes
higher, we regenerate the puriﬁer. To this extent,
P10 gas mixed with hydrogen is made to ﬂow
through the reaction column heated to 150–2001C
in order to deoxidize the nickel catalyst. The
molecular sieves are also heated to 250–3001C forregeneration.

Fig. 9. Block diagram of the gas recirculation system.

250

T. Ishii et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 244–253

1

reconstruction efficiency

0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
without containment cut

0.1
Fig. 10. Muon range in iron vs. muon energy calculated using
the GEANT code.

7. Iron absorber
Iron plates are used not only as an absorber for
range measurements, but also as a target for theneutrino beam. Accurate knowledge of the thickness of the plates is necessary for these measurements. It is guaranteed by the Japanese Industrial
Standard (JIS) that the thickness of the upstream 4
plates is 10071:7 mm and that of downstream 8
plates is 20072 mm: The relationship between themuon energy and the muon range in iron was
calculated using a GEANT based Monte Carlocode [7]. The result is shown in Fig. 10.

with containment cut

0
0

0.5

1

1.5
2
Eµ (GeV)

2.5

3Fig. 11. Reconstruction eﬃciency as a function of the muonenergy. The dotted bars show the eﬃciency without thecontainment cut. The solid-line bars show the eﬃciency withthe containment cut. The vertical bars show the statistical errorof the Monte-Carlo calculation and the horizontal bars show
the bin size.

Tracks which exit the detector are not contained.
The eﬃciency was derived both with and without
the containment cut.
The diﬀerences in the reconstructed and generated muon energies are plotted in Fig. 12a, which
is taken as an energy resolution. The same isplotted for the muon angle, as well as thehorizontal and vertical start positions in Figs. 12b,c and d, respectively.

8. Acceptance and expected resolutions
The acceptance and resolutions of the MRD
were studied by a Monte-Carlo method. In thesimulation, muons with various energies were
injected in the forward direction from randomly
chosen vertices in a radius of 1 m of the 1st ironplate. They were simulated in the detector
considering both ionization and multiple scattering. The hit positions were converted into drift
times. The muon track was reconstructed using the
tracking algorithm explained in the next section.
The reconstruction eﬃciency is plotted as afunction of the muon energy in Fig. 11. In someanalyses, we require a track to be contained in thedetector to make an energy measurement possible.

9. Tracking algorithm
As stated above, one drift-tube module consistsof two half-cell staggered planes. The tracker
treats each layer as two independent drift-tube
planes. The tracking proceeds in the following 4
steps. In steps 1, 2 and 3 the x–z view and the y–zview are treated independently, where the z-axis isdeﬁned by the beam direction and the y-axis isdeﬁned by the upward direction.
(1) Cell ﬁtting: In this step, the drift time isignored and the wire positions are used as hit
positions. A straight line is drawn between
every combination of two hits on diﬀerent

T. Ishii et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 244–253

251

the process is repeated for the remaining
fragments.
(4) 3D ﬁtting: The overlap in the z-direction is
examined between the xz 2D-tracks and the yz
2D-tracks. The pair of ðxz; yzÞ 2D-tracks which
has the longest overlap is taken as a 3D-track.
The used 2D-tracks are removed from the list
and the process is repeated for all the remaining 2D-tracks.
As a result, the shortest track reconstructedconsists of one fragment which has three successive hits in both the xz and yz views. This track
traverses at least one iron plate corresponding toan energy threshold of 150 MeV ð300 MeVÞ for a
10-cm (20-cm) thick iron plate.

Fig. 12. Expected resolutions of the MRD. (a) Energy resolution; (b) angular resolution; (c) horizontal vertex resolution and(d) vertical vertex resolution. The vertical vertex resolution isbetter than the horizontal one because the horizontal tubes arecloser to the 1st iron layer.

planes which are apart by more than threeplanes. The number of hits near the line
(typically closer than 7 cm) is counted. If thenumber of hits is large enough (typically larger
than 60% of the number of planes traversed by
the track), it is regarded as a cell-track.
(2) Fragment ﬁtting: Based on a cell-track, up tosix successive planes are examined. If there
exist 3 hits, it is regarded as a fragment, and alinear ﬁt is done taking into account the drift
times and left=right ambiguity. In this ﬁtting
T0 ; the interaction time, is also varied as one ofthe parameters. Every other plane in a celltrack is taken as the start point of a fragment.
(3) 2D ﬁtting: The most upstream fragment istaken as the start of a 2D-track. A fragment
which can be smoothly connected to theprevious fragment is searched for. Typicalconnection criteria are that the distance
between the two lines is shorter than 10 cm
and diﬀerence of the two slopes is less than
0.15. This process is iterated until noconnecting fragment can be found. This groupof connected fragments is called a 2D-track.
Used fragments are removed from the list, and

10. Performance of the muon range detector
The MRD measures muons produced by the
charged-current neutrino interaction in the ironplates. A typical neutrino–iron interaction event isshown in Fig. 2.
10.1. Tracking eﬃciency and the hit eﬃciency
The tracking eﬃciency as a function of the track
length (number of traversed layers) is derived asfollows using beam data:(1) Tracks longer than the test region by at least
one iron layer both upstream and downstream
are selected as sample tracks in order to assure
the existence of a track in the test region.
(2) All hits except for those in the test region aremasked.
(3) The masked data are ﬁtted by the tracker to
see whether the tracker ﬁnds the track or not.
The tracking eﬃciency is dependent on the hit
eﬃciency of the drift tubes. We determined theeﬀective hit eﬃciency so that the Monte-Carlo
result on the tracking eﬃciency would reproduce
the beam data well. The obtained eﬀective hit
eﬃciency is 97.5%. The ineﬃciency of 2.5%
includes the geometrical eﬀect coming from apossible mis-alignment of the drift tube modules,
as well as the real hit ineﬃciency. The result of the252

T. Ishii et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 244–253

Fig. 14. Average number of reconstructed 3D-tracks from theoﬀ-spill data as a function of time.

Nov.99Fig. 13. Tracking eﬃciency as a function of the traversed ironlayers. The boxes show the Monte-Carlo result, with a size
representing the errors.

tracking eﬃciency is shown in Fig. 13 with theMonte-Carlo result overlaid.

arbitrary unit

average

10.2. Noise rate
The noise rate was estimated from oﬀ-spill data,
which are taken in the time between successive
beam spills. These are one class of randomly
triggered data. The average noise hits in the oﬀspill data amounts to approximately 27 out of6632 total tubes, implying a 0.4% average noise
rate. This rate is composed of all the non-beamrelated background hits, including cosmic-rays.
The main part of this comes from electronic noise,most of which is easily cut using timing information. As a result, noise hits do not change theanalyzed results.
10.3. Detector stability
As of June 2000, the MRD has been alive 91.7%of the time since the start of the K2K data taking
in June 1999. The oﬀ-spill data were analyzed to
study the detector stability. Only cosmic-ray
muons contribute to the oﬀ-spill data as tracks.
The average number of reconstructed 3D-tracks isplotted as a function of time in Fig. 14. The
average number of tracks is about 11=1000 triggers-600θy (degree)

60

Fig. 15. Distribution of number of traversed layers by acosmic-ray track in November 1999 (data points) comparedwith the averaged distribution over all experimental periods(dashed histogram).

and has been stable within the statistical error. The
number of traversed layers by a track is correlatedwith the energy loss of the particle in the MRD.
This distribution is analyzed on a monthly basis.
The distribution in November 1999 is comparedwith the averaged distribution over all experimental periods in Fig. 15. Distributions in othermonths are the same within statistical errors.
Angular distributions, also made on a monthly
basis, in the x–z and y–z views are shown for theNovember 1999 and total data sets in Figs. 16 and17, respectively. Distributions in other months
are the same within statistical errors for bothangular distributions. The reproducibility of theseT. Ishii et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 244–253

distributions shows that the detector has beenworking stably in all experimental periods.

Nov.99average

11. Summary and conclusion

arbitrary unit

-60

253

0
θx (degree)

60

Fig. 16. Angular distribution of cosmic-ray tracks in the x–z
view in November 1999 (data points) compared with theaveraged distribution over all experimental periods (dashedhistogram).

The MRD was constructed as a near detector of
the K2K long-baseline neutrino experiment. It
measures the position, momentum, and directionof muons produced in charged-current neutrino
interactions. The coverage of the detector is
712 mrad with respect to the production target,
and the ﬁducial mass is 329 tons. The energy
acceptance is from 0.3 to 2:8 GeV with a resolution of 0:12 GeV for forward-going muons. The
track angular resolution is about 0:09 rad and thevertex resolution perpendicular to the beam
direction about 2 cm:
The MRD has been working stably since thestart of the K2K experiment.
Acknowledgements
We gratefully acknowledge the cooperation of
all other K2K members for this work. We
appreciate the VENUS group for allowing us toreuse their muon system. Especially we wish to
thank Dr. Y. Asano for sharing with us his
experience with the system.
This work has been supported by the Japanese
Ministry of Education, Science, Sports and Culture (the Monbusho), its grants for ScientiﬁcResearch, the US Department of Energy, theKorea Research Foundation Grants and the
Korea Science and Engineering Foun-dation.

Nov.99arbitrary unit

average

References
1

5

10
NlayerFig. 17. Angular distribution of cosmic-ray tracks in the y–zview in November 1999 (data points) compared with theaveraged distribution over all experimental periods (dashedhistogram). They have a two-peak structure because the tracker
assumes all tracks start from upstream with respect to the beam
(lower z value).

[1] S.H. Ahn, et al., Phys. Lett. B 511 (2001) 178;
K. Nishikawa, et al., KEK-PS proposal (E362) (1995).
[2] K.S. Hirata, et al., Phys. Lett. B 205 (1988) 416.
[3] Y. Fukuda, et al., Phys. Rev. Lett. 81 (1998) 1562.
[4] Y. Asano, et al., Nucl. Instr. and Meth. A 259 (1987) 430.
[5] Y. Asano, et al., Nucl. Instr. and Meth. A 254 (1987) 35.
[6] Y. Ikegami, et al., IEEE Trans. Nucl. Sci. NS 36 (1989) 665.
[7] R. Brun, et al., CERN DD/EE/84-1 (1987).

Nuclear Instruments and Methods in Physics Research A 497 (2003) 450–466

Tracking performance of the scintillating ﬁber detector
in the K2K experiment
B.J. Kima,*, T. Iwashitab, T. Ishidac, E.J. Jeonc, H. Yokoyamad, S. Aokib,
H.G. Bernse, H.C. Bhanga, S. Boyde, K. Fujiib, T. Harab, Y. Hayatoc,
J. Hillf, T. Ishiic, H. Ishinoc, C.K. Jungf, E. Kearnsg, H.I. Kima, J.H. Kima,
J.Y. Kimh, S.B. Kima, T. Kobayashic, G. Kumeb, S. Matsunoi, S. Minej,
K. Nakamurac, M. Nakamurak, K. Nishikawal, M. Onchib, T. Otakib, Y. Oyamac,
H. Parkc,1, M. Sakudac, K. Satob, K. Scholbergg, E. Sharkeyf, J.L. Stoneg,
A. Suzukib, K. Takenakab, N. Tamurak, Y. Tanakab, M. Takatsukib,
C.W. Walterg, J. Wilkese, J. Yooa, M. YoshidamaDepartment of Physics, Seoul National University, Seoul 151-742, South Koreab
Kobe University, Kobe, Hyogo 657-8501, Japanc
High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japand
Department of Physics, Science University of Tokyo, Noda, Chiba 278-0022, Japane
Department of Physics, University of Washington, Seattle, WA 98195-1560, USAf
Department of Physics and Astronomy, State University of New York, Stony Brook, NY 11794-3800, USAg
Department of Physics, Boston University, Boston, MA 02215, USAhDepartment of Physics, Chonnam National University, Kwangju 500-757, South Koreai
Department of Physics and Astronomy, University of Hawaii, Honolulu, HI 96822, USAj
Department of Physics and Astronomy, University of California, Irvine, CA 92697-4575, USAk
Department of Physics, Niigata University, Niigata, Niigata 950-2181, Japanl
Department of Physics, Kyoto University, Kyoto 606-8502, Japanm
Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan
Received 24 June 2002; received in revised form 23 September 2002; accepted 7 November 2002

Abstract
The K2K long-baseline neutrino oscillation experiment uses a Scintillating Fiber Detector (SciFi) to reconstructcharged particles produced in neutrino interactions in the near detector. We describe the track reconstruction algorithm
and the performance of the SciFi after 3 years of operation.
r 2002 Elsevier Science B.V. All rights reserved.
PACS: 14.60.Pq; 29.40.Gx; 29.40.Mc
Keywords: Tracking; Scintillating; Fiber; Neutrino; Long-baseline*Corresponding author. Tel.: +82-2-872-6868; fax: +82-2-884-3002.
E-mail address: bockjoo@neutrino.kek.jp (B.J. Kim).
1
Present address: Department of Physics, Seoul National University, Seoul 151-742, South Korea.
0168-9002/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0168-9002(02)01980-0

B.J. Kim et al. / Nuclear Instruments and Methods in Physics Research A 497 (2003) 450–466

1. K2K experiment
The K2K experiment is the ﬁrst long-baseline
neutrino oscillation experiment in operation.
Muon neutrinos ðnm Þ are produced via the decayof secondary pions from interactions of the KEK
12-GeV proton beam in an aluminum target. The
neutrino beam is directed through a near detector
system at KEK to the Super-Kamiokande underground detector [1] 250 km away. By comparing
the neutrino ﬂuxes and energy spectra measured at
the near detector and at Super-Kamiokande, we
are able to investigate neutrino oscillations. Details of the K2K experiment have been presented
elsewhere [2].
The near detector, located 300 m downstream ofthe pion production point, consists of a 1 kton
water Cherenkov detector, a scintillating ﬁber
tracking detector (SciFi), a hodoscope of plastic
scintillation counters (VETO) around the SciFi, alead glass detector (LG), and a muon range
detector (MRD) [3]. It is illustrated in Fig. 1.
The main purpose of the near detector is tomeasure the proﬁle, ﬂux, and energy spectrum of
the neutrino beam. Among the near detector
components, the SciFi can provide tracking
capability good enough to discriminate neutrino

451

interaction types and to measure energy and crosssection of the muon neutrino.
The K2K experiment started taking data in
April, 1999. This paper will describe the performance of the Scintillating Fiber Detector after 3
years of operation. A description of the K2K
scintillating ﬁber detector is given in Section 2. Hit
reconstruction algorithm and performance aredescribed in Section 3. Track and vertex reconstruction is described in Section 4. Performance ofthe neutrino event reconstruction algorithms arepresented in Section 5. We conclude in Section 6.

2. Scintillating ﬁber detector
The SciFi is a charged particle tracking detector
using scintillating ﬁber technology. Scintillatingﬁber detectors have been used for charged particle
trackers by other experiments [4]. A scintillatingﬁber detector was chosen for this application since
it was necessary to have high tracking efﬁciency
for reconstructing charged particles and theirvertex positions when more than two visible
particles are produced in neutrino interactions.
Using the detector, we can separate quasi-elastic
events from inelastic events, and deﬁne the ﬁducial

Veto/Trigger
counter

Muon Range Detector
Lead Glass
SciFi

1kt
Water Cherenkov
Detector

16 m
Neutrino Beam

24 m
Fig. 1. The layout of the K2K near detector. It consists of a 1-kt water Cherenkov detector, a scintillating ﬁber detector, a hodoscopeof plastic scintillation counters, a lead glass detector, and a muon range detector.

452

B.J. Kim et al. / Nuclear Instruments and Methods in Physics Research A 497 (2003) 450–466

Fig. 2. A schematic view of the K2K scintillating ﬁber detector. The detector has the dimension of 260 cm  260 cm  173 cm inðx; y; zÞ: The central ﬁducial mass region is deﬁned to be 220 cm  220 cm  159 cm in ðx; y; zÞ:volume of the neutrino interactions with 1%
accuracy.
The water is contained in a thin aluminum
structure, and the water layers are interleaved with
scintillating ﬁber layers. Water is chosen as atarget material in order to minimize systematic
uncertainties in neutrino cross-section the event
rates are compared between the SciFi and SuperKamiokande detectors. A scintillating ﬁber layer
provides the hit-position measurement in bothhorizontal and vertical directions.2 An individual
scintillation sheet has a thickness of 1:3 mm;
consisting of two staggered layers of scintillatingﬁbers. Each ﬁber has a diameter of 0:7 mm: The
vertical and horizontal ﬁber sheets are separated
by a 1.6-cm thick honeycomb panel. There are 20
scintillating ﬁber layers in total, and the adjacentlayers are spaced 9 cm apart along the neutrino
beam direction.

2

In the K2K coordinate system, the z-axis is along thenominal neutrino beam direction. y-axis is the vertical direction
pointing upwards, and x-axis is the horizontal direction.

One layer of 240 cm  240 cm  6 cm watertarget, consisting of 15 aluminum tanks, isinterleaved with two ﬁber layers. The wall thickness of the aluminum tank is 1:8 mm: The weights
of water, aluminum, and the honeycomb panel are
6.0, 1.4, and 0:8 ton; respectively. A schematic
view of the SciFi is shown in Fig. 2.
For processing light signals, 11,420 ﬁbers are
glued together to make a single circular shape incross-section which has typically the size of 12 cm
in diameter and are called a ﬁber bundle. Eachbundle is polished using a custom-made polishingmachine. The polished ﬁber bundle is mechanicallyconnected to the surface of an image intensiﬁer
tube (IIT) which is then connected to the opticallenses and a CCD camera.
A total of 24 IITs are used to read out all 274,080
ﬁbers in the SciFi. In order to ﬁnd the correspondence between the hit ﬁber and the CCD images, acertain number of selected ﬁbers are illuminatedperiodically at the opposite side of IITs by anelectro-luminescent (EL) plate. This is referred to as
the EL calibration. A more detailed description of
the SciFi detector is found elsewhere [5].

B.J. Kim et al. / Nuclear Instruments and Methods in Physics Research A 497 (2003) 450–466

3. Hit reconstruction
3.1. Hit finding algorithm
The CCD pixel images need to be handled toreconstruct hits in the SciFi. Fig. 3(a) shows a typical
CCD image of an IIT from a cosmic-ray event. A
ﬁber bundle, consisting of 10 ﬁber sheets, is arranged
to cover a sensitive region of an IIT effectively.
Fig. 3(b) is an expanded view of (a), illustrating atypical ‘‘SciFi hit’’, when a cosmic-ray muon passes
through the ﬁber sheet. The hit reconstruction is
performed in the following four steps:
1. Loading hit pixels: The raw data of the SciFi
consist of hit pixels in the CCD coordinates
ðx; yÞ and 8 bits of ADC brightness. These areall encoded in one byte per pixel. Typically,
there are 3500 hit pixels in a neutrino event.
2. Finding pixel clusters: A pixel cluster is determined by a group of neighboring hit pixels. At
this stage, isolated single pixel hits, comingfrom random electrical noise of CCD, arerejected. Number of single pixel hits is about
twice as many as that of two pixel hits. This wasshown in Fig. 11 of Ref. [5].
3. Identifying hit fibers: A hit ﬁber is identiﬁed byrequiring a cluster of pixles which occupies theﬁber. The hit ﬁber position in the CCD

(a)

453

coordinate is determined from a mapping tablethat is obtained from the EL calibration.
4. Making hit fiber clusters: A ﬁber cluster isdetermined by a group of neighboring hit ﬁbers.
We apply two kinds of ﬁltering algorithms to theﬁber clusters to improve the signal-to-noise ratio:
We require that a ﬁber cluster should consist of
hit ﬁbers in both upper and lower layers of a ﬁber
sheet. This eliminates ﬁber clusters of small pixel
images which come from dark current in an IIT.
* When a big pixel cluster lies on the adjacentﬁber sheets in a bundle and makes multiple ﬁberclusters, we choose only ﬁber clusters containing peak of ADC brightness. This removes fakeﬁber-clusters introduced by the dense arrangement of ﬁber sheets in a bundle.
*

We consider a ﬁber cluster passing through these
two ﬁlters as a SciFi ‘‘hit’’. Every hit position in
the CCD coordinate is then converted into theposition in the detector coordinates (x; z or y; z).
The hit position is deﬁned as the centroid of a ﬁbercluster, weighted by the ADC value of the hit pixel.
3.2. Number of pixels per hitðNpix Þ and its
corrections
A found hit gives the number of pixels for theparent pixel cluster, Npix : LED calibration data(b)

Fig. 3. (a) A typical CCD image of an IIT from a cosmic-ray event. Small distortions of the CCD image are introduced due to theposition dependence of pixel size at the IIT-CCD contact and (b) an expanded view of (a) for a typical SciFi hit. The size of hit pixel is
proportional to brightness in the ADC. Hit ﬁbers are represented by the thick circles. A ‘‘SciFi hit’’ is deﬁned as a cluster ofneighboring hit ﬁbers.

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B.J. Kim et al. / Nuclear Instruments and Methods in Physics Research A 497 (2003) 450–466

show that Npix has good linearity with respect tothe number of photons injected to IIT. Since theintensity of observed light is proportional to theenergy loss in the ﬁber, Npix is expected to be agood estimator of the energy of the parent particle.
Fig. 4 shows an Npix distribution for cosmic-ray
muons. Based on a reconstructed track, Npix canbe corrected to account for both the attenuation oflight in the ﬁber, and the dependence of light yields
on the incident angle ðyin Þ of a particle on a ﬁber
sheet:
F ðl0 Þ cos yin  Npix :Npixcorr ¼
F ðlÞ

ð1Þ

The ﬁrst term is the correction for attenuation of
the scintillation light in the ﬁber and reﬂection at
the far end of ﬁber coated with aluminum [5]. The
correction is normalized by the value in case of thelight from the detector center ðl0 ¼ 130 cmÞ: Thus,F ðlÞ is given byl þ lgF ðlÞ ¼ exp l
2L  l þ lg
þ R exp ;ð2Þlwhere l is the distance from the hit position to thenear end of a ﬁber, lg ð¼ 100 cmÞ is light guidelength from the near end to the IIT, and lð¼
323 cmÞ is the attenuation length. Rð¼ 0:74Þ is thereﬂection coefﬁcient at the far end of a ﬁber, andLð¼ 254 cmÞ is the length of a ﬁber.
The second term in Eq. (1) is the correction forthe dependence of light yields on the incident angle
with respect to the ﬁber sheet. Here, yin ¼ 0 is
perpendicular to the ﬁber sheet. Npixcorr is thus
equivalent to Npix when a particle passes ﬁber sheet
at the center of the detector x=y planes with yin ¼ 0:
Figs. 5(a) and (b) show average values ofNpix ð/Npix SÞ3 as a function of hit position
3
We employ the following Poisson function to ﬁt the Npix
distribution:em  mx;
F ðNpix ; A; a; /Npix SÞ ¼ A
Gðx þ 1Þm  /Npix S=a and x  Npix =a;where A is an overall normalization factor and a is a conversion
factor. /Npix S is the average value of the Npix distribution.

along the ﬁber and as a function of 1=cosðyin Þ;
respectively.
3.3. Simulation of CCD hit pixels
To reproduce SciFi hits accurately in the K2K
Monte Carlo simulation, we include simulation of
hit pixels on CCD due to charged particles andnoise in the following three steps:
1. Charged particles are tracked through thedetector using GEANT [6]. The energy loss ofa particle traversing a ﬁber is computed andthen converted to the number of photoelectronsobserved by the IIT/CCD imaging system,taking into account attenuation along the ﬁber,
reﬂection by the aluminum coating at the end of
a ﬁber, and the quantum efﬁciency of the IIT
photocathodes.
2. The photoelectrons are converted to an image
on the CCD camera surface. Individual photoelectrons are distributed around the ﬁber center
according to a gaussian distribution withparameters determined using data of a single
photoelectrons from LED calibration run. The
size of the pixel cluster corresponding to thesingle photoelectron is then chosen from
distributions of the LED data, and the pixel
cluster is simulated accordingly. The energy
scale is tuned for each IIT using cosmic-ray
muon data.
3. Random noise in a pixel cluster is generated foreach IIT based on neutrino-beam data.
Two kinds of noise pixels are generated. One is theisolated single hit pixel around the main pixel
cluster of a charged track in Step 2. The other isthe random noise pixels in a pixel cluster generated
in Step 3. Using neutrino beam data and cosmicray data, the average number of noise hits is
expected to be 100 per event. We ﬁnd that, amongthe 100 noise hits in an event, 50% is due torandom noise hits, 35% is associated with theneutrino beam, and 15% is coming from the track
associated hits. Since there are a total of 274,080
ﬁbers in the SciFi, this noise rate is considered tobe very low.

B.J. Kim et al. / Nuclear Instruments and Methods in Physics Research A 497 (2003) 450–466

455

Fig. 4. An Npix distribution with the corrections applied for both attenuation and dependence on the incident angle as given in Eq. (1).

(b)

(a)

Fig. 5. An average value of Npix /Npix S; as a function of (a) hit position along the ﬁber and (b) 1=cos yin ; where yin is the incident angle
with respect to the ﬁber sheet, respectively. Solid (open) circles are before (after) the correction. The negative x; y coordinates in (a)correspond to positions close to readout end of the ﬁber, and a larger value of /Npix S is expected before the correction.

3.4. Efficiency and stability of hit reconstruction
We can obtain the hit ﬁnding efﬁciency ehit
which is deﬁned as
ehit ¼

Number of observed hits;
Number of expected hits

ð3Þ

where the expected number of hits (denominator)
is obtained from the information of reconstructed

tracks. Fig. 6 shows ehit for each IIT. Data andMonte Carlo shows a good agreement.
Cosmic-ray data are taken during the normal
data-taking period, roughly once every 2 weeks.
The efﬁciency is monitored using the data as acheck of detector stability.
We have performed a similar check on thestability of hit ﬁnding efﬁciency using the neutrino
data. Clean single track events are selected by

B.J. Kim et al. / Nuclear Instruments and Methods in Physics Research A 497 (2003) 450–466

456

Fig. 6. Hit ﬁnding efﬁciency ehit obtained from cosmic-ray data as a function of IIT number.

was found to be constant stable within 5%. The
average efﬁciency of each IIT was stable within
1.2%.4

1.1

1

4. Reconstruction of track and vertex

0.95
0.9

4.1. Track reconstruction algorithm

July

June

Apr

Feb

june

01Jan

May

Feb

Mar

00Jan

Nov

0.8

99Oct

0.85

May

Beam
Cosmic-ray
mar<Efficiency>

1.05

Fig. 7. Hit ﬁnding efﬁciency as a function of data taking period
for the neutrino data ð3Þ and cosmic ray data ðÞ: The dottedline represents an MC predicted hit ﬁnding efﬁciency.

requiring a track with the number of hit layers
larger than nine, and its matching to activity in thedownstream VETO counter.
Fig. 7 shows hit ﬁnding efﬁciency, which is
obtained by taking average over all IITs, as afunction of data-taking period. The dotted line at
96.8% represents an MC prediction. For thecosmic-ray data, we ﬁnd an average value of ehit
to be 96:570:5% for all IITs and all time periods.
The number of Npix for each IIT was monitored at
on-line and the average number of Npix ; /Npix S;

The track ﬁnding algorithm in the SciFi is
optimized for the charged current interactions
which contain a muon in the ﬁnal state. The track
ﬁnding algorithm consists of three steps.
First, the track ﬁnder searches for two-dimensional (2D) track candidates in the ðx; zÞ or ðy; zÞplanes which traverse at least three layers of SciFi.
A three-dimensional (3D) track is reconstructed by
selecting the best combination of two 2D tracks. In
order to reduce the combinatorial fake tracks, thestarting and ending hit layers are required not tobe different by more than one layer, and theoverlapping of the two 2D tracks is required to beat least two layers.
4This is in good agreement with a simple estimation. When aminimum ionizing particle crossing the ﬁber sheet creates
B8 p:e: (4 p:e: each for upper and lower layers), hit coincidence
probability between upper and lower layers is (1:  e4 Þ2 ¼
0:96: The requirement of the coincidence between two ﬁber
layers is chosen to keep the noise hits at the level of 70 hits per
event.

B.J. Kim et al. / Nuclear Instruments and Methods in Physics Research A 497 (2003) 450–466

Secondly, the combinatorial background ornoise tracks are removed using other componentsof the near detector (VETO and MRD). A track
candidate in the SciFi is required to match withboth a hit in the VETO and a track in the MRD.
Once a good primary track is found, the most
upstream hit-position of the primary track is
considered as a neutrino interaction point. The
track ﬁnder starts searching for extra tracks
near the neutrino interaction point. These tracks
are mostly short tracks with two or more SciFi
hits. This method of extra track ﬁnding turns
out to be an effective way to ﬁnd short tracks
of protons and/or pions from the neutrino
interactions. All the reconstructed tracks are
subjected to a ‘‘global ﬁt’’, a straight line ﬁt to
the found hits.
4.2. Alignment and position resolution
The geometry of the SciFi and its support
structure was surveyed by mechanical means after
their installation was completed. Based on themechanical survey, the alignment of ﬁbers is
estimated to be good to within 5 mm:
A better alignment was obtained by minimizing
the sum of the squared hit-residuals using the
global ﬁt for cosmic-ray tracks. From the mean
shift of the hit residuals, we ﬁnd the accuracy of
the alignment to be B400 mm:
Multiple scatterings occurring in the SciFi is not
negligible compared with the size of a ﬁber. A
Kalman ﬁltering technique [7–9] can be employed
to take the multiple scattering into account and, inprinciple, removes the multiple scattering effectduring track ﬁtting. The technique calculates theerror of a track ﬁt relatively quickly, and avoids
the tedious matrix inversion by the global ﬁt. A
track ﬁt using the Kalman ﬁltering technique isused to estimate the detector resolution moreaccurately and to obtain a better alignment. The
implementation adopts the prescriptions describedin Refs. [9,10].
Some curvature was introduced in the ﬁber
sheets due to bending when they were glued to aﬁber module. Production of the ﬁber modules is
described in Ref. [5]. The curvature of a ﬁber sheet
causes displacement from the aligned ﬁber-posi-

457

tion and results in a rotational misalignment. The
bending of every ﬁber sheet was measured after
each ﬁber module was assembled. The curvature of
a ﬁber sheet was then obtained by ﬁtting themeasured ﬁber bendings to a forth-order polynomial, and used to correct the rotational misalignment.
After the Kalman ﬁltering and the curvaturecorrection, the accuracy of alignment becomes
better than 50 mm; estimated from the mean
residual shift. Fig. 8 shows distributions of themean and standard deviation of the hit residuals
for the mechanical survey alone, the alignment
using the global ﬁt, and the alignment using the
Kalman ﬁltering ﬁt.
The mean of hit residual after the mechanicalsurvey alone shows a variation as large as 1 cm:
After the alignment using the global ﬁt, thevariation becomes smaller than 400 mm: After thealignment using the Kalman ﬁltering ﬁt, thevariation becomes at most 50 mm: The standard
deviations of the hit residual also exhibit such
improvements of ﬁber alignment as the meanvalues. There are a few layers with large standard
deviations of hit residual, compared with otherlayers. These are the outer most layers and indeed
bent more than the rest of inner layers.
The alignment is checked using periodicallytaken cosmic-ray data. Fig. 9 shows the variationin the mean values of hit residuals since May
1999, and the variation is found to be less than
100 mm:The neutrino data with the Kalman ﬁltering ﬁt
are used to obtain the best estimation for the
position resolution. Fig. 10 shows the hit residual
distribution using neutrino data. The position
resolution is 0:73 mm ð0:61 mmÞ for muon energy
Em o1 GeV ðEm > 1 GeVÞ: The overall position
resolution is 0:6470:07 mm: Fig. 11 shows theenergy dependence of the resolution. The effect ofmultiple scattering on the position resolution
decreases as the muon energy increases. The MC
prediction explains the overall energy dependence.
The difference between data and MC, B100 mm; isregarded as the systematic error of our detector.
The position resolution of 0:6 mm is regarded asthat of the SciFi without the multiple scattering
effect.

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B.J. Kim et al. / Nuclear Instruments and Methods in Physics Research A 497 (2003) 450–466

Mean (Hit Residual) (cm)Survey only
Alignment w/Standard Fit
Alignment w/Kalman Fit(a)Layer Number

σ (Hit Residual) (cm)Survey only
Alignment w/Standard Fit
Alignment w/Kalman Fit(b)Layer Number

Fig. 8. (a) Means of hit residual and (b) standard deviations as a function of layers in y  z plane. Points are for the mechanical survey
alone ð%Þ; the alignment using the global ﬁt ð3Þ; and the alignment using the Kalman ﬁltering ðÞ:

4.3. Track finding efficiency
The efﬁciency for ﬁnding a single muon track isestimated from cosmic-ray and MC events. The
cosmic-ray events are selected by requiring associated activities in both upstream and downstream
VETO counters. Furthermore, tracks are required
to have at least 15 SciFi hit layers from the
upstream to the downstream. In order to study
dependence of the track ﬁnding efﬁciency on track
length, we artiﬁcially eliminate hits in the upstreamside of the track. For example, if it is desired toestimate the efﬁciency of the track with 10 SciFi hit
layers using 19-layer long track, the ﬁrst to the19th hit layers are eliminated before applying a
track ﬁnding algorithm. For MC events, a muontrack is required to have at least three hit-layers
within the SciFi ﬁducial volume, and tested by a
track ﬁnding algorithm.
Fig. 12 shows a comparison of single track
ﬁnding efﬁciency as a function of the track
length in unit of number of SciFi layers for theB.J. Kim et al. / Nuclear Instruments and Methods in Physics Research A 497 (2003) 450–466

459∆<Hit Residual>(cm)Nov99-May99
Jun00-May99
Jan01-May99

Layer Number
Fig. 9. The variation in the mean of hit residual, obtained from cosmic-ray data, as a function of layers in x  z plane (1–20) and y  zplane (21–40). The variation is taken relative to May 1999. The dotted line represents the variations of 7100 mm:

Fig. 10. The hit residual distributions using the neutrino beam data with the Kalman ﬁltering ﬁt. The curve is a single Gaussian ﬁt to
the data with s ¼ 0:73 mm ð0:61 mmÞ for muon energy Em o1 GeVðEm > 1 GeVÞ:cosmic-ray data. Track ﬁnding inefﬁciencies comefrom failures in the hit ﬁnding and imperfection of
the track reconstruction. The track ﬁnding efﬁciency is B70% ðB87%Þ for the track length of
three (four) hit SciFi layers and becomes close to100% for longer tracks.

An average track ﬁnding efﬁciency for thecosmic-ray events is estimated by weighting
the track length distribution obtained from
the MC. We ﬁnd the average track ﬁnding
efﬁciency to be 93:670:8% for cosmic-ray
tracks with three hits or more. The average track

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B.J. Kim et al. / Nuclear Instruments and Methods in Physics Research A 497 (2003) 450–466

νµ beam dataσ (Hit Residual) (mm)MCEµ (GeV)

Efficiency

Fig. 11. The energy dependence of position resolution.

1999 cosmic data
2000 cosmic data
2001 cosmic data

Number of SciFi Hit Layers
Fig. 12. The primary track ﬁnding efﬁciency as a function of track length using cosmic-ray data. Solid rectangle, open circle, and solid
circle are for 1999, 2000, and 2001 cosmic-ray data, respectively. The track length is given in units of number of SciFi layers.

ﬁnding efﬁciency is estimated to be 92:770:5% for
the MC events.
4.4. Vertex reconstruction
An accurate ﬁducial volume is crucial formeasuring the cross-section of neutrino in the
SciFi. Therefore, the event vertex has to be
determined precisely.

The vertex position is determined by thereconstructed tracks. The event vertex for a single
track event is found by simply taking the midpoint of the upstream water container nearest to
the ﬁrst hit in the z-direction of the reconstructed
track. The event vertex for a multiple track event isdetermined by taking average of the z positions atintersection of a primary track and the restof shorter tracks. Fig. 13 shows a comparison of

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461

Fig. 13. A comparison of x- and z-vertex distributions between data and MC for both single and multiple track events.

x- and z-vertex distributions between data and MC
for both single and multiple track events. Eventswith tracks ranged out at the MRD are used forthe comparison and the comparison shows a good
agreement between data and MC.
For two track events, z-vertex difference between ðx; zÞ and ðy; zÞ planes can provide a measureof goodness for the vertex reconstruction. Fig. 14
shows a distribution of z-vertex difference between
vertices found in the ðx; zÞ and ðy; zÞ planes forevents with multiple tracks. The event z-vertex isfound by taking the average of the z positions
determined for all such combinations of tracks.
The event vertex in the x and y coordinates iscalculated based on the event z-vertex and theprimary track.
The vertex resolution is estimated by two track
MC events of neutrino charged-current interactions. The vertex resolution is degraded mainly
due to track mis-reconstruction. Fig. 15 shows thedifference between the measured vertex position
and the true vertex position in x; y; and z for thetwo track events. The estimated vertex resolution
for the two track events is found to be 0:6 cm in z
and 0:4 cm in x or y:
4.5. Track counting
An accurate reconstruction of the track
multiplicity of an event is crucial for the measurement of neutrino energy spectrum. Charged
current quasi-elastic events have especiallysimple kinematics which can be used to determine
the incoming neutrino energy. The incoming
neutrino beam is in the z-direction in theK2K coordinate system. The distribution of theangle of the muon candidate track with respect
to the neutrino direction is typically peakedaround 181 and 301 for single and two track
events, respectively. For events with the neutrino
interaction inside the SciFi, the muon momentum
is sensitive above 0:5 GeV=c and the muon
momentum distribution is peaked around
1:2 GeV=c and ranges up to 3 GeV=c: In addition,

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B.J. Kim et al. / Nuclear Instruments and Methods in Physics Research A 497 (2003) 450–466

Number of Events

Data
MC

∆Z (cm)σRMS=0.4cm

Vx(rec)-Vx(true) (cm)Number of events

Number of events

Fig. 14. The z-vertex difference between vertices found in the ðx; zÞ and ðy; zÞ planes for two track events. At least three hits are
required for both primary track and second track. Points are for data and the histogram is for MC.

σRMS=0.3cm

Vy(rec)-Vy(true) (cm)Number of events

σRMS=0.6cm

Vz(rec)-Vz(true) (cm)Fig. 15. The vertex resolution of two track charged-current events.

these events always have one or two tracks
depending on whether the outgoing proton isdetected.

The efﬁciency of ﬁnding multiple tracks is
studied using the MC CCqe events, and deﬁned
to be the fraction of reconstructed events relative

B.J. Kim et al. / Nuclear Instruments and Methods in Physics Research A 497 (2003) 450–466

to the generated events for a given track multiplicity. Both the reconstructed track and thegenerated track are required to have three or more
hits. The efﬁciency is estimated to be 9372% and8674% for single track CCqe events and two track
CCqe events, respectively. For single track events,
the uncertainty in the track counting efﬁciency isdominated by the systematic error due to the gain
variation of the IITs, and estimated to be 1%.
For two track events, the sources of theuncertainty in the track counting efﬁciency are
difference in noise between MC and data, tracks
due to noise hits around the vertex, and noise hits
near the primary track. Due to the difference innoise between data and MC, the track reconstruction may perform differently.

463

In order to estimate the systematic uncertainty
due to difference in noise contamination between
the data and MC, noise contamination is varied in
the MC by 750% from the default value. After thevariation, track ﬁnding is performed again. We
ﬁnd the track counting efﬁciency varies by at most
1.8% with respect to the default noise contamination in the MC for both single track and two track
events.
Single track events can sometimes be reconstructed as two track events because of noise hitsaround the vertex. The size of this effect is studied
using MC generated single track events with three ormore hits. The probability is calculated by counting
number of reconstructed two track events, andfound to be 2:770:1% for the MC CCqe events.

Efficiency

All types of MC events

Number of SciFi Hit Layers

Efficiency

MC CCqe events

Number of SciFi Hit Layers
Fig. 16. The efﬁciency of ﬁnding the second track as a function of number of hits in the SciFi layers. The upper plot is for the all eventsand the lower plot is for the CCqe events.

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B.J. Kim et al. / Nuclear Instruments and Methods in Physics Research A 497 (2003) 450–466

On the other hand, if there are nearby noise
hits along the primary track, single track eventsmight be reconstructed as two track events. This
type of nearby noise hits contributes to theuncertainty in track counting for two track events.
For single track events, the vertex position isrelocated to the middle of the track and the track
ﬁnding is repeated around the new vertex. The
uncertainty in track ﬁnding due to accidental noise
hits around the primary track is found to be lessthan 1%.
The track counting inefﬁciency can be explainedby loss of tracks or additional fake tracks throughreconstruction. The efﬁciency of ﬁnding the second
track is calculated as a function of number of hits

in the SciFi layers using MC. Events with twogenerated tracks are reconstructed, and only thosewith a primary track and a vertex are selected. The
second generated track should have at least two
hits. The efﬁciency of ﬁnding the second track is
deﬁned to be the ratio of number of events with
two reconstructed tracks to the number of eventswith two generated tracks.
Fig. 16 shows the efﬁciency of ﬁnding thesecond track as a function of number of hits in
the SciFi layers. The upper plot is for all types ofMC events, and the lower plot is for the CCqe
events.
For the second tracks with three or more hits,
the inefﬁciency comes from hit ﬁnding inefﬁciency

Fig. 17. A candidate nm CCqe event. The long track is the muon and the short track is most likely a proton.

B.J. Kim et al. / Nuclear Instruments and Methods in Physics Research A 497 (2003) 450–466

and limited performance of the track reconstruction as for the primary track. For the second
tracks with two hits, the inefﬁciency is mainly due
to noise hits around the vertex or near-by noise
hits along the primary track. An average efﬁciency
of ﬁnding the second track with three or more hits
is found to be 82:170:4% for all types of MC
events and 90:170:5% for MC CCqe events.

5. Event reconstruction
In the previous section, we described thereconstruction of tracks and vertices. In thissection, we demonstrate their performances using
distributions of typical physics variables.
Fig. 17 shows a typical nm CCqe candidate
event. The long track is the muon which leaves
hits in the VETO and LG, and ranges out at the
MRD. The short track is most likely a proton.
Most of the protons from neutrino interactions
have relatively low momenta and thus make shortsecond-tracks in the SciFi. Fig. 18 shows the track
length distribution for the second tracks in the
two-track sample. Both Monte Carlo prediction
and data agree well, demonstrating the validity of
the neutrino event generation and the tracking
method.

465

6. Conclusions
The K2K long-baseline neutrino experiment has
taken data for 3 years. The SciFi is used to identify
charged particle tracks in the K2K near detector.
The SciFi hits are calibrated using cosmic-ray
data and electro-luminescent calibration data. MC
simulation reproduces the distribution of thenumber of hit pixels in the cosmic-ray data very
well. The stability of the SciFi is checked
periodically by obtaining the distribution of hit
efﬁciency and calculating the mean of the hit pixel
distribution. The stability is better than 5% and1.2% in the mean of pixel distribution and the hit
efﬁciency, respectively.
A track reconstruction algorithm, optimized forcharged current neutrino interactions, is employed
to ﬁnd tracks and event vertices. The track ﬁnding
efﬁciency is monitored and found to be stablethroughout the data-taking period. Using theestimated vertex resolution, the vertex reconstruction
algorithm is found to be good enough for deﬁning
the event ﬁducial volume with an accuracy better
than 1%. A track ﬁt using a Kalman ﬁltering
technique is used for the best estimation of hit
resolution, and found to improve the ﬁber alignment.
Accurate track counting is crucial for themeasurement of neutrino energy spectrum and in

Number of Events

Data
Monte-Carlo

Track Length (second track) (cm)Fig. 18. A comparison of track length for the proton between neutrino data and MC events in two-track sample.

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B.J. Kim et al. / Nuclear Instruments and Methods in Physics Research A 497 (2003) 450–466

Table 1
Summary of tracking performance of the SciFi
Performance quantity

Data

MC events

Hit ﬁnding efﬁciencyðehit ÞPrimary tack ﬁnding efﬁciencyðetrk ÞPosition resolution

96:570:5%
93:670:8%
0:6470:07 mm

96:870:1%
92:770:5%
0:5770:02 mm

Two track

N/A
N/A
N/A

6 mm (Transverse)
N/A
4 mm (Transverse)
6 mm (Longitudinal)

Tracking counting
1-track efﬁciency
2-track efﬁciency

N/A
N/A

9372%
8674%

Vertex resolution
Single track

Position resolution is measured using the neutrino data. Vertex resolution is estimated using MC events. Track counting efﬁciency is
calculated for the CCqe events. Errors on the hit efﬁciency, tracking efﬁciency, and track counting efﬁciency are the quadratic sums ofstatistical and systematic errors.

achieving the physics goals of the K2K experiment. The track counting uncertainty is found to
be at 2% and 4% level for the single- and twotrack events, respectively. Overall tracking performance of the SciFi is summarized in Table 1.
The performance of the SciFi tracker is welldemonstrated by physics variables of neutrino
interaction. The distributions of variables formuon track and proton track show a good
agreement between data and MC events.

Acknowledgements
The vital contributions of the KEK staff and thetechnical staffs of the participating institutions are
gratefully acknowledged. This work is supportedby the Ministry of Education, Science and Cultureof Japan, the US Department of Energy, theNational Science Foundation of the US, the Korea
Research Foundation, the Korea Science andEngineering Foundation, and the CHEP in Korea.

References[1] Y. Fukuda, et al., Super-Kamiokande collaboration, Phys.
Rev. Lett. 81 (1998) 1158;

M. Nakahata, et al., Super-Kamiokande collaboration,
Nucl. Instr. and Meth. A 421 (1999) 113.
[2] S.H. Ahn, et al., K2K collaboration, Phys. Lett. B 511(2001) 178;
K. Nishikawa et al., KEK-PS proposal (E362)(1995).
[3] T. Ishii, et al., K2K collaboration, Nucl. Instr. and Meth.
A 482 (2002) 244.
[4] R.E. Ansorge, et al., UA2 collaboration, Nucl. Instr. andMeth. A 265 (1988) 33;
P. Annis, et al., CHORUS collaboration, Nucl. Instr. andMeth. A 412 (1998) 19;
D. Adams, et al., IEEE Trans. Nucl. Sci. NS-43 (1996)
1146;
M. Ambrogiani, et al., IEEE Trans. Nucl. Sci. NS-44(1997) 460;
H.B. Dreis, et al., DESY 98-049 (1998).
[5] A. Suzuki, et al., K2K collaboration, Nucl. Instr. andMeth. A 453 (2000) 165.
[6] R. Brun, et al., GEANT3, CERN DD//EE/84-1, revised
1987.
[7] R.E. Kalman, Trans. ASME J. Basic Eng. 82D (1960)
35;
R.E. Kalman, R.S. Bucy, Trans. ASME J. Basic Eng. 83D(1961) 95.
[8] P. Billoir, Nucl. Instr. and Meth. 225 (1984) 352.
[9] R. Fruhwirth, Nucl. Instr. and Meth. A 262 (1987)444.
[10] V. Innocente, E. Nagy, Nucl. Instr. and Meth. A 324(1993) 297;
E.J. Wolin, L.L. Ho, Nucl. Instr. and Meth. A 329 (1993)
493;
R. Harr, IEEE Trans. Nucl. Sci. NS-V 42 (1995) 134.

E369 T. Nagae
Structure of 89ΛY via the (!+ ,K+) reaction
H. Hotchi et al.
Spectroscopy of medium-heavy Λ-hypernuclei via ("+,K+) reaction,
Phys. Rev. C 64 (2001) 044302.
● J.H. Kim et al.
Neutron energy spectra from the nonmesonic weak decay of 12ΛC and 89ΛY hypernuclei
Phys. Rev. C 68 (2003) 065201.
● T. Nagae
Progress in hypernuclear physics
Eur. Phys. J. A 18 (2003) 283.
★ H. Hotchi
Spectroscopy of medium-heavy Λ-hypernuclei via the ("+,K+) reaction
University of Tokyo, 2000.
★ J.H. Kim
Nonmesonic Weak Decay of 12ΛC and 89ΛY hypernuclei
Seoul National University, 2003.
◇ T. Nagae
Structure of a Λ-hypernucleus 89ΛY measured with the SKS spectrometer
Nucl. Phys. A 670 (2000) 269c.
◇ T. Nagae
Overview of the results on Λ-hypernuclei spectroscopy with the SKS
Nucl. Phys. A 691 (2001) 76c.
◇ H.C. Bhang et al.
Overview of Λ-hypernuclear Weak decay results with the SKS
Nucl. Phys. A 691 (2001) 156c.
◇ T. Nagae et al.
Properties of Λ in 89ΛY
Proc. Int. Nucl. Phys. Conf. INPC2001, Berkeley, USA, July 2001.
●________________________________________________________________________● Physics papers published in refereed journal.
○★
◇＊

Technical papers.
PhD theses.
Conference and Symposium.
Internal Report and others.

[attached][attached][attached]PHYSICAL REVIEW C, VOLUME 64, 044302

Spectroscopy of medium-heavy ⌳ hypernuclei via the „ ␲¿ ,K ¿ … reaction
H. Hotchi,1,* T. Nagae,2 H. Outa,2 H. Noumi,2 M. Sekimoto,2 T. Fukuda,2 H. Bhang,3 Y. D. Kim,3,† J. H. Kim,3 H. Park,3,‡
K. Tanida,1 O. Hashimoto,4 H. Tamura,4 T. Takahashi,4 Y. Sato,4 T. Endo,4 S. Satoh,4 J. Nishida,4 T. Miyoshi,4
T. Saitoh,5 T. Kishimoto,6 A. Sakaguchi,6 S. Ajimura,6 Y. Shimizu,6 T. Mori,6 S. Minami,6 M. Sumihama,6 R. Sawafta,7,§and L. Tang8
1

Department of Physics, University of Tokyo, Tokyo 113-0033, Japan
Institute for Particle and Nuclear Physics, KEK, Tsukuba, Ibaraki 305-0801, Japan3
Department of Physics, Seoul National University, Seoul 151-742, Korea
4
Department of Physics, Tohoku University, Sendai 980-8578, Japan
5
Laboratory of Nuclear Science, Tohoku University, Sendai 980-0826, Japan
6
Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan
7
Physics Department, North Carolina A&T University, Greensboro, North Carolina 27411
8Department of Physics, Hampton University, Hampton, Virginia 23668
共Received 23 April 2001; published 19 September 2001兲
2

89
51
12
Excitation energy spectra of ⌳Y, ⌳V, and ⌳C have been measured via the ( ␲ ⫹ ,K ⫹ ) reaction by using the89
SKS spectrometer at the K6 beam line in the KEK 12-GeV Proton Synchrotron. In the ⌳Y spectrum, obtainedwith an energy resolution of 1.65 MeV 共FWHM兲 and in the highest statistics so far, we have succeeded inclearly observing a characteristic fine structure in heavy ⌳ hypernuclear systems and precisely obtaining aseries of ⌳ single-particle energies in the very wide excitation energy range of more than 20 MeV, for the first51
89
12
V spectrum, a similar structure to that of ⌳Y was observed. In the ⌳C spectrum, newtime. Also in the ⌳core-excited states were clearly resolved thanks to the best energy resolution of 1.45 MeV so far achieved byusing the SKS spectrometer.

DOI: 10.1103/PhysRevC.64.044302

PACS number共s兲: 21.80.⫹a, 25.80.Hp

I. INTRODUCTION

Among various hypernuclei, ⌳ hypernuclei are the mostfamiliar hypernuclear system so far, and allow us to carry out
precise spectroscopic investigations thanks to the narrow intrinsic widths 关1–3兴.
In light ⌳ hypernuclei, in particular, the spin-dependentparts of the ⌳-N interaction, such as spin-spin, spin-orbit,
and tensor interactions, play an important role on the finelevel structure. A high energy resolution of much less than
100 keV would be necessary to resolve the fine splittings,
because the spin-dependent ⌳-N interactions are known to
be much weaker than those of the N-N case 关4 – 6兴. In thisregard, recent attempts at hypernuclear ␥-ray spectroscopy
appear promising.
On the other hand, heavy ⌳ hypernuclei can provide us
with better information on the behavior of a hyperon in the
nuclear medium, because deeper ⌳ bound states are formed
for heavier systems. The basic quantities such as the ⌳ effective mass and the ⌳ potential depth in the nuclear medium
can be obtained from a series of ⌳ single-particle energies inthe very wide excitation energy range of more than 20 MeV
in heavy systems. In astrophysics, it has been discussed that
hyperons in the high-density nuclear matter at the core ofneutron stars play a significant role in their formation andthermal and structural evolution 关7兴. They are basic and important parameters for a realistic discussion on various properties of neutron stars. In addition, heavy ⌳ hypernuclei havesignificant merit in the measurement of the ⌳ spin-orbit splitting, because ⌳ orbits with higher angular momenta, inwhich the spin-orbit splitting would be enhanced, are boundin going to heavier systems. It is very interesting to compare
the splitting enhanced in the higher ⌳ orbit of heavy ⌳ hypernuclei with the small splitting in p-shell ⌳ hypernuclei.

Nuclear systems including strangeness provide us with aunique playground for studying new forms of hadronic
many-body systems and various hadronic interactions extended in the flavor SU共3兲. A hyperon implanted in a nucleus
can move around free from the Pauli blocking by nucleons,probing deep inside of the nucleus. Thus hypernuclei canprovide invaluable information concerning the behavior of abaryon deep inside a nucleus not accessible in ordinary nuclei. A hypernucleus is also an excellent tool to extract information on the hyperon-nucleon interaction. At this moment,
it is not easy to carry out a direct measurement on hyperonproton scattering at low energy, because it is hard to obtain ahigh-intensity hyperon beam in this energy due to its short
lifetime. Therefore it is a unique opportunity to obtain information on the hyperon-nucleon interaction by observing thefine structures of hypernuclei caused by the specific spindependent hyperon-nucleon interactions. For a unified understanding of baryon-baryon interactions in terms of themeson-exchange mechanism and/or the quark-gluon picture,
basic experimental data on the hyperon-nucleon interaction
as well as the nucleon-nucleon interaction have been longawaited.

*Present address: Brookhaven National Laboratory, Upton,
NY 11973.
†
Present address: Department of Physics, Sejong University,
Seoul 114-747, Korea.
‡
Present address: Institute for Particle and Nuclear Physics, KEK,
Tsukuba, Ibaraki 305-0801, Japan.
§
Present address: TransTech Pharma, Inc., High Point, NC 27265.
0556-2813/2001/64共4兲/044302共15兲/$20.00

64 044302-1©2001 The American Physical Society

H. HOTCHI et al.

PHYSICAL REVIEW C 64 044302

The two-body ⌳-N spin-dependent interactions may mainly
contribute to the splitting in p-shell ⌳ hypernuclei, while inheavy ⌳ hypernuclei the splitting due to higher-order manybody effects might contribute additionally.
For a unified understanding of ⌳ hypernuclear systems,
high-quality and systematic spectroscopy is indispensable.
However, the existing data are mostly limited to light ⌳ hypernuclei, and our understanding of heavy ⌳ hypernuclear
systems has been limited by available energy resolution andpoor statistics. So far, experimental studies of hypernuclei
have been carried out with three major reactions;⫺, ␲ ⫺ ), in-flight (K ⫺ , ␲ ⫺ ), and ( ␲ ⫹ ,K ⫹ ). The pro(K stopped
duction of hypernuclei by the (K ⫺ , ␲ ⫺ ) reaction in both
stopped and in-flight kaon processes is known to be suitable
for light hypernuclear systems in the s and p shells, but notfor heavy systems. The ( ␲ ⫹ ,K ⫹ ) reaction was proposed to
be an effective tool for investigations of heavy ⌳ hypernuclei
by Thiessen 关8兴 and Dover et al. 关9兴 in 1980. This reaction ischaracterized by a large momentum transfer of 350 MeV/c,
which is comparable to the nuclear Fermi momentum. Therefore this reaction efficiently populates configurations with anouter neutron hole and a ⌳ hyperon in a series of orbits
covering all the bound ⌳-major shells. In particular, angularmomentum stretched states with maximum spin are selectively excited. The richness of spectroscopic information on⌳ hypernuclear bound states obtained in the ( ␲ ⫹ ,K ⫹ ) reaction was demonstrated in experiments at BNL 关10–12兴 and at
51
89
V and ⌳
Y data measured with
KEK 关13兴. In particular, the ⌳an energy resolution of 3 MeV 关full width at half maximum
共FWHM兲兴 at BNL disclosed the independent single-particle
nature of a ⌳ hyperon deep inside a nucleus. Recently, ahigh-resolution ( ␲ ⫹ ,K ⫹ ) experiment was performed for awide mass number range of A⫽10– 208 by using the SKS
spectrometer 共KEK-PS E140a 关14,15兴兲. One of the surprises
89
139
Y, ⌳
La, andin the experiment was the smoothness of the ⌳208⌳ Pb spectra. Each bump structure which reflects the ⌳ shellstructure had a wider width than the energy resolution of 2.3
MeV, and the global shapes were smoother than expectedfrom theoretical calculations. These structures in heavy systems were, in fact, smeared out in the BNL data because of
the limited energy resolution. A part of the smoothness could
be ascribed to the fragmentation of neutron-hole states due tothe complexity of nuclear interactions. For example, one way
to explain the smoothness is to enhance the contributions
from deep neutron-hole states more than twice to fill up thevalleys between the major bumps. However, such a large
enhancement does not seem to be reasonable considering theselectivity of the ( ␲ ⫹ ,K ⫹ ) reaction. As another possible way
to reproduce the smooth spectral shape, a contribution of thelarger spin-orbit splittings than typical values in the ⌳ orbits
was discussed 关16兴. However, the energy resolution and thestatistical significance were not adequate to produce a convincing argument. Thus high quality experimental data on
heavy ⌳ hypernuclei in statistics and energy resolution havebeen long awaited.
In the present paper, we report on our spectroscopic in89
51
vestigation of medium-heavy ⌳ hypernuclei, ⌳Y, ⌳V, and
12
⫹
⫹
C,
via
the
(
␲
,K
)
reaction
with
the
best
energyresolu⌳FIG. 1. Schematic view of the experimental setup.

tion so far achieved by using the SKS spectrometer. In par89
ticular, the ⌳Y spectrum has been measured with an improved energy resolution of 1.65 MeV and about eight times
better statistics than that of E140a. In the spectrum, we havesucceeded in clearly observing a characteristic fine structurein heavy systems and precisely obtaining a series of ⌳single-particle energies in the very wide excitation energy
range of more than 20 MeV, for the first time.
II. EXPERIMENTAL APPARATUS

The experiment 共KEK-PS E369兲 was carried out via the( ␲ ⫹ ,K ⫹ ) reaction at the K6 beam line in the North Experimental Hall of the KEK 12-GeV Proton Synchrotron using
the Superconducting Kaon Spectrometer 共SKS兲. The central
momentum of the incident pion beam was set at 1.05 GeV/c.
Then, the outgoing kaon was produced with a momentum
around 0.72 GeV/c. Figure 1 shows a schematic view of theexperimental setup. The incident momentum of the pion wasmeasured with a beam line spectrometer which was comprised of a dipole magnet and four quadruple magnets
共QQDQQ兲, while the momentum of the kaon was measured
with the SKS spectrometer which was comprised of a large
superconducting dipole magnet. Details of the present detector system are given elsewhere 关17兴. The two spectrometers
were designed to achieve a momentum resolution of 0.1%
共FWHM兲 at 1 GeV/c. Prior to this experiment, the best energy resolution achieved in this spectrometer system was 1.9
12
C reaction with a 0.9-g/cm2 carbon
MeV for the ( ␲ ⫹ ,K ⫹ ) ⌳target 关14兴. In the present experiment, the drift-chamber configuration in the SKS spectrometer was modified and also

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SPECTROSCOPY OF MEDIUM-HEAVY ⌳ HYPERNUCLEI . . .

PHYSICAL REVIEW C 64 044302

special care was taken in the drift-chamber position calibrations in the whole spectrometer system in order to reduce
systematic errors. Consequently, the energy resolution
achieved in the present experiment was 1.45 MeV for thesame target, which is significantly better than before.

TABLE I. Specifications of the experimental targets. The quoted
errors of the thickness come from target size measurements.

Targets
89

Y
V
12
C
12
C

A. K6 beam line

51

The K6 beam line is a general-purpose high-intensity
beam line that can supply mass-separated secondary beams
in the momentum range between 0.5 and 2.0 GeV/c 关18兴.
The secondary pions were produced by the primary 12-GeV
proton beam in a platinum production target located at theentrance of the K6 beam line. After momentum and mass
selections, the pion beam was achromatically focused on theexperimental target. The measured beam size at the experimental target was typically 5.5 mm共horizontal兲⫻8.2 mm
共vertical兲 in rms. The central beam momentum was set at
1.05 GeV/c and its intensity was adjusted to be about 1.5⫻106 per spill at the experimental target.
B. Beam spectrometer

The end part of the K6 beam line was used as a spectrometer in order to analyze the beam momentum. It consisted of
a QQDQQ magnet system with four sets of high-rate drift
chambers 共BDC1– 4兲, a freon-gas Čerenkov counter 共eGC兲,and two sets of segmented plastic scintillation counters 共BH1
and BH2兲.
eGC(n⫽1.002 45) vetoed the positron contamination of
20% in the beam with a rejection efficiency of better than
99.9%. BH1, with seven vertical segments, was installed justdownstream of eGC, while BH2, with three vertical segments, was located 30 cm upstream of the experimental target to define the beam hitting the experimental target. By
requiring timing coincidence between BH1 and BH2, theproton contamination of 10% in the beam was rejected together with other background particles which could not passthrough the QQDQQ system. The beam trigger was definedas BEAM⬅BH1⫻BH2⫻eGC.
12
C events
In the experiment, we always had the ( ␲ ⫹ ,K ⫹ ) ⌳coming from BH2 共72 mm wide, 40 mm high, and 6 mm
thick兲 during the data taking, in addition to the ( ␲ ⫹ ,K ⫹ )12
C spectrum from BH2
events coming from the target. The ⌳was useful for the calibration of the energy scale and theestimation of the energy resolution, as described below.
BDC1– 4 共drift space: ⫾2.5 mm兲 were installed upstream
and downstream of the QQDQQ system. The beam track wasmeasured with a position resolution of 300 ␮m in rms via
BDC’s. The beam momentum was obtained particle by particle using a third-order transport matrix calculated with
ORBIT 关19兴. In order to minimize multiple-scattering effects
on the momentum resolution, the QQDQQ system was designed so as to make the 具 x 兩 ␪ 典 term of the transport matrix
sufficiently small. In addition, the beam pipe in the QQDQQ
system was evacuated with a Kapton window of 100 ␮m
thickness, and the drift chambers were made as thin as possible. The magnetic field of the dipole magnet 共D2兲 wasmonitored during the data acquisition for every spill with ahigh-precision Hall probe in order to correct its fluctuation in

Thickness
共g/cm2兲

Purity共%兲

2.223⫾0.016
2.816⫾0.042
0.859⫾0.006
1.739⫾0.012

99.9
99.7
99.5
99.5

the off-line analysis. The fluctuation was within ⫾0.05%.
C. SKS spectrometer

The SKS spectrometer, comprised of a superconducting
dipole magnet with four sets of drift chambers 共SDC1– 4兲
and three kinds of trigger counters 共TOF, AC1•2, and LC兲,
was use to analyze scattered particles. This unique apparatus
with good momentum resolution 共0.1% FWHM兲 and large
acceptance 共100 msr兲 has a short flight path 共5 m兲 to minimize kaon decay and is provided with powerful kaon identification capability.
TOF comprised 15 vertical scintillation counters and wasused as a time-of-flight wall for scattered particle identification. A typical timing resolution between BH2 and TOF was
200 ps in rms, which was good enough to separate scatteredpions, kaons, and protons. AC1 and AC2 are threshold-typesilica aerogel Čerenkov counters (n⫽1.06) for pion veto. LC
comprised 14 vertical threshold-type lucite Čerenkov
counters (n⫽1.49), and discriminated protons from pions
and kaons. The ( ␲ ⫹ ,K ⫹ ) trigger was defined as
PIK⬅BEAM⫻TOF⫻AC1⫻AC2⫻LC. The trigger rate wastypically 180 counts for a beam rate of 1.5⫻106 per spill.
SDC1 and SDC2, which were installed at the entrance ofthe magnet, had the same drift-cell structure as that of
BDC’s, because they were exposed to the beam. SDC3 andSDC4, which were placed at the exit of the magnet, had alarge drift space of ⫾21 mm. The particle track was measured with a position resolution of 300 ␮m in rms via SDC’s.
In the ( ␲ ⫹ ,K ⫹ ) runs, the magnet was excited at 2.2 T 共272A兲, where the central trajectory corresponded to 0.72 GeV/c.
The scattered-particle momentum was obtained particle byparticle with the Runge-Kutta method 关20兴 using a precisely
measured magnetic field map. In order to reduce multiple
scattering effects on the momentum resolution, the drift
chambers were made as thin as possible, and the spaces
along the particle trajectory were filled with helium bags
instead of air. The magnetic field was monitored with an
NMR probe during the data acquisition to correct its fluctuation in the off-line analysis. In fact, it was quite stable during
the experiment; the fluctuation was within ⫾0.02%.
D. Experimental targets and data summary

Table I shows specifications of the experimental targets,
Y, 51V, and 12C 共thin and thick兲, used in the present experiment. The thickness was chosen so as to make theenergy-loss straggling sufficiently small in order to keep theenergy resolution better than 2 MeV 共FWHM兲. The 12C tar89

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H. HOTCHI et al.

PHYSICAL REVIEW C 64 044302

TABLE II. Data summary, where N beam is the number of pions
incident on each target.
CyclesDate

97-1

Nov. 11–16, 1997

Targets

N beam(⫻109 )

Y
C
89
Y
12
C 共thin兲
12
C
89
Y
12
C
89
Y
12
C
89
Y
51
V
12
C 共thin兲
12
C

31
52
454
41
83
269
38
5
32
134
144
43
158

89
12

97-2

Nov. 19–Dec. 07, 1997

97-3

Dec. 10–18, 1997

98-1

Jan. 22–31, 1998

98-2

Feb. 06 –22, 1998

FIG. 2. Scattered-particle mass spectrum obtained for theC( ␲ ⫹ ,K ⫹ ) data. The selected region is shown as arrows.

12

A. Momentum reconstruction

gets were used to adjust an absolute energy offset in thebinding energy scale and to check the energy resolution; the12⌳ C spectrum is well known to have two well-separated narrow peaks and the width of the ground-state peak directlyshows the experimental energy resolution. Also the yield for12
the ⌳C ground-state peak was used for checking a stabilityof the overall detection efficiency.
The data were taken in five separated experimental cycles
from November 1997 to February 1998, as listed in Table II.
The total number of pions incident on all the targets was
about 1012.
III. DATA ANALYSISThe hypernuclear mass (M HY ) is obtained as a missing
mass in the ( ␲ ⫹ ,K ⫹ ) reaction as follows:
M HY ⫽ 冑共 E ␲ ⫹M A ⫺E K 兲 2 ⫺ 共 p ␲2 ⫹p K2 ⫺2 p ␲ p K cos ␪ 兲 ,共3.1兲
where E ␲ and p ␲ are the total energy and momentum of apion, E K and p K are those of a kaon, M A is the mass of atarget nucleus, and ␪ is the scattering angle of the reaction.
Then the binding energy (B ⌳ ) of a ⌳ hyperon in the hypernucleus is obtained asB ⌳ ⫽M A⫺1 ⫹M ⌳ ⫺M HY ,共3.2兲

where M A⫺1 is the mass of a core nucleus at its ground stateand M ⌳ is the mass of a ⌳ hyperon. Thus there were threekinematic variables to be measured via the momentum reconstruction; p ␲ , p K , and ␪. The procedure of the analysis
was comprised of 共i兲 event selection by using the counters’
information, 共ii兲 momentum reconstruction for beam particles, 共iii兲 momentum reconstruction for scattered particles,
共iv兲 identification of kaons, 共v兲 reconstruction of the scattering angle and vertex point, and 共vi兲 calculation of the hypernuclear mass.

At the first stage of the analysis, a large background eventin the PIK trigger was rejected only using the counters’ information. An incident pion was selected by using the timeof-flight information between BH1 and BH2, while a scattered kaon was roughly selected by using the ADC’s andTDC’s information of TOF and LC. At this stage, the numberof events to be analyzed was reduced by about a factor of 10.
Then the pion momentum and the kaon momentum were
determined by reconstructing particle trajectories from thehit positions in BDC’s and SDC’s, respectively. In the tracking process, straight-line tracks were first defined locallyboth at the entrance and the exit of each spectrometer using aleast-squares method. Then we assigned the best combination of the straight-line tracks which gave a chi-square minimum in the momentum reconstruction. In the beam spectrometer, a third-order transport matrix was used for the
momentum reconstruction, while, in the SKS spectrometer,
the momentum was calculated by the Runge-Kutta methodwith a measured magnetic field map.
B. „ ␲ ¿ ,K ¿ … event selection

The main background in the PIK trigger was fast protons
which fired LC, while pions were well suppressed in thetrigger level because of the excellent performance of AC1
and AC2. After the momentum reconstruction, the mass of ascattered particle (M scat) was calculated as
M scat⫽p
冑1⫺ ␤ 2 ,
␤

共3.3兲

where ␤ is the velocity of a scattered particle obtained fromthe time-of-flight measurement between BH2 and TOF, and pis the momentum determined by SKS. Figure 2 shows a typical scattered-particle mass spectrum obtained for the12C( ␲ ⫹ ,K ⫹ ) data. Kaons, for which the selected region isindicated with arrows, were clearly separated from pions andprotons.
The scattering angle and vertex point were obtained fromtwo tracks; a local straight-line track obtained from BDC3•4 and a track obtained in the momentum reconstruction of
SKS. Figure 3 shows a scatter plot between the horizontal
scattering angle and the horizontal position difference of an

044302-4

SPECTROSCOPY OF MEDIUM-HEAVY ⌳ HYPERNUCLEI . . .

FIG. 3. Scatter plot between the horizontal scattering angle andthe horizontal position difference of an incident pion and an outgoing kaon at the target, obtained for the 12C( ␲ ⫹ ,K ⫹ ) data. The
events scattered on the target, BH2, and SDC1 are clearly distinguished as different images.

incident pion and an outgoing kaon at the target point, obtained for the 12C( ␲ ⫹ ,K ⫹ ) data. In the figure, events scattered on the target, BH2 and SDC1 can be distinguished as
different images, and events in the gated region were selected as good events which had a reaction on the target. The
backgrounds from BH2 and SDC1 around forward angles
were sufficiently rejected by additionally applying the scattering angle cut ( ␪ ⭓2°).
C. Energy calibration and precision

In calibrating the binding energy scale, we corrected the
12Ckaon momentum so as to adjust the binding energy of the ⌳ground state at 10.76 MeV. This value was taken from theother experimental data obtained in emulsion experiments
89
51
Y and ⌳
V data, the energy scale was关21兴. As for the ⌳12adjusted by using the ⌳ C spectrum from BH2 obtained at the12same time during the data taking. As an example, the ⌳C
spectrum from BH2, which was obtained during the89
Y( ␲ ⫹ ,K ⫹ ) data taking, is shown in Fig. 4. The validity of
12
C spectrum from BH2, lothe energy calibration with the ⌳cated 30 cm upstream of the target, was examined with the12C( ␲ ⫹ ,K ⫹ ) data; we checked the consistency between the12
C spectra from the carground-state peak positions of the ⌳bon target and from BH2. They were consistent within thestatistical errors. The binding energy scale was calibrated
cycle by cycle within a error of ⫾0.17 MeV, which came
12
C ground-state peak position.
from a fitting error of the ⌳The linearity of the SKS momentum at 2.2 T was examined by sending pion beams through the two spectrometers atseveral central momenta from 0.65 to 0.79 GeV/c. The linearity was found to be better than ⫾0.04 MeV/c

PHYSICAL REVIEW C 64 044302

12
FIG. 4. Mass spectrum in the ( ␲ ⫹ ,K ⫹ ) ⌳C reaction on the BH2
counter, obtained at the same time during the 89Y( ␲ ⫹ ,K ⫹ ) datataking, which was obtained by selecting the hatched region in theinset that shows a Z projection 共beam direction兲 of the vertex point.

(⫾0.04␤ K MeV) in the momentum range. Here we assumed
that the central momentum of the beam spectrometer wasexactly proportional to the magnetic fields of the beam line
magnets.
Taking account of the two errors mentioned above, thetotal systematic error of the binding energy scale became⫾ 冑(0.04␤ K ) 2 ⫹0.172 ⫹⌬E⫽⫾0.17⫹⌬E MeV for the12C( ␲ ⫹ ,K ⫹ ) data, where ⌬E is the error based on the defi12
C ground-state binding energy of 10.76 MeV.
nition of the ⌳As for the other data, the error arising from the energyloss corrections for pions and kaons based on the calculation
should be taken into account additionally. From comparisons
between the measured energy losses with pion beams 共0.72
and 0.90 GeV/c兲 and the corresponding calculated values,
the error was estimated to be better than ⫾0.15 MeV. Thus
89
51
Y and ⌳
V data was obthe total systematic error for the ⌳22tained to be ⫾ 冑0.17 ⫹0.15 ⫹⌬E⫽⫾0.23⫹⌬E MeV.
D. Energy resolution

The experimental energy resolution consists of the momentum resolutions of the two spectrometers and the energyloss fluctuations in the target and in the BH2 counter. As forthe 12C( ␲ ⫹ ,K ⫹ ) data, the energy resolution was directly obtained by fitting the ground-state peak with a Gaussian form;
1.45⫾0.06 MeV 共FWHM兲 for the thin target. As shown in
Table III, the energy resolution was stable during the experiment.
89
Y spectrum was estimated
The energy resolution of the ⌳12by using the ⌳ C spectrum from BH2 obtained at the sametime during the data taking. It was calculated to be 1.65
⫾0.10 MeV from the value (1.69⫾0.10 MeV) directly ob12
C ground-state peak, by considering thetained from the ⌳difference of the energy-loss fluctuations in the ( ␲ ⫹ ,K ⫹ )044302-5

H. HOTCHI et al.

PHYSICAL REVIEW C 64 044302

12
TABLE III. Widths and cross sections of the ⌳C ground-statepeak obtained in the different experimental cycles, where the crosssections were obtained in the scattering angles from 2 to 14 ° byintegrating the binding energy region from 6.5 to 13 MeV. The
quoted errors are statistical.

Thick target

The accidental coincidence rate between BH1 and BH2
was estimated to be 19.0⫾0.7% by using the TDC spectrumwith the BEAM trigger.
In total, the beam normalization factor was typically76.0⫾2.1%.

Thin target

CyclesFWHM
共MeV兲Cross sections
共␮b/sr兲

FWHM
共MeV兲

Cross sections
共␮b/sr兲

97-2-1
97-2-2
97-3
98-1
98-2-198-2-2

1.75⫾0.08
1.77⫾0.08
1.74⫾0.07
1.72⫾0.08
1.80⫾0.06
1.85⫾0.06

9.6⫾0.4
10.0⫾0.4
10.0⫾0.4
9.5⫾0.5
9.6⫾0.3
9.1⫾0.3

1.45⫾0.09

9.8⫾0.6

1.48⫾0.08

9.5⫾0.6

reaction on the target and on BH2. Because the kaons from
BH2 also passed through the 89Y target, we could reliably
determine the energy resolution for the 89Y( ␲ ⫹ ,K ⫹ ) reac51
V spectrum was similarlytion. The energy resolution of the ⌳estimated to be 1.95⫾0.10 MeV. The validity of the estimation was examined with the 12C( ␲ ⫹ ,K ⫹ ) data; we checked
the consistency between the energy resolution directly obtained from the 12C( ␲ ⫹ ,K ⫹ ) spectrum and the correspond12
C spectrum from BH2.
ing resolution estimated from the ⌳They were consistent within the statistical errors.
E. Cross section

The cross section was calculated from the experimental
yields as

冉 冊

d␴
A
1
NK
,
⫽
•
•
d⍀
共 ␳ x 兲 •N A 共 N beam• f beam兲 ␧ exp•d⍀

共3.4兲

where A is the target mass number, ␳ x the target thickness ing/cm2, N A the Avogadro’s number, N beam the scaler counts of
the BEAM trigger, N K the number of the ( ␲ ⫹ ,K ⫹ ) events,d⍀ the effective solid angle of SKS, f beam the beam normalization factor, and ⑀ exp represents several experimental efficiency factors; detector and analysis efficiency, dataacquisition efficiency, K ⫹ decay factor, and K ⫹ absorption
factor.
1. Beam normalization factor

The beam normalization factor represents the fraction ofpions in the beam. The positron contamination was rejectedwith eGC. Also the proton contamination was rejected byrequiring timing coincidence between BH1 and BH2. Butmuons in the beam, which were the decay products of pions,
could not be separated from pions. The ␮ ⫹ contamination
rate was estimated to be 6.2% by using DECAY-TURTLE 关22兴.
The validity of the estimation had been examined in the previous experiment by measuring the contamination rate with ahigh-pressure freon-gas Čerenkov counter 关23兴; the calculation agreed with the measurement within ⫾2.0%.

2. Detector and analysis efficiency

The efficiency in the beam momentum reconstruction was
estimated using the BEAM trigger, which was typically
94.5⫾1.5% in total.
The TOF efficiency was measured to be 99.9⫾0.1% using a pion beam of 072 GeV/c. The LC efficiency was similarly estimated to be 99.4⫾0.2%. In the analysis of thecounters in SKS, events with more than two hits on TOF or
LC were rejected in order to reduce a background level. The
corresponding cut efficiency was estimated to be 97.0
⫾1.0%.
The efficiency of SDC1•2 including the analysis efficiency was estimated using the BEAM trigger. Since thebeam counting rate per wire was quite high at SDC1•2 dueto the beam focused at the target, a small degradation of theefficiency was observed near the beam spot. Therefore theefficiency as a function of the horizontal incident position
was used for the correction event by event. The averaged
value of the SDC1•2 efficiency was typically 91.0⫾1.5%.
The efficiency of SDC3•4 including the analysis efficiency was estimated using the ( ␲ ⫹ ,p) events in the PIK
trigger because of the negligible decay in flight. Since it had
a dependence on the incident position due to some noisy
channels, the efficiency as a function of the horizontal incident position was used for the correction event by event. The
averaged value of the SDC3•4 efficiency was typically
81.0⫾1.5%.
The kaon-tracking-␹ 2 cut efficiency was estimated to be
97.0⫾1.4%. The cut efficiency in the kaon identification
共the scattered-particle mass cut and the TOF-ADC, LC-ADC,
and LC-TDC cuts兲 was estimated to be 90⫾1.5% in total.
The event-vertex cut efficiency was estimated to be 97.0
⫾1.0%.
3. Other factors

The data-acquisition efficiency, caused by the dead timeof the data-acquisition system, was determined to be 89.4
⫾0.1%.
Due to a high counting rate of AC1•2, some of the PIK
triggers were deadened accidentally. The corresponding correction factor was calculated to be 91.5⫾1.5% from thecounting rate 共typically 1.5⫻106 s⫺1兲 and the coincidence
width between AC1•2 and BEAM⫻TOF⫻LC(56⫾5 ns).
The kaon decay rate was studied in detail with a Monte
Carlo simulation by T. Hasegawa 关23兴. The corresponding
correction factor was calculated to be 40⫾2% in typical andit was corrected event by event taking account of the momentum and the flight path length.
The K ⫹ N inelastic cross section is less than 0.5 mbaround 0.7 GeV/c 关24兴. The kaon absorption rate was calculated to be less than 0.4%.

044302-6

SPECTROSCOPY OF MEDIUM-HEAVY ⌳ HYPERNUCLEI . . .

PHYSICAL REVIEW C 64 044302

4. Acceptance of the SKS spectrometer

The effective solid angle of SKS (d⍀) was calculatedwith a Monte Carlo simulation code GEANT 关25兴. The effects
of energy loss and multiple scattering through a trajectorywere included in this calculation. The effective solid angle
was averaged on the distribution of the beam profile obtained
from the experimental data. It was calculated as a function of
scattering angle 共␪兲 and momentum 共p兲 as follows:d⍀ 共 ␪ ,p 兲 ⫽冕␪ ⫹ 共 1/2兲 ⌬ ␪␪ ⫺ 共 1/2兲 ⌬ ␪⫻

d cos ␪

冕 ␾
2␲

d

0

number of events accepted,
number of events generated

共3.5兲

where events were generated uniformly from ␪ ⫺ 21 ⌬ ␪ to ␪
⫹ 12 ⌬ ␪ in the polar angle, from 0 to 2␲ in the azimuthal
angle, and from p⫺ 12 ⌬ p to p⫹ 21 ⌬ p in the momentum.
5. Total systematic errors

The error on the beam normalization and the experimental
efficiency factors was obtained to be ⫾7% by adding in
quadrature assuming no correlations among the factors. As
for the effective solid angle of SKS, the possible change
caused by the long-term fluctuation of the beam profile wastaken into account as a systematic error, which was estimatedto be ⫾1%. The error on the target thickness is shown in
Table I. The total systematic error on the cross section foreach target was obtained combining these errors; ⫾9% for89
1251⌳ Y and ⌳ C, and ⫾10% for ⌳ V.
The consistency among the cross sections obtained in the
different experimental cycles was examined by using the12C( ␲ ⫹ ,K ⫹ ) data. As shown in Table III, the cross sections
12
C ground-state peak, calculated separately for eachof the ⌳experimental cycle, agreed quite well within the statisticalerrors.

89
FIG. 5. Hypernuclear mass spectra of ⌳Y without 共up兲 and with
共down兲 fitting curves described in the text. The quoted errors are
statistical.

¯␴ 2° – 14° ⬅

F. Background level

The background levels for all the spectra were examinedby looking at the events in the region where the binding
energy is larger than that for the ground state of a produced⌳ hypernucleus. The backgrounds were almost uniform andfound to be less than 0.03 ␮b/sr MeV for all the spectra.
The target-empty ( ␲ ⫹ ,K ⫹ ) data were analyzed using thesame analysis program as that for the normal ( ␲ ⫹ ,K ⫹ ) data.
The background was almost uniform and estimated to be lessthan 0.04 ␮b/sr MeV.
On the basis of the analyses, we assumed the backgrounds
around the bound regions of the obtained spectra were negligible and uniform.
IV. EXPERIMENTAL RESULTS
89
51
12
The hypernuclear mass spectra of ⌳Y, ⌳V, and ⌳C 共thintarget兲 are shown in Figs. 5, 6, and 7. The vertical scale isshown in the average cross section obtained in the scattering
angles from 2 to 14 ° in the laboratory frame, which is defined as follows:冕 冉 ␴冊 冒 冕␪ ⫽14°

␪ ⫽2°

d
d⍀
d⍀

␪ ⫽14°

␪ ⫽2°

d⍀.

共4.1兲

The horizontal scale is shown in the binding energy calculated by Eq. 共3.2兲. For convenience, they are shown in thetabular form in Tables IV, V, and VI.
Qualities of the spectra discussed in the last section aresummarized in Table VII.
89
A. ⌳
Y

89
The ⌳
Y spectrum showed characteristic bump structures
which reflect the major shell structure of the ⌳ orbits⫺1
neutron-hole state. The widths for the p,
coupled to the 0g 9/2
d, and f orbits were significantly wider than expected fromthe energy resolution of 1.65 MeV 共FWHM兲 and became
wider for the ⌳ orbits with higher angular momenta; thewidths were obtained to be 2.4⫾0.2, 3.0⫾0.2, and 4.6⫾0.5 MeV for the p, d, and f orbits by fitting each major
bump with a single Gaussian. In particular, the widest bumpof the f orbit appears to split into two peaks. In the present
experiment, the energy resolution can be accurately esti-

044302-7

H. HOTCHI et al.

PHYSICAL REVIEW C 64 044302

12
FIG. 7. Hypernuclear mass spectrum of ⌳C obtained with thethin carbon target. The quoted errors are statistical.

the energy regions near the tops of the major bumps. Therefore the energy regions to be fitted were carefully selected
and each of the bumps was fitted with a single Gaussian forthe s orbit or two Gaussians for the other orbits (F 1 ), where
the Gaussian widths were fixed to be the energy resolution( ␴ exp). The whole bound region were fitted well by additionally introducing three Gaussians (F 2 ) representing extra
yields in between the bumps. The fitting function (F⫽F 1⫹F 2 ) is written as follows:
51
FIG. 6. Hypernuclear mass spectra of ⌳V without 共up兲 and with
共down兲 fitting curves described in the text. The quoted errors are
statistical.

3

F 1 ⫽G 共 a 0 ,b 0 , ␴ exp兲 ⫹⫹G 共 a Rl ,b Rl , ␴ exp兲 其 ,12
mated by using the ⌳C spectrum from the BH2 counter ob-

tained at the same time during the data taking, as describedin the last section. The large width of the bumps cannot be
attributed to the spreading widths, because a series of ⌳single-particle states coupled to an outer neutron hole, including even highly excited states, is expected to maintainnarrow widths of less than 100 keV 关1–3兴. Thus we conclude
that each of the broad bumps contains at least two peaks.
In between the major bumps, there exist contributions⫺1⫺1
and 0 f 7/2,
from deeper neutron-hole states such as 0 f 5/2
while contributions from neutron-hole states lying at lower⫺1⫺1
and 1p 3/2, would be negexcitation energies, such as 1p 1/2
ligible due to angular momentum mismatch and smaller
spectroscopic factors. In fact, in the neutron pickup⫺190Zr(p,d) 89Zr reaction 关26兴, it is known that the 0 f 5/2
and⫺1
0 f 7/2 neutron-hole states are populated at mean excitationenergies of 3.27(5/2⫺ ) and 10.3(7/2⫺ ) MeV with intensitiesof 65–70 % relative to the ground state. However, the contributions from the deep neutron-hole states would besmeared out due to the expected large spreading widths 共3– 6
MeV兲. Since they are just located between the major bumps,
their contributions should be not so large and almost flat in

兺 兵 G 共 a Ll ,b Ll , ␴ exp兲l⫽1

2

F 2 ⫽G 共 A 0 ,b 0 ⫹ ␦ B, ␴ 兲 ⫹兺 G 共 A l ,b Ll ⫹ ␦ B, ␴ 兲 , 共4.2兲

l⫽1

where G(a,b, ␴ )⬅⌬E bin关 (a/ 冑2 ␲␴ 2 )exp兵⫺(x⫺b)2/2␴ 2 其 兴
and ⌬E bin is a spectrum bin width. In the fitting, the fitting
parameters of F 1 were fixed to be the values obtained fromthe simple fitting applied to the regions near the tops of the
major bumps. The energy shift ( ␦ B) from F 1 to F 2 was
obtained to be 4.18⫾0.07 MeV, which was almost consistent
with the mean excitation energy of the 5/2⫺ excited state of89
Zr obtained in the neutron pickup 90Zr(p,d) 89Zr reaction
关26兴. The results of the fitting are summarized in Table VIII.
51
B. ⌳
V

51
The ⌳
V spectrum showed a similar structure to that of the89⌳ Y spectrum. Again, the bumps for the p and d orbits are

significantly wider than expected from the energy resolutionof 1.95 MeV 共FWHM兲 and the widths are getting wider as
the angular momentum of the ⌳ orbit increases. The spec89
Ytrum can be well reproduced in the same way as the ⌳044302-8

SPECTROSCOPY OF MEDIUM-HEAVY ⌳ HYPERNUCLEI . . .

PHYSICAL REVIEW C 64 044302

89
TABLE IV. List of the data points of the ⌳Y spectrum shown in Fig. 5. The quoted values of ⫺B ⌳ show the centers of the bins.

⫺B ⌳
共MeV兲

¯␴ 2 – 14 °( ␮b/sr•0.25 MeV)

⫺B ⌳
共MeV兲

¯␴ 2 – 14 °( ␮b/sr•0.25 MeV)

⫺B ⌳
共MeV兲

¯␴ 2 – 14 °( ␮b/sr•0.25 MeV)

⫺B ⌳
共MeV兲

¯␴ 2 – 14 °( ␮b/sr•0.25 MeV)

⫺29.875⫺29.625⫺29.375⫺29.125⫺28.875⫺28.625⫺28.375⫺28.125⫺27.875⫺27.625⫺27.375⫺27.125⫺26.875⫺26.625⫺26.375⫺26.125⫺25.875⫺25.625⫺25.375⫺25.125⫺24.875⫺24.625⫺24.375⫺24.125⫺23.875⫺23.625⫺23.375⫺23.125⫺22.875⫺22.625⫺22.375⫺22.125⫺21.875⫺21.625⫺21.375⫺21.125⫺20.875⫺20.625⫺20.375⫺20.125

0.000
0.000
0.012
0.003
0.000
0.007
0.000
0.009
0.018
0.003
0.010
0.016
0.014
0.000
0.005
0.003
0.000
0.006
0.000
0.040
0.017
0.023
0.016
0.038
0.049
0.090
0.079
0.028
0.096
0.057
0.090
0.047
0.073
0.086
0.026
0.013
0.060
0.007
0.039
0.014

⫺19.875⫺19.625⫺19.375⫺19.125⫺18.875⫺18.625⫺18.375⫺18.125⫺17.875⫺17.625⫺17.375⫺17.125⫺16.875⫺16.625⫺16.375⫺16.125⫺15.875⫺15.625⫺15.375⫺15.125⫺14.875⫺14.625⫺14.375⫺14.125⫺13.875⫺13.625⫺13.375⫺13.125⫺12.875⫺12.625⫺12.375⫺12.125⫺11.875⫺11.625⫺11.375⫺11.125⫺10.875⫺10.625⫺10.375⫺10.125

0.033
0.011
0.023
0.033
0.016
0.070
0.057
0.141
0.169
0.155
0.373
0.293
0.335
0.332
0.345
0.207
0.304
0.260
0.174
0.176
0.190
0.142
0.203
0.155
0.069
0.144
0.172
0.148
0.101
0.252
0.139
0.152
0.213
0.257
0.247
0.380
0.624
0.594
0.692
0.799

⫺9.875⫺9.625⫺9.375⫺9.125⫺8.875⫺8.625⫺8.375⫺8.125⫺7.875⫺7.625⫺7.375⫺7.125⫺6.875⫺6.625⫺6.375⫺6.125⫺5.875⫺5.625⫺5.375⫺5.125⫺4.875⫺4.625⫺4.375⫺4.125⫺3.875⫺3.625⫺3.375⫺3.125⫺2.875⫺2.625⫺2.375⫺2.125⫺1.875⫺1.625⫺1.375⫺1.125⫺0.875⫺0.625⫺0.375⫺0.125

0.736
0.753
0.521
0.633
0.612
0.456
0.538
0.518
0.449
0.611
0.465
0.457
0.463
0.412
0.405
0.485
0.399
0.350
0.303
0.355
0.479
0.485
0.480
0.614
0.687
0.859
0.762
0.967
0.975
1.043
1.164
0.926
0.901
1.048
0.937
0.829
0.867
0.815
0.791
0.689

0.125
0.375
0.625
0.875
1.125
1.375
1.625
1.875
2.125
2.375
2.625
2.875
3.125
3.375
3.625
3.875
4.125
4.375
4.625
4.875
5.125
5.375
5.625
5.875
6.125
6.375
6.625
6.875
7.125
7.375
7.625
7.875
8.125
8.375
8.625
8.875
9.125
9.375
9.625
9.875

0.725
0.841
0.739
0.743
0.733
0.674
0.915
0.894
0.751
0.912
1.118
0.982
0.862
1.108
1.473
1.283
1.316
1.295
1.245
1.242
1.385
1.169
1.595
1.323
1.052
1.214
1.383
1.077
1.218
1.129
1.162
1.040
1.096
1.238
1.208
1.250
1.401
1.277
1.376
1.173

spectrum; we assumed each of the broad bumps was made⫺1
neutron-hole
up of two peaks. In the fitting, the major 0 f 7/2⫺1⫺1
state (F 1 ) and the 0d 3/2 and 1s 1/2 neutron-hole states (F 2 )were assumed to mainly contribute to the spectrum. In fact,
in the neutron pickup 51V(p,d) 50V reaction 关27兴, the 3/2⫹
and 1/2⫹ excited states of 50V are populated at the same
excitations of ⬃3 MeV with a strength of about 30% in total
relative to the ground state. The widths of Gaussians corresponding to these neutron-hole states were fixed to be theenergy resolution ( ␴ exp). In addition, we assumed two Gaus-

sians (F 3 ) representing extra yields from deeper neutronhole states, where the widths were set to be 3.46 MeV
共FWHM兲 by considering the spreading width. The fitting
function (F⫽F 1 ⫹F 2 ⫹F 3 ) is expressed as follows:

044302-9

再冉

2F 1 ⫽G 共 a 0 ,b 0 , ␴ exp兲 ⫹
⫹G

冉

al R
,b , ␴
2 l expa兺 G 2l ,b Ll , ␴ exp
l⫽1

冊冎

,

冊

H. HOTCHI et al.

PHYSICAL REVIEW C 64 044302

51
TABLE V. List of the data points of the ⌳V spectrum shown in Fig. 6. The quoted values of ⫺B ⌳ show
the centers of the bins.

⫺B ⌳¯␴ 2 – 14 °¯␴ 2 – 14 °¯␴ 2 – 14 °¯␴ 2 – 14 °
⫺B ⌳
⫺B ⌳
⫺B ⌳
共MeV兲 ( ␮b/sr•0.5 MeV) 共MeV兲 ( ␮b/sr•0.5 MeV) 共MeV兲 ( ␮b/sr•0.5 MeV) 共MeV兲 ( ␮b/sr•0.5 MeV)
⫺29.75⫺29.25⫺28.75⫺28.25⫺27.75⫺27.25⫺26.75⫺26.25⫺25.75⫺25.25⫺24.75⫺24.25⫺23.75⫺23.25⫺22.75⫺22.25⫺21.75⫺21.25⫺20.75⫺20.25⫺19.75⫺9.25⫺18.75⫺18.25⫺17.75⫺17.25⫺16.75⫺16.25⫺15.75⫺15.25⫺14.75⫺14.25⫺13.75⫺13.25⫺12.75⫺12.25⫺11.75⫺11.25⫺10.75⫺10.25

0.000
0.035
0.00
0.000
0.000
0.010
0.000
0.014
0.000
0.019
0.021
0.00
0.007
0.000
0.028
0.007
0.135
0.145
0.184
0.195

⫺9.75
⫺9.25
⫺8.75
⫺8.25
⫺7.75
⫺7.25
⫺6.75
⫺6.25
⫺5.75
⫺5.25
⫺4.75
⫺4.25
⫺3.75
⫺3.25
⫺2.75
⫺2.25
⫺1.75
⫺1.25
⫺0.75
⫺0.25

0.262
0.163
0.153
0.167
0.115
0.135
0.169
0.200
0.129
0.054
0.207
0.137
0.349
0.466
0.382
0.769
0.809
0.740
0.833
0.665

2

F 2 ⫽G 共 A 0 ,b 0 ⫹ ␦ B, ␴ exp兲 ⫹兺 G 共 A l ,b Ll ⫹ ␦ B, ␴ exp兲 ,l⫽1

F 3 ⫽G 共 A ⬘0 ,b 0 ⫹ ␦ B ⬘ , ␴ 兲 ⫹G 共 A 1⬘ ,b L1 ⫹ ␦ B ⬘ , ␴ 兲 , 共4.3兲
where, in F 1 , the strength ratios between the two peaks ineach broad bump were set to be 1:1, and for F 2 and F 3 , thestrengths were constrained to be (A l ⫽const⫻al) and (A ⬘l⫽const⬘⫻al) to that for F 1 . The energy shift ( ␦ B) from F 1
to F 2 was obtained to be 3.3⫾0.2 MeV, which was consistent with the excitation energies of the 3/2⫹ and 1/2⫹ excitedstates of 50V obtained in the neutron pickup 51V(p,d) 50V
reaction 关27兴. The results of the fitting are given in Table IX.
12
C. ⌳
C

12
The ⌳
C spectrum have two prominent peaks at B ⌳ ⬇11
and 0 MeV. They were already observed in previous experi⫺1,⌳s)ments and interpreted as being configurations of ( ␯ p 3/2⫺1
and ( ␯ p 3/2 ,⌳p) 关10–13兴. In addition, we have succeeded inclearly resolving four satellite peaks in the spectrum obtained with the thin target, thanks to the best energy resolution of 1.45 MeV 共FWHM兲. The microstructure between thetwo prominent peaks was discovered in the KEK-PS E140a
experiment 关14兴; two small peaks were observed at 2.6 and
6.9 MeV in the excitation energy, in which the higher one
had a broader width than expected from the energy resolution of 2 MeV. In the present experiment, the broad peak was
resolved into two peaks.

0.883
0.367
0.509
0.412
0.526
0.497
0.428
0.665
0.650
0.633
1.147
1.303
1.436
1.032
1.281
1.350
1.168
1.518
1.363
1.216

0.25
0.75
1.25
1.75
2.25
2.75
3.25
3.75
4.25
4.75
5.25
5.75
6.25
6.75
7.25
7.75
8.25
8.75
9.25
9.75

0.975
1.218
1.060
1.020
1.223
1.595
1.485
1.513
1.658
1.859
2.344
2.069
1.810
1.598
1.822
1.812
2.676
1.548
2.003
2.133

The spectrum was fitted with six Gaussian peaks 共#1–#6兲
and a quasifree component. In the fitting, the Gaussian
widths were constrained to be the same. The quasifree part,
representing a continuum spectrum in the unbound region,
was fitted with a first order polynomial which is an adequate
approximation in the narrow region near B ⌳ ⫽0; it was constrained to be 0 at B ⌳ ⫽0 and folded with the energy resolution. The results of the fitting are listed in Table X.
The angular distributions of the cross sections for the ob12
C specserved peaks were derived from the high-statistics ⌳trum obtained with the thick carbon target. Though it was
unfortunate the small peaks, which were resolved in the thintarget data, were somewhat smeared out in the thick target
data due to a little worse energy resolution of 1.8 MeV as
shown in Fig. 8共a兲, we fitted it with six Gaussian peaks in thesame way as the thin target data. The two results from thethin and thick target data on both excitation energy and crosssection were consistent with each other within the statisticalerrors. The obtained angular distributions are shown in Fig.
8共b兲 and in Table XI.
V. DISCUSSION
89
51
A. ⌳
Y and ⌳
V

1. Structure of heavy Λ hypernuclei
89
In ⌳Y, the energy separations ( ␦ E l ⬅b Rl ⫺b Ll ) between
the two peaks in each broad bump were determined to be
1.37⫾0.20, 1.63⫾0.14, and 1.70⫾0.10 MeV for the p, d,and f orbits. The systematic errors on the energy separations
were estimated to be less than ⫾0.1 MeV considering two

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PHYSICAL REVIEW C 64 044302

12
TABLE VI. List of the data points of the ⌳C spectrum shown in Fig. 7. The quoted values of ⫺B ⌳ show the centers of the bins.

⫺B ⌳
共MeV兲

¯␴ 2 – 14 °( ␮b/sr•0.25 MeV)

⫺B ⌳
共MeV兲

¯␴ 2 – 14 °( ␮b/sr•0.25 MeV)

⫺B ⌳
共MeV兲

¯␴ 2 – 14 °( ␮b/sr•0.25 MeV)

⫺B ⌳
共MeV兲

¯␴ 2 – 14 °( ␮b/sr•0.25 MeV)

⫺24.875⫺24.625⫺24.375⫺24.125⫺23.875⫺23.625⫺23.375⫺23.125⫺22.875⫺22.625⫺22.375⫺22.125⫺21.875⫺21.625⫺21.375⫺21.125⫺20.875⫺20.625⫺20.375⫺20.125⫺19.875⫺19.625⫺19.375⫺19.125⫺18.875⫺18.625⫺18.375⫺18.125⫺17.875⫺17.625⫺17.375⫺17.125⫺16.875⫺16.625⫺16.375⫺16.125⫺15.875⫺15.625⫺15.375⫺15.125

0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.010
0.000
0.000
0.000
0.000
0.025
0.000
0.037
0.020
0.008
0.008
0.000
0.000
0.000
0.000
0.008
0.000
0.000
0.018
0.000
0.000
0.000
0.000
0.016
0.000
0.011
0.017
0.000
0.034
0.000
0.009
0.000

⫺14.875⫺14.625⫺14.375⫺14.125⫺13.875⫺13.625⫺13.375⫺13.125⫺12.875⫺12.625⫺12.375⫺12.125⫺11.875⫺11.625⫺11.375⫺11.125⫺10.875⫺10.625⫺10.375⫺10.125⫺9.875⫺9.625⫺9.375⫺9.125⫺8.875⫺8.625⫺8.375⫺8.125⫺7.875⫺7.625⫺7.375⫺7.125⫺6.875⫺6.625⫺6.375⫺6.125⫺5.875⫺5.625⫺5.375⫺5.125

0.008
0.009
0.000
0.000
0.009
0.037
0.000
0.000
0.031
0.022
0.047
0.068
0.214
0.467
1.098
1.179
1.229
1.259
1.147
0.625
0.597
0.294
0.393
0.222
0.178
0.139
0.148
0.122
0.210
0.221
0.054
0.059
0.159
0.062
0.026
0.052
0.128
0.022
0.163
0.152

⫺4.875⫺4.625⫺4.375⫺4.125⫺3.875⫺3.625⫺3.375⫺3.125⫺2.875⫺2.625⫺2.375⫺2.125⫺1.875⫺1.625⫺1.375⫺1.125⫺0.875⫺0.625⫺0.375⫺0.125⫺0.125⫺0.375⫺0.625⫺0.875⫺1.125⫺1.375⫺1.625⫺1.875⫺2.125⫺2.375⫺2.625⫺2.875⫺3.125⫺3.375⫺3.625⫺3.875⫺4.125⫺4.375⫺4.625⫺4.875

0.294
0.253
0.168
0.239
0.174
0.116
0.186
0.205
0.236
0.108
0.207
0.130
0.086
0.200
0.537
0.369
0.477
0.652
1.533
1.409
1.303
1.065
0.727
0.807
0.556
0.649
0.882
0.601
0.826
0.508
0.281
0.248
0.228
0.387
0.521
0.671
0.348
0.449
0.448
0.475

5.125
5.375
5.625
5.875
6.125
6.375
6.625
6.875
7.125
7.375
7.625
7.875
8.125
8.375
8.625
8.875
9.125
9.375
9.625
9.875
10.12510.37510.62510.875
11.12511.375
11.62511.87512.12512.37512.62512.87513.12513.37513.62513.87514.12514.37514.62514.875

0.714
0.528
0.748
0.553
0.439
0.972
0.771
0.826
0.765
0.692
0.583
0.580
0.749
0.633
1.071
0.746
0.753
0.677
0.626
0.900
0.628
0.902
0.708
0.631
0.639
0.800
1.042
1.361
0.949
0.819
1.031
0.526
0.589
1.028
0.724
1.287
1.077
1.054
0.908
1.128

TABLE VII. Characteristics of the hypernuclear mass spectra, where ⌬E is the error of the definition of12
the ⌳C ground-state binding energy of 10.76 MeV.
Systematics
Scattering
angle

Energy precision
共MeV兲

Energy resolution
FWHM 共MeV兲

Cross section

SpectraTargetthickness
共g/cm2兲

Background level( ␮b/sr•MeV)

89
⌳Y
51
⌳V
12
⌳C

2.22
2.82
0.86

2⫺14°
2–14°
2–14°

⫾0.23⫹⌬E⫾0.23⫹⌬E⫾0.17⫹⌬E

1.65⫾0.10
1.95⫾0.10
1.45⫾0.06

⫾9%
⫾10%
⫾9%

⬍0.02
⬍0.02
⬍0.03

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PHYSICAL REVIEW C 64 044302

89
TABLE VIII. Results of the fitting for the ⌳Y spectrum. The
quoted errors are statistical.

Peaks
(F1)

B⌳
共MeV兲

l⫽0

23.11⫾0.10

l⫽1⫺L
l⫽1⫺R
l⫽2⫺L
l⫽2⫺R
l⫽3⫺L
l⫽3⫺R

17.10⫾0.08
15.73⫾0.18
10.32⫾0.06
8.69⫾0.13
3.13⫾0.07
1.43⫾0.07

Peaks
(F2)

Energy shift ( ␦ B)
共MeV兲

l⫽0
l⫽1
l⫽2

冎

FWHM
共MeV兲

冧

Cross sections
共␮b/sr兲0.60⫾0.06
2.00⫾0.22
1.38⫾0.19
5.10⫾0.31
3.52⫾0.25
6.87⫾0.33
6.79⫾0.31

1.65共 fixed兲

FWHM
共MeV兲

冎

4.18⫾0.07

Cross sections
共␮b/sr兲
0.18⫾0.06
1.83⫾0.14
6.17⫾0.28

3.24⫾0.15

12TABLE X. Results of the fitting for the ⌳C spectrum obtainedwith the thin target, where E x shows the excitation energy. The
quoted errors are statistical.

Ex
B⌳
ErrorsPeaks 共MeV兲 共MeV兲 共MeV兲
#1
#2
#3
#4
#5
#6

2.51
6.30
8.06
10.66
12.37

10.76 共fixed兲
8.25 ⫾0.17
4.46 ⫾0.11
2.70 ⫾0.19
0.10 ⫾0.04⫺1.61 ⫾0.09

FWHM
共MeV兲

冧

1.44⫾0.05

Cross sections
共␮b/sr兲
8.07⫾0.38
1.04⫾0.14
1.29⫾0.21
0.99⫾0.17
7.71⫾0.45
3.01⫾0.40

1/3with the parameters, V ⌳fm,
0 ⫽⫺30 MeV, R⫽1.1(A⫺1)⌳
a⫽0.60 fm, and V LS ⫽4 MeV, where the strength of the ⌳⌳) of 4 MeV gives rise to the splitting
spin-orbit potential (V LS
89
of about 1.5 MeV in the f orbit of ⌳Y. The global structure

effects: the possible systematic error in estimating the energy
resolution, and the linearity of the energy scale. The fact thatthe energy separations are getting wider as the angular momentum of the ⌳ orbit increases suggests that they are due to89
Y spectrum was
the spin-orbit splitting. In this regard, the ⌳compared with a theoretical one 关1,28兴 based on a distortedwave impulse approximation 共DWIA兲 calculation with asimple ⌳ single-particle potential of the Woods-Saxon form,as⌳2
U ⌳ 共 r 兲 ⫽V ⌳0 f 共 r 兲 ⫹V LS 共 ប/m ␲ c 兲

1 d f 共r兲ជl •sជ ,r dr

f 共 r 兲 ⫽ 兵 1⫹exp关共 r⫺R 兲 /a 兴 其 ⫺1 ,

共5.1兲

51
TABLE IX. Results of the fitting for the ⌳V spectrum, where
the quoted ratios show the strength ratios of F 2 and F 3 to F 1 . The
quoted errors are statistical.

Peaks
(F1)

B⌳
共MeV兲

FWHM
共MeV兲

Cross sections
共␮b/sr兲冧

1.15⫾0.10l⫽019.97⫾0.13

l⫽1⫺L
l⫽1⫺R
l⫽2⫺L
l⫽2⫺R

11.90⫾0.17
10.57⫾0.15
3.55⫾0.14
1.55⫾0.11

Peaks
(F2)

Energy shift ( ␦ B)
共MeV兲

FWHM
共MeV兲

Ratio (A l /a l )3.31⫾0.18

1.95共fixed兲

0.45⫾0.06

Energy shift ( ␦ B ⬘ )
共MeV兲

FWHM
共MeV兲

Ratio (A l⬘ /a l )6.57⫾0.21

3.46共fixed兲

1.13⫾0.23

Peaks
(F3)

1.95共 fixed兲

4.20⫾0.38共sum兲(L:R⫽1:1)
8.48⫾0.83共sum兲(L:R⫽1:1)

12
FIG. 8. 共a兲 ⌳C spectrum obtained with the thick carbon target.
共b兲 Angular distributions of kaons leading to the observed peaks forthe 12C( ␲ ⫹ ,K ⫹ ) reaction, derived from the above high-statistics
spectrum. The quoted errors are statistical only. The DWIA calculations by Itonaga are also shown as solid lines.

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PHYSICAL REVIEW C 64 044302

TABLE XI. List of the differential cross sections for the 12C( ␲ ⫹ ,K ⫹ ) reaction shown in Fig. 8共b兲. The
quoted errors are statistical only.
d ␴ /d⍀ 共␮b/sr兲

␪ lab 共deg兲

#1

#2

#3

#4

#5

#6

3
5
7
9
11
13

14.92⫾0.56
14.18⫾0.46
11.66⫾0.46
8.19⫾0.44
5.36⫾0.37
2.11⫾0.26

2.39⫾0.25
2.34⫾0.22
2.05⫾0.22
0.96⫾0.18
0.45⫾0.15
0.52⫾0.17

2.12⫾0.22
2.26⫾0.19
1.24⫾0.17
0.90⫾0.17
0.59⫾0.16
0.34⫾0.16

1.79⫾0.24
1.90⫾0.21
1.24⫾0.18
0.61⫾0.16
0.43⫾0.16
0.05⫾0.05

16.18⫾0.68
13.76⫾0.54
11.44⫾0.54
7.28⫾0.47
5.83⫾0.47
3.77⫾0.44

2.59⫾0.55
3.17⫾0.46
3.23⫾0.46
2.02⫾0.40
1.18⫾0.35
1.85⫾0.38

of the experimental spectrum is qualitatively reproduced wellby the calculation. However, in careful comparison, the intensity ratios (R l ⬅a Rl /a Ll ) between the two peaks in eachbroad bump determined to be 0.69⫾0.12, 0.69⫾0.06, and0.99⫾0.07 for the p, d, and f orbits from the fitting, weredifferent from those 共1, 1.6, and 1.9, respectively兲 expectedin the calculation which were mainly determined from thekinematics of the ( ␲ ⫹ ,K ⫹ ) reaction. The difference could be
explained with a small mixing between the two states,⫺1⫺1,⌳l l⫹1/2) and ( ␯ g 9/2,⌳l l⫺1/2), due to some residual( ␯ g 9/2
interaction 关29兴. However, we need more detailed theoreticalanalyses.
A possible alternative would be to interpret the peak on
the right side in each ⌳ orbit as a contribution of a neutron⫺1
neutron-hole state.
hole state different from the major 0g 9/2
In this case, it is a naive expectation that the energy separations ( ␦ E l ) would be a fixed amount corresponding to theneutron-hole energy difference. However, the fitting results
show the increase of the energy separations for higher angular momenta of the ⌳ orbits. In addition, the peak intensity
ratios (R l ) seem to be too large in this case, because thespectroscopic factors obtained from the neutron pickup90
Zr(p,d) 89Zr reaction 关26兴 show us that the strength of the
5/2⫺ excited state at 1.46 MeV in the excitation energy is
only ⬃30% relative to the ground state. Therefore it seems
that the observed double-peak structures cannot be explained
only by the assumption that they are formed from twoneutron-hole series.
51
V spectrum also had a similar structure to that ofThe ⌳89
the ⌳ Y spectrum. The widths of the bumps could be ex89
plained in the same way as the ⌳Y spectrum. Therefore thesplitting of ⌳ orbits with large angular momentum might bea general trend in heavy ⌳ hypernuclear systems. However,it should be noted that recent hypernuclear ␥-ray measure13
9
C and ⌳
Be strongly suggest the two-body ⌳-Nments in ⌳spin-orbit interaction, which causes the ⌳ single-particle
spin-orbit splitting, is too weak to explain the observed struc89
51
tures in ⌳Y and ⌳
V simply by the spin-orbit splitting
关30,31兴. The splittings obtained in the present analysis show
a somewhat weaker l dependence than expected from theoretical calculations, in assuming that the observed doublepeak structures are fully originated from the two-body ⌳-Nspin-orbit interaction.
In Table XII, the binding energies and cross sections of

89
51
the ground states of ⌳Y and ⌳V are compared with those⫹obtained from previous ( ␲ ,K ⫹ ) experiments at BNL 关12兴and at KEK 关15兴. It is noted that the binding energy of the89⌳ Y ground state obtained in the present analysis is about 1
MeV larger than those in the previous data. This apparent
difference is understandable by considering that the small⫺1,yields lying between the s and p bumps, coming from 1p 1/2⫺1⫺1
1 p 3/2 , and 0 f 5/2 , are separated from the ground-state peak in
the present measurement.

2. Λ effective mass in the nuclear medium
89
In the present measurement of ⌳Y, a series of ⌳ singleparticle energies in the very wide excitation energy range ofabout 24 MeV was obtained with high precision for the firsttime. The impact of the data was theoretically investigatedby Yamamoto 关32兴. He calculated the ⌳ single-particle ener89
Y based on a Brückner-Hartree-Fock-type calculagies of ⌳tion with density-dependent G matrices obtained for two
kinds of one-boson exchange potentials 共NHC-F and
NSC97f 关33,34兴兲, assuming the ⌳ spin-orbit splitting to reproduce the observed double-peak structures. In the calcula* ) at the normal nuclear mattertion, the ⌳ effective mass (M ⌳density and the ⌳ potential depth (U ⌳ ) are the main parameters to be fitted to reproduce the experimental energy levels.
From the fitting, the parameters were obtained to be* /M ⌳ ⫽0.72 and U ⌳ ⫽30.5 MeV for the case of theM⌳

TABLE XII. Comparisons of the obtained binding energies and89
51cross sections of ⌳Y and ⌳
V ground states with those in previous( ␲ ⫹ ,K ⫹ ) experiments. The quoted errors of the present results are
statistical.

89
⌳Y

B⌳
共MeV兲

Cross sections
共␮b/sr兲Present 共2–14 °兲
BNL 共10°兲 关12兴
KEK 共2–14 °兲 关15兴23.11⫾0.10
22.1⫾1.6
22.0⫾0.5

0.60⫾0.06
0.54⫾0.38
0.21⫾0.05

51
⌳V

B⌳
共MeV兲

Cross sections
共␮b/sr兲19.97⫾0.13
19.9⫾1.0

1.15⫾0.10
1.00⫾0.56

Present 共2–14 °兲
BNL 共10°兲 关12兴

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H. HOTCHI et al.

PHYSICAL REVIEW C 64 044302

NHC-F. Similar results were obtained for the case of the* /M ⌳ ⫽0.76 and
NSC97f with slightly different values of M ⌳U ⌳ ⫽30.8 MeV. The difference between the ⌳ effective
masses is caused by that between the space-exchange partsof the two potentials. Although there still remains a model
dependence of the determination of these parameters, the error coming from the experimental data is remarkably small
共⬃⫾0.01兲.
12
B. ⌳
C

The obtained binding energies and cross sections werecompared with a DWIA calculation based on aconfiguration-mixing shell model 关35,36兴. In addition to thetwo prominent peaks, the calculation predicts that two 1 ⫺states and a 2 ⫹ state are populated with non-negligible intensities, which are related to the 11C excited states at 2.0* ⫺ ⫻⌳s)1 ⫺(1/2⫺ ) and 4.8 (3/2⫺ ) MeV 关37兴; ( 11CJ⫽1/2
2 ,
⫺
⫹
11 *
11 *
( CJ⫽3/2⫺ ⫻⌳s)1 3 , and ( CJ⫽1/2⫺ ⫻⌳ p)2 3 . The obtained angular distributions of the cross sections are compared with the calculation 关38兴 in Fig. 8共b兲. The distribution
newly obtained for the #6 peak is in good agreement with thecalculation. The other distributions are also well reproducedby the calculation except for the #4 peak, as already discussed in the KEK-E336 data 关39兴. While the theoreticalcurve (1 ⫺3 ) for the #3 peak fits the measured distribution for
the #4 peak equally well, the #4 peak is peculiar in terms of
the excitation energy. As for the #2, #3, and #6 peaks, theenergy shifts from the corresponding core-excitation energies, which reflect the influence of a ⌳ hyperon on the core
nucleus, can be possibly explained with an appropriate ⌳-Ninteraction. However, the energy level of the #4 peak is toohigh to be explained as a simple core-excited state shiftedwith conventional ⌳-N interactions. A state that can correspond to the #4 peak does not appear within the calculationin the conventional shell-model space.
Recently, an idea of a parity-mixing intershell couplinghas been proposed by several theorists 关40– 43兴. In thismodel, core-nucleus excited states with different parities
coupled to different ⌳ orbits are mixed in 1ប␻ excited configurations. Although this extended shell-model calculation
12
C is still in progress, it has been reported by Motoba
for ⌳that a new core-excited state can be obtained at the position
consistent with the #4 peak 关43兴.

关1兴 T. Motoba, H. Bandō, R. Wünsch, and J. Žofka, Phys. Rev. C
38, 1322 共1988兲.
关2兴 H. Bandō, T. Motoba, and Y. Yamamoto, Phys. Rev. C 31, 265
共1985兲.
关3兴 A. Likar, M. Rosina, and B. Povh, Z. Phys. A 324, 35 共1986兲.
关4兴 W. Brückner et al., Phys. Lett. 62B, 481 共1976兲.
关5兴 M. May et al., Phys. Rev. Lett. 47, 1106 共1981兲.
关6兴 M. May et al., Phys. Rev. Lett. 51, 2085 共1983兲.
关7兴 M. Prakash and J. U. Lattimen, Nucl. Phys. A639, 433c
共1998兲.
关8兴 H. A. Thiessen, AGS proposal, 758, 1980.

VI. SUMMARYHigh quality excitation energy spectra of medium-heavy
89
51
12
Y, ⌳
V, and ⌳C, have been measured via⌳ hypernuclei, ⌳the ( ␲ ⫹ ,K ⫹ ) reaction with the best energy resolution so far
achieved by using the SKS spectrometer.
89
Y spectrum, characteristic bump structures reIn the ⌳flecting the ⌳ major-shell orbits were clearly observed. The
widths for the p, d, and f orbits were significantly wider than
expected from the energy resolution of 1.65 MeV 共FWHM兲
and became broader for the ⌳ orbits with higher angularmomenta. In particular, it seems that the widest bump in thef orbit split into two peaks. The energy separation in the forbit was determined to be 1.70⫾0.10⫾0.10 MeV. The
spectrum was reproduced reasonably well assuming thateach of the broad bumps was composed of two peaks. Froma fitting, a series of ⌳ single-particle energies from theground state to the higher ones in the very wide excitationenergy range of about 24 MeV was obtained with high precision. The obtained binding energy of the ground state was⬃1 MeV larger than those in the previous measurements.
51
89
V spectrum, a structure similar to that of ⌳Y
Also in the ⌳was observed; the bumps for the p and d orbits were significantly wider than expected from the energy resolution of
1.95 MeV and the widths became broader for the ⌳ orbits
with higher angular momenta. The spectrum was reproduced
89
Y spectrum.
well in the same way as the ⌳In order to get a clear interpretation on the observed finestructures in heavy ⌳ hypernuclear systems, further theoretical investigations are necessary.
12
In the ⌳C spectrum, new fine structure peaks related tocore-excited states were clearly resolved owing to the bestenergy resolution of 1.45 MeV.

ACKNOWLEDGMENTS

We are grateful to Professor K. Nakamura and the
KEK-PS staff for their support of the experiment. We also
acknowledge Professor T. Motoba, Professor Y. Yamamoto,
Professor K. Itonaga, Professor D. Lanskoy, and Dr. D. J.
Millener for valuable discussions on the experimental results. The authors thank Dr. R. E. Chrien and Dr. A. Rusek
for a careful reading of this manuscript.

关9兴 C. B. Dover, L. Ludeking, and G. E. Walker, Phys. Rev. C 22,
2073 共1980兲.
关10兴 C. Milner et al., Phys. Rev. Lett. 54, 1237 共1985兲.
关11兴 R. E. Chrien, Nucl. Phys. A478, 705c 共1988兲.
关12兴 P. H. Pile et al., Phys. Rev. Lett. 66, 2585 共1991兲.
关13兴 M. Akei et al., Nucl. Phys. A534, 478 共1991兲.
关14兴 T. Hasegawa et al., Phys. Rev. Lett. 74, 224 共1995兲.
关15兴 T. Hasegawa et al., Phys. Rev. C 53, 1210 共1996兲.
关16兴 T. Nagae, in Proceedings of the 23rd INS International Symposium, Tokyo, Japan, 1995, edited by S. Sugimoto and O.
Hashimoto, p. 175.

044302-14

SPECTROSCOPY OF MEDIUM-HEAVY ⌳ HYPERNUCLEI . . .

PHYSICAL REVIEW C 64 044302

关17兴 T. Fukuda et al., Nucl. Instrum. Methods Phys. Res. A 361,
485 共1995兲.
关18兴 K. H. Tanaka et al., Nucl. Phys. A450, 533c 共1986兲.
关19兴 S. Morinobu 共private communication兲.
关20兴 J. Myrhein and L. Bugge, Nucl. Instrum. Methods 160, 43
共1979兲.
关21兴 D. H. Davis, Nucl. Phys. A547, 369c 共1992兲.
关22兴 K. L. Brown et al., CERN 74-02, 1974.
关23兴 T. Hasegawa, Ph.D. thesis, University of Tokyo, INS-IM-15,
1994.
关24兴 C. B. Dover and G. E. Walker, Phys. Rep. 89, 1 共1982兲.
关25兴 CERN Program Library Entry W5013, GEANT.
关26兴 J. Kasagi et al., Phys. Rev. C 28, 1065 共1983兲.
关27兴 H. Ohmura et al., Phys. Lett. 73B, 145 共1978兲.
关28兴 T. Motoba, in Proceedings of the 23rd INS International Symposium 共Ref. 关16兴兲, p. 187.
关29兴 D. E. Lanskoy 共private communication兲.
关30兴 A. Sakaguchi, in Proceedings of the APCTP Workshop on
Strangeness Nuclear Physics (SNP’99), Seoul, Korea, 1999,
edited by Il-T. Cheon, S. W. Hong, and T. Motoba 共World
Scientific, Singapore, 2000兲, p. 231.

关31兴 H. Tamura et al., Nucl. Phys. A663, 481 共2000兲.
关32兴 Y. Yamamoto, in Proceedings of the APCTP Workshop on
Strangeness Nuclear Physics (SNP’99) 共Ref. 关30兴兲, p. 60.
关33兴 M. M. Nagels, T. A. Rijken, and J. J. deSwart, Phys. Rev. D
20, 1633 共1979兲.
关34兴 T. A. Rijken, V. G. J. Stoks, and Y. Yamamoto, Phys. Rev. C
59, 21 共1999兲.
关35兴 K. Itonaga, T. Motoba, and H. Bandō, Prog. Theor. Phys. 84,
291 共1990兲.
关36兴 K. Itonaga, T. Motoba, O. Richter, and M. Sotona, Phys. Rev.
C 49, 1045 共1994兲.
关37兴 F. Ajzenberg-Selove, Nucl. Phys. A248, 1 共1975兲.
关38兴 K. Itonaga 共private communication兲.
关39兴 T. Takahashi et al., Nucl. Phys. A670, 265 共2000兲.
关40兴 D. J. Millener, in New Vistas in Physics with High-Energy Pion
Beams, edited by B. F. Gibson and J. B. McClelland 共World
Scientific, Singapore, 1993兲, p. 19.
关41兴 A. Gal, in Proceedings of the 23rd INS International Symposium 共Ref. 关16兴兲, p. 23.
关42兴 L. Majling et al., Phys. Part. Nucl. 28, 101 共1997兲.
关43兴 T. Motoba, Nucl. Phys. A639, 135c 共1998兲.

044302-15

PHYSICAL REVIEW C 68, 065201 (2003)
89
Neutron energy spectra from the nonmesonic weak decay of 12⌳ C and ⌳ Y hypernuclei

J. H. Kim,* H. Bhang, M. J. Kim, Y. D. Kim,† and H. Park‡
Department of Physics, Seoul National University, Seoul 151-742, Korea

S. Ajimura, T. Kishimoto, S. Minami, T. Mori, A. Sakaguchi, Y. Shimizu, and M. Sumihama
Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan储

T. Endo, O. Hashimoto, T. Miyoshi,§ J. Nishida, T. Saito, Y. Sato, S. Satoh, T. Takahashi, and H. Tamura
Department of Physics, Tohoku University, Sendai, 980-8578, JapanT. Fukuda,¶ T. Nagae, H. Noumi, H. Outa,** and M. SekimotoHigh Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan

H. Hotchi†† and K. Tanida**
Graduate School of Science, University of Tokyo, Tokyo 113-0033, Japan

R. Sawafta
Department of Physics, North Carolina A&T State University, Greensboro, North Carolina 27411, USA

L. I. Tang
Department of Physics, Hampton University, Hampton, Virginia 23668, USA(Received 1 April 2002; revised manuscript received 23 May 2003; published 16 December 2003)
1289
We have measured the neutron spectra emitted in the weak decay of ⌳C and ⌳Y over the energy region
above 10 MeV in the 共␲+, K+兲 reaction with better statistics and an improved signal-to-background ratio. The
neutron yields in the nonmesonic weak decays were obtained from the spectra. By using the proton yield of
12⌳ C, the ⌫n/⌫p ratio was estimated to be 0.51±0.15 共stat兲 from the neutron-to-proton yield ratio for the firsttime, which suggests the ⌫n/⌫p ratio is significantly less than unity.

DOI: 10.1103/PhysRevC.68.065201

PACS number(s): 21.80.⫹a, 13.30.Eg, 13.75.Ev

I. INTRODUCTION

The study of the weak decay properties of ⌳ hypernuclei
is an important subject in strangeness nuclear physics. Forvery light hypernuclei, the mesonic decay mode 共⌫m:⌳→ ␲N, q⬃100 MeV/c兲 is the dominant process. As the mass
number increases, the nonmesonic weak decay (NMWD)
which consists mainly of the two one-nucleon 共1N兲 induced
channels (⌫p:⌳p→np and ⌫n:⌳n→nn, q⬃400 MeV/c) becomes dominant because of the Pauli blocking. In such aflavor-changing weak process, both parity-conserving and*Present address: Department of Physics, Chung-Ang University,
Seoul 156-756, Korea.
†
Present address: Department of Physics, Sejong University, Seoul
143-747, Korea.
‡
Present address: KRISS, Daejeon 305-600, Korea.
§
Present address: Department of Physics, University of Houston,
Houston, TX 77204-5506.
储
Present address: High Energy Accelerator Research Organization(KEK), Tsukuba, Ibaraki 305-0801, Japan.
¶
Present address: Laboratory of Physics, Osaka ElectroCommunication University, Neyagawa, Osaka 572-8530, Japan.
**Present address: RIKEN, Wako 351-0198, Japan.
††
Present address: Center for Proton Accelerator Facilities, Japan
Atomic Energy Research Institute, Tokai 319-1195, Japan.
0556-2813/2003/68(6)/065201(5)/$20.00

parity-violating partial widths can be measured since there isno possible contribution from the strong interaction. In the
nucleon-nucleon weak interaction, in contrast, the parityconserving partial width is masked by the overwhelming
strong interaction. Thus, the NMWD plays an important roleand gives us a unique opportunity for the study of baryonbaryon weak interactions.
One of the major issues in the study of the NMWD has
been the large discrepancy between the experimental values
and the theoretically calculated ones based on the one-bosonexchange model of the ⌫n/⌫p ratio. The experimental results
give values which are close to unity or larger [1–3], while
theoretical calculations predict rather small ratios 共0.1–0.5兲.
The difficulties in understanding the ⌫n/⌫p ratio have stimulated many theoretical approaches to solve this puzzle: theone-boson-exchange model including heavier mesons, thetwo-pion exchange model, and the quark model [4–7].
As for the experimental data, there have been large uncertainties mainly because ⌫n was not estimated directly fromthe neutron measurement, but indirectly from the proton
measurement. Furthermore, the energy threshold is rather
high 共30⬃40 MeV兲 because of the thick target used to enhance the coincidence rate. ⌫n was extracted from ⌫nm, as⌫n =⌫nm −⌫p, which in turn was estimated from the lifetime
measurement ⌫tot = ␶−1 subtracting the mesonic decay rate ⌫m,as ⌫nm =⌫tot −⌫m. Therefore, if the omitted mechanisms be-

68 065201-1©2003 The American Physical Society

PHYSICAL REVIEW C 68, 065201 (2003)

J. H. KIM et al.

low the detection threshold are not properly taken into account, any proton missed from this detection threshold tends
to be taken as neutron emission, which increases the ⌫n/⌫pratio. In this regard, there exist two important processes to beconsidered. One is the effect of final state interaction (FSI)
on the emitted nucleons in the NMWD. FSI reduces thehigh-energy component and enhances the low-energy component, and becomes more important in heavier hypernuclei.
The other process is the two-nucleon 共2N兲 induced process
共⌳NN→NNN兲 predicted in the theoretical calculation, whose
contribution was estimated as 20–30% of the free ⌳ width[8]. However, so far we have no experimental evidence of
this 2N process.
Therefore, an accurate measurement of neutron over thefull dynamic range is imperative. Further, we could reducethe detection energy threshold to as low as ⬃10 MeV since
neutrons suffer no energy loss in the target. It is quite important to obtain the whole energy spectral shape of the emitted
neutrons in order to estimate the contribution of FSI and the2N process, quantitatively. One consideration we should pay
attention to is the signal-to-background 共S/B兲 ratio in theneutron measurement when we reduce the energy threshold.
Szymanski et al. [1] measured the neutron spectra from the5
12
He and ⌳C in the 共K−, ␲−兲 reaction. However, it
NMWD of ⌳suffered from poor statistics and a large background since alarge number of ␲−’s associated with the K− beam and produced from K− decay in flight can easily produce the background neutron through ␲− absorption in the materials
around the target. In this regard, the 共␲+, K+兲 reaction is superior to the 共K−, ␲−兲 reaction to study the neutron yield of
the NMWD. We designed the present experiment KEK-PS
E369, the measurement of the neutrons in the NMWD of ⌳hypernuclei using the 共␲+, K+兲 reaction, aiming for an orderof magnitude improvement in statistics, and a similar improvement in S/B ratio keeping the detection energy threshold as low as possible.

II. EXPERIMENTThis is the second of our series of experimental investigations with the superconducting karon spectrometer (SKS)
spectrometer: so far we have reported the results on lifetimes[9] and proton energy spectra [3]. The measurement was carried out at the K6 beam line of the KEK 12-GeV PS with the共␲+, K+兲 reaction at 1.05 GeV/c on 12C and 89Y targets, thethicknesses of which are 1.739 g/cm2 and 2.22 g/cm2, re1289
C and ⌳
Y
spectively. The hypernuclear mass spectra of ⌳were produced by reconstructing momenta and trajectories
with the beam line spectrometer and the SKS spectrometer[10].
The neutral decay particles (neutron and ␥) emitted from⌳ hypernuclei were measured with a neutral detection system as shown in Fig. 1. It consisted of a time-zero counter
T0, charged particle veto counters T1 and T2, and neutron
counter arrays T3. For the neutron measurement, four sets ofarrays composed of six layers of scintillators with a total
thickness of 30 cm were installed 68 cm from the center of
the target. Table I shows the specifications of each detector.

FIG. 1. Schematic view of the neutral detection system (topview) used in the neutron measurement.
III. DATA ANALYSIS

We set the gates for decay measurement in the hyper12
89
nuclear mass spectra for ⌳C and ⌳
Y [10] as shown in Fig. 2.
These gates represent the ⌳-bound regions. The particle
identification of neutron and ␥ was performed by using thetime-of-flight (TOF) technique. The neutron TOF from thedecay vertex to T3 is
TOFn = 共T3 − T0 − ␶HY 兲 − TOFb ,共1兲

where ␶HY is the lifetime of ⌳ hypernuclei, T3 is the timingof T3, T0 is the time zero, and TOFb is the TOF of the12
C is 231± 15 ps and we takebeam pion. The lifetime of ⌳89
the lifetime of ⌳ Y as 215 ps, which is the saturated lifetime of heavy ⌳ hypernuclei 关9兴. The neutron velocity ␤and the neutron kinetic energy En = 关1/冑共1 − ␤2兲 − 1兴mn wereobtained using TOFn.
12
89
C and (b) ⌳Y are shown in Fig. 3
The 1/␤ spectra of (a) ⌳with 2 MeVee (MeV electron equivalent) threshold. The
neutron gate corresponds to 5⬍En ⬍150 MeV. The neutronenergy resolution is about 10 MeV (full width at half maximum) at 75 MeV. The background was estimated in the region below the ␥ peak and above the neutron gate in the
TABLE I. Specifications of the decay counters.
Active area Thickness Distancea
共cm2兲
(cm)
(cm)
T0
T1
T2
T3
a

12⫻12
48⫻25
140⫻10080⫻100

0.6
0.6
2.0
30.0

Distance from the target.

065201-2

30.0
15.0
60.0
68.0

Three segments
Eight segments
14 Segments
Four arrays with six layers

PHYSICAL REVIEW C 68, 065201 (2003)

NEUTRON ENERGY SPECTRA FROM THE NONMESONIC…

FIG. 2. Hypernuclear mass spectra as a function of ⌳ binding
1289
C and (b) ⌳Y with the neutral particle coincienergy −B⌳ for (a) ⌳dence.

TOFn spectrum where the random background should be uniform. The estimated backgrounds within the neutron gate
1289
C and ⌳Y, respectively. The
were about 2% and 3% for ⌳S/B ratio was greatly improved compared to the previous
BNL experiment [1] [Fig. 3(c)].
The number of neutrons per NMWD, Nn, can be writtenas
Nn = Y n/共Y HY bnm␧n⍀n兲,

共2兲where Y HY is the number of hypernuclei produced in thegated region and Y n is the number of neutrons measured
by T3. The acceptance of T3, ⍀n, was estimated as
共11.4± 0.3兲% by GEANT-based Monte Carlo simulation.
12
C was evaluThe nonmesonic branching ratio bnm for ⌳ated as 0.727±0.059 from the mesonic branching ratios
89[11,12] and that for ⌳Y 0.996 from the theoretical ratio (SeeTable III in Ref. [13]). There is a non-negligible contribution

12
FIG. 4. Neutron energy spectra per NMWD of (a) ⌳C and (b)89
⌳ Y. Errors are statistical.

of the background neutrons produced through the absorptionof ␲− in the materials around the target emitted from mesonic12
decays of ⌳C. It was estimated using the yield of neutrons
produced in the same way through the absorption of ␲− emitted from the quasifree ⌳ decay 共⌳→ ␲−p兲. Here, we assumed
that the neutron energy spectra are same. The neutron efficiency ␧n was calculated by Monte Carlo simulation DEMONS[14], which is applicable to a multielement neutron detector
based on CECIL [15]. We have compared the calculations
with various existing experimental results [16–19] and foundthat the integrated yields in the energy region above 10, 20,
30, and 40 MeV were well reproduced within 6% error level.
12
The neutron energy spectra per NMWD of (a) ⌳C and (b)89
⌳ Y are shown in Fig. 4. We should note that the two spectral
shapes are very similar above 30 MeV and decline smoothly
toward the high-energy end, while an enhancement in the89Y
low-energy region below 30 MeV is observed in the ⌳spectrum. In Table II, we summarize the number of neutrons
1289
C and ⌳Y in the energy region above 10,
per NMWD of ⌳20, 30, and 40 MeV. As the threshold energy increases, the89number of neutrons per NMWD of ⌳Y becomes smaller
12
compared to that of ⌳ C. This may be interpreted as the effectof FSI.
IV. DISCUSSION

Hashimoto et al. [3] recently reported the energy spectra
1228
of emitted proton in the weak decay of ⌳C and ⌳
Si which
12
89
TABLE II. Number of neutrons per NMWD of ⌳C and ⌳Y.
Errors are statistical and systematic. Systematic errors are due to theerror in neutron efficiency and that of bnm.

1289FIG. 3. 1/␤ spectra of (a) ⌳C and (b) ⌳Y with 2 MeVee thresh12
old. (c) shows the previous BNL results [1] for ⌳C ground-stateregion with 10 MeVee threshold.

En ⬎10 MeV
En ⬎20 MeV
En ⬎30 MeV
En ⬎40 MeV

065201-3

Number of neutrons per NMWD
12
⌳C

89
⌳Y

1.36±0.11±0.16
1.12±0.10±0.13
0.92±0.09±0.10
0.80±0.08±0.09

1.36±0.08±0.09
0.89±0.06±0.06
0.67±0.06±0.04
0.58±0.05±0.04

PHYSICAL REVIEW C 68, 065201 (2003)

J. H. KIM et al.

were compared to the FSI model calculations [intranuclear
cascade (INC) calculations] of Ramos et al. [20]. The ⌫n/⌫p
12
28+0.09+0.20
ratio was obtained for ⌳C and ⌳
Si to be 1.17−0.08−0.18
and+0.13+0.27
1.38−0.11−0.25, respectively, assuming that the NMWD occurs
through the 1N process only. Unfortunately, the erratum of
the Ref. [21] on INC calculation was reported later and theresults were modified to be 0.87±0.09±0.21 and+0.13+0.25
1228[22] for ⌳C and ⌳
Si, respectively. Since we
0.79−0.11−0.24
12
now have both the neutron and proton spectra of ⌳C, itwould be desirable to estimate ⌫n/⌫p ratio directly from theexperimental data avoiding such theoretical model dependence.
Thus, we would like to estimate the ⌫n/⌫p ratio from the12neutron-to-proton yield ratio in the case of ⌳C. Here, we
assume the dominance of the 1N process 共⌳N→NN兲 in thenonmesonic weak decay. Then, the emitted neutron (proton)
numbers per NMWD, Nn共p兲, can be written as
Nn = 共2rn + r p兲f n + r pgn ,共3a兲

N p = 共2rn + r p兲g p + r p f p ,共3b兲

where rn共=⌫n/⌫nm兲 and rp共=⌫p/⌫nm兲 are the fraction ratios of
the neutron and proton induced channels out of the NMWD
and rn +rp =1. f n共p兲 is the loss factor of neutron (proton) fromthe region counted due to FSI and gn共p兲 the crossover influxof neutron (proton) from proton (neutron) due to FSI. If there
were no FSI, f n共p兲 would be one and gn共p兲 zero. Here, we haveassumed f n = f p = f and gn =gp =g, considering the similar spectral shapes in the energy region, say above 40 MeV, of theneutron and proton spectra, which reflect the isospin independence of the strong interaction and isospin symmetric
12
C. By using Eq. (3b), the ⌫n/⌫p ratio
propagating medium ⌳has been estimated from proton measurements comparedwith some help of theoretical estimations for the factors f
and g. However, the estimation would be largely affected by
the uncertainty of theoretical calculations and also experimentally by the error on ⌫nm arising from the errors on ⌫total
and the mesonic decay branching ratio.
Using the ratio between Nn and Np, the ⌫n/⌫p ratio can be
written as⌫n 共Nn/N p − 1兲共1 + ␣兲=.
⌫p
2共1 − ␣Nn/N p兲

共4兲

It is noted that we have only one parameter, ␣ = g/f, toobtain the ⌫n/⌫ p ratio. Further, this method has an advantage that the ambiguity on ⌫nm is canceled out.
Now, we can estimate the ⌫n/⌫p ratio from the yield ratioof neutron to proton, Nn/Np. Unfortunately, the proton energy
in Ref. [3] is only shown in the detected energy scale, whichmeans that the energy of the emitted protons is degraded due
to energy losses in the target material and other detectormaterials. Thus, part of low-energy protons below the detection threshold were lost from the data. In contrast, thepresent neutron energy is practically the emission energy ofthe neutron coming out of the nucleus, and the energy range
is much wider than that of the proton spectrum in Ref. [3]. In
order to compare the two spectra, we need to transform either the proton spectrum into the bare energy scale of neu-

TABLE III. Experimental results and recent calculations of the12⌫n/⌫p ratio for ⌳C. The “1N only” and “1N and 2N” stand for theprocesses considered to estimate the ratios.
⌫n/⌫p
1N only

Refs.
1N and 2N
Experiment0.51±0.15 共stat兲
0.45±0.14 共stat兲+1.12
1.33−0.81+0.32
1.87±0.59−1.00+0.09+0.20
1.17−0.08−0.18
0.87±0.09±0.21+0.10+0.22
0.96−0.09−0.21+0.11+0.23
0.60−0.09−0.21

Present 共␣ =0.11兲
Present 共␣ =0.076兲
Szymanski et al. [1]Noumi et al. [2]Hashimoto et al. [3]Sato et al. [22]Theory0.288⬃0.341
0.53
0.368

Parreño and Ramos [4]Jido et al. [5]Itonaga et al. [6]tron or neutron spectrum to the degraded energy scale ofproton. It would be conceptually simpler to unfold the material effect from the proton spectrum in order to compare
directly to that of neutron. However, due to the thick target
used and the limited statistics of the proton spectrum in Ref.
[3], it was impossible to do such an inverse transformation
uniquely without knowing the whole shape of the protonenergy spectrum before the energy losses. We need to rely on
the spectral shapes from some model calculation, such as
INC, of FSI effect on the emitted nucleons.
In this discussion, we would like to take the maximum
advantage of the availability of both spectra of proton andneutron in order to avoid the model dependence on the conclusion. What we need is just one number, Nn/Np, the ratio of
the integrated neutron-to-proton numbers per NMWD in thesame dynamic range. Therefore we degrade the energy ofneutrons as if they were protons, we can adjust the energy
scale same as the proton energy scale in Ref. [3], and takeaccount of the same detection condition. Then, the neutron
number per NMWD over 40 MeV in the degraded energy
scale became 0.69±0.08 共stat兲 which is to be compared withthe corresponding proton number 0.40±0.02 共stat兲 [3]. Thus,
we have obtained the Nn/Np ratio in a common energy threshold to be 1.73±0.22 共stat兲, although we could not tell thethreshold energy explicitly.
In order to determine the ⌫n/⌫p ratio, we need to know ␣.
It is about 0.11 which we adopted from the FSI calculationon the proton energy spectrum by Ramos [23]. Then, the⌫n/⌫p ratio is obtained to be 0.51±0.15 共stat兲 from Eq. (4).
In order to see the effect of ␣ magnitude on the ⌫n/⌫p ratio,
we obtained the ratio of 0.45±0.14 共stat兲 with a different ␣,
0.076, estimated from the INC calculation [24]. We note thata 30% decrease of ␣ changes the ratio by 0.06 which is muchsmaller than the current statistical uncertainty of 0.15. Table
III summarizes the present result of the ⌫n/⌫p ratio alongwith the recent theoretical calculations.
This is the first experimental result suggesting the dominance of the proton channel over the neutron channel in the
NMWD of ⌳ hypernucleus. Recent theoretical calculations

065201-4

NEUTRON ENERGY SPECTRA FROM THE NONMESONIC…

PHYSICAL REVIEW C 68, 065201 (2003)

TABLE IV. List of the data points of the neutron energy per
1289
NMWD of ⌳C and ⌳Y as shown in Fig. 4. The quoted values of En
show the centers of the bins and the errors are statistical.

⌫n/⌫p requires further studies on the FSI effect and the contribution from the 2N process. In the experimental side, coincidence measurements of the two emitted nucleons 共n+p/n+n兲 would provide very useful information. In fact,
such experiments are now in progress at KEK [25,26].

En
(MeV)

Number of neutrons per NMWD
12
89
⌳C
⌳Y

15
25
35
45
55
65
75
85
95
105
115
125
135
145

0.24±0.04
0.20±0.04
0.13±0.03
0.15±0.03
0.14±0.03
0.13±0.03
0.11±0.03
0.10±0.03
0.07±0.03
0.05±0.02
0.03±0.02
0.02±0.01
0.01±0.01–

V. SUMMARY

0.46±0.05
0.22±0.03
0.09±0.02
0.12±0.02
0.09±0.02
0.09±0.02
0.08±0.02
0.07±0.02
0.06±0.02
0.02±0.01
0.01±0.01
0.02±0.01

We have measured the neutron spectra emitted in the
1289
weak decay of ⌳C and ⌳
Y with better statistics and an improved signal-to-background ratio. Such improvement of theneutron spectrum and the recent accurate measurement of the
proton spectrum [3] made it possible to estimate the ⌫n/⌫pratio to be 0.51±0.15 共stat兲, from the neutron-to-proton yieldratio of 1.73±0.22 共stat兲. It suggests that the ⌫n/⌫p ratio of
NMWD is significantly less than unity.
ACKNOWLEDGMENTS

0.01±0.01

The authors would like to express their deep gratitude toProfessor K. Nakamura and the KEK-PS staff for their support of the experiment. We are also grateful to Professor Y.
Doi, K. Aoki, and the Cryogenics group. Authors (J.H.K. andH.B.) acknowledge support from KOSEF (Grant No. 20002-11100-004-4 ) and KRF (Grant No. 2000-015-dp0084).

[4–6] on the NMWD reported greatly increased values of the⌫n/⌫p ratio reaching 0.5 and now agree reasonably well with
our measured result of proton dominance. We note that the⌫n/⌫p ratio rederived from the proton spectrum comparing
with the recently corrected INC calculation [21] also showeda value smaller than unity, namely, 0.87±0.09±0.21 [22].
However, we note that the low-energy portion of the spectra has not been well reproduced in the existing theoretical
calculations. Further conclusive determination of the ratio

Table IV is the list of the data points of the neutron energy
1289
spectra per NMWD of ⌳C and ⌳Y as shown in Fig. 4. The
quoted values of En show the centers of the bins and theerrors are statistical.

[1] J. J. Szymanski et al., Phys. Rev. C 43, 849 (1991).
[2] H. Noumi et al., Phys. Rev. C 52, 2936 (1995).
[3] O. Hashimoto et al., Phys. Rev. Lett. 88, 042503 (2002).
[4] A. Parreño and A. Ramos, Phys. Rev. C 65, 015204 (2001).
[5] D. Jido, E. Oset, and J. E. Palomar, Nucl. Phys. A694, 525(2001).
[6] K. Itonaga, T. Ueda, and T. Motoba, Phys. Rev. C 65, 034617(2002).
[7] K. Sasaki, T. Inoue, and M. Oka, Nucl. Phys. A669, 331(2000).
[8] A. Ramos, E. Oset, and L. L. Salcedo, Phys. Rev. C 50, 2314
(1994).
[9] H. Bhang et al., Phys. Rev. Lett. 81, 4321 (1998); H. Park etal., Phys. Rev. C 61, 054004 (2000).
[10] H. Hotchi et al., Phys. Rev. C 64, 044302 (2001).
[11] Y. Sato et al., Nucl. Phys. A691, 189c (2001).
[12] A. Sakaguchi et al., Phys. Rev. C 43, 73 (1991).
[13] W. M. Alberico, A. De Pace, G. Garbarino, and A. Ramos,
Phys. Rev. C 61, 044314 (2000).
[14] R. C. Byrd, P. L. McGaughey, W. C. Sailor, R. C. Hammock,and Y. Yariv, Nucl. Instrum. Methods Phys. Res. A 313, 437(1992).

[15] R. A. Cecil, B. D. Anderson, and R. Madey, Nucl. Instrum.
Methods 161, 439 (1979).
[16] M. W. Mcnaughton, F. P. Brady, W. B. Broste, A. L. Sagle, andS. W. Johnsen, Nucl. Instrum. Methods 116, 25 (1974).
[17] J. C. Young, J. L. Romero, F. P. Brady, and J. R. Morales,
Nucl. Instrum. Methods 68, 333 (1969).
[18] D. G. Crabb, J. G. Mcewen, E. G. Auld, and A. Langsford,
Nucl. Instrum. Methods 48, 87 (1967).
[19] G. Betti, A. DeL Guerra, A. Giazotto, M. A. Giorgi, A. Stefanini, D. R. Botterill, D. W. Braben, D. Clarke, and P. R.
Norton, Nucl. Instrum. Methods 135, 319 (1976).
[20] A. Ramos, M. J. Vicente-Vacas, and E. Oset, Phys. Rev. C 55,
735 (1997).
[21] A. Ramos, M. J. Vicente-Vacas, and E. Oset, Phys. Rev. C 66,
039903(E) (2002).
[22] Y. Sato et al., Phys. Rev. C (submitted).
[23] A. Ramos (private communication).
[24] H. Bhang, M. J. Kim, and J. H. Kim, in Hadrons and Nuclei,edited by Il-Tong Cheon, Taekeun Choi, Seung-woo Hong, andSu Houng Lee, AIP Conf. Proc. No. 594 (AIP, Melville, NY,
2001), p. 171.
[25] H. Outa, KEK PS experiment E462 (2000).
[26] H. Bhang, KEK PS experiment E508 (2002).

APPENDIX

065201-5

Eur. Phys. J. A 18, 283–287 (2003)
DOI 10.1140/epja/i2002-10217-3

THE EUROPEAN
PHYSICAL JOURNAL A

Progress in hypernuclear physics
T. Nagaea
High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
Received: 30 September 2002 /c Società Italiana di Fisica / Springer-Verlag 2003
Published online: 22 October 2003 – 
Abstract. The recent progress in hypernuclear physics is discussed in this paper. The topics include recent
results on hypernuclear gamma-ray spectroscopy, non-mesonic weak decay, and double-Λ hypernuclei.
Future prospects at the 50 GeV PS are also discussed.
PACS. 21.80.+a Hypernuclei – 25.80.-e Meson- and hyperon-induced reactions

1 Introduction
Since the 1980s, the Alternating-Gradient Synchrotron(AGS) in Brookhaven National Laboratory (BNL) and the
12 GeV proton synchrotron (PS) in KEK have played avery important role in Hypernuclear Physics by providing
high-intensity K − and π ± beams. The (π + , K + ) and (K − ,π − ) reactions were used for the production of Λ hypernuclei, and the (K − , K + ) reaction for the investigation on
the strangeness S = −2 systems, such as double-Λ hypernuclei, Ξ − hypernuclei, and the hypothetical H dibaryon.
These experiments at BNL-AGS and KEK-PS have extended the scope of hypernuclear physics to the physicsof hadron many-body systems with strangeness degrees of
freedom, i.e. Strangeness Nuclear Physics.
Recently, Jeﬀerson Laboratory (JLab) started hypernuclear spectroscopy with a diﬀerent reaction of (e, e K + ).
In 2007, the new 50 GeV proton synchrotron, now in construction in Japan, will be in operation. Then, it will be
the opening of a new era in Strangeness Nuclear Physics.

2 Recent topics
2.1 Spectroscopy of Λ hypernuclei
The spectroscopy of Λ hypernuclei has been successfully
carried out in KEK by using the SKS spectrometer [1].
An overview of the measurements with the (π + , K + ) reactions was given in ref. [2]. The SKS spectrometer improved
the energy resolution in Λ hypernuclear spectroscopy from
3–4 MeVFWHM to ∼ 2 MeV, and further improved it to
1.45 MeV in the recent measurement [3]. The energy resolution resolved the major shell structure of Λ-orbits up++
to 208
Λ Pb [4] owing to the selectivity of the (π , K ) reaction due to the high-momentum transfer (∼ 350 MeV/c)ae-mail: tomofumi.nagae@kek.jp

of the reaction. It also revealed, for the ﬁrst time, several
core-excited states of 12
Λ C [4,3]. Some of them were notsimply explained in naive weak-coupling structure, and it
was suggested that the parity-mixing intershell couplings
mediated by a Λ-particle should be taken into account [5].
The great progress in Λ hypernuclear spectroscopy hastaken place with a success of a hypernuclear γ-ray measurement by using a germanium (Ge) detector system,
called Hyperball, in KEK [6]. The detector consisted offourteen N -type coaxial Ge detectors of 60% relative eﬃciency. It covered 15% of the 4π sr. The energy resolutionof 3–4 keVFWHM was achieved at 1.33 MeV. It is a great
improvement. So far, a few hypernuclear γ-rays were wellidentiﬁed with NaI counters in worse energy resolutions.
++
The measurement observed the spin-ﬂip M 1( 32 → 12 )7
γ-ray at 691.7±0.6(stat)±1.0(syst) keV in Λ Li, for the ﬁrst
time. The energy splitting provided a stringent constraint
to the strength of the ΛN spin-spin interaction.
++
It also observed the E2( 52 → 12 ) γ-ray transition at
2050.1 ± 0.4(stat)±0.7(syst) keV. Because of the good energy resolution of the Hyperball, we could measure the life+
time of the 52 state from the peak shape analysis, the socalled Doppler-shift attenuation method [7]. The large momentum transfer of the (π + , K + ) reaction (∼ 350 MeV/c)makes the stopping time of the recoiling 7Λ Li (∼ 13 ps)+comparable to the expected lifetime (3–10 ps) of the 52
state. The observed γ-ray peak at 2050 keV had two components; a sharp peak and a Doppler-broadened tail part.
The sharp peak corresponds to the γ-rays emitted after
the stopping of the recoiling 7Λ Li. From the two-component+
ﬁtting by changing the lifetime of the 52 state, the life+0.9
time was obtained to be 5.8−0.7± 0.7 ps, from which the
reduced transition probability, B(E2), was estimated to+0.5 2be 3.6 ± 0.5−0.4
e fm4 . This value is about one-third of
the B(E2) of the corresponding E2(3+ → 1+ ) transitionin 6 Li. The B(E2) is very sensitive to size contraction; in

284

The European Physical Journal A# of neutrons/# of hypernuclei

Ethr = 2 MeVee
10x100x2 cm3

R=60 cm

4x25x0.6 cm3
R=15 cmπ+Preliminary

KEK E369
89

LY

12

LC

Neutron Kinetic Energy (MeV)K+

Fig. 2. The neutron energy spectra with a 2 MeVee threshold89
for 12
Λ C and Λ Y. The vertical scale is normalized to the numberof hypernuclei in the bound region.
n

20x100x5 cm3(6 layer)

Fig. 1. Detector setup for the coincidence measurement of
neutrons and protons emitted from non-mesonic weak decays
in KEK-PS E369.

this case, the shrinkage of the 6 Li core in the 7Λ Li hypernucleus.
The further γ-ray measurements in p-shell Λ hypernuclei are in progress at BNL-AGS with the (K − , π − )reactions by using the Hyperball.
2.2 Weak decay of Λ hypernuclei
The weak decay properties of a Λ-hyperon in hypernuclei
is another interesting subject. It is well known that themesonic free Λ decay(Λ → πN ) is strongly suppressed because of the Pauli blocking and the dominant decay modeis the non-mesonic decay(ΛN → N N, ΛN N → N N N ).
However, the present experimental data have too large
ambiguities on the total decay rate and the neutron emission probability to pin down the reaction mechanism.
In the study of non-mesonic weak decays of Λ hypernuclei, a large ratio of Γn (Λn → nn)/Γp (Λp → np) ∼1 has
been a puzzle not to be simply explained. The importanceof two-nucleon–induced weak decay (ΛN N → N N N ) was
suggested in ref. [8] related to this puzzle. However, available data were not enough to distinguish the one-nucleon–
induced decay and two-nucleon–induced one. For example, the energy spectra of emitted nucleons should havediﬀerent shapes in two decay processes [9]. An enhancement of the low-energy component is expected in the two-nucleon–induced process. The proton measurements so
far done, however, usually had high energy thresholds of
40–50 MeV, which lost sensitivity to detect the low-energy
component from the two-nucleon–induced process, if it existed. Therefore, we carried out the neutron measurement,
in which the detection energy threshold could be reduced
to ≤10 MeV. The quality of the neutron measurements
so far has been very bad in statistics and signal-to-noise
ratio.
The experimental setup for coincidence measurementsof neutrons and protons emitted from non-mesonic weak
decays of 89
Λ Y is shown in ﬁg. 1. Since the mesonic weakdecay rate is expected to be negligibly small, all kinds of
weak-decay products were measured in this setup.
The emitted timing of protons was measured with the
T1 counter located 15 cm apart from the target. The ﬂight
time diﬀerence of protons was corrected with the velocity
measured from the time of ﬂight between the T1 and T2
counters. The overall timing resolution for protons wasfound to be ∼ 150 ps (rms).
Neutral particles, such as γ’s and neutrons, were measured in the T3 counter with T1 and T2 as veto counters.
The T3 counter consisted of six layers of a plastic scintillator (BC420) of 20[W]×100[L]×5[T] cm3 . The velocity
of a neutral particle was obtained with a time-of-ﬂight
measurement between T0 and T3 counters. The relative
timing of all 24 neutron counters was adjusted with theprompt γ’s from the (π + , pX) reaction. The typical resolution for the prompt γ’s was ∼ 200 ps (rms).
The time-of-ﬂight spectra of neutral particles in coinci89
dence with bound states of 12
Λ C and Λ Y showed that then/γ separation was very clean and we had a very goodsignal-to-noise ratio for neutrons. Gated to the neutroncomponent, the energy spectra of neutrons from 12
Λ C and89
Λ Y are obtained as shown in ﬁg. 2.
Here, we notice that there exists no prominent bump
structure around ∼ 75 MeV expected from the onenucleon–induced decay process (ΛN → N N ) even for 12
Λ C,

T. Nagae: Progress in hypernuclear physics

while the total shape seems to be very similar to that for
the two-nucleon–induced process [9]. The recent measurement of the neutron spectrum from 5Λ He [10] also shows avery similar shape with that for 12
Λ C. This suggests that
rescattering eﬀects after the one-nucleon–induced decay
were unexpectedly large and/or the two-nucleon–induced
decay plays a dominant role in the non-mesonic weak decay.

285

A letter of intent for experiments on strangeness nuclear physics [16] well summarizes the initial experimental programs in this ﬁeld. Among the various topics discussed in the letter of intent, I introduce two interesting
programs: the investigation of new hadronic many-bodysystems with strangeness S = −2 and high-resolution γray spectroscopy.
3.1 Spectroscopic study of S = −2 systems

2.3 Double-Λ hypernuclei
There has been three emulsion events which showed theexistence of double-Λ hypernuclei [11–13]. However, these
events had some ambiguities on interpretation etc., andthe ground-state binding energy was not determined definitely.
In 2001, two new observations were reported from
BNL-AGS and KEK-PS.
The KEK-PS E373 experiment [14] reported a doublehyperfragment event in a hybrid-emulsion measurement.
The event is uniquely identiﬁed as the sequential decay of6−ΛΛ He emitted from a Ξ hyperon nuclear capture at rest,
12

C + Ξ − → 6ΛΛ He +4 He + t ,65−ΛΛ He → Λ He + p + π .

The binding energy of 6ΛΛ He has been measured to be+0.18
MeV, for the ﬁrst time. It corresponds to
7.25±0.19−0.11+0.18
MeV, where ∆BΛΛ is deﬁned
the ∆BΛΛ = 1.01±0.20−0.11
A
Z)
=
B
(
Z)
− 2BΛ (A−1
Z). It means theby ∆BΛΛ (A
ΛΛ ΛΛ
ΛΛ
Λ
Λ-Λ interaction is weakly attractive. Two of the past measurements gave a value of ∆BΛΛ to be about 4.5 MeV.
Thus, the new value is smaller than these.
The other experiment at BNL-AGS [15] demonstrated
the production of double-Λ hypernuclei in the (K − , K + )reaction on 9 Be. It used a quite new technique to identify
double-Λ hypernuclei by using a Cylindrical Drift chamber System (CDS). In the CDS, two π − ’s emitted throughsequential weak decays of hypernuclei were detected in agood momentum resolution. The result indicated the production of a signiﬁcant number of the double-Λ hypernucleus 4ΛΛ H and the twin hypernuclei 4Λ H and 3Λ H.

3 Future prospects at the 50 GeV PS
The 50 GeV proton synchrotron (PS) is now under construction in the Joint Project between KEK and Japan
Atomic Energy Research Institute (JAERI). The average beam current is 15.7 µA, which will produce theworld’s highest-intensity K − beams. The construction will
be completed early in 2007.
In the experimental area for the slow-extracted beam,
two secondary beam lines, K1.8 and K1.1, are proposed
for the studies of strangeness nuclear physics; the former
for the S = −2 systems, and the latter for the S = −1
systems.

The high-intensity K − beam at ∼ 1.8 GeV/c available
at the 50 GeV PS is quite unique to open a new frontier of Strangeness Nuclear Physics in the spectroscopic
studies of strangeness S = −2 systems. This is not only
a step forward from the S = −1 systems as a naturalextension, but also a signiﬁcant step to explore the multistrangeness hadronic systems; in the course of the limit,
strange hadronic matter (S = −∞) in the core of a neutron star is our concern. Also, it is important to extract
some information on ΞN and Λ-Λ interactions from thespectroscopic data.
The (K − , K + ) reaction is one of the best tools to
implant the S = −2 through an elementary process
K − +p → K + +Ξ − , the cross-section of which in the forward direction has a broad maximum around this energy.
At present, the experimental information on the S =−2 systems mainly comes from several emulsion data inlimited statistics. As for the Ξ hypernuclei, there exist
some hints of emulsion events for their existence. However, it is still not conclusive. Some upper limits on the
Ξ-nucleus potential have been obtained from the production rate in the bound region of a Ξ hypernucleus via the(K − , K + ) reaction.
The energy diﬀerence between the (Ξ − p) system andthe (ΛΛ) system is only 28.3 MeV in free space. Therefore,a relatively large conﬁguration mixing between Ξ − +Aand ΛΛ+(A − 1) states is suggested. It is very interesting to investigate whether the single-particle picture of
Ξ − is valid or not in such a system.
It is expected that the spectroscopy of the Ξ hypernuclei is promising. Here we use the (K − , K + ) reactionin which we can use the same method for the (π + , K + )reaction in the Λ-hypernuclear spectroscopy. In fact, tworeactions have very similar characteristics of large recoil
momentum of a produced hyperon: pΞ − ∼ 500 MeV/c andpΛ ∼ 350 MeV/c. Therefore, even for heavy targets wellseparated peak structures are expected in spite of many
possible excitations, because the spin-stretched conﬁgurations with p + Ξ + J = even are strongly populated asin the case of the (π + , K + ) reaction, or more strongly.
For the spectroscopy of the (K − , K + ) reaction, we
need two spectrometers as in the (π + , K + ) reaction: abeam line spectrometer for the incident K − and a K +spectrometer.
A 2 GeV/c kaon beam line for the JHF was designed
by J. Doornbos [17]. A beam line spectrometer is installedin the last part of the beam line. It consists of a QDQDQsystem. It is estimated that the momentum resolution of
2×10−4 could be achievable.

286

The European Physical Journal A

90%
10%

KEK-SKS
MeV

6
SDC3
SDC4
TOF

e f
18%
82%

2

CERENKOVbc

TARGET

1200 MeV/ca
BEAM1100 MeV/c
0

i j

12C
Λ

−

Fig. 3. Schematic layout of the SKS spectrometer for the (K ,K + ) reaction at 1.65 GeV/c.
singleFor the K spectrometer, we will use the existing SKS
spectrometer with some modiﬁcations. In ﬁg. 3, the setupof the SKS spectrometer for the (K − , K + ) reaction isshown schematically.
Since the radius for the central momentum is larger
than that for the (π + , K + ) reaction, the target point is
moved away from the magnet. So that, the acceptance ofthe spectrometer is reduced to be ∼ 50 msr. The overall
energy resolution is estimated to be 2 MeV(FWHM) fora 2 g/cm2 target thickness.
The production cross-section of the Ξ hypernuclei in the (K − , K + ) reaction is calculated to be∼ 0.1 µb/sr/MeV around the middle of the bound region
for various types of potentials. Thus, the yield for the208
Pb target with 2 g/cm2 thickness is estimated to be 6 events/MeV/day. So, even for the heaviest case, we
could get enough statistics within ∼ 20 days to obtain
spectroscopic information. For lighter targets such as 28 Si
and 58 Ni, the yields are several times higher with the normalized target thickness of 2 g/cm2 .
3.2 High-resolution hypernuclear γ-ray spectroscopy
High-resolution hypernuclear γ-ray spectroscopy will be
extended further and exciting new physics ﬁelds will be
opened up at the 50 GeV PS.
The (K − , π − ) reaction at 1.1 GeV/c is used in order toproduce Λ hypernuclei. This reaction has a large spin-ﬂip
amplitude and allows population of various hypernuclear
states including spin-ﬂip states with unnatural parities.
We require a secondary beam line whose intensity is
optimized to 1.1 GeV/c. It should have a double-stage
mass separator to obtain pure K − beams in order tominimize the counting rates of Ge detectors and tracking devices in the spectrometer. The beam intensity of
1.1 GeV/c K − is expected to be 1.9 × 107 K − per spill(2 × 1014 protons) in 3.4 s cycle.
The momentum of the K − beam is measured eventby-event with a beamline spectrometer having < 0.2%

b
c
d
e
f
g

2213-

5.9

01-2

2.6

112000

0.201

4.5
9*

0.663

9
18*

0.07

25*
50*

m

112000
2111-

0

a

l

k

a

1 m

+

(µb/sr)

8

8.3

d

100%

LUCITEh100%

4

AEROGELCERENKOV

g2+

1.1 GeV/c (K,π)

0+

4%
36%
60%

33%
7%
25%
35%

8

dσ/dΩ (θ=10o)

Ex
(MeV)

h
2600 i
12000 j7500 k1100 l
240 m
1300

0

Exp
Calc
(E369) (Millener)

Expected Yield (5 days)
γγ coincidnece
with awith c1700
c1900
a 1900
64
f
50
g
39
580
g
150
k
50
1400
j
94
2000
k
170
230
m
31

*Calc
(Itonaga)with dh 55

Fig. 4. Top: Expected level energies and γ transitions of 12
Λ C.
Bottom: Expected yields of γ transitions of 12
Λ C for 5 days’ run.
Yields of γ-γ coincidence events are also shown.

FWHM resolution. The outgoing π − is measured with aspectrometer similar to SKS, which is required to have an
acceptance of more than 50 msr and momentum resolutionof < 0.2% FWHM. An overall mass resolution better than
3 MeV is necessary.
Around the target, we install a new Ge detector system, which is similar to the present Hyperball but has amuch larger eﬃciency. In the present design, we expect
to use 14 sets of “Segmented Super Clover Ge detectors”,
which has recently become commercially available. One
detector set consists of four Ge crystals of 7 cm × 14 cm,
and the electrode of each crystal is segmented into 4 readout channels. Such a ﬁne segmentation is necessary for
Doppler-shift correction. The Ge crystals cover about 40%of the total solid angle. The Ge detector system has aphoto-peak eﬃciency of 12% at 1 MeV in total. Each of
the Ge detectors is surrounded by a set of BGO or GSO
counters, which are used to veto Compton scattering andhigh-energy γ-rays from π 0 .
An example of the γ-ray measurement in the case of12
Λ C for 5-day data taking is shown in ﬁg. 4. The produc+tion cross-sections of the 12
Λ C states (except for the ﬁrst 2
state) were calculated by Itonaga et al. for the 1.1 GeV/c(K − , π − ) reaction [18]. The energy levels are taken fromthe experimental values from KEK-E369 [3], but the doublet spacing energies are taken from the new parameterset of the spin-dependent interactions by Millener [19].
We will have quite enough yields for almost all theγ transitions. We can expect good statistics even for γ-γ

T. Nagae: Progress in hypernuclear physics

coincidence spectra, which enable us to completely reconstruct the level scheme.

4 Summary
There has been a lot of progress in Hypernuclear Physics
in the last decade. The recent success of hypernuclear γray spectroscopy with the Hyperball detector opened a
new stage of hypernuclear spectroscopy in precision. As
for non-mesonic weak decays, new neutron spectra fromseveral Λ-hypernuclei gave a new information to investigate the reaction mechanism. There was also important
progress in double-Λ hypernuclei.
We hope these eﬀorts will be in full bloom at the
50 GeV PS with high-intensity K − beams.

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E373 K. Nakazawa
Study of Double Strange Nuclei by Emulsion-Scintllating fiber Hybrid Methods
A. Ichikawa et al.
[attached]
Production of twin Λ-hypernuclei from Ξ hyperon capture at rest,
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● H. Takahashi et al.
[attached]
Observation of a 6ΛΛ He Double Hypernucleus
Phys. Rev. Lett., 87 (2001) 212052.
○ H. Takahashi et al.
[attached]
Scintillating-fiber-block detector for studying double-strangeness nuclei,
Nucl. Instr. Meth., A483 (2002) 689.
★ A. Ichikawa
Study of S=-2 Nuclear Systems with Hybrid Emulsion Method.
Kyoto University, 2001.
★ H. Takahashi
Study of Double-Hypernuclei with Hybrid Emulsion Method.
Kyoto University, 2003.
◇ K. Nakazawa
Study of Double-Λ hypernuclei
Proc. of The 1st International Conference on Nuclear Physics; Dec.10-12, 2003, Mandalay
University, Myanmar, shall be printed.
◇ K. Nakazawa
The experimental production of S=-2 systems
Proc. of The 8th International Conference on Hypernuclear & Strange Particle Physics (HYP2003);
Oct.14-18, 2003, Jefferson Lab, Newport News, shall be printed.
◇ K. Nakazawa
The nuclear double-strangeness achievements and perspectives
Proc. of The XVIIth International workshop on High Energy Physics and Quantum Field Theory(QFTHEP2003); Sep.4-11, 2003, Samara-Saratov, Russia, shall be printed.
◇ H. Takahashi
Observation of Double-Hypernuclei and Λ-Λ Interaction
Proc. of the XVIth international Conference on Particle and Nuclei (PANIC02) (ed. by Toki, et al.); Osaka,
Nucl. Phys. A721 (2003) pp.951c-954c.
◇ K. Nakazawa
Systematic Study of Double Strangeness System by Emulsion-Counter Hybrid Method
Proc. of 2nd Asia Pacific Conference on Few-Body Problems in Physics (ed. by H.Q.Song et al.); Shanghai,World Scientific, Modern Physics Lett. A18,No.2-6, (2003) pp.116-119.
Soryushiron-Kenkyu Vol.106 No.2 (2002) pp.B32-B34.
◇ K. NakazawaΛΛ interaction indicated by Lambpha (6ΛΛ He Double hypernucleus)
Proc. of 8th in the series of meetings on MESONS & LIGHT NUCLEI, Prague, AIP press, CP603(2001) pp.463-466.
◇ K. Nakazawa
Doubly strange nuclei by a hybrid-emulsion experiment E373 at KEK
Proc. of Int’l Symp. on Hadron & Nuclei, Seoul, AIP press, CP594 (2001) pp.180-188.
◇ A. Ichikawa
Study of Double-Strangeness Nuclei with Hybrid-Emulsion Method (KEK-PS E373)
Proc. of The 7th International Conference on Hypernuclear & Strange Particle Physics (HYP2000);
Oct.23-27, 2000, Torino, Italy, Nucl. Phys. A691 (2001) pp.246c-249c.
◇ K. Nakazawa
An experiment to study S=-2 nuclear system; PS-E373 (KEK)
Proc. of the APCTP Workshop (SNP99),(ed. by I.T.Cheon, S.W.Hong & T.Motoba); Seoul,●________________________________________________________________________● Physics papers published in refereed journal.
○★
◇＊

Technical papers.
PhD theses.
Conference and Symposium.
Internal Report and others.

World Scientific [Strangeness Nuclear Physics] (2000) pp.193-198.
A. Ichikawa
Study of Double-Strangeness Nuclei with Hybrid-Emulsion Method (KEK-PS E373)
Proc. of the APCTP Workshop (SNP99),(ed. by I.T.Cheon, S.W.Hong & T.Motoba); Seoul,
World Scientific [Strangeness Nuclear Physics] (2000) pp.188-192.
◇
A. Ichikawa
Study of Double-Strangeness Nuclei with Hybrid-Emulsion Method (KEK-PS E373)
Proc. Of the KEK-Tanashi International Symposium on Hadrons and Nuclei; Tokyo,
Nucl. Phys. A670 (2000) pp.289c-292c.
◇________________________________________________________________________● Physics papers published in refereed journal.
○★
◇＊

Technical papers.
PhD theses.
Conference and Symposium.
Internal Report and others.

15 February 2001

Physics Letters B 500 (2001) 37–46
www.elsevier.nl/locate/npe

Production of twin -hypernuclei from − hyperon capture
at restA. Ichikawa a,l,∗ , J.K. Ahn a,1 , H. Akikawa a , S. Aoki b , K. Arai c , S.Y. Bahk d ,K.M. Baik e , B. Bassalleck f , J.H. Chung g , M.S. Chung e , K. Hoshino h , M. Ieiri i ,K. Imai a , Y.H. Iwata j , Y.S. Iwata j , H. Kanda a,2 , M. Kaneko j , T. Kawai h , C.O. Kim e ,J.Y. Kim g , S.J. Kim g , S.H. Kim k , Y. Kondo a,3 , T. Kouketsu j , Y.L. Lee m ,J.W.C. McNabb n , M. Mitsuhara j , Y. Nagase j , C. Nagoshi o , K. Nakazawa j , H. Noumi i ,S. Ogawa c , H. Okabe p , K. Oyama c , H.M. Park j,4 , I.G. Park k , J. Parker n , Y.S. Ra j,5 ,J.T. Rhee m , A. Rusek q , H. Shibuya c , K.S. Sim e , P.K. Saha i , D. Seki a , M. Sekimoto i ,J.S. Song k , H. Takahashi a , T. Takahashi r , F. Takeutchi s , H. Tanaka t , K. Tanida u ,J. Tojo a , H. Torii a , S. Torikai j , N. Ushida t , K. Yamamoto a,6 , N. Yasuda v , J.T. Yang e ,C.J. Yoon a , C.S. Yoon k , M. Yosoi a,7 , T. Yoshida w , L. Zhu aa Department of Physics, Kyoto University, Kyoto 606-8502, Japanb Faculty of Human Development, Kobe University, Kobe 657-8501, Japanc Department of Physics, Toho University, Funabashi 274-8510, Japand Wonkwang University, Iri 570-749, South Korea
e Department of Physics, Korea University, Seoul 136-701, South Korea
f Department of Physics and Astronomy, University of New Mexico, Albuquerque, NM 87131, USA
g Chonnam National University, Kwangju 500-757, South Korea
h Department of Physics, Nagoya University, Nagoya, 464-8601, Japani KEK, High Energy Accelerator Research Organization, Tsukuba 305-0801, Japanj Physics Department, Gifu University, Gifu 501-1193, Japank Department of Physics, Gyeongsang National University, Jinju 660-701, South Korea
l Korea National University of Education, South Korea
m Institute for Advanced Physics, Konkuk University, Seoul 143-701, South Korea
n Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213, USAo Higashi Nippon International University, Iwaki 970-8023, Japanp Osaka Prefectural Education Center, Osaka 558-0011, Japanq Brookhaven National Laboratory, New York, NY 11973, USAr Department of Physics, Tohoku University, Sendai 980-8578, Japans Faculty of Science, Kyoto Sangyo University, Kyoto 603-8555, Japant Aichi University of Education, Kariya 448-8542, Japanu Department of Physics, University of Tokyo, Tokyo 113-0033, Japanv Department of Quantum Engineering and Systems Science, University of Tokyo, Tokyo 113-8656, Japanw Department of Physics, Osaka City University, Osaka 558-8585, Japan

Received 9 October 2000; received in revised form 18 December 2000; accepted 20 December 2000
Editor: J.P. Schiffer
0370-2693/01/$ – see front matter  2001 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 0 - 2 6 9 3 ( 0 1 ) 0 0 0 4 9 - 1

38

A. Ichikawa et al. / Physics Letters B 500 (2001) 37–46

Abstract
A hybrid emulsion experiment was carried out to study double-strangeness nuclei produced via − hyperon capture atrest with the expectation of ten times larger statistics than previous experiments. We have analyzed 5% of the total emulsion
and found one “twin-hypernuclei” event involving the emission of two single- hypernuclei and a nuclear fragment from a −
hyperon stopping point. The event is interpreted as the decay of a − + 14 N atomic system to 5 He + 5 He + 4 He + neutron. The
species of the − -atom and the fragmentation products are uniquely identified for the first time for twin-hypernuclei events.
Combined with the results from a past hybrid-emulsion experiment, one double- hypernucleus and three twin-hypernuclei
events have been found from − captures on light emulsion nuclei. The ratio of the detected rate of double- hypernuclei tothat of twin-hypernuclei is compared with theoretical estimates.  2001 Elsevier Science B.V. All rights reserved.
PACS: 21.80.+a; 25.80.Pw; 13.75.Cs
Keywords: Double- hypernucleus; Twin  hypernuclei; − capture at rest; Nuclear emulsion

1. Introduction
Nuclear capture of a − hyperon at rest is themost efficient way to produce nuclei with strangenessS = −2. This was demonstrated by the KEK experiment E176 [1–4] which reported about 80 events of− hyperon capture at rest. Until now three different emulsion experiments, including E176, have reported events in which double- hypernuclei were
emitted from − hyperon stopping points [2,5–7].
However, in the event reported by Danysz et al. [5,6]and Prowse [7], the positive identification of the incoming track as that of a − hyperon could not be
made. Dalitz et al. [6] estimated that the number of− hyperons brought to rest was one or two in theexperiment of Danysz et al. and less than one for theexperiment by Prowse. For the latter event, only the* Corresponding author. Tel: +81-75-753-3832, fax: +81-75-

753-3887.
E-mail address: ichikawa@nh.scphys.kyoto-u.ac.jp(A. Ichikawa).
1 Present address: RCNP, Osaka University, Ibaraki 567-0047,
Japan.
2 Present address: Department of Physics, Tohoku University.
3 Present address: KEK, High Energy Accelerator Research
Organization.
4 Present address: Chonnam National University.
5 Present address: Wonkwang University.
6 Present address: Department of Physics, Osaka City University.
7 Present address: RCNP, Osaka University, Ibaraki 567-0047,
Japan.

drawing is left, the emulsion having been lost, andthere was no study or analysis of that event independent of the sole author. In the event of E176, the −
hyperon was well identified. However there remained
10 Be ortwo interpretations for that event, namely, 
13 B [2]. From the mass of a double- hypernucleus,
A Z) =the  interaction energy defined by B (A−1
A
B ( Z) − 2B (  Z) can be extracted. The valueof B was reported to be 4 ∼ 5 MeV by Danysz
et al. [5,6] and Prowse [7], suggesting an attractive  interaction. Experiment E176 reported that10
B is −4.8+0.7−0.8 MeV if the interpretation is  Be
13
and +4.9 ± 0.8 MeV in the case of  B. Dover et al.
argued that the latter one is more favorable based on
the consideration about the production mechanism [8].
For all the past experiments, the mass of the double-
hypernucleus was calculated by kinematically reconstructing it’s decay assuming that the daughter single hypernucleus was produced in the ground state. For
13 B case), there
the events of Danysz et al. and E176 (is the possibility that the single- hypernucleus produced from the decay of the double- hypernucleus
might be in an excited state, in which case the valueof B must be decreased from the reported values
by 3.0 MeV and 4.9 MeV, respectively. The value ofB is, therefore, still unknown and our knowledge
is limited to it’s upper limit, i.e., 4 ∼ 5 MeV.
On the other hand, E176 reported two so-called“twin-hypernuclei” events, in which two single- hypernuclei were emitted from a − stopping point [3,4].
The binding energy of the − -atom, which can pro-A. Ichikawa et al. / Physics Letters B 500 (2001) 37–46

vide the information on the − N interaction, may be
determined by measuring the mass of the system fromwhich twin-hypernuclei are emitted. The informationof the atomic level in which − absorption can occur
is also important. The two events reported by E176
were identified as the − hyperon captures on lightemulsion nuclei. However, the species of the − -atomand the fragmentation products are not uniquely identified.
A hybrid emulsion experiment (E373) was carriedout at the KEK proton synchrotron using a 1.66 GeV/cseparated K − beam. The purpose of this experiment is
to study double-strangeness nuclei produced via −
hyperon capture at rest. By adopting a diamond blockas the − production target instead of emulsion, it was
enabled to obtain ten times as many events as E176
did with the emulsion twice as much as that of E176.
Until now 5% of the total emulsion has been analyzed
and one event showing the topology of twin single-
hypernuclei has been found. This event is described indetail in this Letter.

2. Experiment
The experimental setup is shown in Fig. 1. The
incident K − meson beam was focused on a 3 cm

39long diamond target. Typical intensity of the beam
was 1.1 × 104 per 2 seconds spill and the K − /π −ratio was 1/3. Downstream of the target a stackof emulsion sheets [9] was placed perpendicular tothe beam direction. Each stack was composed of 12
emulsion sheets of 250 × 245 mm2 area. Each sheet
consisted of a 500 µm coating of emulsion gel (FujiET7C) on both sides of an acrylic film of thickness
50 µm except for the most upstream plate which had
70 µm thick emulsion gel on both sides of a 200 µm
thick acrylic base.
The − hyperons were produced in the diamondtarget via the p(K − , K + )− quasi-free reaction.
Some of them were slowed down and brought to restin the nuclear emulsion stack. In order to measure theposition and angle of the − hyperon track, a highresolution tracking detector using coherent bundlesof scintillating micro fibers (fiber-bundle) had beendeveloped [10]. The diameter of one fiber was 45 µm,
and the thickness of the detector was 3 mm. It was
installed between the target and the emulsion stack.
The identification of the incident particles was performed with two scintillation counters (T1, T2), andtwo aerogel Čerenkov counters (BAC1, BAC2). The
outgoing K + mesons were detected by a spectrometer which consisted of a dipole magnet (1.01 T m), twoaerogel Čerenkov counters (BVAC, FAC), four plastic-

Fig. 1. Schematic view of the E373 experimental setup.

40

A. Ichikawa et al. / Physics Letters B 500 (2001) 37–46

scintillator hodoscopes (VH, CH, YH and FTOF),
three drift chambers (DC1, DC2 and DC3), and alimited-streamer chamber (LST). The on-line firstlevel trigger distinguished K + mesons from pions andK − mesons using the aerogel Čerenkov counters andthe scintillator hodoscopes. The second-level trigger
used additional TDC information to distinguish K +mesons from protons. In the off-line analysis the massof the outgoing particle was reconstructed from themomentum and TOF information. The obtained mass
spectrum is shown in Fig. 2. A particle having a reconstructed mass between 0.4 GeV/c2 and 0.6 GeV/c2
and a momentum between 0.4 GeV/c and 1.4 GeV/c
was considered to be a K + meson.
Fast protons and π − s from the decay of hypernuclei can be detected with the range counter of scintillating mono-fiber blocks (SciFi-blocks) located both
upstream and downstream of the emulsion stack [11].
The SciFi-blocks were also used to determine the vertex position of the (K − , K + ) reaction and to reject− hyperon decay events. The signals from the fiberbundle detector and the SciFi-blocks were intensified
and read out by image intensifier tubes and charge coupled devices.
About 9.0 × 104 (K − , K + ) events were obtainedwith 100 emulsion stacks made of 69 litres of emulsion

Fig. 2. Mass distribution of scattered particles. The arrow indicatesthe value for the twin-hypernuclei event.

gel. From a Monte Carlo simulation based on the E176
result, about 1% of the − hyperons were expected tocome to rest in the emulsion stack. For the events identified as a (K − , K + ) reaction, the images obtainedwith the scintillating fiber detectors were scanned byeye. Events which showed the topology of no thick
track in the fiber-bundle image, wrong vertex position, or the decay of a − hyperon were rejected. For
the events which survive this rejection, the tracks aresearched and traced in the emulsion plates with thehelp of an automatic track scanning system [12]. Several track candidates were found in the searched region
for each prediction by fiber-bundle. Main backgrounds
are protons and π − mesons. The latter can be distinguished from their ionization in the emulsion. Therefore, there is almost no accidental background for the
event having at least one evaporation track from thestopping point of the traced particle.

3. Description of the event
Until now, 5% of the total emulsion has been
analyzed, and one event showing the topology oftwin single- hypernuclei was found. In this event,a − hyperon was produced in the diamond target
in conjunction with a K + meson of momentum
1.10 GeV/c. The reconstructed mass of the K + meson
is 0.483 GeV/c, and is indicated by an arrow in Fig. 2.
The level of background due to misidentificationby the spectrometer data analysis is 3% for thisreconstructed mass and momentum region. The −
hyperon left the diamond target and went throughthe fiber-bundle detector into the emulsion stack.
Using the position and angle information from thefiber-bundle detector, tracks were searched for in
a 600 µm × 450 µm area in the emulsion. Eighttracks were found in the region and traced. Amongthem, one track shows the topology of twin single hypernuclei emission at the capture point after
traversing about 5 mm in the emulsion. A large angle
deflection near the capture point was observed for thistrack. Observation of clearly separated three vertices,
i.e., a capture and two delayed decays proves that an
S = −2 particle was captured at that point. No track
was observed in the SciFi-blocks image, meaning thatno charged particle left the emulsion and went into
them in this event.

A. Ichikawa et al. / Physics Letters B 500 (2001) 37–46

Fig. 3 shows a picture and a schematic drawingof the event. It was obtained by summing up images
taken at various focusing depths of the microscope.
To obtain a better position resolution a small area
(15 mm × 15 mm) of the emulsion plate, centeredon the event, was cut and swollen by water and sugar

41

mixture to the original thickness before development.
It was sliced to a tiny piece of 0.5 mm width to view
from an orthogonal direction. The − was captured
by a nucleus at point A. Three charged particles (track#1, #2, #3) were emitted from point A. Track #1 shows
the topology of a decay into three charged particles

Fig. 3. Picture and schematic drawing of the event viewed from the vertical (a) and the horizontal (b) direction.

42

A. Ichikawa et al. / Physics Letters B 500 (2001) 37–46

Table 1
Ranges and emission angles of the tracks. Angles are expressed by a zenith angle (θ ) and an azimuthal angle (φ) in respect to the angle
perpendicular to the plate. For track #4 and #7, the ranges in an acrylic base were converted to those in the emulsion

point A

point B

point C

Track

range (µm)

θ (degree)φ (degree)#1

19.1 ± 0.4

113 ± 7262 ± 7

hypernucleus#2

5.2 ± 0.566 ± 797 ± 8hypernucleus#323.0 ± 0.4

66 ± 378 ± 2#412517

87.14 ± 1.2

48.19 ± 1.2#5286 ± 4150.0 ± 1.0

153.1 ± 1.2#67.7 ± 0.6

40 ± 4265 ± 4#7

 14884

88 ± 1.5

257.1 ± 0.7#88.8 ± 0.477 ± 581 ± 3at point B. Track #2 also shows a topology of adecay into two charged particles at point C. Table 1
summarizes the ranges and the emission angles ofthese tracks. The ranges were measured prior toswelling the emulsion. The effect on the measurementof the angles by any distortion introduced by swelling
was checked and found to be negligible within thequoted errors. A particle (track #4) emitted from point
B traveled 12514 µm before stopping. It is identified
as a π − meson from a measurement of its ionizationand the observation of a track of a charged particle
emitted from its stopping point. A particle (track #7)
emitted from point C traversed more than 14 mm andwas lost at the acrylic base of the emulsion plate. The
grain density of this track near point C is 1.51 ± 0.06
times larger than that of track #4 near point B. Fromthe ionization and the lower limit of the range, theparticle of track #7 was identified as a proton.

4. Analysis
Once the particle species has been assigned to a
track, the kinetic energy can be calculated by meansof a range-energy relation. A package “SRIM2000”[13–15] was used to calculate a kinetic energy froma range. The calibration for the range-energy relation
was made by measuring the ranges of α-particles emitted from 212Po (8.784 MeV) and 228 Th (5.423 MeV) toan accuracy of 0.6%. The mean ranges were 48.4 µm

π − (26.5 MeV)

p ( 64 MeV)

and 23.6 µm, respectively. From these values, the density of the emulsion of this plate was determined as
3.55 ± 0.07 g/cm3 using SRIM2000, which is consistent with the density 3.60 ± 0.07 g/cm3 deduced
from the measurement of the weight and volume ofthe emulsion plates. The two ranges at two different
energies agree well with the range-energy relation calculated with SRIM2000. Table 2 summarizes the possible kinetic energies for the particles of tracks #1, 2,
4, 5, 6 obtained from the range-energy relation assuming their species.
The event is interpreted as containing two single-
hypernuclei originating from the reaction − + p → +  after the − absorption. Track #1 is a single-
hypernucleus which decayed mesonically at point B.
Since the Q-value of the reaction is 28.2 MeV, only
light hypernuclei are capable of traversing the lengthof track #1 (19.1 µm) from this energy release. Hencemesonic decay modes with three charged particles
were examined for all known hypernuclei whose
masses are lighter than 18
 O. For each decay modes,
the total kinetic energy was calculated by assigning
the particle species to each track. Listed in Table 3 aremesonic decay modes whose Q-value is large enoughto account for the sum of the kinetic energies ofcharged decay products (Evisi ). For the decay modes
with no neutron, the momentum sum of three charged
particles (psum ) must be zero within the error. For
the modes with a neutron, the kinetic energy of theneutron was calculated using momentum balance for

A. Ichikawa et al. / Physics Letters B 500 (2001) 37–46

43

Table 2Kinetic energies (MeV) for the tracks #1, 2, 3, 5, 6 particles calculated from the range-energy relation assuming their species
3 H)
d (

t (4 H)

3 He ( 4 He)

#1(1.75)(1.88)(4.73)(4.98)(5.18)#2(0.58)(0.56)

(1.6)

(1.6)

(1.6)

Track

p

4 He ( 5 He)


#3

1.39

1.74

1.96

4.80

5.33

#5

6.62

8.70

10.2

23.5

26.4

#6

0.67

0.79

0.84

2.20

2.30

(6 He)Table 3
Decay modes for track #1 whose Q-value is large enough compared to the visible energy (Evisi ). Here Evisi and psum are the sum of the kinetic
energy and momenta of three charged particles, respectively. Etotal is the total kinetic energy of decay particles
Decay mode3−
H → π + p + p + n3−
H → π + p + d3−
H → π + d + p
4 H → π− + p + t4−3
 He → π + p + He
5−
4
 He → π + p + He
6−
4
 He → π + p + He + n
6 He → π − + d + 4 He8−4
 He → π + t + He + n

Q (MeV)

Evisi (MeV)psum (MeV/c)

Etotal35.49

33.8 ± 0.5

95.6 ± 2.2

38.6 ± 0.7

rejected

37.71

33.9 ± 0.5

76.7 ± 2.7

33.9 ± 0.5

rejected

37.71

35.8 ± 0.5

152.8 ± 2.5

35.8 ± 0.5

rejected

35.80

34.0 ± 0.5

67.8 ± 3.2

34.0 ± 0.5

rejected

35.45

35.3 ± 0.5

23.0 ± 5.0

35.3 ± 0.5

rejected

34.72

35.4 ± 0.5

13.0 ± 8.7

35.4 ± 0.5

acceptable

34.56

35.4 ± 0.5

13.0 ± 8.7

35.5 ± 0.6

acceptable

36.78

37.5 ± 0.5

63.1 ± 6.0

37.5 ± 0.5

rejected

38.64

39.0 ± 0.5

120.9 ± 6.0

46.7 ± 1.3

rejected

the charged particles. The total kinetic energy ofcharged particles and a neutron (Etotal ) was comparedwith the Q-value. Only 5 He and 6 He were foundto be acceptable for track #1 candidates. The particleof track #2 decayed in a non-mesonic mode and its
species cannot be assigned from its decay topology.
We assert that the − hyperon was absorbed bya light nucleus (12 C (, 13 C), 14 N or 16 O) in theemulsion for the following reasons. The kinetic energyof the track #1 particle is 5.0 MeV or 5.2 MeV in
the case of 5 He or 6 He, respectively. With thesevalues they are very unlikely to escape through the
Coulomb barrier of a Ag or Br nucleus. This is true
even if the hypernucleus of track #1 is lighter than5 He. Furthermore the absence of Auger electrons also
supports this.
All the decay modes from − + 12 C/13 C/14 N/16 O
to two hypernuclei and a non-strange nucleus were
examined. The total kinetic energy of three nuclei wascalculated and was compared with the Q-value. Only
two decay modes,
− + 14 N → 5 He + 5 He + 4 He + n,
− + 14 N → 5 He + 6 He + 4 He,
have Q-values either larger than or equal within errors
to the calculated total kinetic energy. Again, only
5
6
 He or  He is acceptable as the hypernucleus for
track #1. It should be noted that neither 5 He nor 6 He
has any excited states which can decay via gamma
emission. Therefore, there is no uncertainty arisingfrom excited states. For each of these three modes,
the total momentum of the three nuclei was calculated.
The results are summarized in Table 4. For the modes
− + 14 N → 5 He(#1) + 6 He(#2) + 4 He(#3) and− + 14 N → 6 He(#1) + 5 He(#2) + 4 He(#3), the sumof the momenta of the three nuclei deviates from zeroby 10 and 6 standard deviations, respectively.

44

A. Ichikawa et al. / Physics Letters B 500 (2001) 37–46

Table 4
Decay modes to two hypernuclei (track #1 and #2) and one nucleus (track #3) whose Q-value is larger than or equal to the visible energy(Evisi ). Here Evisi and psum are the sum of the kinetic energy and momenta of three nuclei, respectively. Etotal is the total kinetic energy ofdecay particles
Target nucleus

14 N
14 N
14 N

Track

Number of neutrons

Q (MeV)

Evisi

psum

Etotal

#1

#2

#3

5
 He
5
 He
6
 He

5
 He
6
 He
5
 He

4 He

1

14.70

11.89 ± 0.21

101 ± 9.8

17.3 ± 1.2

4 He

0

14.86

11.85 ± 0.21

111 ± 1111.85 ± 0.21

4 He

0

14.86

12.08 ± 0.21

76 ± 1212.08 ± 0.21

We conclude, therefore, that this event can be
assigned as
− + 14 N → 5 He(#1) + 5 He(#2) + 4 He(#3) + n.
The binding energy of the − is then calculated
to be −2.6 ± 1.2 MeV. Here the quoted error is
one standard deviation. The negative binding energy
would correspond to an unbound state, however within
the error bar it is certainly consistent with a capture
from an atomic, i.e., bound level. The species of the
− -atom and the fragmentation products are uniquely
identified for the first time for twin-hypernuclei events

5. Discussion about production rate
There are several theoretical calculations estimating the production probability of double- hypernuclei [16–19] and of twin-hypernuclei events [17–19].
These calculations use an attractive  interaction,

except Ref. [17], in which both attractive and repulsive interactions were considered. The results of thesecalculations are summarized in Table 5. The ratio of
the production rate of double- hypernuclei to twinhypernuclei is larger than 5 with an attractive  interaction. This characteristics is naively explained byconsidering the phase space available for each channel. In the case of B = 4.9 MeV (attractive), theQ-values to produce double- hypernuclei are muchlarger than those for twin-hypernuclei. However, ifB = −4.9 MeV (repulsive), the Q-values for thetwo types become comparable and the ratio is decreased.
Combined with the results of the E176 experiment, one double- hypernucleus and three twinhypernuclei events have been found from − hyperon captures on light emulsion nuclei. The ratio ofthe detected rate for double- hypernuclei to twinhypernuclei events is therefore 0.33 with the 90% confidence interval lying from 0.07 to 2.5. This value
deviates markedly from the theoretical estimates us-

Table 5
Theoretical estimations for the production probability of double- hypernuclei and twin-hypernuclei, and their ratio. In Ref. [17], those from
13 B∗ , 15 C∗ and 17 N∗ , formed after a − hyperon capture had been calculated. Quoted here are the values averaged
the compound states, with the weights of the molar ratio of 12 C, 14 N and 16 O in the emulsion. The values from Refs. [18,19] are those from − + 12 C atomic states.
Only two body decay channels are considered in [18]. The parenthesized values are those excluding the fragmentation channels in which a
12 B + ndouble- hypernucleus and only neutrons are emitted, like − + 12 C → Model

Double-hyp.
(exc. one charge frag.)Twin-hyp.

Ratio

Statistical decay (attractive) [17]70% (57%)12%

5.8 (4.8)

Statistical decay (repulsive) [17]30% (21%)28%

1.1 (0.75)

Doorway decay [18]4.8% (3.3%)

0.11%

43 (30)

AMD-QL [19]11.4% (9.8%)

1.2%

9.5 (8.2)

A. Ichikawa et al. / Physics Letters B 500 (2001) 37–46

ing an attractive  interaction (B = 4.9 MeV)
given in Table 5. However, to compare with thetheoretical estimates, the difference of the detection
efficiency between double- hypernuclei and twinhypernuclei events must be considered. Following fourcases are considered to be the main origin of the detection inefficiency.
(a) If a hypernucleus decays in a neutral mode ( →π 0 n or n → nn) and only one or no trackof the residual nucleus is visible, the identification is difficult. Both a double- hypernucleus
and a twin-hypernuclei emission have two decay vertices. The inefficiencies due to the neutral decay are, therefore, considered to be thesame.
(b) If the distance between two decay vertices is very
short, the separation is difficult. With the attractive B (4.9 MeV), Q-values for double-
hyperfragment channels are large and the range
of a double- hypernucleus is expected to belonger than that of a hypernucleus of the same
mass number from a twin-hypernuclei event. As
for the range of a daughter hypernucleus froma double- hypernucleus, the range can become
short when the parent nucleus decays in a mesonicmode. The branching ratio for the mesonic decay modes for such hypernuclei is expected to beabout 20% from the data on single- hypernuclei [20]. The present discrepancy cannot be explained this way.
(c) If a double- hypernucleus and only neutrons
are emitted from a − stopping point like − +12 C → 12 B + n, the detection is also diffi
cult. In Table 5, the estimated production rates
without these modes are also listed. Even without these modes, there is still a large discrepancy between the present result and the theoretical estimations using an attractive  interaction.
(d) When a double- hypernucleus decays in a nonmesonic mode, the remaining  can escape fromthe nucleus due to the large Q-values of nonmesonic modes. In such a case, the identification as a double- hypernucleus is almost impossible. From the data on single- hypernuclei, the non-mesonic decay rate was estimatedto be about 80% on the average. If the escap-

45

ing probability for the remaining  is larger than
70%, the ratio of the detected rates doesn’t contradict the theoretical estimates by the statistical decay model with an attractive  interaction.
Consequently, if the  interaction is attractive andB = 4.9 MeV, then the escaping probability of
the remaining  at the first decay of a double- hypernucleus in a non-mesonic mode might be as highas 70%.
In summary, a hybrid-emulsion experiment KEKPS E373 has observed a new example of twin single hypernuclei emission from a − stopping point.
It has been interpreted as the fragmentation from a− + 14 N atomic state to 5 He +5 He + 4 He + n. The
species of the − -atom and the fragmentation products are uniquely identified for the first time for twinhypernuclei events. Combined with the results of aprevious experiment, E176, the ratio of the detected
rate of double- hypernuclei to twin-hypernuclei iscompared to theoretical calculations. It’s value is consistent with a theoretical estimation using a repulsive
 interaction, and is much smaller than those using
an attractive  interaction. The deviation might be
explained by the detection inefficiency for double-
hypernuclei due to the escape of the remaining  at
the first decay in a non-mesonic mode.

Acknowledgements
We would like to express our thanks to staffs of
KEK PS for their support during the experiment andanalysis. We are grateful to Professor D.H. Davis andDr. D.N. Tovee for valuable discussions. We thankProfessors Y. Yamamoto, T. Yamada and A. Ohnishi
and Dr. Y. Hirata for discussions on theoretical aspects. This work was supported partially by Grantsin-Aid for Scientific Research on the Priority Area
“Strangeness Nuclear Physics” (No. 08239103) fromthe Japan Ministry of Education and Korea Research
Foundation Grant (KRF-99-005-D00004). One of theauthors (A.I.) is grateful to JSPS Research Fellowships
for Young Scientists. Another author (B.B.) also appreciates the support of a JSPS Fellowship.

46

A. Ichikawa et al. / Physics Letters B 500 (2001) 37–46

References[1] S. Aoki et al., Prog. Theor. Phys. 85 (1991) 951.
[2] S. Aoki et al., Prog. Theor. Phys. 85 (1991) 1287.
[3] S. Aoki et al., Prog. Theor. Phys. 89 (1993) 493.
[4] S. Aoki et al., Phys. Lett. B 355 (1995) 45.
[5] M. Danysz et al., Nucl. Phys. 49 (1963) 121.
[6] R.H. Dalitz et al., Proc. R. Soc. Lond. A 426 (1989) 1.
[7] D.J. Prowse, Phys. Rev. Lett. 17 (1966) 782.
[8] C.B. Dover, D.J. Millener, A. Gal, D.H. Davis, Phys. Rev. C 44
(1991) 1905.
[9] H. Akikawa et al., submitted to Nucl. Instrum. Methods.
[10] A. Ichikawa et al., Nucl. Instrum. Methods A 417 (1998) 220.
[11] H. Takahashi et al., to be submitted to Nucl. Instrum. Methods.
[12] J.K. Ahn et al., KEK-PS-E373, Nucl. Phys. A 670 (2000) 289.

[13] J.F. Ziegler, J.P. Biersack, Srim-2000, http://www.research.
ibm.com/ionbeams/home.htm[14] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping andRange of Ions in Solids, Pergamon Press, 1985.
[15] J.P. Biersack, L.G. Haggmark, Nucl. Instrum. Methods 174(1980) 257.
[16] D. Zhu, C.B. Dover, A. Gal, M. May, Phys. Rev. Lett. 67(1991) 2268.
[17] M. Sano, M. Wakai, Y. Yamamoto, Prog. Theor. Phys. 87(1992) 957.
[18] T. Yamada, K. Ikeda, Phys. Rev. C 56 (1997) 3216.
[19] Y. Hirata, Y. Nara, A. Ohnishi, T. Harada, J. Randrup, Prog.
Theor. Phys. 102 (1999) 89, nucl-th/9711063.
[20] J.P. Lagnaux et al., Nucl. Phys. 60 (1964) 97.

VOLUME 87, NUMBER 21

PHYSICAL REVIEW LETTERS

19 NOVEMBER 2001

Observation of a LL6 He Double Hypernucleus
H. Takahashi,1, * J. K. Ahn,1, † H. Akikawa,1 S. Aoki,2 K. Arai,3 S. Y. Bahk,4 K. M. Baik,5 B. Bassalleck,6 J. H. Chung,7M. S. Chung,5 D. H. Davis,8 T. Fukuda,9, ‡ K. Hoshino,10 A. Ichikawa,1 M. Ieiri,9 K. Imai,1 Y. H. Iwata,7 Y. S. Iwata,7
H. Kanda,1, § M. Kaneko,7 T. Kawai,10 M. Kawasaki,7 C. O. Kim,5 J. Y. Kim,11 S. J. Kim,11 S. H. Kim,12 Y. Kondo,1, k
T. Kouketsu,7 Y. L. Lee,13 J. W. C. McNabb,14 M. Mitsuhara,7 Y. Nagase,7 C. Nagoshi,15 K. Nakazawa,7 H. Noumi,9S. Ogawa,3 H. Okabe,16 K. Oyama,3 H. M. Park,7, ¶ I. G. Park,12 J. Parker,14 Y. S. Ra,7, ** J. T. Rhee,13 A. Rusek,17
H. Shibuya,3 K. S. Sim,5 P. K. Saha,9 D. Seki,1 M. Sekimoto,9 J. S. Song,12 T. Takahashi,18 F. Takeutchi,19 H. Tanaka,20
K. Tanida,21 J. Tojo,1 H. Torii,1 S. Torikai,7 D. N. Tovee,8 N. Ushida,20 K. Yamamoto,1, †† N. Yasuda,22 J. T. Yang,5C. J. Yoon,1 C. S. Yoon,12 M. Yosoi,1 T. Yoshida,23 and L. Zhu1
1
Department of Physics, Kyoto University, Kyoto 606-8502, Japan
Faculty of Human Development, Kobe University, Kobe 657-8501, Japan3
Department of Physics, Toho University, Funabashi 274-8510, Japan
4
Wonkwang University, Iri 570-749, Korea
5
Department of Physics, Korea University, Seoul 136-701, Korea
6
Department of Physics and Astronomy, University of New Mexico, Albuquerque, New Mexico 87131
7
Physics Department, Gifu University, Gifu 501-1193, Japan
8
Department of Physics and Astronomy, University College London, London WC1E 6BT, United Kingdom
9Institute of Particle and Nuclear Studies, KEK, Tsukuba 305-0801, Japan10
Department of Physics, Nagoya University, Nagoya 464-8601, Japan
11
Chonnam National University, Kwangju 500-757, Korea
12
Department of Physics, Gyeongsang National University, Jinju 660-701, Korea
13
Institute for Advanced Physics, Konkuk University, Seoul 143-701, Korea
14
Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213
15
Higashi Nippon International University, Iwaki 970-8023, Japan
16
Osaka Prefectural Education Center, Osaka 558-0011, Japan
17
Brookhaven National Laboratory, Upton, New York 11973
18
Department of Physics, Tohoku University, Sendai 980-8578, Japan
19
Faculty of Science, Kyoto Sangyo University, Kyoto 603-8555, Japan
20
Aichi University of Education, Kariya 448-8542, Japan
21
Department of Physics, University of Tokyo, Tokyo 113-0033, Japan
22
National Institute of Radiological Science, Chiba 263-8555, Japan
23
Department of Physics, Osaka City University, Osaka 558-8585, Japan(Received 17 July 2001; published 2 November 2001)
2

A double-hyperfragment event has been found in a hybrid-emulsion experiment. It is identified
uniquely as the sequential decay of LL6 He emitted from a J2 hyperon nuclear capture at rest. The
mass of LL6 He and the L-L interaction energy DBLL have been measured for the first time devoid of
10.18
MeV.
the ambiguities due to the possibilities of excited states. The value of DBLL is 1.01 6 0.2020.11
This demonstrates that the L-L interaction is weakly attractive.
DOI: 10.1103/PhysRevLett.87.212502

PACS numbers: 21.80. +a, 21.10.Dr, 25.80.Pw, 27.20. +n

There are three reports on the observations of double-L hypernuclei in nuclear emulsion. About three decades ago, an
6He was presented
example of the double-hypernucleus LL[1]. However, only a schematic drawing of the event wasgiven in the Letter, and measured angles were not presented. The authenticity of it was considered doubtful[2]. The other two double-hypernucleus events [2–5] have
either more than one interpretation for the species or the
possibility of production of excited states. The production
4of LL
H hypernuclei was recently reported in a counterexperiment [6], but the statistics were limited and a valueof DBLL was not presented.
Theoretical calculations of the binding energies ofdouble hypernuclei have been made since the 1960s,
aiming to obtain information on the L-L interaction
6[7 –9]. Among possible double-L hypernuclei, LL
He hasbeen considered to be important because it gives information not only on the L-L interaction but also on the6cluster structure of hypernuclei. The LL
He hypernucleus
constitutes the lightest closed shell structure containing p,212502-1© 2001 The American Physical Society

The observation of double-L hypernuclei gives important information about the L-L interaction. The binding
energy of two L hyperons, BLL , and the L-L interactionenergy, DBLL , can be obtained from the measurement of
the masses of double-L nuclei, where DBLL is defined byAAZ兲 苷 BLL 共LL
Z兲 2 2BL 共A21L Z兲 .
DBLL 共LL(1)0031-9007兾01兾 87(21)兾212502(5)$15.00

212502-1

VOLUME 87, NUMBER 21

PHYSICAL REVIEW LETTERSn, and L baryons [10]. Double-L hypernuclei are closely
related to the existence of the H dibaryon [11]. If themass of the H dibaryon, MH , was less than twice the L
hyperon mass in a nucleus, two L hyperons in the nucleus
would be expected to form the H. With this assumption,
the lower limit of the mass of the H dibaryon can becalculated from the following relation:
MH . 2ML 2 BLL ,(2)

where ML is the mass of a L hyperon in free space.
In order to study such systems, an emulsion/
scintillating-fiber hybrid experiment (E373) has been
carried out at the KEK proton synchrotron using the1.66 GeV兾c separated K 2 meson beam [12,13]. The
schematic view around the target is given in Fig. 1. J2hyperons were produced via the quasifree 共K 2 , K 1 兲 reactions in a diamond target [14] and brought to rest in Fuji
ET-7C emulsion. The 共K 2 , K 1 兲 reactions were tagged
by a spectrometer system. The positions and angles ofentry of the J2 hyperons at the emulsion were measured
with a scintillating microfiber-bundle detector [15] placed
between the diamond target and the emulsion stack. The
tracks of the J2 hyperons were scanned and traced in theemulsion by a newly developed automatic track scanningsystem [16]. An emulsion stack consisted of a thin emulsion plate located upstream followed by eleven thickemulsion plates [17]. The thin plate had 70-mm-thickemulsion gel on both sides of a 200-mm-thick acrylic basefilm, and each thick plate had 500-mm-thick emulsion gel
on both sides of a 50-mm-thick acrylic film.
Although we have analyzed only 11% of the total emulsion, we have found an event of seminal importance, amesonically decaying double hypernucleus emitted froma J2 capture at rest [18]. A photograph and schematic
drawing of the event are shown in Fig. 2. We named thisevent “NAGARA.” A J2 hyperon came to rest at point

FIG. 1. Schematic view of the experimental setup.

212502-2

19 NOVEMBER 2001

A, from which three charged particles (tracks No. 1, No. 3,
and No. 4) were emitted. One of them decayed into a p 2
meson (track No. 6) and two other charged particles (tracks
No. 2 and No. 5) at point B. The particle of track No. 2
decayed again to two charged particles (tracks No. 7 andNo. 8) at point C.
The measured lengths and emission angles of thesetracks are summarized in Table I. The particle of track
No. 7 left the emulsion stack and entered the downstream
scintillating-fiber block detector (D-Block) [19]. Track
No. 5 ended in a 50-mm-thick acrylic base film. The tracksof the three charged particles emitted from point A are
coplanar within the error as are the three tracks from point
B. The kinetic energy of each charged particle was calculated from its range, where the range-energy relation wascalibrated using a decays of thorium series in the emulsion and m1 decays from p 1 meson decays at rest.
The single hypernucleus (track No. 2) was identified
from event reconstruction of its decay at point C. Mesonicdecay modes of single hypernuclei were rejected because
their Q values are too small. The decay mode of thesingle hypernucleus is nonmesonic with neutron emission.
If either track No. 7 or No. 8 has more than unit charge,
the total kinetic energy of the two charged particles ismuch larger than the Q value of any possible decay mode
because of the long ranges of tracks No. 7 and No. 8.
Therefore, both tracks No. 7 and No. 8 are singly charged,
and only L He isotopes are acceptable.
The kinematics of all possible decay modes of the double hypernucleus (track No. 1) which decays into L He
(track No. 2) and p 2 (track No. 6) were checked, and
BLL and DBLL were calculated. Since track No. 5 ended
in the base film, only the lower limit of the kinetic energy

FIG. 2. Photograph and schematic drawing of NAGARA
event. See text for detailed explanation.

212502-2

VOLUME 87, NUMBER 21

19 NOVEMBER 2001

PHYSICAL REVIEW LETTERSTABLE I. Lengths and emission angles of the tracks. Angles are expressed by a zenith angle 共u兲 with respect to the direction
perpendicular to the plate and an azimuthal angle 共f兲. The indicated errors are the measurement errors only. For tracks No. 4,
No. 6, No. 7, and No. 8, the ranges in an acrylic base were converted to those in emulsion. The lengths of tracks No. 5 and No. 7
are the visible ones in the emulsion only. The length of track No. 7 inside the D-Block is 13.1 mm.
Point

Track No.

Length 关mm兴u [degree]f [degree]

A

1
3
4
2
5
6
7
8

8.1 6 0.3
3.2 6 0.4
88.3 6 0.5
9.1 6 0.3
82.1 6 0.6
13 697742.6 6 0.6
5868 6 2044.9 6 2.0
57.7 6 5.2
156.2 6 0.5
77.7 6 1.6
122.8 6 1.0
81.1 6 0.8
138.5 6 0.2
52.2 6 1.2

337.5 6 1.8
174.9 6 2.9
143.0 6 1.0
115.9 6 0.8
284.2 6 0.7
305.5 6 0.1
322.1 6 0.3
123.7 6 0.7

BC

can be determined. For the decay modes without neutron emission, the range of the particle of track No. 5 wasincreased to minimize the missing momentum. If the sumof the momenta of the three charged particles (tracks No. 2,
No. 5, and No. 6) deviated from zero by more than threestandard deviations even after the range of track No. 5 wasincreased from the missing momentum, that decay mode
was rejected. For the decay modes with neutron emission,
the upper limits of BLL and DBLL were obtained. Only
the results for DBLL . 220 MeV are listed in Table II.
The cases of double hypernuclei with more than two unitsof charge are not given because their values of DBLL wereless than 220 MeV.
Kinematical analysis of the production reaction wasmade by assuming that the J2 hyperon was captured by alight nucleus in the emulsion (12 C, N14 , or 16 O). This assumption is reasonable, taking into account the existence

Double hypernucleus

Single hypernucleus
Stopped in basep2
Stopped in D-Block
Scattered before stoppingof the short track No. 3 and the Coulomb barrier of the target nucleus. For each of the modes without neutron emission, if the sum of momenta deviated from zero by morethan three standard deviations, the mode was rejected. For
the modes with one neutron emission, the momentum of
the neutron was assigned to the missing momentum of thethree charged particles (tracks No. 1, No. 3, and No. 4).
For the modes with more than one neutron emission, thelower limits of the total kinetic energy of the neutrons were
calculated from the missing momentum. The results for
DBLL , 20 MeV are presented in Table III. The values
of BLL and DBLL were calculated with the J2 hyperon
binding energy BJ2 set to zero. Hence these values arelower limits of BLL and DBLL , and their true values arelarger, depending on the actual value of BJ2 .
A comparison of the values of BLL and DBLL obtained
from both points A and B was made. After rejecting theTABLE II. Possible decay modes of the double hypernucleus which include L He as a decay daughter. The errors on BLL and
DBLL do not include those of the binding energies of single hypernuclei. Only the cases of DBLL . 220 MeV are listed.
Double hypernucleus

No. 2

No. 5

No. 6

BLL [MeV]DBLL [MeV]

5
LL He
6
LL He
7
LL He
7
LL He
8
LL He
8
LL He
8
LL He
8
LL He
9
LL He
9
LL He
9
LL He
9
LL He
9
LL He
9
LL He
9
LL He
9
LL He

4
L He
5
L He
5
L He
6
L He
5
L He
5
L He
6
L He
7
a
L He
5
L He
5
L He
5
L He
6
L He
6
L He
7
a
L He
7
a
L He
8
L He

p
p
p
p
p
d
p
p
p
d
t
p
d
p
d
p

p2
p2
p2
p2
p2
p2
p2
p2
p2
p2
p2
p2
p2
p2
p2
p2

7.1 6 0.5
6.9 6 0.6
,8.6
6.3 6 0.7
,6.8
,7.4
,6.6
7.7 6 0.8
,7.2
,8.2
,11.2
,7.2
,8.4
,11.2
13.4 6 0.5
6.4 6 0.8

2.4 6 0.5
0.6 6 0.6
,0.3
22.0 6 0.7, 2 7.2, 26.6, 27.426.3 6 0.8, 27.1, 26.1, 23.1, 27.1, 25.9, 23.1
20.9 6 0.5
27.9 6 0.8

1n
2n
1n
1n
3n
2n
1n
2n
1n
1n

We took the value of 7.0 MeV for the upper limit of the L hyperon binding energy in 7L He, because it has not yet been averaged[21].
a212502-3

212502-3

VOLUME 87, NUMBER 21

PHYSICAL REVIEW LETTERSTABLE III. Possible production modes of the double hypernucleus. The errors on the mass of J2 hyperon and the binding
energies of single hypernuclei are not included in the errors on
BLL and DBLL . Only the cases of DBLL , 20 MeV are listed.
Target No. 1 No. 3 No. 4
12

C
C
12
C
12
C
14
N
14
N
14
N
14
N
14
N
16
O
12

6
LL He
6
LL He
6
LL He
7
LL He
6
LL He
6
LL He
6
LL He
7
LL Li
9
LL Li
8
LL Li

4

He
He
4
He
4
He
7
Li
6
Li
4
He
4
He
p
4
He
4

p
d
t
p
p
d
4
He
t
4
He
4
He

BLL [MeV] DBLL [MeV]
2n
1n
1n
1n
1n
1n
1n
1n
1n

.16.9
14.5 6 0.7
7.3 6 0.2
21.6 6 1.3
24.4 6 2.1
25.8 6 1.3
17.9 6 1.5
26.2 6 0.9
31.5 6 1.8
31.1 6 0.9.10.6
8.2 6 0.7
1.1 6 0.2
13.3 6 1.3
18.2 6 2.1
19.6 6 1.3
11.7 6 1.5
17.2 6 0.9
17.9 6 1.8
19.9 6 0.9

modes which have inconsistent values, only one interpretation remained,
126
C 1 J2 ! LL
He 1 4 He 1 t
6
52
LL He ! L He 1 p 1 p .

The fact that the tracks of the reaction products were coplanar at both points A and B also suggests that no neutrons
were emitted from either vertex. The decay mode of L5 He
is nonmesonic but undetermined.
The possibilities that the double hypernucleus or the
single hypernucleus was produced in an excited state
can be rejected for the following reasons. If the doublehypernucleus or the other fragments emitted from theJ2 stopping point had been produced in an excited state,
the value of DBLL calculated at the production point A
would be increased by the excitation energy. On the other
hand, if the single hypernucleus or the residual particles
emitted from the decay of the double hypernucleus had
been created in an excited state, the value of DBLL
calculated at the decay point B would be decreased by the
excitation energy. In both cases, the difference between
DBLL calculated at point A and at point B would beenlarged and the consistency of the values of DBLL
would not be satisfied. Hence, our event, NAGARA, hasbeen interpreted uniquely as the sequential weak decay of
6
6
LL He. Moreover, in the production and decay of LL He,
no particle-stable excited states are known or expected
for any of the reaction products. Therefore, there are no
ambiguities arising from excited states.
The value of DBLL was obtained as 0.62 6 0.61 MeV
from the decay vertex B of the double hypernucleus, while
its lower limit was determined as 1.08 6 0.22 MeV fromthe production point A. These errors also include theuncertainties in the values of the mass of the J2 hyperon (0.13 MeV) [20] and the binding energy of L5 He(0.02 MeV) [21]. A kinematic fit was applied at each vertex independently using the kinematic constraints of con212502-4

19 NOVEMBER 2001

servation of momentum and energy. In the fit at vertex B
the momentum of the proton (track No. 5) was constrained
to have a value consistent with the particle entering theacrylic base film but not emerging from it, whereas themass of the double hypernucleus was a free parameter inboth the fit at vertex A and the fit at vertex B. By minimizing the x 2 , we obtained DBLL 苷 0.69 6 0.54 MeV
from the decay vertex B, and DBLL 2 BJ2 苷 0.92 6
0.21 MeV from the production point A. The value of
BJ2 was obtained experimentally from these values as
20.24 6 0.58 MeV. The fitted momentum of the proton(track No. 5) was 87.9 6 3.0 MeV兾c and the corresponding range was 127 6 15 mm, which agrees with the factthat the proton entered but did not emerge from the basefilm.
The values of BLL and DBLL were determined uniquelyfrom vertex B with large errors, whereas the values obtained from vertex A were more precise but depend on
BJ2 . In order to obtain their most probable values, we
combined the two independent determinations for several
fixed values of the J2 hyperon binding energy BJ2 . The
results, expressed as a function of BJ2 (MeV), were
BLL 苷 7.13 1 0.87 BJ2 共60.19兲 MeV ,(3)

DBLL 苷 0.89 1 0.87 BJ2 共60.20兲 MeV .

(4)

According to theoretical calculations for the nuclear
absorption rate of J2 hyperons [22–24], J2 hyperon
capture from an atomic 3D state in 12 C is dominant, but
capture from a 4F or 2P state is not negligible. The
value of BJ2 of the 2P state varies with the J2 hyperonnucleus potential well depth, whereas the energy levelof the 3D state is better known because it depends
overwhelmingly on the Coulomb interaction rather
than the J2 hyperon-nucleus strong interaction. The
value of BJ2 of the 3D state is 0.13 MeV, whichis consistent with the present experimental result of
20.24 6 0.58 MeV.
Adopting the value BJ2 苷
0.13 MeV as the most probable value, the weighted meanvalues are BLL 苷 7.25 6 0.1910.18
20.11 MeV and DBLL 苷
10.18
MeV, where the systematic errors are
1.01 6 0.2020.11
determined from the fact that the value of BJ2 is uncertainin the range from 0 to 0.34 MeV in our measurement.
Two of the past experiments [2–5] gave a value of
DBLL to be about 4.5 MeV. As mentioned above, there
remains the possibility in both events that the single hypernuclei was produced in excited states. In such cases,
DBLL would be about 1 MeV and not in contradiction with
our result.
From the relation (2), the E176 experiment presented alower limit on the mass of the H dibaryon as 2203.7 6
0.7 MeV兾c2 [4]. Several counterexperiments have put theupper limits on the production rate of the H dibaryon[25–27], which were below the theoretical calculation [28]in the mass region below 2200 MeV兾c2 , and indicate thenonexistence of a deeply bound H dibaryon. Using the212502-4

VOLUME 87, NUMBER 21

PHYSICAL REVIEW LETTERS

present result of BLL from the decay vertex, the lower
limit of the H mass was found to be 2223.7 MeV兾c 2 at a90% confidence level, which is much closer to the two-L
threshold.
In summary, a double-hypernucleus event, NAGARA,
has been observed in an emulsion stack exposed to the1.66 GeV兾c K 2 meson beam. It is interpreted uniquely
6
as the sequential decay of LL
He emitted from a J2 hyperon nuclear capture at rest. The mass has been measured,and the binding energy of the two L hyperons, BLL , andthe L-L interaction energy, DBLL , have been determined
for the first time without the ambiguities arising from thepossibilities of excited states. The value of DBLL obtained
from the decay kinematics is 0.69 6 0.54 MeV. By using
both the production and decay kinematics, we obtained
10.18
MeV, where the J2 binding
DBLL 苷 1.01 6 0.2020.11
energy of an atomic 3D state in 12 C, 0.13 MeV, is used as
the most probable value, and the systematic error is determined from the error of the BJ2 obtained from this event.
It established that the L-L interaction energy is attractive
but considerably smaller than that previously estimated experimentally. The violent disagreement between our result
for DBLL and that reported in Ref. [1] confirms the doubtson the authenticity of the previous event. In addition, thelower limit of the mass of the H dibaryon has been obtained as 2223.7 MeV兾c2 at a 90% confidence level.
The authors thank the staff of the KEK proton synchrotron. They also express their thanks to Professor
T. Motoba, Professor Y. Yamamoto, and Professor
T. Yamada for useful discussions. The E373 experiment
was supported partly by a Grant-in-Aid for Scientific Research on the Priority Area “Strangeness Nuclear Physics”(No. 08239103) from the Ministry of Education, Science
and Culture in Japan and Korea Research Foundation
Grant (KRF-99-005-D00004).

*Email address: thitoshi@nh.scphys.kyoto-u.ac.jp†
Present address: Department of Physics, Pusan National
University, Pusan 609-735, Korea.
‡
Present address: Laboratory of Physics, Osaka ElectroCommunication University, Neyagawa 572-8530, Japan.
§
Present address: Department of Physics, Tohoku University, Sendai 980-8578, Japan.
kPresent address: Japan Atomic Energy Research Institute,
Tokai 319-1195, Japan.
¶
Present address: Chonnam National University, Kwangju
500-757, Korea.

212502-5

19 NOVEMBER 2001**Present address: Wonkwang University, Iri 570-749, Korea.
††
Present address: Department of Physics, Osaka City University, Osaka 558-8585, Japan.
[1] D. J. Prowse, Phys. Rev. Lett. 17, 782 (1966).
[2] R. H. Dalitz, D. H. Davis, P. H. Fowlder, A. Motwill,
J. Pniewski, and J. A. Zakrzewski, Proc. R. Soc. London
A 426, 1 (1989).
[3] M. Danysz et al., Nucl. Phys. 49, 121 (1963).
[4] S. Aoki et al., Prog. Theor. Phys. 85, 1287 (1991).
[5] C. B. Dover, D. J. Millener, A. Gal, and D. H. Davis, Phys.
Rev. C 44, 1905 (1991).
[6] J. K. Ahn et al., Phys. Rev. Lett. 87, 132504 (2001).
[7] Y. C. Tang and R. C. Herndon, Phys. Rev. Lett. 14, 991(1965).
[8] A. R. Bodmer, Q. N. Usmani, and J. Carson, Nucl. Phys.
A422, 510 (1984).
[9] X.-c. Wang, H. Takaki, and H. Bando, Prog. Theor. Phys.
76, 865 (1986).
[10] Bando et al. indicated its importance as a multihypernuclear cluster system, and proposed the name
“Lambpha 共â兲” for LL6 He by analogy to the a particle intheir paper [H. Bando et al., Prog. Theor. Phys. 66, 1344
(1981)].
[11] R. L. Jaffe, Phys. Rev. Lett. 38, 195 (1977).
[12] A. Ichikawa et al., Phys. Lett. B 500, 37 (2001).
[13] A. Ichikawa, Ph.D. thesis, Kyoto University, 2001
(unpublished).
[14] D. E. Alburger and M. May, Nucl. Instrum. Methods Phys.
Res., Sect. A 443, 27 (2000).
[15] A. Ichikawa et al., Nucl. Instrum. Methods Phys. Res.,
Sect. A 417, 220 (1998).
[16] A. Ichikawa et al. (to be published).
[17] H. Akikawa et al. (to be published).
[18] We have also found four candidate events which include a
double-strangeness system. Some of them are discussed inRef. [12] and in Refs. [13,29].
[19] H. Takahashi et al., Nucl. Instrum. Methods Phys. Res.,
Sect. A (to be published).
[20] Particle Data Group, D. E. Groom et al., Eur. Phys. J. C
15, 1 (2000).
[21] M. Juric et al., Nucl. Phys. B52, 1 (1973).
[22] C. J. Batty, E. Friedman, and A. Gal, Phys. Rev. C 59, 295(1999).
[23] D. Zhu, C. B. Dover, A. Gal, and M. May, Phys. Rev. Lett.
67, 2268 (1991).
[24] T. Koike (private communication).
[25] J. K. Ahn et al., Phys. Lett. B 378, 53 (1996).
[26] R. W. Stotzer et al., Phys. Rev. Lett. 78, 3646 (1997).
[27] K. Yamamoto et al., Phys. Lett. B 478, 401 (2000).
[28] A. T. M. Aerts and C. B. Dover, Phys. Rev. D 28, 450(1983). The modified calculation is presented in Ref. [27].
[29] J. K. Ahn et al., in the Proceedings of the Conference on
Hadron and Nuclei, Seoul, 2001 [AIP (to be published)].

212502-5

Nuclear Instruments and Methods in Physics Research A 483 (2002) 689–697

Scintillating-ﬁber-block detector for studying
double-strangeness nuclei
H. Takahashia,*, H. Akikawaa, S. Aokib, A. Ichikawaa, M. Ieiric, K. Imaia,
H. Kandaa,1, Y. Kondoa,2, K. Nakazawad, J.T. Rheee, J. Tojoa,
K. Yamamotoa,3, T. YoshidafaDepartment of Physics, Faculty of Science, Kyoto University, Kyoto 606-8502, Japanb
Faculty of Human Development, Kobe University, Kobe 657-8501, Japanc
KEK, High Energy Accelerator Research Organization, Tsukuba 305-0801, Japand
Physics Department, Gifu University, Gifu 501-1193, Japane
Institute for Advanced Physics, Konkuk University, Seoul 143-701, South Korea
fDepartment of Physics, Osaka City University, Osaka 558-0022, Japan

Received 16 May 2001; received in revised form 6 August 2001; accepted 11 August 2001

Abstract
Scintillating-ﬁber-block detectors have been constructed using 0:5 mm square ﬁbers for the KEK-PS E373
experiment, which searches for double-strangeness nuclei. The main aim of the detector is to measure the energies ofprotons and pions produced from decay of double- or single-hypernuclei using their ranges. It is also used to detect L
hyperons or the H-dibaryon, and to make event selection. The performance of the detectors was studied with lowenergy proton and pion beams as well as minimum-ionizing particles. The energy resolution determined by the range
measurements was 0:86 MeV for 37 MeV p ’s and 1:4 MeV for 47 MeV protons. The excellent capability of p=p
separation has been also demonstrated. r 2002 Elsevier Science B.V. All rights reserved.
PACS: 29.40.Gx; 29.40.Mc; 21.80.+aKeywords: Scintillating ﬁber; Image-intensiﬁer tube; Double-L hypernucleus

1. Introduction*Corresponding author. Tel.: +81-75-753-3842; fax: +8175-753-3887.
E-mail address: thitoshi@nh.scphys.kyoto-u.ac.jp(H. Takahashi).
1
Present address: Department of Physics, Tohoku University, Sendai 980-8578, Japan.
2
Present address: Japan Atomic Energy Research Institute,
Tokai 319-1195, Japan.
3
Present address: Department of Physics, Osaka City
University, Osaka 558-0022, Japan.

The history of tracking detectors using scintillating optical ﬁbers began in the late 1950s [1]. In
the early 1980s, the technologies of ﬁber optics,
image-intensiﬁer tubes (IITs) and charge-coupleddevices (CCDs) were established [2,3] and ﬁber
detectors have drawn much attention as tracking
detectors with good spatial resolution in highenergy physics experiments [4–6]. This technique0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 9 4 6 - 5

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H. Takahashi et al. / Nuclear Instruments and Methods in Physics Research A 483 (2002) 689–697

has also been used to detect low-energy hyperons
in strangeness nuclear physics experiments, such as
the H-dibaryon search experiments [7,8] andhyperon–nucleon scattering experiments [9,10].
We have constructed scintillating-ﬁber-blockdetectors (SciFi-Blocks) for the E373 experiment,
which was carried out in the K2 beam line of the
12 GeV proton synchrotron at KEK. The purposeof this experiment is to search for double-strangeness nuclei, such as LL-nuclei, X-nuclei or the
H-dibaryon, with a counter-emulsion hybrid
method [11]. This method was used in the former
E176 experiment with great success, such as to
conﬁrm the existence of LL-nuclei which are stableagainst strong decay [12]. However, only one pmesonic decay event was found, and there
remained two kinds of interpretations for the
species. The L–L interaction has not been
determined uniquely yet. In E373, we search for
double-strangeness nuclei with 10 times higher
statistics than that in E176.
A schematic view of the target region of theE373 experiment is shown in Fig. 1. In order toobtain such high statistics with limited amount of
nuclear emulsion, the thickness of the emulsion
stack has been reduced to about one-ﬁfth of thatfor E176 [13]. For this reason, pions and protons
emitted from the process of weak decay ofhypernuclei may not be stopped in the emulsion.
It is very important, however, to measure theenergy of such particles in identiﬁcation of thenuclide and in determination of the mass of the
double-hypernucleus and the L–L interaction.
Therefore, we used the SciFi-Blocks as a range
counter for pions and protons.

The X particles, produced via the quasi-free‘‘p’’ (K  ; K þ ÞX reactions in a diamond target,
are slowed down in the target, and some of them
are stopped in a nuclear emulsion. These(K  ; K þ ) reactions are tagged using a magnetic
spectrometer system. Hyperfragments emitted
from the X absorption points are observed in
the emulsion. The positions and directions of theX particles are measured with a scintillatingmicroﬁber-bundle detector (SciFi-Bundle), which
is located between the diamond target and thenuclear emulsion. Details of the SciFi-Bundle
detector are described in Ref. [14].
We have used two SciFi-Blocks, an upstream
block (U-Block) located upstream of the SciFiBundle and a downstream block (D-Block) placeddownstream of the emulsion chamber. The main
aim of the SciFi-Blocks is to determine the energyof particles, such as pions and protons, produced
from decay of hypernuclei by measuring theirranges in the blocks. The second purpose is todetect L or the H-dibaryon emitted from thestopping points of X ’s or (K  ; K þ ) reaction
vertices by observing their decay. In addition, they
are used for event selection to reduce the numberof events to be analyzed in the emulsion, whichneeds huge amount of time. For example, theevent in which X decays before stopping in theemulsion is rejected by the human eye scanning.
In this paper, we present an evaluation of the
SciFi-Blocks using low-momentum pion and proton beams and minimum-ionizing particle’s tracks.
In the next section we describe in detail the SciFiBlock detectors; their performances are presented
in Section 3.

2. SciFi-Block detectors
We have used 0:5 mm  0:5 mm square ﬁbers(KURARAY SCSF-784). The core is made ofpolystyrene with a refractive index of 1.59 and adensity of 1:06 g=cm3 : It is surrounded by a 10 mmthick cladding made of polymethylmethacrylate(PMMA) with an index of 1.49 and a density ofFig. 1. Schematic view of the target region.

4
KURARAY Co. Ltd., Kurashiki 2-28, Nakajo, Kitakanbara, Niigata 959-2653, Japan.

H. Takahashi et al. / Nuclear Instruments and Methods in Physics Research A 483 (2002) 689–697

1:18 g=cm3 : The attenuation length was measured
with a radioactive source of b-rays [15].
Since the position resolution of a SciFi-Blockdepends on the precision of the packing of ﬁber
sheets, we required a precision of 0:2 mm in thepacking. The number of ﬁbers per sheet is 144 for
the U-Block and 120 for the D-Block. The UBlock consists of 98 ﬁber sheets and the D-Block
90 ﬁber sheets.
The structure of the U-Block is shown in Fig. 2.
There is a square hole (20 mm  20 mm) in thecenter of the U-Block in order to insert the
diamond target and to pass the beam through.
For this reason, it has a very complicatedstructure. To make the hole without making
unreadable dead spaces, one sheet was divided
into two partial sheets. One is extended to the leftside IIT and the other to the right-side IIT. The
partial sheets from the u- and v-direction were
piled up alternately at the input window of each
IIT chain (Fig. 3(a)). Therefore, two images of uzand vz-plane are read out as one overlapped
image. It is thus necessary to divide one obtained
image to each sheet and reconstruct the original
two images. At the readout-end of the ﬁber sheets,
0:3 mm thick black vinyl chloride sheets wereinserted between ﬁber sheets to identify each sheet

Fig. 2. Schematic view of the U-Block.

691

Fig. 3. Basic structure of the U-Block (a) and the D-Block (b).
In the U-Block, u sheets and v sheets are piled up alternately. In
the D-Block, each sheet is divided into upper and lower parts
and they are stacked by turns.

easily. At the detection area, 0:3 mm thick Al
degraders were inserted between ﬁber sheets inorder to increase the acceptance of the block as arange counter. In order to increase the number ofdetected photoelectrons, one side of each ﬁber
sheet for the U-Block which is opposite to thereadout-end was polished and evaporated withaluminum to reﬂect the photons. The eﬀect of thismirror was studied by measuring the yield ofscintillation light induced by the passage of b-rays.
It was found that the light yield was increased by
1.6 times with the mirror.
The structure of the D-Block is shown in Fig. 4.
In order to utilize the full aperture of the IIT, each
sheet was divided into two partial sheets, and they
were piled up alternately at the input of each IIT(Fig. 3(b)). Hence, images of upper part and lower
part are overlapped to each other. Black vinyl
chloride sheets of 0:2 mm in thickness and Al
degraders of 0:3 mm were inserted at the readoutend of the ﬁber sheets and at the detection area,
respectively, like the U-Block. Black acrylic plates

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H. Takahashi et al. / Nuclear Instruments and Methods in Physics Research A 483 (2002) 689–697

Fig. 4. Schematic view of the D-Block.

evaporated with aluminum were attached as
mirrors to the end of the block opposite to thereadout-end.
Each IIT chain of the U-Block consists of four
image-intensiﬁer tubes and a CCD (Fig. 5(a)). The
input window of the ﬁrst stage IIT is 100 mmf indiameter. The ﬁrst (HAMAMATSU V4440PX5)
and second (DEP PP0030X6) stages of the IIT
chain are the electrostatically focusing type. The
third and fourth (PROXITRONIC BV2583EG,
see footnote 6) are the Micro Channel Plate(MCP)
type. The ﬁrst-stage IIT has a magniﬁcation factorof 25=100 and the other stages have a magniﬁcation of 1. A tapered-optical-ﬁber plate with amagniﬁcation of 7=25 was inserted between the
fourth-stage IIT and the CCD chip, giving thetotal magniﬁcation of 7=100: The phosphor ofeach stage is PS-5, P46, P20 and P20 in that order.
The IIT chain of the D-Block consists of three
image-intensiﬁer tubes (DEP PP0040C, see footnote 6) and a CCD (Fig. 5(b)). The input windowof the ﬁrst-stage IIT is 80 mmf : The ﬁrst stage of
the IIT chain is the electrostatically focusing typewith a magniﬁcation of 16=80: The second andthird IITs are the MCP type. A tapered-opticalﬁber plate with a magniﬁcation of 11=18 wasinserted between the third-stage IIT and the CCD.
The total magniﬁcation is 11=90: The phosphors of5
Hamamatsu Co. Ltd., Toyooka Factory, Shimokanzo 3145, Toyooka, Iwata, Shizuoka 438-0193, Japan.
6
DHT Co. Ltd., 1-636 Maruko-doori, Nakahara, Kawasaki,
Kanagawa 211-0006, Japan.

Fig. 5. Schematic drawing of the structure of IIT chains for the
U-Block (a) and the D-Block (b).

the ﬁrst, second and third stages are P24, P20 and
P20, respectively.
The CCD chips have 768  493 pixels, where thesize of each pixel is 11 mm  13 mm: The acquisition of images from the CCD cameras was
performed with three types of modules; a clockand-coordinate generator (CCG) module, ﬂash
ADC (FADC) modules and ﬁrst-in–ﬁrst-out (FI/FO) memory modules. The CCG module provides
a common clock (14 MHz) to synchronize thewhole system, and generates two-dimensional
coordinates of each CCD pixel. It initiates asequence of digitization when the second-level
trigger is ﬁred. The FADC modules digitize thepulse height of the video signals into 8-bit brightness data for each pixel. For those pixels having
brightness above some threshold, the write-strobe
signals to the corresponding FI/FO module are
generated. Then, the data on the brightness (8-bit),the coordinates (H:10-bit, V:10-bit) and the event
number (4-bit) are stored in the memory of the FI/
FO module. Each of the FI/FO modules has four
FIFO chips, IDT-7207 for the SciFi-Bundle andthe U-Block and IDT-7208 for the D-Block. The
total memory size of a module is 128 kB (IDT7207  4) or 256 kB (IDT-7208  4).
Images are distorted mainly due to the pin-holedistortion caused by the electrostatic lens of IITs.

H. Takahashi et al. / Nuclear Instruments and Methods in Physics Research A 483 (2002) 689–697

Since these SciFi-Block systems were installed very
close to the spectrometer, their images were also
distorted by the magnetic ﬁeld (about 0.01 or
0:02 T). We minimized the distortion of images bycovering each IIT chain with a double magnetic
shield made of iron and m-metal. The eﬀect due tothe remaining magnetic ﬁeld was corrected during
an oﬄine analysis. The correction parameters were
determined using the images of a plate with a gridpattern holes, which was attached to the input
window of the IITs and illuminated by a LED.

693

The error of the correction was estimated from thediﬀerences between the corrected positions of eachpoint of the grid and the real positions. The
standard deviation of the diﬀerences was 130 mm
for the U-Block and 145 mm for the D-Block.
After this correction, image reconstruction was
performed. Fig. 6 shows an example of a reconstructed image, where the decay of a X hyperon
can be clearly seen.

3. Performance of the SciFi-Block detectors
The SciFi-Block detectors were installed in theK2 beam line of KEK-PS. We employed lowenergy proton and pion beams as well as
minimum-ionizing beams for calibrating the SciFi-Blocks. A K  beam of about 1:66 GeV=c wasused to evaluate the position resolution. For
calibrating the range–energy relation, data with aproton beam of about 600 MeV=c and a p ofabout 400 MeV=c were analyzed. The capability ofthe separation of stopped-p /stopped-proton was
also evaluated with the data of the low-momentum
beams.

3.1. Position resolution

Fig. 6. Example of image reconstruction. (a) Images before
reconstruction (after calibration) and (b) those after reconstruction. A X hyperon decayed in the D-Block can be clearly seen.
The incident K  was not observed because it had passed
through the center hole of the U-Block.

The position precision and resolution of the
SciFi-Blocks were evaluated with the values of‘‘cluster residual’’ and ‘‘track width’’, respectively.
The cluster residual is deﬁned as the distribution of
the length from a ﬁtted straight-line to the centerof each photon cluster. The track width is the
distribution of the distance from one track to eachpixel weighted by the brightness of the pixel. The
results are summarized in Table 1.
The worse resolution of the U-Block comesfrom the diﬃculty of packing the ﬁber sheets
because of its complicated structure. Taking into
account the precision of the packing, these results
are consistent with the expected values, which were
calculated based on the size of the ﬁbers, thedistortion of the images, the resolution of the
CCDs and the cluster size of one photon.

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H. Takahashi et al. / Nuclear Instruments and Methods in Physics Research A 483 (2002) 689–697

Table 1
Position precision and resolution (r.m.s.) for straight tracks ofminimum-ionizing particles

U-Block
D-Block

Cluster residualðmmÞTrack width
ðmmÞ

290
250

350
290

3.2. Stopped-p /stopped-proton separation
We must determine the stopping position of thep ’s in order to measure the range. When a pmeson comes to rest in a material, it is absorbed bya nucleus, and then some evaporated particles are
emitted. In the case of an event which shows a kinktrack in the SciFi-Blocks, it is necessary to
distinguish whether the track is a scattered p oran evaporated particle from the stopping point ofa p : If an evaporated particle is an a or heavier
particle, its range is less than the thickness of one
ﬁber. An evaporated particle whose range was
longer than the thickness of a ﬁber can be regarded
as a proton, deuteron or triton. Therefore, it isnecessary to separate p ’s from protons to
distinguish p scattering from particle evaporation. Furthermore, it is important to distinguishwhether a decay daughter of a hypernucleus is apion or not in order to determine the decay modeof the hypernucleus, i.e., mesonic decay or nonmesonic decay.
The separation capability was studied using the
low-momentum p ’s and protons. Identiﬁcation of
the beam particles was made based on the time of
ﬂight (TOF) through the beam line. The peaks of
the TOF for p ; m and e were clearly separated.
Fig. 7 shows an example of an image of a
stopped p : Since the brightness of light emitted
from a ﬁber is proportional to the energy loss of aparticle in the ﬁber, the brightness distribution
varies depending on the particle species, like the‘‘Bragg curve’’. It is possible to distinguish p ’sfrom protons by comparing the mean brightness
per track length near the stopping point. Because
the brightness of the stopping point of p can bevery high due to the short-range evaporation ofparticles, the brightnesses of tracks of stopping

Fig. 7. Image near the stopping point of a p : Since thebrightness of the stopping point of a p can be very high, thebrightnesses of the stopping tracks of p ’s and protons arecompared without including the stopping points.

p ’s and protons were compared without including the stopping points. Since the gains of the IITs
are diﬀerent from each other, the brightness was
obtained by calibrating the ADC value using the
track of minimum-ionizing particles.
Fig. 8 shows the result in which the mean
brightness was obtained using 8 ﬁber sheets. The
threshold brightness to separate p ’s from protons, T; was determined as a point of interior
division between the peaks of p ’s and protons:
sp mp þ sp mp
T¼
sp þ spwhere sp and mp are the standard deviation and themean of the brightness distribution of the p ’s,
respectively; sp and mp denote those for protons.
For the U-Block, the distance from the threshold
T to the p ’s peak is 2:0sp and that to the peak ofprotons is 2:0sp : This means that 97.7% of thep ’s or protons are distributed below/above thethreshold. In the case of the D-Block the distance
between the brightness peak and a threshold is also
2:0sp for the p ’s and 2:0sp for the protons.
The distances of the peaks and the ratio of the
distribution are summarized in Table 2. Even
using only four ﬁber sheets, more than 94% of the

H. Takahashi et al. / Nuclear Instruments and Methods in Physics Research A 483 (2002) 689–697

695

Fig. 8. Mean brightness (per mm) near the stopping points of p ’s and protons calculated from eight ﬁber sheets.

Table 2
Distances from the peaks of the mean brightness of p ’s andprotons to the threshold values, and the ratio of the distribution
Total numberof ﬁber sheets
4
6
8

U-Block

D-Block

Distance

Ratio

Distance

Ratio

1:8s
1:9s
2:0s

96.2%
97.3%
97.7%

1:6s
1:8s
2:0s

94.0%
96.4%
97.7%

straight track in the image were selected. Eachproton track was ﬁtted to a straight line and its
range was determined by automatic tracking
software. For p ’s, however, all the events were
scanned by human eyes because the event topology
might be more complicated.
The results are shown in Fig. 9. The curves on
the plots are the results of the ﬁtting to the wellknown relation between range R and energy T;
R ¼ a  Tbp ’s and protons are distributed below and above
the threshold, respectively.
3.3. Range resolution
As mentioned in Section 1, the main purposeof the SciFi-Blocks is to measure the energyof a particle from its range in the blocks. The
range resolution of the SciFi-Blocks is very important for precise measurements of the mass of
hypernuclei.
An analysis for the range resolution was madefor the U-Block. The incident energy at the SciFiBlock as well as particle identiﬁcation wasdetermined from the TOF. The relation between
the TOF and the incident energy, and the error of
the incident energy were estimated with a simulation. Only those events which included a simple

where a and b are the ﬁtting parameters.
The range resolution was estimated from the
distance from the ﬁtted curve to each data pointand from the error of the incident energy. It is
1:1 mm for 37 MeV p ’s and 0:57 mm for 47 MeV
protons. These values are almost same as theresults of a simulation which included the eﬀect ofrange straggling. The values from the simulation
are 1:1 and 0:55 mm; respectively. These range
resolutions are equivalent to the energy resolutionsof 0:86 MeV for 37 MeV p ’s and 1:4 MeV for
47 MeV protons. These values satisfy the requirement from the E373 experiment.
Finally, we have reconstructed the mass of L
hyperons which decayed to p ’s and protons in the
SciFi-Blocks using the obtained range–energy
relation. Fig. 10 shows the mass distribution
for L’s reconstructed from the ranges of the p ’s

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H. Takahashi et al. / Nuclear Instruments and Methods in Physics Research A 483 (2002) 689–697

Fig. 9. Range–energy curve for p and proton.

SciFi-Blocks, an upstream block (U-Block) and adownstream one (D-Block), to determine theenergy of pions and protons emitted from decay
of hypernuclei by measuring their ranges. The
performance of the SciFi-Blocks was evaluatedusing low-momentum pion and proton beams aswell as minimum-ionizing particle beams. The
position precisions are 290 and 250 mm for the UBlock and the D-Block, respectively. The relations
between the range and the energy for the p ’s andprotons were obtained using low-momentum p
and proton beams. The energy resolution was
0:86 MeV for 37 MeV p ’s and 1:4 MeV for
47 MeV protons. It was found that the SciFiBlocks had a capability for p/p separation. For
more than 94% of the p ’s/protons the mean
brightness near the stopping points was distributed
below/above some threshold.
Fig. 10. The mass distribution for L’s calculated from therange and the angle of the tracks of p and proton.

Acknowledgements
and protons, both of which stopped in the SciFiBlock. The mean value of the mass is1115:771:4 MeV=c2 ; which demonstrates the validity of our range–energy relations.

4. Summary
We have studied scintillating-ﬁber-block detectors (SciFi-Blocks) for the double-strangenessnuclei experiment, KEK-PS E373. We used two

We would like to thank the accelerator crew, thebeam-channel group and other staﬀ of the KEK
12 GeV proton synchrotron for their continuous
support. We are very grateful to Nisseidenki Co.
Ltd. for the construction of the SciFi-Blocks. This
work was supported in part by a Grant-in-Aid for
Scientiﬁc Research on the Priority Area ‘‘Strangeness Nuclear Physics’’ (No. 08239103) from the
Ministry of Education, Science and Culture in
Japan.

H. Takahashi et al. / Nuclear Instruments and Methods in Physics Research A 483 (2002) 689–697

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[6] C. Angelini, et al., Nucl. Instr. and Meth. A 289 (1990)
342.
[7] J.K. Ahn, et al., Phys. Lett. B 378 (1996) 53.
[8] K. Yamamoto, et al., Nucl. Phys. A 639 (1998) 371.
[9] J.K. Ahn, et al., Nucl. Phys. A 648 (1999) 263.
[10] Y. Kondo, et al., Nucl. Phys. A 676 (2000) 371.
[11] J.K. Ahn, et al., Nucl. Phys. A 670 (2000) 289.
[12] S. Aoki, et al., Prog. Theor. Phys. 85 (1991) 1287.
[13] H. Akikawa et al., Nucl. Instr. and Meth. A, accepted.
[14] A. Ichikawa, et al., Nucl. Instr. and Meth. A 417 (1998)
220.
[15] J.K. Ahn, et al., Nucl. Instr. and Meth. A 457 (2001) 137.

E391a T. Inagaki_
Measurement of the KL → π0νν Decay
T. Inagaki et al.
[attached]
Development of CeF-3 crystal for high-energy electromagnetic calorimetry
Nucl. Inst. Meth. A 443, (2000) 126.
○ H. Watanabe et al.
[attached]
Scintillator-Lucite sandwich detector for n/gamma separation in GeV energy region
Nucl. Inst. Meth. A 484, (2002) 118.
○ M. Itaya et al.
[attached]
Development of a new photomultiplier tube with high sensitivity for a wavelength-shifter fiber
readout
Nucl. Inst. and Meth., A522, (2004) 477.
★ H. Watanabe_
K0 beam line for the study of the KL → π0νν decay
Saga University 2001.
◇ M. Yamaga et[attached]_ al.
KL → π0νν experiment at KEK 12-GeV PS, E391a
Nucl. Phys. A721 (2003) 449c.
◇ Y. Sugaya et al.
_
DAQ system for the KL → π0νν search at KEK-PS,
Proc. of 13th IEEE-NPSS Real Time Conference 2003, RT-034.
＊ K. Abe et al.
_
Status of the KL → π0νν Experiment at KEK (E391a)
KEK Preprint 2000-89 (2000).
＊ T. Inagaki_
Status of the KL → π0νν Experiment (KEK-E391a),、Report on the engineering run
HE News Vol21, No3 (2003) (in Japanese).
＊ T. Inagaki et al.
_
Status of the KL → π0νν Experiment (KEK-E391a),、Report on the detector construction
HE News Vol23, No1 (2004), (to be published in Japanese).
○________________________________________________________________________● Physics papers published in refereed journal.
○★
◇＊

Technical papers.
PhD theses.
Conference and Symposium.
Internal Report and others.

Nuclear Instruments and Methods in Physics Research A 443 (2000) 126}135

Development of CeF crystal for high-energy electromagnetic
3
calorimetry
Takao Inagaki!,*, Yoshio Yoshimura!, Yukio Kanda", Yoshikazu Matsumoto#,
Kazuhiro Minami#!Institute of Particle and Nuclear Studies, High Energy Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan"Applied Research Laboratory, High Energy Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan#Ohyo Koken Kogyo Co., Ltd (OKEN), 88-77 Shingou, Higashimatsuyama, Saitama 355-0071, Japan
Received 10 March 1999; received in revised form 16 August 1999; accepted 18 August 1999

Abstract
An R&D study has been performed to develop a long CeF crystal for electromagnetic calorimetry in high-energy
3physics. The successful growth of samples with good quality implies that application could be near. ( 2000 Elsevier
Science B.V. All rights reserved.

1. Introduction
A new crystal, CeF , is one of the heavy and fast3
scintillators suitable for various high-energy physics experiments. Since most experiments, regardlessof the collider or the "xed target, are carried out ina high-rate environment in order to obtain highsensitivity, there is a special demand for a heavy
and fast scintillator. When Anderson [1,2] andMoses}Derenzo [3] discovered the good properties
of CeF on small samples in 1989, it was thought
3
that most crystal calorimeters in high-energy physics would soon be replaced by CeF from other3
crystals so far available. Many attempts have been
carried out to grow long CeF crystals for high3energy calorimeters since then. However, long

* Corresponding author.
E-mail address: takao.inagaki@kek.jp (T. Inagaki).

CeF crystals having the same properties as those3of short samples have not yet been stably grown.
The growth on long, transparent crystals has beenfound to be not easily achieved.
Here, we report the status of our study, which
was started soon after the discovery, and which isstill being continued by a small, local collaboration.
In Section 2 we summarize the properties of CeF ,3and the fabrication of long crystals is discussed in
Sections 3 and 4.

2. Properties of CeF crystal
3
Table 1 shows the properties of CeF to3
gether with those of another crystal of undoped-CsI(CsI), which is one of the best available
scintillators with a fast response. Most of thevalues are those reported in Refs. [1,2] andsome are obtained from measurements on small
samples.

0168-9002/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 9 6 2 - 6

T. Inagaki et al. / Nuclear Instruments and Methods in Physics Research A 443 (2000) 126}135

127

Table 1
Properties of CeF and undoped-CsI crystals
3

Density (gr/cm3)
Radiation length (cm)Moliere radius (cm)Light yield (in ratio with NaI, %)Temperature dependence of light yield (%/3C)
Decay time (ns)
Refraction index at 400 nm
Hygroscopicity
Tensil strength (dyn/cm2)
Young modulus (dyn/cm2)
Thermal expansion coe$cient (/3C)
Thermal-neutron cross section (barn)(a) CeF is as strong as CsI in terms of the stopping3
power for high-energy gammas, because CeF
3
is a high-density material. The z-number of
CeF is smaller than that of CsI. It is compen3
sated by a higher density, and CeF possesses
3
a slightly shorter radiation length than CsI. The
low-z and high-density cause the shape of a cascade shower to be thin. The Moliere radius of
CeF is considerably smaller than that of CsI,
3
where the Moliere radius is de"ned as theradius of a cylinder in which 90% of the energy
can be captured for a gamma injected in thecenter. A small Moliere radius, which meansa small tranverse spread of the cascade shower,
is very important to avoid signal overlap that isalways harmful in a high-rate experiment.
(b) The light output of CeF is greater than that of3
CsI by about 50%. Fig. 1 shows the responses of
CeF and CsI to gamma rays from 137Cs and3
60Co. The CeF crystal has an emission peak at3
a wavelength of 300 nm, which is similar to CsI
crystal. Fig. 2 shows the emission spectrum
from a CeF crystal irradiated by gamma rays
3
from 137Cs. The light output of CeF depends
3
only slightly on the temperature, while the large
temperature dependence of CsI is always
troublesome in applications.
(c) There is no very slow component in the scintillation decay time of CeF , unlike that in other3
crystals, such as 1 ls in CsI and 10 ls in BaF .
2This is important not only for good timing, but

CeF
3

CsI

6.16
1.7
2.6
4}5
0.05
5, 30
1.62
none
3.1]1091.1]1012
1.3]10~5
0.65

4.53
1.9
3.8
3}4
!1.5
10, 30, '1000
1.75
slight*(2]10104.8]10~535also for preventing the signal base-line fromshifting due to an overlap of slow components
in a high-rate experiment, which is very important especially for an experiment to manage
small signals. Fig. 3 shows the decay time distribution.1(d) The refractive index of CeF is small and near3
to the glass value. This means that special selections of a re#ector and an adhesive, which always cause trouble in an assembly of crystals
with a high refractive index, are not necessary toobtain good light collection and transmission
for CeF . Moreover, it implies that a long CeF
3
3
crystal could be built by gluing together smaller
pieces. A small refractive index reduces the timing spread due to a depth #uctuation of shower
development.
(e) The CeF crystal does not show hygroscopicity,3
which is painful in any application of NaI, CsI
and BaF . The mechanical properties of CeF
2
3
are similar to those of metal. A large di!erencein the thermal expansion between CsI andmetals causes some problems for the calorimeter assembly, for example, a metal attachment to support readout devices. There is nosuch problem in CeF . Fig. 4 shows the elastic3
linearities of both CeF and CsI. Although
3
1 The decay time distribution was measured by Norio Tamuraand Hiroyuki Maeohmichi at Okayama University in 1993.

128

T. Inagaki et al. / Nuclear Instruments and Methods in Physics Research A 443 (2000) 126}135

Fig. 1. Light yields of CeF ((a) and (b)) and CsI ((c) and (d)) for gamma rays from 137Cs and 60Co, respectively. They are measured using3
the same photomultiplier with a quartz window for samples having the same size of 20 mm( and 40 mml. The CeF sample is a piece3
being cut from the central part of a 35 cm long ingot. ADC gain was adjusted for each measurement to get the photo-absorption peak inthe middle of ADC range.

Fig. 2. Emission spectrum from CeF with irradiation by 137Cs
3
gamma rays.

CeF shows elastic deformation, CsI does not.
3
The arrow in Fig. 4(b) indicates the pressure
loaded under a pile of 2 m high CsI. Since CsI is
already inelastically deformed at the pressure, it
raises a concern about the long-term stability ofthe scintillation property of CsI in a large assembly.
(f) The cross section to capture thermal neutrons issmall for CeF , since Ce has the smallest ther3
mal-neutron cross section in all lanthanide

Fig. 3. Decay-time distribution of CeF scintillation light. The
3
solid line is a "t to the data between 5.4 and 70 ns by a function
having two exponentials: a e~t@q1 #a e~t@q2 , where the best-"t1
2
values of q , q and a /a are 2.53, 30.85 ns and 14.4%.
1 2
1 2

atoms. This would be helpful for any experiment with a thermal-neutron background.
(g) The following are other things which require
more quantitative investigations and are notlisted in Table 1. The internal radioactivity of
CeF is small and less than that of BaF by at3
2
least a factor of three. We are now preparing

T. Inagaki et al. / Nuclear Instruments and Methods in Physics Research A 443 (2000) 126}135

129

tained in the popular rare-earth ores, such as Monazite, BastnaK site and Xenotime. It is very interesting that almost all lanthanide atoms are distributedin each ore [5]. The richest atom is Ce. Some rareatoms (Y, Sm, Nd, Eu, etc.) are high-tech materials
and are of high demand in commercial applications. The large production of Ce is due to this highmarket demand for Y, Sm, Nd, Eu, etc. Thus, theuse of Ce means a kind of resource preservation onearth.

3. For long crystals
3.1. Problems

Fig. 4. Strain vs. stress curves for (a) CeF and (b) CsI crystals.
3
The scale for stress is quite di!erent from each other. The arrowin (b) indicates the stress (pressure) under the CsI pile of 2 m.

a low-background set-up in order to measure
the internal radioactivity conclusively. CeF
3
was reported to be highly radiation-resistive[4]. A high radiation resistance is very crucial
for many high-rate experiments. However, wehave not yet measured a tolerable dose rate for
CeF , because the radiation resistance (or radi3
ation damage) may be very sensitive to theimpurities contained and the crystal imperfections. In any case, the radiation tolerance aswell as the internal radioactivity should bechecked soon after a successful crystallizationof practical size.
As described above, CeF is better than CsI con3
cerning the many properties demanded for applications to high-energy physics. In addition to them,
CeF is expected to be cheaper than CsI when3
a cheap method for crystal growth is established.
The raw-material Ce is abundant on the earth. It is
more common than cobalt and zinc. Ce belongs to
the rare-earth, lanthanide, and is maximally con-

Many groups have tried to develop a long crystal
that is useful for high-energy physics, since thediscovery of the good properties of CeF , using3
small (1-cm cubic) samples in 1989 [1}3], The largest group is the Crystal Clear collaboration at
CERN [6]. They wanted to use CeF for electro3
magnetic calorimeters in the LHC experiments,and started their R&D in 1991, but gave up in 1996.
The main reason for this was said to be due toa lack of reproducibility [7].
We started R&D in 1992 [8,9]. During the "rstfew years we concentrated on a study of the crystal
properties using small samples (about one cubicinch), and then moved to trials for long-size crystals. We have stuck to the Bridgeman}Stockbarger
method grown in a vacuum furnace. This is one of
the simplest methods for making long crystals, andmany factories have vacuum furnaces for the
Bridgeman}Stockbarger method. We believe thatmass production in low cost could be obtained onlyby this method.
In the mean time some important characters
concerning the growth of long crystals have beenfound.
The "rst thing is that the thermal-expansion coe$cient of CeF depends on the direction with3
respect to the crystal axis. Fig. 5 shows the thermal-expansion (linear) rate measured for samples
being cut out in three orthogonal directions. The
thermal-expansion coe$cient values, which wereobtained by averaging over the temperature range
from 253C to 3003C, are di!erent by 30% between

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T. Inagaki et al. / Nuclear Instruments and Methods in Physics Research A 443 (2000) 126}135

Fig. 5. Temperature dependence of the linear thermal-expansion rate. Three samples of 2 mm squared and 10 mm long were
cut out from an ingot in three orthogonal directions as shown ina small "gure. Each rate was measured in the direction of thelength.

1 and 3 samples. A polycrystal which has various
crystal axes in piece may break up during cooldown from the melting point (&14003C) in the"nal stage. CeF is required to be grown as a single
3
crystal. For the growth of a single crystal, we
have found several requirements for the heater,
crucible and furnace, such as a steep temperature
derivative at the crystallization surface, a small
temperature di!erence between the top and bottomof the crucible and a seed which determines theinitial direction of crystallization. However, optimization has not been achieved, because some upgrades of the present system including the furnace
were required. Long crystals are presently grown ina crucible with three holes, where each hole is 34 cm
long and 6 cm in diameter without a tip for a seed.
The size of the crucible matches the dimensions of
the furnace (heater), which we can use for our study.
The yield rate is only about 30% for crystals morethan 10 cm long. We have a plan to change thecrucible to one with a seed-tip and to upgrade theheater arrangement. The upgrade plan will soon
start, because a method for crystallization has almost been established, as described below.
The second character is a kind of speculative
conclusion, that contamination by other lanthanide atoms may not be the main problem against thegood quality of the crystal. We at "rst thought that
quality of crystal would largely depend on the impurities in CeF powder (raw material for crystalli3
zation), especially a contamination by otherlanthanides, because chemical properties are similar among all lanthanide atoms. Certainly, a veryde"nitive result was obtained by the Crystal Clear
Collaboration [10]. Contamination by 200 ppm
Nd3` drastically changed the optical transmission.
Sharp drops in the transmission were observed atseveral wavelengths in the region between 300 and
600 nm. We also observed that mixtures of BaF
2and NdF cause a redshift of the optical absorption
3edge by 15 nm. (Even such a small shift in theabsorption edge reduces the light yield remarkably,
because the absorption edge is very near to theemission wavelength peak of scintillation light.)
However, the impurities being discussed are at
a level of 1000 ppm, and the impurity in commercial powder is no more than 10 ppm for all lanthanide atoms. This has been examined by inductively
coupled plasma mass spectrometry (ICP-MS), an
analyzing method of moderate cost. The impurity
level is stable batch to batch, and no large di!erence is observed between vendors. Other lanthanides are already well removed from commerical
CeF powder. However, even if a small amount of3
impurity seriously deteriorated the crystal property, companies could not easily check and eliminate it using the present technology. Since to remove
it might require a drastic and costly improvementof a large chemical plant which the tiny production
demanded by us could not justify.
A third thing is that CeF powder shows an
3
unexpected behavior at high temperature. It has
been said that when CeF powder is heated in air,3
almost all of the powder vanishes with nothingremaining [11]. We also observed a mass decreaseof 60% after 8 h at 13503C in an atmosphere ofhelium gas with 20% oxygen.
Fig. 6 shows a test result of the thermogravimetry and simultaneous mass spectrometry of
the out-gas by constantly heating up a raw CeF
3
powder. The mass spectrometry is a measurementof m/e using an electromagnetic mass separator,
where m and e are the mass and charge of the gasion, respectively. The test was carried out in an
atmosphere of 1-atm pure-helium #owing from

T. Inagaki et al. / Nuclear Instruments and Methods in Physics Research A 443 (2000) 126}135131

chamber by an attack of the gas, which is a free
radical at high temperature. Fluorine emission
from CeF powder was con"rmed by another test3
using a graphite furnace, argon as a carrier gas andion chromatography in a temperature range
between 12003C and 15003C. The emission ofa considerable amount of #uorine (0.29% of the#uorine in a CeF sample for 2 min heating at
312003C) and its increase with temperature wereobserved. The #uorine liberation from CeF pow3
der suggests the action of the other free radical,
oxygen, at high temperature. An oxygen e!ect in
the crystallization of rare-earth #uorides is discussed in Ref. [12].

3.2. Behavior at high temperature

Fig. 6. (a) Mass de"ciency with increasing temperature by
103C/min (solid) and its derivative (dashed). The mass de"ciency
was measured using a balance method. (b) Out-gas intensities for
m/e"18 (solid) and 85 (dashed). The m/e was measured withmass spectrometry.

a furnace (chamber for thermogravimetry) to themass separator. Two prominent peaks have been
observed in a derivative mass de"ciency, whichcoincide with emission peaks of water (m/e"18:
H O`). Both peaks are higher than the normal
2
boiling point of water (1003C). The water vapor
comes from combined water which is bound tosmall crystals in the powder. Another rapid phenomenon in both mass decrease and gas emission isobserved at high temperature. The gas corresponds
to the SiF` ion (m/e"57), which can be considered3
to be a fragment of SiF . SiF` is also observed with4
4
a less intensity. It can be assigned that #uorine
emitted from the powder attacks a glass window on
the way to the mass separator, and forms SiF gas
4using Si in the glass.
The existence of #uorine is evident by observing
the breakdown of thermometers installed in the

The oxygen in the thermogravimetry test using
pure helium must be supplied from the sample. Thisis also the same in the case of actual crystallization,
because the furnace is evacuated to a high vacuum
during crystal growth. One of the oxygen sources iscombined water.
Our "rst powder contained water by a few wt-%,
as shown in Fig. 6. The powder was produced by
a #uorine exchange reaction from cerium carbonate (Ce (CO ) ) by an aqueous solution of #uoric
233acid (HF). The problem lies in the di!erence of thematerial phase, Ce (CO ) of solid and aqueous
2
33
solution of HF. They cannot be equivalently mixed
with each other for a perfect reaction, and some
excess of liquid HF remains after the reaction. The
HF excess should be neutralized by ammonium.
Then, the remaining acid}ammonium-#uoride(NH F ) HF) must be washed out by water and the4
powder must "nally be dried. It does not look easy
to control the water contamination at low levels.
After heating the powder in a vacuum for crystallization, it turned black. We then tried to "nda chemical compound to be mixed with CeF pow3
der, a so-called scavenger. It should decompose andsupply #uorine so as to compensate the #ush ofwater. Eventually a scavenger, which covers thetemperature of the water #ush, was found and crystals were changed to colorless by its use. A similar
but lesser progress was seen for a powder in whichacid}ammonium-#uoride remained by skipping the

132

T. Inagaki et al. / Nuclear Instruments and Methods in Physics Research A 443 (2000) 126}135

water-washing process. The remaining acid}ammonium-#uoride may play a kind of scavenger,
however, such a large amount of acid}ammonium#uoride with poor control was found to be risky to
the furnace.
In the mean time we obtained the second powder
produced by another company using cerium dioxide (CeO ) as a starting material. The powder is2made through a dry method using HF gas. It does
not require water washing. No prominent peaks
due to water emission at relatively low temperatures were observed in a thermogravimetry andsimultaneous out-gas analysis. However, at hightemperature, a rapid mass de"ciency equal to thatfor the "rst powder appeared.
There was no remarkable di!erence in the crystal
quality between the second powder and the "rst
one using the scavenger. For both powders, theshort crystals using a small crucible are transparent, but long ones using a large crucible are cloudy.
There are many needle-like thin, white scatterers
that are a few mm long. The scatterers can be seenin the background of the strong light being transmitted. Although scatterers exist even in short crystals, the density of scatterers in short crystals ismuch less than that in long ones. This seems to berelated to the rapid mass decrease and liberation of#uorine near to the melting point of CeF . Some3chemical reaction is presumed to arise at that temperature. The di!erence in the density between
short and long crystals can be considered to occur
due to a di!erence in the crystallization time. Thelong ones spend more time at high temperaturethan the short ones.
A material test using X-ray di!raction was performed for two samples of the second powder,a raw powder of CeF and that after a melt. We
3found several di!raction peaks corresponding to
CeO for the powder before a melt, while none of2them remained and other peaks corresponding to
CeOF or CeF appeared for the sample after2
a melt. CeO seems to be replaced by CeOF or
2
CeF upon heating.
2
Fig. 7 shows the result of a pair of thermogravimetry tests for the second powder in a di!erent
atmosphere, pure helium gas and a mixture of helium (80%) and oxygen (20%). The temperature
was constantly raised up to near the melting point,
and then held at 13503C. Mass decrease starts fromthe time slightly before the temperature hold in
both cases; however, the decrease is much steeper inthe case of the oxygen-containing helium gas thanFig. 7. Mass de"ciency with time of the temperature-rise in two di!erent atmospheres of pure helium (solid) and a mixture of helium(80%) and oxygen (20%) (dotted). The dashed curve shows the temperature, which was held constant at 13503C after having risen by
103C/min.

T. Inagaki et al. / Nuclear Instruments and Methods in Physics Research A 443 (2000) 126}135

pure helium. This clearly shows the role of oxygen
at high temperature.
An interesting behavior indicated in Fig. 7 is thatthe mass de"ciency arises constantly after the temperature hold, and that it reaches 10 wt% even in
the case of pure helium. Oxygen contamination to
the second powder was known to be 0.1 wt% by an
X-ray #uorescence analysis in advance using twoother samples of the same powder. A gap between
10 and 0.1 wt% seems to be too large if #uorine in
CeF is simply replaced by the initial oxygen con3
tained in the sample. The very large mass decrease
which was observed in the case of the oxygenmixed gas also cannot be explained by the simple
replacement. With the free oxygen existing in the
powder, the mass de"ciency is expected to be only
0.18 wt% due to the #uorite emission through thereaction CeF #O PCeO #3F using 0.1 wt%
3
2
2
oxygen. The reaction CeF #CeO P2CeOF#
3
2
F, where the oxygen existed as a molecule of CeO ,2would decrease the mass by only 0.06%.
It seems that oxygen should be used many times,
like a catalyser, and also that Ce ion should be
liberated in order to cause such a large and constant mass de"ciency. Based on this information,
we speculatively conclude that a series of reactions
occur as shown below.
At "rst, CeOF would be produced as
CeF #CeO P2CeOF#FC.
3
2
Then, CeOF, which is distributed in the powder,
would interact with the surrounding CeF through3
a repeated chain reaction as
CeOF#CeF P2CeF C(?)#O
3
2
andO#CeF PCeOF#2FC,
3
where the upward-pointing arrow means liberation(not clear for CeF ). The CeOF produced by the2third reaction would be re-used in the second reaction.
CeF and CeO are composed of cerium ions,3
2
Ce3` and Ce4`, and the hydroxides of two ions arequite di!erent in pH from each other, 6.83 forCe(OH) and 2.7 for Ce(OH) [13]. The mixture of3
4
Ce3` and Ce4` is likely to cause the "rst reaction
at high temperature, because both CeF and CeO
3
2

133

are very stable at room temperature. The liberationof Ce (here, CeF ) is consistent with the large mass
2de"ciency at high temperature as shown in Fig. 7.
Liberation of only #uorine cannot account for sucha large mass de"ciency. After normal crystal
growth we always found material cohering in various places inside the furnace, where the temperature is relatively low. The cohering material was
found to be CeF through an X-ray di!raction test.
3
It can be naturally explained as the evaporated
CeF recombining with the #uorine to the more
2stable compound, CeF , at a place of relatively low
3temperature.

4. Present status of our R&D for long crystals
We have not found any reports on the behaviorof CeF at high temperature or the chemical prop3
erties of CeOF and CeF . Although the chain reac2
tion above described is speculative, it provides
a guide to reduce the oxygen e!ect at high temperature, that is to break the chain.
Another guide is the promising fact that short
crystals are already clear. The di!erence in thequality between short and long crystals would
come from the di!erence in the crystallization time.
Since the ratio of the crystallization time is only oneof the many factors and a mass decrease occurs at
a constant rate after the temperature hold without
any exponential enhancement, the required levelfor oxygen reduction may not be more than one
order of magnitude. The goal to reduce oxygen in
the powder or to suppress the chain reaction seems
possible.
We asked the second company for a careful
prodution decreasing the particle size of the starting material of CeO . A small particle size of CeO
2
2
should enhance the e$ciency of the #uorine-exchange reaction and reduce any CeO remaining in2
CeF powder. No remarkable improvement was3
observed in crystal quality by the change. Then, inorder to purify the powder, we inserted a process of
pre-heating in vacuum prior to crystallization. Although the pre-heating did not bring about any
remarkable improvement in the crystal quality, thepre-heating is necessary to reduce the volume
loaded to the "nal crystallization crucible.

134

T. Inagaki et al. / Nuclear Instruments and Methods in Physics Research A 443 (2000) 126}135

Among various trials addressing suppression of
the high-temperature reaction, mixtures of BaF
2and NdF were e!ective to eliminate the scatterers,
3
although it brings about a redshift of the opticalabsorption edge by more than 15 nm and reduces
the scintillating light yield, as previouly described.
Crystal growth in argon gas with SF also caused
6
a redshift. In this case, evacuation of the furnace
was minimized to keep the SF which supplies
6#uorine at a high temperature. On the other hand,
the gas pressure was upper limited by heater power,
because the heat leak due to convection increases
with pressure.
Accidentally, we applied such a technique (theminimized evacuation to the normal crystallization
by mixing nothing but two scavengers. One scavenger (Polytetra#uoroethylene: 0.1%) covered a relatively low temperature, which was used tocompensate the water #ush from the "rst powder;
the other (Lead-#uoride: 3%) covered a high temperature of around 10003C. The thus-grown crystal
has little scatterers without any redshift, and thefurnace after crystal growth was cleaner than in theprevious cases with little material cohering to thewall and support. A short sample taken out froma long ingot shows a nice response for isotope
gammas as shown in Figs. 1(a) and (b). The transmission also exhibits a good shape as shown in
Fig. 8. All three ingots in the crucible were equallytransparent and the second attempt using the same
method reproduced the same good quality. Fromthese reproductions we may say that the method
used for long transparent crystals has been established.
However, there remains another subject to raise
the yield rate for long crystals. As described in
Section 3.1, CeF is required to be grown as
3
a single crystal and the present yield rate is only
about 30% for crystals longer than 10 cm. We
are going to start an upgrade plan of the systemof heater, crucible and furnace in order to meetthe requirements for productively growing single
crystals.

Acknowledgements
The authors would like to express sincere
acknowledgments to the directors of KEK (H.
Sugawara and S. Iwata) for their continuous support and encouragement to this long-term work.
We have had much continuous support and help
from the members of OKEN, especially from R.
Nakajima, S. Takano and M. Ohbuchi, and would
like to express our sincere thanks to them. We arealso grateful to the members of TORAY ResearchCenter, Inc. for their cooperative measurements ofmaterial characters, especially those at high temperature. This work was supported in part by
a Grant-in-Aid for Scienti"c Research fromJapanese Ministry of Education, Science andCulture.
References

Fig. 8. Wavelength dependence of light transmission. The
sample is the same as the one used for the light yield measurement shown in Fig. 1.

[1] D.F. Anderson, IEEE Trans. Nucl. Sci. NS-36 (1989)
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[2] D.F. Anderson, Nucl. Instr. and Meth. A 287 (1990)
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[3] W.W. Moses, S.E. Derenzo, IEEE Trans. Nucl. Sci. NS-36(1989) 173.
[4] M. Kobayashi et al., Nucl. Instr. and Meth. A 302 (1991)
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[5] T. Kanow, H. Yanagita, Rare Earth, Properties and Applications, Gihoudou Shuppan, 1980, p. 367 (in Japanese).
[6] Crystal Clear Collaboration, CERN/DRDC p27/91-15,
CERN-PPE/91}124, 1991.
[7] E. Au!ray et al., Nucl. Instr. and Meth. A 383 (1996) 367.
[8] Y. Matsumoto, Proceedings of the Worshop on Scintillating Crystals at KEK in April 1997, KEK Proc. 97}9 (1997)100.

T. Inagaki et al. / Nuclear Instruments and Methods in Physics Research A 443 (2000) 126}135[9] T. Inagaki, Proceedings of the Worshop on Scintillating
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[10] S. Anderson et al., Nucl. Instr. and Meth. A 332 (1993)
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[11] M. Ishii, private communication, 1996.
[12] W. Korczak, P. Mikolajzak, J. Crystal Growth 61 (1983)
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[13] T. Moeller, The Chemistry of the Lanthanides, Reinhold
Pub. Corp., New York, 1963.

Nuclear Instruments and Methods in Physics Research A 484 (2002) 118–128

Scintillator–Lucite sandwich detector for n=g separation in the
GeV energy region
H. Watanabea,*, K. Abea, E. Haradaa, S. Inouea, T. Inagakib, S. Kobayashia,
A.S. Kurilind, G.Y. Limb, I. Ogawaa, H. Okunob, K. Omatab, T. Satob,
T. Shinkawac, Z. Tsamalaidzed, T. Tsukamotoa, Y. YoshimurabaDepartment of Physics, Saga University, Honjou 1, Saga 840-8502, Japan
High Energy Accelerator Research Organization, KEK, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japanc
National Defense Academy in Japan, 1-10-20 Hashirimizu, Yokosuka, Kanagawa 239-8686, Japand
Joint Institute for Nuclear Research, Joliot-Curie 6, 141980 Dubna, Moscow region, Russian Federationb

Received 6 August 2001; accepted 15 August 2001

Abstract
A Scintillator–Lucite Sandwich Detector (SLSD) has been developed for n=g separation in the GeV energy region.
An eﬃcient n=g separation is achieved by measuring a correlation between the scintillation and Cherenkov light yields.
The basic performance of the detector has been tested with e, p and p beams with momenta between 0.5 and 2:0 GeV=c;
the results were compared with a Monte-Carlo simulation. The n=g separation capability of this detector has beenstudied by simulations in the energy range from 5 MeV to 12 GeV: The SLSD detector was successfully used for a beam
survey of the new K0L beam line built for a K0L -p0 n%n experiment at the KEK 12-GeV proton synchrotron. r 2002
Elsevier Science B.V. All rights reserved.
Keywords: n=g separation; K0L beam line

1. Introduction
A new K0L beam line has been constructed at the
12-GeV proton synchrotron of KEK for the E391a
experiment, which aims at a search for K0L -p0 n%n
decay [1,2]. Since the branching ratio of this decay
mode is predicted to be 3  1011 by the Standard
Model, it is essential to reduce all kinds of*Corresponding author. Fax: +81-952-28-8545.
E-mail address: nabe@hep.phys.saga-u.ac.jp(H. Watanabe).

background in the experiment. The backgroundoriginating from beam particles is the mostserious.
In order to suppress the beam halo, the neutralparticle beam is tightly collimated with multi-step
active and passive collimators to a beam size of
5 cm in diameter at the exit of the beam line, 10 m
away from the primary target. The beam halo ﬂux
is reduced to less than 104 of the beam core ﬂux
due to this tight collimation. However, in thisneutral-particle beam line, a much larger amountsof the n’s and g’s are contained compared with

0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 9 6 9 - 6

H. Watanabe et al. / Nuclear Instruments and Methods in Physics Research A 484 (2002) 118–128

the K0L ’s. Therefore, it is very important toknow the energy distributions, proﬁles and ﬂuxesof the n=g components. Especially, since thefeatures of backgrounds caused by n and g arequite diﬀerent, separate measurements of theirﬂuxes are required for a reliable estimation of
the backgrounds. Considering the simulation
result that the g ﬂux is estimated to be 8-times
larger than the neutron ﬂux in the beam coreregion and 10-times less than in the halo region,
the mis-identiﬁcation probability should be lessthan 102 for each particle. In addition, a compact
detector is required for measuring the beam
proﬁle.
A method for n=g separation is to use a PulseShape Discrimination (PSD) with a pure-CsI
scintillator or a liquid scintillator [3,4]. However,since a pulse-shape diﬀerence results from thelarge diﬀerence between dE=dx for the lowenergy proton produced by n and that for theeþ =e produced by g; the PSD method is dif
ﬁcult to be applied in the energy region above
100 MeV:
Another method for n=g separation is to use aheterogeneous combination of scintillation and
Cherenkov detectors, which was ﬁrst introducedby Degtyarenko et al. [5]. We constructed thistype, a Scintillator–Lucite Sandwich Detector(SLSD), for measuring the n and g components
in the GeV energy K0L beam line.
The basic structure of the SLSD is a sandwich ofplastic scintillator plates and Lucite Cherenkov
radiator plates. With the SLSD, n and g are
separated as follows. When g is converted into eþand e in the SLSD, both scintillation light andCherenkov light are emitted. On the other hand,for neutrons, only scintillation light is emitted by
low-energy protons produced at n–p or n–C
collisions.
In this report, we ﬁrst describe the structure of
the SLSD detector, a calibration experimentusing e; p and p beams and a comparison withthe simulation. We then describe a Monte-Carlostudy for n=g separation in Section 5. An
application of the SLSD to the survey of the K0L
beam line is described in Section 6. A possible
improvement and other applications are also
discussed.

119

2. SLSD detector
A side view of the SLSD is shown in Fig. 1. It
consists of 10 layers of 3:7 mm-thick plastic
scintillators and 10 layers of 5:5 mm-thick Lucite
Cherenkov radiators sandwiched with each other.
The total radiation length and the total nuclear
collision length are 0:23X0 and 0:17lT ; respectively. The cross-section of the active area is
10:0  10:0 cm2 : Lucite light guides, 10:0  12:3 
10:0 cm3 ; are glued to the top and bottom sides ofthe scintillator–Lucite sandwich with opticalcement (BC600). A 3-in. PMT with a UV glass

Fig. 1. Side view of the SLSD. The SLSD consists of 10 repeatsof a 3:7 mm scintillator plate and a 5:5 mm Lucite plate. The
scintillator plates and the Lucite plates are optically separated.

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H. Watanabe et al. / Nuclear Instruments and Methods in Physics Research A 484 (2002) 118–128

window (Hamamatsu R5056) is used for thereadout of Cherenkov light through a siliconrubber. Scintillation light is read by a 3-in. PMT(Hamamatsu R1652). All of the surfaces of thescintillator plates, Lucite plates and light guides
were optically polished. The scintillator plates andthe light guides were wrapped with white Millipore
ﬁlm and aluminized Mylar, respectively. The
Lucite plates were wrapped with a black sheet inorder to absorb Cherenkov light emitted at small
angles. Scintillator plates and Lucite plates are
optically separated from each other.

3. Calibration experiment
In order to calibrate the scintillation and
Cherenkov light yields, the SLSD was irradiatedby p and p beams with momenta of 0:5;
0:8; 1:0; 1:2; 1:5 and 2:0 GeV=c and electrons
with momentum of 0:8 GeV=c at the KEK-PS T1
beam line. Fig. 2 shows a schematic view of theexperimental setup. Plastic scintillation counters(S1, S2 and S3) were used for triggering. CounterS3 deﬁned the beam cross-section to be 3  3 cm2and the FWHM momentum spread was Dp=p ¼
1%: Particle identiﬁcation was made by a time-ofﬂight measurement between S1 and S3 and by agas Cherenkov counter ﬁlled with air at atmospheric pressure.
The experimental results are shown in Figs. 3–5,
where the distributions of scintillation and Cherenkov light yields are plotted as a function of theFig. 2. Schematic view of the experimental setup in the T1
beam line. S1, S2 and S3 are plastic scintillation counters fortriggering. S1 and S3 are used as a start and a stop counter forthe time-of-ﬂight measurement. The gas Cherenkov counter GC
ﬁlled with air at atmospheric pressure is used for electron
identiﬁcation.

Fig. 3. Scintillation light yields for e, p and p with momenta of
0:5; 0:8; 1:0; 1:2; 1:5 and 2:0 GeV=c: The histograms show
the results of the Monte-Carlo simulation.

Fig. 4. Cherenkov light yields for e, p and p with momenta of
0:5; 0:8; 1:0; 1:2; 1:5 and 2:0 GeV=c: The histograms show
the results of the Monte-Carlo simulation.

H. Watanabe et al. / Nuclear Instruments and Methods in Physics Research A 484 (2002) 118–128

121

The number of photoelectrons for p averaged overthe momentum region between 0.8 and 1:5 GeV=c
is 11:070:4: Hereafter, this value is used as acalibration constant of the Cherenkov light yieldfor the minimum-ionization particle.

4. Comparison with simulations
In order to understand the beam-test results, wehave developed a light-yield simulation code basedon the GEANT 4 program. In the simulation, ﬁrst,
the energy deposit in the scintillator and thecharged-particle track length in the Lucite radiatorwere calculated by the GEANT 4 code. These werethen converted to the light outputs in the followingway.
4.1. Scintillation light yield
Generally, the scintillation light yield ðdL=dxÞ isexpressed by the Birk’s equation [6] as
dL
S
dE
¼

dx 1 þ kB  ðdE=dxÞ dx
Fig. 5. Correlations between the scintillation light yield and theCherenkov light yield for e, p and p with momenta of 0:8; 1:0
and 2:0 GeV=c:beam momentum. Since the total thickness of the
SLSD is relatively small compared to the unit
radiation length and the unit collision length,
almost all the light yields are due to the primarybeam. As a result, both spectra of the scintillation
light yields and the Cherenkov light yields show atypical peak structure. The position and the widthof the peak are characterized by the beam species
and by the type of radiation.
The mean value of the scintillation light yield isconstant for e and p at all momenta, while for p itincreases as the momentum decreases. The spectrum shows a typical Landau distribution. The
width of the peak for p is 10.4% in s: In the case of
the Cherenkov light yield, the mean value for p
with momentum below 1:2 GeV=c is almost zero,as expected. The number of photoelectrons due tothe Cherenkov light yield for p is estimated from s
obtained by a Gaussian ﬁtting of the spectrum.

ð1Þ

where dL=dx is the scintillation light yield per cm,
dE=dx is the energy loss per cm, S is an absolute
scintillation eﬃciency and kB is a Birk’s factor,
which should be determined for each scintillator
by measurement. In Fig. 6, we plot the scintillation
light yield ðdL=dxÞ for p and p as a function ofdE=dx; where dE=dx is an averaged value of theenergy loss in the SLSD obtained by the simulation. In this ﬁgure, dL=dx is given by the unit of
ADC counts per cm. When the absolute scintillation yield is calibrated for the minimum-ionization
particle, dE=dx ¼ 1:8 MeV=cm; dL=dx is given by
the unit of MeV-equivalent/cm, as shown on theright-hand side of the ﬁgure. By ﬁtting theexperimental data with Eq. (1), Birk’s factor was
obtained as 0:024 cm=MeV; which is consistent
with 0:0207 cm=MeV in Ref. [7]. In the following
simulation, the energy loss obtained by the
GEANT 4 calculation is converted to the lightyield by using Eq. (1). The results are shown by thehistogram in Fig. 3. The agreement between theexperimental data and the simulation results isgood for e, p and p at all momenta.

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H. Watanabe et al. / Nuclear Instruments and Methods in Physics Research A 484 (2002) 118–128

pedestal distribution approximated by the Gaussian function is also folded on the simulated
distribution of the light yield. In Fig. 4, thesimulation results are shown by the histograms.
The experimental data are well reproduced by thesimulation.
4.3. Probability of proton mis-identiﬁcation

Fig. 6. Scintillation light yield ðdL=dxÞ as a function of dE=dx
in the SLSD scintillator. The energy deposit in the SLSD iscalculated with the GEANT simulator. The scintillation lightyield is obtained from the test results for p and p. The solid lineis the result of ﬁtting with Birk’s equation.

4.2. Cherenkov light yield
Using the charged-particle track information
obtained by GEANT 4, the Cherenkov light yield
was calculated by the following steps: ð1Þ togenerate Cherenkov photons for each particle in
the Lucite radiator, ð2Þ to propagate each photon
in the Lucite radiator and light guide, where thecut-oﬀ of the transmission wavelength in Lucite
was set to 340 nm; and ð3Þ to convert the numberof arriving photons to the number of photoelectrons by multiplying the PMT quantum eﬃciency,
in which the wavelength dependence was taken
into account.
For photon propagation, only the total reﬂection on the inner surface of the Lucite plate wastaken into account. Further, we tuned otherparameters, such as the light attenuation lengthin Lucite, to obtain 11.0 for the average number ofphotoelectrons for p at 1 GeV=c: Since the numberof Cherenkov photons generated in the SLSD for
p at 1 GeV=c is calculated to be 1900, the lightcollection eﬃciency is estimated to be about 3%.
For a comparison with the experimental data, theFor a performance check, we investigated theprobability of mis-identiﬁcation of p to p=e: Whenthe momentum of p becomes larger than the
Cherenkov threshold of 0:85 GeV=c; the Lucite
radiator starts to emit Cherenkov photons. However, when the emission angle is smaller than the
total reﬂection angle of Lucite ð42:21Þ; photons
cannot be propagated. In this case, the threshold is
calculated to be 2:0 GeV=c: The mis-identiﬁcation
probability of p to p=e is estimated from themeasured spectrum of the Cherenkov light yield.
Since the detection eﬃciency of Cherenkov lightdepends on the threshold of the number ofphotoelectrons, the mis-identiﬁcation probabilityof p to p=e is plotted as a function of the
momentum in Fig. 7, where the threshold is set
at 3 photoelectrons. At this threshold, the p=edetection eﬃciency is calculated to be 98.5%. It isseen that the mis-identiﬁcation probability starts

Fig. 7. Mis-identiﬁcation probability of p to p=e as a functionof the momentum. The solid line indicates the results of theMonte-Carlo simulation.

H. Watanabe et al. / Nuclear Instruments and Methods in Physics Research A 484 (2002) 118–128

to increase at 1:2 GeV=c: This energy-dependentfeature is also reproduced by the simulation asshown by the solid line in the ﬁgure.

5. Monte-Carlo study on n=c separation with the
SLSD
Since the experimental data of the SLSD for
charged particles could be well understood by thesimulation, we extended the Monte-Carlo study
estimating the SLSD responses to n and g beams.
The procedure for the simulation is the same as
described in the preceding section. Compared tothe case for charged particles, calculations forneutral particles are diﬀerent only in the fact thatcharged-particle tracks are produced from theirinteractions with the SLSD material.

123

The simulation results are shown in Fig. 8,
where the parameter in this case is the kinetic
energy of n and g: E ¼ 0:1; 0:5; 2:0 and
10:0 GeV: In these plots, the scintillation lightyield is expressed by the MeV-equivalent unit
and the Cherenkov light yield is expressed by the
number of photoelectrons. Due to the diﬀerent
interaction positions in the SLSD, both thescintillation and the Cherenkov light yields arewidely scattered compared to those for the
charged-particle beams. For g’s, there is a linear
relation between two yields, and no clear energy
dependence exists in both spectra. However, forthe n’s, the distribution shows a strong energy
dependence, especially at high energies, due
to the multi-pion production at the n–pcollision, where the Cherenkov light yield starts
to increase.

Fig. 8. Correlations between the scintillation light yield and the Cherenkov light yield for n and g obtained by the Monte-Carlosimulation. The solid lines indicate the boundaries for n=g selection. The numbers in the ﬁgures are the slope parameters deﬁned in the
text.

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H. Watanabe et al. / Nuclear Instruments and Methods in Physics Research A 484 (2002) 118–128

In order to separate n and g; we set a boundary
in the two-dimensional scatter plot as
YCherenkov ¼ slope  ðYscinti  Yoffset Þ

ð2Þ

Fig. 11. g detection eﬃciency as a function of the energy forthree Cherenkov photoelectron thresholds. The slope parameter
is ﬁxed at 1.0.

Fig. 9. Neutron detection eﬃciency as a function of the kinetic
energy of neutrons for three slope parameters.

Fig. 10. Mis-identiﬁcation probability of g to n as a function of
the energy for three slope parameters.

Fig. 12. Mis-identiﬁcation probability of n to g as a function of
the energy for three Cherenkov photoelectron thresholds. The
slope parameter is ﬁxed at 1.0.

H. Watanabe et al. / Nuclear Instruments and Methods in Physics Research A 484 (2002) 118–128

where YCherenkov is the Cherenkov light yield, Yscinti
is the scintillation light yield and Yoffset is a
threshold for the scintillation light yield.
The oﬀset is determined to be 3 MeV-equivalent
in order to reject any event with a small energy
deposit in the detector. In Fig. 8, the distributionsof slope ¼ YCherenkov =ðYscinti  Yoffset Þ for simulated n and g events are shown by the histograms.
The parameter ‘‘slope’’ is varied as 0:5; 1:0 and 1.5
photoelectron-number/MeV-equivalent for theneutron selection, as indicated in the ﬁgure. For
g detection, the parameter ‘‘slope’’ is ﬁxed to be
1.0, but the threshold for the Cherenkov light yieldis varied from 4 to 7 photoelectrons.
The results for the detection eﬃciencies and misidentiﬁcation probabilities of the SLSD for n and g

125

are shown in Figs. 9–12. The neutron detection
eﬃciency starts to increase at 10 MeV and reaches
7% level at 30 MeV: Beyond 100 MeV; theeﬃciency increases again to the 10% level at
5 GeV due to the opening of inelastic channels in
the n–p collision. The neutron detection eﬃciency
above 10 GeV is suppressed because the Cherenkov light yield increases due to the secondary p’s,
and n is mis-identiﬁed as g: The mis-identiﬁcation
probability of g to n is found to be 102 –104 ;depending on the slope parameter, as shown in
Fig. 10. In Fig. 11, the g detection eﬃciency
increases with the g energy and reaches the 12%level at 500 MeV: As shown in Fig. 12, the misidentiﬁcation probability of n to g increases from103 to 4  102 as the neutron energy increases.

Fig. 13. Top ﬁgure shows a schematic view of the K0L beam line and the bottom ﬁgure shows the setup of the SLSD and the lead-glass
Cherenkov counter for the beam survey.

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H. Watanabe et al. / Nuclear Instruments and Methods in Physics Research A 484 (2002) 118–128

6. Use of the SLSD in the K0L beam line
We used the SLSD to the beam survey in the K0L
beam line at the KEK 12-GeV PS. Schematic
ﬁgures of the beam line and the detector setup
are shown in Fig. 13. The primary proton
beam hits the production target of Pt with athickness of 60 mm: The ﬁrst beam-deﬁning
collimator is set at 41 with a half-cone angleof 2 mrad: The charged particles are removedby the magnetic ﬁeld, and further collimationof beam-halos is made by a series of collimators.
In the K0L beam line, removable absorbers,
5 cm-thick Pb ð8:9X0 ; 0:49lT Þ and 30 cm-thick Be
ð0:85X0 ; 0:99lT Þ; can be set on request.
The n and g ﬂuxes were measured 10 m
downstream from the production target. The
beam core size was 5 cm in diameter. For thisbeam survey, the proton ﬂux on the production
target was maintained at 1011 protons/pulse, where
the repetition rate was 0:25 Hz and the beam spill
length was 2 s:
For a measurement of the n=g components inthe beam line, the SLSD was used in combination
with the charged-particle veto counter and thetotal absorption-type Cherenkov counter made oflead-glass SF-5 with a dimension of 15  15 
30 cm3 : The radiation length of the lead-glass was
12:7X0 and the total nuclear collision length was
1:3lT :In a beam survey, the neutral-particle eventswere triggered by requesting the lead-glass signals
vetoed by the veto counter. Fig. 14 shows twodimensional plots of the scintillation light yieldand the Cherenkov light yield from the SLSD forthree beam-line conditions: (a) with no absorber,(b) with the Pb absorber and (c) with the Be
absorber. The number of events are normalized by
the primary proton number on the target. In the
low-energy region, many events are concentrated.
Since the events in this region are quite hard to be
identiﬁed, we set the 3 MeV threshold for the
scintillation light yield and the 5 photoelectron
threshold for the Cherenkov light yield. When weplot slope parameters, we can identify n and g; asshown in Fig. 14. From the ﬁgures, we can observe
that, by inserting the Pb absorber in the beam line,
the g component is greatly reduced, but the nFig. 14. SLSD data for the beam survey in the K0L beam line.
The ﬁgures on the left show the correlations between the
scintillation light yield and the Cherenkov light yield, the ﬁgureson the right show the projection to the slope parameter. Three
ﬁgures correspond to the beam-line conditions: (a) with noabsorber, (b) with Pb absorber and (c) with Be absorber. Eventsin the two regions in the left ﬁgures are assigned as g and n.

component is not so much reduced. On the other
hand, by inserting the Be absorber, both n=g
components are decreased at a similar ratio.
In combination with the SLSD, we measured
the energy spectra with the lead-glass Cherenkov
counter. The results are shown in Fig. 15, where
the horizontal scale is shown by the unit of GeV
for g: Fig. 15(a) shows the events triggered only by
the lead-glass Cherenkov counter. Fig. 15(b) and(c) displays g events and n events selected with the
SLSD. Without using the SLSD, the diﬀerence
between the eﬀects of the Pb and Be absorbers is
not clear. On the other hand, with the SLSD,
the eﬀects of the Pb absorber and the Be absorber are clearly measured. By using a 5 cm-thickPb absorber, the g component decreased by afactor of about 20, while the n components
decreased only by about 70% in the energy region

H. Watanabe et al. / Nuclear Instruments and Methods in Physics Research A 484 (2002) 118–128

127

Fig. 15. Distributions of the signal amplitude from the lead-glass Cherenkov counter in the K0L beam line. (a) for self-trigger events, (b)for g events and (c) for neutron events, where the selection criteria are shown in Fig. 14. The beam line conditions are indicated in theﬁgures.

above 1 GeV: While, using a 30 cm-thick Be
absorber, both the g and n components decreased
by a factor of 4–6. These eﬀects of two types ofabsorbers are in good agreement with a ﬂux
calculation of the K0L beam line.
7. Summary and discussion
We have developed a compact and simple
n=g separation detector SLSD, which utilizes
simultaneous measurements of scintillation
light and Cherenkov light. The SLSD is foundto be an eﬀective detector from the viewpointof n=g separation in the energy region from20 MeV to 12 GeV: The neutron detection

eﬃciency is obtained as 7–10% with a g
mis-identiﬁcation
probabilityofo102 :The SLSD was successfully used for the
beam survey of the K0L beam line in the GeV
energy region.
One problem of the present design concerns thesmall light collection eﬃciency for Cherenkov
photons. Since the mis-identiﬁcation probability
is dominated by a ﬂuctuation of the Cherenkov
light yield, the light-collection eﬃciency is crucial
for a better n=g separation. This could be achievedby an improved design of the light guide for
Cherenkov photons.
We note that the SLSD can also be applied for ag detector under a large neutron background. The
lead plates will be added as a converter in order to128

H. Watanabe et al. / Nuclear Instruments and Methods in Physics Research A 484 (2002) 118–128

obtain a high detection eﬃciency for g: One
example of such an application of the SLSD is touse it as a beam catcher for the KEK-E391a
experiment, where a high g detection eﬃciency isrequired together with a good n=g separation.
Many other applications of the SLSD will beconsidered for n=g detection in the GeV energy
region.

Acknowledgements
The authors would like to thank all the membersof the E391a collaboration for their kind help andvaluable discussions. This work was partly supported by a Grant-in-Aid for Scientiﬁc Researchof the Ministry of Education, Science, Sports andCulture of Japan.

References[1] T. Inagaki, et al., Measurement of the K0L -p0 n%n; KEK
Internal, 96-13, 1996.
[2] K. Abe, et al., Status of the K0L -p0 n%n experiment at KEK,
KEK preprint, 2000-89, 2000.
[3] S. Usudat, et al., Nucl. Instr. and Meth. A 321 (1992)
247.
[4] T. Yorita, et al., Proceedings of the Workshop on
Scintillation Crystals, KEK Proceedings 97-9 (1997) 11.
[5] P. Degtyarenko, et al., Proceedings of the International
Symposium on Coordinate Detectors, Dubna, USSR, 1988,
p. 316.
[6] G. Knoll, Radiation Detection and Measurements, Wiley,
New York, 1989.
[7] L. Torrisi, Nucl. Instr. and Meth. B 170 (2000) 523.

ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research A 522 (2004) 477–486

Development of a new photomultiplier tube with highsensitivity for a wavelength-shifter ﬁber readout
M. Itayaa,1, T. Inagakib, T. Iwataa, G.Y. Limb, H. Oishic, H. Okunob, Y. Tajimaa,
H.Y. Yoshidaa,*, Y. YoshimurabaDepartment of Physics, Yamagata University, 1-4-12 Kojirakawa-machi Yamagata 990-8560, Japanb
High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japanc
Hamamatsu Photonics K.K., Shizuoka 438-0193, Japan
Received 24 July 2003; accepted 5 November 2003

Abstract
We have developed a new photomultiplier tube (PMT) with high sensitivity for a wavelength-shifter ﬁber readout ofplastic scintillators. Improvements of the photoelectron yield were made on a 2-in. PMT (Hamamatsu R329) byadopting (1) a prism-shaped photocathode, (2) an extended-green photocathode material, and (3) an optical mirror
surface of the electrodes. As a result, the number of photoelectrons obtained by the new PMT has increased to be 1.8times that by R329. Other PMT characteristics, such as uniformity of sensitivity, time response and darkcurrent are notso much different from those of R329 for practical applications.
r 2003 Elsevier B.V. All rights reserved.
PACS: 29.90.þr; 29.40.Mc; 29.40.Vj
Keywords: Photomultiplier tube (PMT); Prism photocathode; Wavelength-shifter (WLS) ﬁber readout; Quantum efﬁciency (QE);
Time response; Darkcurrent

1. Introduction
KEK-PS E391a is an experiment to study direct
CP violation through a measurement of theKL -p0 nn% decay branching ratio [1,2]. The signature of the decay mode is 2g; of which the invariant
mass is consistent with p0 and nothing other than*Corresponding author. Tel.: +81-23-628-4570; fax: +8123-628-4567.
E-mail address: yoshida@sci.kj.yamagata-u.ac.jp(H.Y. Yoshida).
1
Present address: NEC TOKIN Corp., 7-1, Kohriyama
6-chome, Taihaku-ku, Sendai-shi, Miyagi 982-8510, Japan

one p0 : This requires a photon veto counter systemwith a high efﬁciency that covers the whole KL
decay region.
Lead-scintillator sandwich-type counters were
designed for this purpose [2]. One of them is acentral-barrel veto counter with a length of 5:5 m;
another is a front-barrel veto counter with a lengthof 2:75 m: They are made of 60 layers of 5 mmthick extruded plastic scintillators and thin lead
sheets. Wavelength-shifter (WLS) ﬁbers of 1 mm
in diameter embedded in grooves on the plastic
scintillator surface are employed for a scintillation
light readout. A similar system was adopted and0168-9002/$ - see front matter r 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.nima.2003.11.201

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developed in the BNL experiment E926(KOPIO)
which has the same physics goal [3,4]. There areseveral advantages in this choice. The convenient
readout with the WLS ﬁbers leads to reducing thenumber of photomultiplier tubes (PMTs), hence
resulting in low cost. Such a readout allows auniform detector response over the whole detector
area, because the attenuation length of the WLS
ﬁbers is sufﬁciently longer than that of the plastic
scintillators. However, there is a drawback of apoor light yield at the PMT. To secure the strengthof the counter structure, we adopted a MS
(methyl-methacrylate styrene) resin based scintillator produced by the extrusion technique. It
provides a less light yield than that of other plastic
scintillators commonly used, e.g. Bicron BC408[5]. In addition to this, such a WLS ﬁber readout
system has a lower light collection efﬁciency thanthat of an ordinary acrylic light guide system. In
order to overcome this, we made a new attempt toincrease the photoelectron yield by making improvements on the PMT. We report on these
improvements and the application of the new
PMT for the WLS ﬁber readout.

The improvements made on this PMT were as
follows:(1) Prism-shaped photocathode: The inner surface of the photocathode window was molded intoa prism-shape, as shown in Fig. 1. This processing
gives a longer passage of the light through thephotocathode material, resulting in an increase of
the chance for photoelectric emission. The thickness of the photocathode material was kept thephoto-cathodephoto-cathode

Dy1

Dy1

2. Improvements on the R329 PMT
We employed a 2-in. PMT, Hamamatsu R329[6], considering cost, easy handling and the size of
the WLS ﬁber bundles. The speciﬁcations of theR329 PMT are summarized in Table 1.

Dy3(a) R329EGPX(b) R329

1.5 mmTable 1
Speciﬁcations of the Hamamatsu R329 PMT [6]Spectral response
Wavelength of maximum
response
Photocathode material
Photocathode minimum
effective area
Window material
Window shape
Dynode structure
Dynode number of stages
Gain
Maximum supply voltage
Electron transit time
Transit time spread

3002650 nm
420 nm

1.5 mmBialkali
46 mm fBorosilicate glass
Plano-concave
Linear focused12
1:1  106 at 1500 V
2700 VDC
48 ns
1:1 ns

Dy2

Dy2
Dy3

0.75 mm

1.5 mm(c) Structure of prism photo-cathodeFig. 1. Schematic structure of (a) the new PMT (R329EGPX);(b) the standard PMT (R329). The photocathode of the new
PMT has 827 prisms on its surface. The thickness ofphotocathode material is the same for both PMTs; (c) structureof each prism.

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100

(a)

Quantum Efficiency (%)

101Standard PMTs (R329)
Extended Green PMTs (R329EG)
New PMTs (R329EGPX)

0.1

Amplitude

0.01(arbitrary unit)

same as that of the standard PMT. Since the angleof the surface of the prisms is inclined by 45 ; it isexpected that the chance of photoelectric
emissionpﬃﬃﬃ
is increased by a factor of 2 for light with anormal incidence on the window surface of thePMT.
(2) Extended-green photocathode: The photocathode material was changed into the so-called
extended-green one. By changing the mixture of
the contents of the photocathode material, its
work function was lowered. This extends its
sensitive region to 4502600 nm; which matches
the emission spectrum of the WLS ﬁber.
(3) Well-polished electrodes inside the PMT:The surface of the focusing electrode, the ﬁrst andthe second dynodes, was polished to have better
light reﬂection. The light, having passed throughthe photocathode, and hence not having contributed to the photoelectric effect, is reﬂected on thesurface back to the photocathode. Although theprobability of the photoelectric effect is expected
to be somewhat increased by this improvement, aquantitative evaluation was not performed.
The quantum efﬁciencies (QE) of the PMTs atdifferent wavelengths were measured by Hamamatsu Photonics K.K., as shown in Fig. 2(a) [7].
The emission spectrum of the WLS ﬁber (KurarayY-11) is also shown in Fig 2(b) as a reference. The
standard bialkali-photocathode PMTs, R329, givethe maximum QE at around 400 nm: The PMTs
with the extended-green photocathode (hereafter
denoted as ‘‘extended-green PMTs’’), R329EG,
give higher values at longer wavelengths, as
expected. They give an improvement by a factorof 1.0–1.4 at 4502600 nm; which corresponds tothe wavelength of the emission from the WLS
ﬁber. The value of QE of the new PMTs,
R329EGPX, is increased compared to those of
the extended-green ones over the whole region
above 400 nm: In particular, above 600 nm; anunexpected enhancement is seen. This is one ofpreferable features for applications with WLS
ﬁbers. Below 400 nm; however, the new PMTs
are not as good as expected; the value at 350 nm is
about one order of a magnitude less than those of
the other PMTs. This is due to the effect of thedifferent window material. The window of the new
PMT is made of a kind of borosilicate glass, but is

479

(b)

200

Kuraray Y-11

300

400

500

600

700

800

Fig. 2. (a) The QEs measured at different wavelengths. The
open triangles represent the data for the standard PMTs
(R329), the open squares for the extended-green ones
(R329EG) and the open circles for the new ones (R329EGPX).
Note that the data represent the averaged values obtained with
19 pieces of R329, 3 pieces of R329EG and 9 pieces of
R329EGPX; (b) the emission spectrum of the WLS ﬁber,
Kuraray Y-11, to be used in the E391a experiment. The
spectrum was obtained for excitation at a wavelength of 400 nm
using a ﬂuorescence spectrometer, Hitachi F-4500. It was
conﬁrmed that another WLS ﬁber to be used in the experiment,
Bicron BCF-91A, gives a similar emission spectrum.

slightly changed from that of ordinary R329 to besuitable for making the prism structure in a
molding press. Also, this material has a worsetransmission below 400 nm compared with thatused in the standard PMTs. However, it will notcause any problems in our application with theWLS ﬁbers (Bicron BCF-91A and Kuraray Y-11),
having a peak in the region 4502600 nm in theiremission spectrum, as shown in Fig. 2(b).
Regarding the above-mentioned improvements,
we expect an increase of the photoelectron yield byat least a factor of 1.7 in the region of the emission
spectrum of the WLS ﬁber. To conﬁrm thisincrease of the photoelectron yield, we made tests
with the actual counter in the WLS ﬁber readout.

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480

We report on the methods of the tests and theirresults in subsequent sections.

700

600

3. Photoelectron yield in a WLS ﬁber readout%eN N% N1
pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
N!
2pN s
N¼1


ðx  pN  qÞ2
 exp 2Ns2

F ðxÞ ¼ A

N¼N
Xmax

where x is the ADC channel, A is the normalization factor, N% is the average number ofphotoelectrons (p.e.), s is the single-photoelectron
peak resolution, p is the peak interval and q is the% s and ppedestal channel. In these ﬁttings, A; N;

Trigger counterβ
Plastic scintillator
WLS fiber

106

Ru

PMT on test

Fig. 3. Experimental setup for measuring the number ofphotoelectrons.

counts/channel

The photoelectron yield of the new PMTs wasinvestigated for an actual read out with the WLS
ﬁber in the setup shown in Fig. 3, and comparedwith that of the standard PMTs. A WLS ﬁber
(Kuraray Y-11) of 1 mm diameter, 5:2 m in lengthis embedded in the groove on the surface of the
Bicron BC408 plastic scintillator ð100 mm 
50 mm  5 mmÞ: It guides the secondary light tothe center of the PMT’s window. The plastic
scintillator is exposed to b-rays from 106 Ru; andthe signal pulse height is registered by ADC, as
shown in Fig. 4. The ADC is gated by the signal
from a trigger counter mounted above the plastic
scintillator.
The ADC spectrum was ﬁtted by the resolution
convoluted Poisson distribution [8],N p.e. =2.0

500

400

300

200

100

0
200

400
600
ADC channel

800

1000

ADC (ZK0682)

Fig. 4. Typical ADC spectrum obtained with the new PMT.
The solid curve is the result of the ﬁt described in the text. The
dotted curves are decomposed Gaussians for N ¼ 1; 2; 3; 4; 5:

are the free parameters to be determined from theobtained spectrum. From the single-photoelectron
peak measurement for each PMT, we have also
conﬁrmed the s and p values. The obtained valueof N% has an uncertainty of B3%; besides the ﬁtting
error of 1–2%. The uncertainty is due to selectionof the ﬁtting range; in particular, the position of
the lower edge affects the ﬁtting, because thespectrum in the lower channel range may beslightly contaminated with the background due toelectrical noises, and so on.
The averaged numbers of photoelectrons obtained with the PMTs operated at 2:2 kV areshown in Fig. 5. It turned out that the new PMTs
give about a 1.8-times larger photoelectron yieldthan the standard ones. The results for theextended-green PMTs show that the photoelectron
yield is about 1.5-times larger than that of thestandard PMT. Hence, the adoption of a prismshaped photocathode and the polished electrodes
improve the photoelectron yield by a factor of 1.2.
This improvement is crucial as well as theextended-green photocathode in order to overcome the disadvantage of using the MS resin
based scintillator which provides a light yield of

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the repetition of the prism structure, as shown in
Fig. 6. Note that the broad dip seen at around
10 mm on the y-axis in Fig. 6(b) is supposed to be
due to the asymmetric arrangement of thedynodes. The ﬁne variation may sometimes cause

2.0

1.5

100

p.e.

N

481

1.0

0.5

RD0717RD0644RD0292

ZK1320

ZK1316

ZK1310Extended-green PMTs
(R329EG)Standard PMTs
(R329)

Fig. 5. Averaged numbers of photoelectrons obtained with the
new PMTs (ZK0679, ZK0680, ZK0682), the extended-green
ones (ZK1310, ZK1316, ZK1320) and the standard ones
(RD0292, RD0644, RD0717).

70–80% of the Bicron BC408. Comparing with thestandard PMTs in combination with the Bicron
BC408, the new PMTs are expected to give amore photoelectron yield, at least by a factor of
1:26 ð¼ 0:7  1:8Þ; in combination even with the
MS resin-based scintillator.
In addition to the uncertainty in the ﬁtting
procedure mentioned above, another uncertaintyof B4% is caused by the reproducibility of theexperimental condition, e.g. positioning of the
PMTs and the WLS ﬁber in the setup. Taking this
into account, the total uncertainty in the photoelectron yield is estimated to be at most 8%. Even
taking into account the uncertainty, the observed
differences of the photoelectron yield among thedifferent types of PMT are signiﬁcant.

The structure of the prism-shaped photocathodemay inﬂuence the uniformity of the sensitivity. As
reported by Hamamatsu Photonics K.K. [9], a ﬁnevariation in its sensitivity is seen corresponding to40

Ref[6]
our measurements

0
-30

-20

-10

(a)

0

10

20

30

10

20

30

Position (mm)along x-axis
100

80

60

40

20
Ref[6]
our measurements

0
-30

(b)

4. Uniformity of sensitivity

60

20

Relative Output (%)

New PMTs(R329EGPX)

ZK0682

ZK0680

ZK06790.0

Relative Output (%)

80

-20

-10

0

Position (mm)along y-axis
Fig. 6. Position dependence of the sensitivity for the new PMT(R329EGPX). The deﬁnition of the axes is the same as
described in Fig. 7. The solid lines represent the data taken
by Hamamatsu Photonics K.K. at 420 nm: The closed squares
represent our data for the ﬁber readout with the WLS ﬁber of
1 mm in diameter.

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M. Itaya et al. / Nuclear Instruments and Methods in Physics Research A 522 (2004) 477–486

Y

2.5mm

Dy1

X

trons emitted at different positions on the photocathode give different trajectories to the ﬁrst
dynode. Due to their low velocity, the different
trajectories lead to a difference in the electrontransit time, depending on the photon incident
position. They also affect the spread in the electrontransit time when the photocathode is uniformly
illuminated.

Dy2

5.1. Electron transit time difference (TTD)

Fig. 7. Fiber positions of photon incidence in the measurementof TTD and uniformity.

problems, particularly when the ﬁber readout isapplied. We studied the uniformity of the sensitivity for the ﬁber readout with the WLS ﬁber of
1 mm in diameter with the same setup employed
for the photoelectron yield measurement. The ﬁber
positions of the photon incidence are shown in
Fig. 7. The measurements were carried out at thepositions located along two lines, which areparallel and perpendicular to the dynode arrangement. Taking account of the interval of therepetition of the prism-structure, namely 1:5 mm;
the pitch of the positions was chosen to be 2:5 mm:
The behavior of our data, shown in Fig. 6, follows
the data obtained by Hamamatsu Photonics K.K..
It also turned out that the ﬁne variation was
smeared out in the case of the ﬁber readout.
Therefore, it will not cause any problem in ourapplication for the lead-scintillator sandwich
counters in E391a.

5. Time response
The time response is one of the importantcharacteristics of PMTs. The prism-shaped photocathode, which gives a non-uniform electric ﬁeld,
may affect the time response. It can be observed in
the electron transit time, the time for the electrons
leaving from the photocathode to reach the anode.
Usually, it strongly depends on the incident
photon position on the photocathode. Photoelec-

We measured the TTD, the relative electrontransit time, at different positions of photon
incidence. The measurement was carried out using
a pulse laser, Hamamatsu PLP-02, as shown in
Fig. 8. It provides photons at a wavelength of
408 nm within a short duration of 58 ps: One ofthe positions on the photocathode was irradiatedthrough a hole of 1:0 mm in diameter. The timedifference between the synchronous trigger outputof the pulse laser and the output signal of the PMT
was measured by a TDC.
The data of TTD are shown in Fig. 9. The
deﬁnition of the axes is the same as described inFig. 7. The data sets along the x-axis show a more
ﬂat behavior compared with those along the yaxis. This is because the dynodes are symmetricallyarranged along the x-axis. When we compared theresults for the standard and new PMTs, the data
set of the latter showed a zigzag behavior, which is
supposed to be originated from the prism structure, although the data points of the standard
PMT are smoothly connected. The data points of
the latter ones spread over a slightly wider range
than those of the former ones. However, thespreading is not signiﬁcant and the overall
Pulse
Laser

Wave length = 408 nm
PMT

Synchronizedtrigger output
Delay

Discriminator

Start

TDC

Stop

Fig. 8. Setup for the TTD measurement.

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483

1.5
Pulse
Laser

1.0

PMT
ND filter
Glass
diffuser

T.T.D. (ns)0.5Constant Fraction
Discriminator

0.0
Delay

TDC
Start

Delay

-1.5
-30

-20

-10

(a)

0

10

20

30

20

30

Position (mm)

T.T.D. along x-axis
1.5

1.0

0.5

0.0

-0.5

-1.0

-1.5
-30

Stop

ADC
Gate

Input

Fig. 10. Setup for the TTS measurement.

-1.0

T.T.D. (ns)Amp

Discriminator

-0.5

(b)

Holographic
diffuser

-20

-10

0

10

Position (mm)

T.T.D. along y-axis
Fig. 9. TTD along (a) x-axis and (b) y-axis for the standard
PMT (open circles) and the new PMT (closed circles). The
PMTs are operated at 2:5 kV: The error bars are the root-meansquares of the TDC distributions.

behaviors of the data for both PMTs are similar toeach other.
5.2. Electron transit time spread (TTS)The time response of PMTs can also becharacterized by the TTS, the spread of theelectron transit time. It is an important parameter,
which dominates the time resolutions in time-ofﬂight counters as well as the number of scintillating photons. We evaluated the TTS by a measurement of the electron transit time for ‘‘single
photoelectron events’’ under the condition thatthe whole surface area of the photocathode was
uniformly irradiated by laser photons. Such amethod is usually employed to obtain the catalog
values of TTS by major suppliers of PMTs,
including Hamamatsu Photonics K.K. [10].
The measurement setup is shown in Fig. 10. The
laser beam, of which the intensity was signiﬁcantly
reduced by two ﬁlters,2 was diffused by two optical
diffusers3 in such a way that only a few photons wereincident on the photocathode with a uniform
intensity distribution. We used the same pulse laserthat was used in the TTD measurement. The timedifference between the synchronous trigger output of
the laser and the output of the PMT was registeredby a TDC. We employed a constant-fraction
discriminator for the PMT signal to eliminate thetime spread due to the difference in pulse heights.
Registering the PMT signal by an ADC, we selected
only the ‘‘single photoelectron events’’ with a pulse
height in the range of 170:5 p.e..
The TDC distributions obtained with the standard and the new PMTs are shown in Fig. 11.
We evaluated the spreads of the distributions in
2
Products of Edmund Industrial Optics. The ND ﬁlters withtransmittance of 0.3%.
3
Products of Edmund Industrial Optics. The holographic
diffuser and the opal glass diffuser.

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484

Table 2Time spreads observed in the TDC distributions

counts/channel

10 3

Type of PMT

Serial number

Spread (FWHM)

10 2Standard PMTs
(R329)

RD0292RD0644RD0717
Average

1:6970:03 ns
1:3970:03 ns
1:4470:02 ns
1:5070:03 ns

10

New PMTs(R329EGPX)

ZK0679
ZK0680
ZK0682Average

1:9070:02 ns
1:9470:03 ns
1:8270:04 ns
1:8970:03 ns

1

2400

2500

(a)

2600

2700

2800

2900

2800

2900

TDC channel

Standard PMT

counts/channel

10 3

10 2

10

the laser to the PMT, time spreads due to multiple
scattering of the photons in the ﬁlters and the
diffusers and so on. Thus, the observed spreads
are regarded as upper limits of the TTS. Since theexperimental conditions are the same, it can bestated that the new PMTs give a larger TTS than
the standard PMTs. In other words, the time
response of the new PMTs, investigated in the single
photoelectron range, is signiﬁcantly worse than thatof the standard ones. However, in actual applications in TOF counters, their time resolutions are
determined not only by the time response mentioned
above, but also by the statistics of the photoelectrons. Thanks to the ability to give a better
photoelectron yield of the new PMTs, their worse
time response can be compensated so that the time
resolutions of the TOF counters might be maintained.

1

6. Darkcurrent
2400

(b)

2500

2600

2700

TDC channel

New PMTFig. 11. TDC distributions obtained with (a) the standard
PMT and (b) the new PMT. The scale of the horizontal axis is
25 ps=channel:

full-width at half-maximum (FWHM), as shown in
Table 2. The standard PMTs gave 1:50 ns as theaveraged value, while the new ones gave 1:89 ns:
The observed spreads are reasonably supposed to
be dominated by the TTS of the PMTs. And it is
difﬁcult to eliminate other small contributions tothe spreads, e.g. difference in the photon path fromIt was of concern that the new PMTs have aproblem due to the darkcurrent, for the following
reasons:(1) The photocathodepﬃﬃﬃ surface area was extended
by a factor of 2:(2) The work function was lowered for an
extended-green treatment.
(3) The electric ﬁeld above the photocathodesurface is locally strong, particularly, aroundthe apex of the prisms.
An increase in the darkcurrent leads to a highthreshold level, and spoils the efﬁciency of the

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6

10RD0292
PMTs
RD0644 Standard(R329)
RD0717

5Counting Rate (cps)10ZK0679
ZK0680

New PMTs(R329EGPX)

ZK0682
4

10

3

10

2

10

-10

0

10

20

30

40

50

60

Temperature (oC)Fig. 12. Background noise counting rates as a function of thetemperature in the darkcurrent measurement. The thresholdvoltage value corresponding to 0.5 p.e.. The open symbols
denote the results for the original PMTs, and the closed
symbols are for the new ones.

counters. We evaluated the darkcurrent at several
temperatures (0 C; 23 C; 50 C) by measuring thecounting rates at a threshold level of 0.5 p.e. with
the PMTs not being irradiated. The results areshown in Fig. 12. We found that the new and thestandard PMTs give similar darkcurrents at room
temperature ð23 CÞ: In standard applications of
the new PMTs, the darkcurrent is not a problem.
At a higher temperature ð50 CÞ; the new PMTs
give a few-times more darkcurrent than thestandard ones. Therefore, when one wants tocarefully avoid darkcurrent, their use at relatively
low temperatures is recommended for the new
PMTs, even in the room-temperature range. At a
still lower temperature ð0 CÞ; the darkcurrent of
the new PMTs is signiﬁcantly reduced, by at mostone order of magnitude compared with that atroom temperature.

7. Conclusion
We have made improvements on the 2-in. PMT(Hamamatsu R329) to increase the photoelectron

485

yield with a prism-shaped, an extended-green
photocathode and polished electrodes. Consequently, about 1.8-times larger photoelectron yield
was obtained with the new PMTs. A similar
behavior in the TTD was found for the standard
and new PMTs. However, the TTS of the new
PMTs is signiﬁcantly larger than that of thestandard PMTs. The darkcurrent of the new
PMTs is comparable to that of the standard PMTs
at room temperature.
More extensive studies are required for the new
PMTs, especially concerning the photoelectron
yield at different photon incident angles. It is also
desirable to improve the photoelectron yield in the
short-wavelength region. Technical developments
in making the prism structure will make it possible
to employ a window material with better transmission. These changes will lead to new applicationsof the PMTs for calorimetry with scintillating
crystals operated at shorter wavelengths. Anotherissue is an enlargement of the photocathodesurface area, which is necessary for applicationsin Cherenkov counters.

Acknowledgements
We would like to thank Profs. S. Kato, H.
Shimizu and the group members of Yamagata
University, and all other E391a collaborators, fortheir discussions, encouragement and support. We
are indebted to the engineers of Hamamatsu
Photonics K.K. for their kind discussions concerning the PMTs.

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preprint 2000–89, 2000.
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M. Itaya et al. / Nuclear Instruments and Methods in Physics Research A 522 (2004) 477–486[6] Hamamatsu Photonics K.K. web catalog ‘‘R32902 TPMH1254E01-1.pdf’’, available at http://www.hpk.
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ELSGIER

Nuclear Physics A721 (2003) 449~452~
www.elsevier.com/locate/npe

KL ---t nod experiment at KEK 12-GeV PS -E391a -

M. Yamaga’“*, M. Dorochenkoa, T. Inagaki”, N. Ishihara”, G.Y. Lim”, T. Morimoto’, H.
Okuno’, K. Omataa, T. Sato”, M. Sekimoto”, Y. Yoshimura”, A. Ajimurab, T. Ikeib, Y.
Ikemotob, M. Nomachib, T. Obab, K. Sakashitab, Y. Sugayab, T. Yamanakab, M. Itaya’,T. Iwata”, T. HariuC, M. Moriya”, Y. Tajima’, M. Yamamotoc, H. Yoshida”, Y.
Yoshida”, A. Lednevd, 3. Nixd, G. Perdued, E. Podd, M. Rawtajizakd, Y. Wahd, H.
Watanabed, V. Baranove, N. Khomouove, A. Kurilin”, G. Macharashvili”, A. Moissenko” ,2. Tsamalaidzee, Y. Akune’, Y. Fujiokaf , N. Kawakubo’, S. Kobayashi’, T. Kojimaf ,J.K. Ah+, H.S. Leeg, S.Y. Leeg, T. Shinkawah, Y.B. Hsiung’ and T. Nakan$“High Energy Accelerator Research Organization (KEK)bOsaka University“Yamagata University
dUniversity of Chicago, USAeJoint Institute for Nuclear Research, Russia‘Saga University
gPusan National University, Korea
hNational Defense Academy in Japan‘Fermi National Accelerator Laboratory, USA
jResearch Center for Nuclear Physics, Osaka University
The KL ---f ~~OVV
decay is one of the most attractive phenomena for the study of the CP
violation. The branching ratio can be calculated with a small theoretical ambiguity, which
will provide a clean determination of a basic parameter of the present particle physics.
It will also play an important role for the search of the new physics as well as for thefundamental understanding of the CP-violation. We will start a kind of pilot experimentusing a present 12-GeV proton synchrotron (PS) at KEK from early 2004, which is thefirst dedicated experiment searching for the KL + r”vii decay.
1. Introduction

The KL --f ~~OVV
decay is a rare decay process of flavor-changing-neutral-current (FCNC)
with direct CP-violation dominated. In the Standard Model, the decay amplitude of this*electric address: yamaga@post.kek.jp0375-9474/03/$ - see front matter 0 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0375-9474(03)01096-O

M. Yamaga et al. /Nuclear Physics A721 (2003) 449~452~

45oc

mode is proportional to a CKM matrix element, Im(V,dV,‘,), which corresponds to theheight of the unitarity-triangle.
Since the uncertainty in the theoretical calculation isestimated to be only a few % [l], the pure and clean information from the KL -+ XOVV
decay will be the most critical check for extra-effects beyond the Standard model as wellas the B meson system.
The present experimental upper-limit of its branching ratio is 5.9 x lo-’ [2], which is
far above the Standard Model prediction of 3 x lo- ll. Instead of leaping over this large
gap at once, we planned to carry out a kind of pilot experiment at KEK 12-GeV PS,
E391a, which will reach the sensitivity of 3 x lo-lo. The E391a experiment is scheduled
to start the physics run in early 2004. It will enable us to search for the decay beyond
the indirect experimental limit deduced from the K+ -+ x+vb decay. It will also gives us
many important informations about the situation in the tiny branching ratio region, which
will guide us to the next stage of the experiment using high-intensity proton synchrotron(50-GeV PS) [3] that will start operation at 2007.
2. Measurement
2.1.

Detection

methodprinciple

Experimentally, the KL + YT’UDdecay will be observed by an in-flight decay of KL
by a signal of two photons and no additional activation in the whole detector elementscompletely surrounding the decay region. Among the KL decay mode, KL + n”vD isthe unique mode of the two-photon final state except for the KL --+ yy which is easily
rejected by the kinematics of two-body decay. Since the decay is quite rare, we have tosuppress the huge amount of the backgrounds. KL -+ 7r01rodecay is considered as themain source of the background from the KL decay, since the branching ratio is relatively
high (9.27 x lo-*) and it is easily mis-identified as a signal by missing the two photons.
In order to suppress such background, very high efficiency of photon detection is requiredfor our detector. The charged particles from the KL decay will be suppressed without any
difficulty since the charged particles are always produced as a pair. Decay region should
be highly evacuated in order to suppress the 7r” production by the neutron interaction to
the residual gas. Contribution from the hyperons is not serious for our low-energy beam.
Double decay-chamber structure will suppress the KL to multi-gamma decay by detecting
the photons escaped to the upstream. Narrow beam size is required to apply the high-Pr
selection of # as well as to reduce the direct hit of beam to the detector.
2.2.

Experimental

apparatus

We have constructed a new neutral beam line at the KEK-PS in March 2000, as shownin Figure 1. The neutral beam is produced at the platinum target of 8mm in diameterand 60mm in length by 12 GeV protons, and collimated by five stages of heavy-metal
collimators. Charged particles are sweeped out by two stages of bending magnets. The
beam aperture is defined at 2 mrad by the 10 m long collimators.
So far the series of the beam survey has been carried out and its characteristics havebeen understood well. Figure 2 shows the beam profile measured by the gamma andneutron. The beam-core size of 10 cm in diameter is narrow enough, and the halo part issmaller than the core by the order of four as expected from the Monte Carlo simulation.
This will be the cleanest neutral beam-line so far.

M. Yamaga et al. /Nuclear Physics A721 (2003) 449~452~

Figure 1. Neutral beam line for the E391a experiment.

451c

Figure 2. Beam profile.

Figure 3 shows the schematic view of the E391a detector composed of the electromagnetic calorimeter and perfect veto system. The energy and the position of two photons from 7r”are measured by the electromagnetic calorimeter consisting of 576 pure CsI
crystals. The decay vertex and the PT of the r” are reconstructed by the two photons
assuming the r” mass. The signal will be identified by requiring that the vertex is in
the decay fiducial-volume and the PT is larger than 120 MeV/c. The decay fiducial volume is evacuated to 10e5 Pa, which is realized by the differential pumping method. The
detector region is separated from the highly evacuated decay volume, and evacuated to
10-l Pa. Events accompanying with charged particles are rejected by the scintillation
counters in front of the CsI. The veto system for additional particles, which consists ofthe central-barrel calorimeter, front-barrel calorimeter, beam-ant#i counter and five-stageof the beam-collar counters, are designed to cover whole decay region without any deadregion. The veto calorimeters consist of the alternating scintl?stor and lead or tungsten
plate with wave-length-shifter fiber read-out.

3. ‘Present

statusThe E391a experiment is now in the phase of detector construction. The downstream
section containing the electromagnetic calorimeter was already fabricated as shown in
Figure 4. It consists of 576 of CsI crystals and one of the beam-collar counter at thecenter of the calorimeter. The other detector components are being fabricated and will
be assembled into our detector system by the autumn 2003.
The engineering run with the downstream section is scheduled in the beam time thisautumn. Since the 7r”will be reconstructed by the electromagnetic calorimeter, obtained
data during this engineering run will be very important to develop the analysis method,which bring us to the stage of the off-line analysis. We will do the precise calibrationof each detector as well as the overall check of our experimental apparatus including theelectronics and data acquisition system.

452~

M. Yamaga et al. /Nuclear Physics A721 (2003) 449~~452~

Figure 3. Schematic view of the E39la detector.

Figure 4.
section.

Downstream

4. Summary

KL -+ 7r”vg decay is often called as a ‘Gold plated mode’ to access the CP violation.
However, there is still a large gap between the experimental achievement and the Standard
Model prediction.
KEK PS-E391a is the first dedicated experiment to the KL -P TT~VZ~
decay with a high-sensitivity
close to the Standard Model prediction.
It also serves asa pilot experiment for the future experiment at the 50-GeV PS. The E391a will give
us meaningful data to understand the situation at the tiny branching-ratio
region. The
engineering run using a part of our detectors is scheduled this autumn. The E391a will
start the physics run in early 2004.
REFERENCES
1.
2.
3.

G. Buchalla and A.J. Buras, Phys. Rev. D54 (1996) 6782.
A. Alavi-Harati
et. al., Phys. Rev. D61 (2000) 072006.
The Joint Project for High-Intensity
Proton Accelerators, KEK Report99-4.

E419 H. Tamura
Measurement of B(E2) Transition Rate in 7ΛLi Hypernucleus
H. Tamura et al.
[attached]
Observation of a spin-flip M1 transition in 7ΛLi,
Phys. Rev. Lett. 84 (2000) 5963-5966.
● K. Tanida et al.
[attached]
Measurement of B(E2) of 7ΛLi and shrinkage of the hypernuclear size,
Phys. Rev. Lett. 86 (2001) 1982-1985.
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[attached]7
Li
Ground-State
Spin
Determined
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of
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Subsequent
to
Weak
Decay,
L
Phys. Lett. B 579 (2004) 258-264.
★ K. Tanida
Gamma-Ray Spectroscopy of 7ΛLi,
Ph.D. Thesis, University of Tokyo, 2000.
◇ H. Tamura
Gamma Spectroscopy of Light Hypernuclei,
Workshop on Hypernuclear Physics, INS Univ.of Tokyo, December 7-8, 1996,
Genshikaku Kenkyu vol.41 No.6, 55.
◇ H. Tamura et al.
High-resolution gamma-ray spectroscopy of 7LLi
KEK-Tanashi Int. Symp. on “Physics of Hadrons and Nuclei”, Tokyo, December 1998,
Nucl. Phys. A670 (2000) 249c-256c.
◇ K. Tanida et al.
Hypernuclear Gamma-Ray Spectroscopy Experiments with Germanium Detectors,
APCTP Workshop on Strangeness Nuclear Physics (SNP'99), Seoul, February 1999,"Strageness Nuclear Physics", World Scientific (2000) p.98.
◇ H. Tamura
Future Plans of High-Resolution Hypernuclear Gamma-Spectroscopy,
APCTP Workshop on Strangeness Nuclear Physics (SNP'99), Seoul, February 1999,"Strageness Nuclear Physics", World Scientific (2000) p.411.
◇ H. Tamura
High-resolution spectroscopy of hypernuclei,
Workshop on JHF K-arena Physics, KEK, March 17-19, 1999, KEK proceedings 99-5, p.73.
◇ H. Tamura et al.
High-resolution hypernuclear gamma-ray spectroscopy,
15th Int. Conf. of Particles and Nuclei (PANIC'99), Uppsala, June 1999,
Nucl. Phys. A 663&664 (2000) 481c-484c.
◇ H. Tamura et al.,High-resolution gamma-ray spectroscopy of 7ΛLi and 9ΛBe,
Asia Pacific Conf. on “Few Body Problems in Physics”, Noda/Kashiwa, August 1999,
Few-Body Systems Suppl. 12 (2000) 342-346.
◇ K. Tanida, et al.
ΛN Spin Dependent Interactions Studied by γ-Ray Spectroscopy of Hypernuclei,
RCNP-TMU Symposium on ``Spins in Nuclear and Hadronic Reactions'', Tokyo Metropolitan
University, October 26-28, 1999, World Scientific (2000) p.215.
◇ K. Tanida et al.
High-Resolution Gamma-Ray Spectroscopy of Hypernuclei with Germanium Detectors,
16th Int. Conf. on “Few Body Problems”, Taipei, March, 2000, Nucl. Phys. A 684 (2001) 560c.
◇ H. Tamura et al.
Hypernuclear gamma spectroscopy: recent results with HYPERBALL,
Int. Conf. on “Structure of the Nucleus at the Dawn of the Century” (Bologna 2000), Bologna, May
2000, “Hadrons, Nuclei and Applications”, Ed. G.C. Bonsignori et al., World Scientific, p.106-109.
●________________________________________________________________________● Physics papers published in refereed journal.
○★
◇＊

Technical papers.
PhD theses.
Conference and Symposium.
Internal Report and others.

K. Imai
Recent Progress of Spectroscopy of Light Hypernuclei,
8th meeting in Mesons and Light Nuclei (Prague 2001), Prague, July 2001,
AIP Conf. Proc. 603 (2001) 69-78.
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Impurity Nuclear Physics –Hypernuclear γ spectroscopy and future plans for neutron-rich
hypernuclei,
Int. Symp. on “Perspectives in Physics with Radioactive Isotope Beams” (RIB00), Hayama,
November 2000, Eur. Phys. J. A 13 (2002) 181-187.
◇ K. Tanida et al.
High-resolution gamma-ray spectroscopy of 7ΛLi,
Int. Conf. on Hypernuclear and Strange Particle Physics, (HYP2000), Torino, October 2000, Nucl.
Phys. A 691 (2001) 115.
◇ H. Tamura
High resolution spectroscopy of Λ hypernuclei: present status and perspectives,
Int. Conf. on “Hypernuclear and Strange Particle Physics” (HYP2000), Torino, October 2000,
Nucl. Phys. A 691 (2001) 76.
◇ H. Tamura
Hypernuclear Studies at KEK and BNL: Gamma Spectroscopy of Λ Hypernuclei,
29th International Workshop on “Gross Properties of Nuclei and Nuclear Excitations” (Hirschegg'01), Hirschegg, Austria, January 14-20, 2001, "Hirschegg 2001--Structure of hadrons" GSI, (2001)
290-299.
◇ H. Tamura
Hypernuclear gamma spectroscopy and ΛN interactions,Int. Symp. on ``Hadrons and Nuclei'', Seoul, March 2001, Ed. Il-T. Cheon et al.,AIP Conf. Proc. 594 (2001) 147-154.
◇ H. Tamura
Hyperball and a new frontier in hypernuclear physics,
Int. Workshop on “development of Ge detector array and frontiers of gamma-ray spectroscopy”,
CNS, University of Tokyo, December 11-13, 2001, ISSN 1343-2230 CNS report CNS-REP-43,
p.159.
◇ H. Tamura
Structure of Light Hypernuclei Studied by Gamma-Ray Spectroscopy,
Int. Symp. on ``Clustering Aspects of Quantum Many-Body Systems'' (POSTYK01), Kyoto,
November 2001, Ed. Ohnishi et al., World Scientific (2002) p.49-57.
◇ H. Tamura
Hypernuclear Gamma-Ray Spectroscopy --A Breakthrough in Strangeness Nuclear Physics,
4th Italy-Japan Symposium on ``Perspectives in Heavy Ion Physics'', RIKEN and University ofTokyo, September 26 - 29, 2001, Ed. K. Yoshida et al., World Scientific (2003) p.401-412.
◇ H. Tamura
Matter with Strangeness,
16th Int. Conf. on Particles and Nuclei (PANIC02), Osaka, September 30-October 4, 2002,
Nucl. Phys. A 721 (2003) 84c-93c.
◇ H. Tamura
Experimental Progress in Hypernuclear Physics,
18th Nishinomiya Yukawa Memorial Symposium on “Strangeness in Nuclear Matter”,
Nishinomiya, December 4-5, 2003, Prog. Theor. Phys. in press.
＊ The Incredible Shrinking Nucleus,
Physical Review Focus, 1 March 2001.
＊ Quark Quirk Triggers Nuclear Shrinkage,
Science 291, 9 March 2001, p.1877.
＊ Teeny-weeny --Japanese scientists unveil their incredible shrinking nucleus,
New Scientist 24, March 2001, p.13.
◇________________________________________________________________________● Physics papers published in refereed journal.
○★
◇＊

Technical papers.
PhD theses.
Conference and Symposium.
Internal Report and others.

VOLUME 84, NUMBER 26

PHYSICAL REVIEW LETTERS

26 JUNE 2000

Observation of a Spin-Flip M1 Transition in L7 Li
H. Tamura,1 K. Tanida,2 D. Abe,1 H. Akikawa,3 K. Araki,1 H. Bhang,4 T. Endo,1 Y. Fujii,1 T. Fukuda,5 O. Hashimoto,1
K. Imai,3 H. Hotchi,2 Y. Kakiguchi,5 J. H. Kim,4 Y. D. Kim,6 T. Miyoshi,1 T. Murakami,3 T. Nagae,5 H. Noumi,5H. Outa,5 K. Ozawa,1 T. Saito,7 J. Sasao,1 Y. Sato,1 S. Satoh,1 R. I. Sawafta,8 M. Sekimoto,5 T. Takahashi,1
L. Tang,9 H. H. Xia,10 S. H. Zhou,10 and L. H. Zhu3,10
1

Department of Physics, Tohoku University, Sendai 980-8578, Japan
Department of Physics, University of Tokyo, Tokyo 113-0033, Japan3
Department of Physics, Kyoto University, Kyoto 606-8502, Japan
4
Department of Physics, Seoul National University, Seoul 151-742, Korea
5
Institute of Particle and Nuclear Studies, KEK, Tsukuba 305-0801, Japan
6
Department of Physics, Sejong University, Seoul 143-747, Korea
7
Laboratory of Nuclear Science, Tohoku University, Sendai 980-0826, Japan8
Physics Department, North Carolina A&T State University, Greensboro, North Carolina 27411
9Department of Physics, Hampton University, Hampton, Virginia 23668
10
Department of Nuclear Physics, CIAE, P.O. Box 275(80), Beijing 102413, China(Received 20 March 2000)
2

Using a large-acceptance germanium detector array (Hyperball), we have observed a spin-flip M1 g
transition between the ground-state spin doublet of 7L Li ( 32 1 ! 12 1 ). The observed energy of 691.7 6
0.6共stat兲 6 1.0共syst兲 keV provides crucial information on the strength of the spin-spin interaction between a L and a nucleon. This is the first observation of well-identified hypernuclear g transitions using
germanium detectors.
PACS numbers: 21.80. + a, 21.30.Fe, 23.20.Lv, 25.80.Hp

High-resolution g-ray spectroscopy using germanium(Ge) detectors has played a remarkably important rolein the development of nuclear physics. Detailed levelstructures of hundreds of nuclei have been precisely measured with an energy resolution of an order of a few keV.
However, high-resolution g-ray spectroscopy has not beensuccessfully used for L hypernuclei because of technicaldifficulties in the operation of Ge detectors under highcounting-rate conditions. Hypernuclear structure has been
investigated mostly with the (K 2 , p 2 ) and (p 1 , K 1 ) reactions by means of magnetic spectrometers having a few
MeV (FWHM) resolution.
Observations of well-identified hypernuclear g transitions have been reported for only a few cases, in which
NaI counters having much worse energy resolution thanGe detectors were used [1–3]. The only previous attemptwith Ge detectors could not identify hypernuclear g transitions [4]. Recently, we constructed a large-acceptanceGe detector array dedicated to hypernuclear g-ray spectroscopy (Hyperball) and started a series of experiments.
Physics motivations and experimental plans are describedin Ref. [5]. In this paper, we report on the first experimentwith Hyperball, in which g transitions in 7L Li were successfully observed.
One of the motivations for hypernuclear g spectroscopy
is to obtain information on the interactions between a L
and a nucleon from hypernuclear level structures, which isthe first step toward unified understanding of the baryonbaryon interactions. In light L hypernuclei, level structures can be understood from the weak coupling of the L
to the nuclear core. For low-excitation levels where the L
0031-9007兾00兾84(26)兾5963(4)$15.00

occupies the 0s orbit, each level of the nuclear core having spin J (ﬁ 0) is split into two levels with spins J 1 12
and J 2 12 , the spacing of which is determined by the LN
spin-dependent (spin-spin, spin-orbit, and tensor) interactions. The level structures and g transitions of p-shell
hypernuclei have been theoretically studied in relation to
the spin-dependent interactions [6–9]. Since these interactions are weak, such a doublet spacing is generally expected to be smaller than spacings of low-lying levels of
the core nucleus for p-shell hypernuclei. By measuring
such doublet spacings, we can obtain information on the
LN spin-dependent interactions, which are extremely difficult to study by LN scattering experiments. In the caseof the ground-state doublet of a hypernucleus, the upper
member of the doublet is deexcited via g emission to thelower member by flipping the L spin, unless the doublet
spacing is smaller than 0.1 MeV when the weak decay of
the upper member dominates the g transition.
Such spin-flip M1 transitions between hypernuclear
doublet states have been observed only for the A 苷 4hypernuclei, 4L H and 4L He, where the ⬃1.1 MeV 11 ! 01
transitions were detected with NaI counters [1]. Although
this energy naively gives the strength of the LN spinspin interaction, an effective three-body LNN interaction
arising from S-L coupling [10] also contributes, and thus
doublet spacing data for p-shell hypernuclei are required
to quantify the LN spin-spin interaction strength. The
two-body pN sL effective interaction is expressible interms of five radial integrals, which can be determinedfrom phenomenological analysis of hypernuclear data
[6,9]. The four spin-dependent parameters are denoted© 2000 The American Physical Society

5963

VOLUME 84, NUMBER 26

PHYSICAL REVIEW LETTERS

by D, SL , SN , and T , corresponding to the operators
sN sL, lNLsL, lNLsN , and S12 . The spin-spin term, D,
was deduced as 0.40 , D , 0.64 MeV from the A 苷 4
spacing and D 苷 0.50 6 0.11 MeV from the binding
energy difference between 7L Li and 7L Be [9]. On the other
hand, a subsequent experiment for 10
L B did not observe
the spin-flip M1 transition of the ground-state doublet(22 ! 12 ); this was attributed to a small doublet spacingof less than 0.1 MeV and was interpreted in terms ofa small spin-spin strength, D , 0.22 MeV [4]. In thispuzzling situation, measurements of spin-doublet spacingsof other p-shell hypernuclei have been awaited. 7L Li is thebest hypernucleus for this purpose, because the groundstate doublet ( 32 1 , 12 1 ) spacing is determined almost
entirely by the spin-spin interaction due to the almost pure3
S1 configuration of the core nucleus 6 Li共11 兲.
The experiment (E419) was performed at the 12 GeV
Proton Synchrotron (PS) in the High Energy Accelerator
Research Organization (KEK). Bound states of 7L Li were
produced by the 7 Li共p 1 , K 1 兲 reaction with a 1.05 GeV兾cpion beam, employing the K6 beam line and the Superconducting Kaon Spectrometer (SKS), and g rays weredetected with Hyperball in coincidence. The intensity ofthe p 1 beam was typically 1.8 3 106 per spill (0.7 s duration) occurring every 3 s. The trajectory of each beam
particle was measured with the K6 beam spectrometer. The
beam pions hit a 25 cm thick 98% enriched 7 Li target andoutgoing K 1 mesons were identified and momentum analyzed by the SKS. Detailed descriptions on the K6 beamline and the SKS are to be found in Refs. [11,12].
Hyperball was installed surrounding the target. It
consisted of fourteen N-type coaxial Ge detectors of 60%
relative efficiency (crystal size of about 70 mmf 3
70 mm). The distance between the detector end cap andthe beam axis was about 15 cm. It had a photo-peakefficiency of about 2.5% at 1 MeV. The signal from each
Ge detector was processed with a transistor-reset preamplifier and a gated-integrator amplifier. Each Ge
detector is surrounded by six bismuth germanate (BGO)
counters which provide veto signals to suppress backgrounds from Compton scattering, high-energy g raysfrom p 0 decays, and high-energy charged particles. Data
from all the detectors in Hyperball and the K6兾SKS
system were recorded with a (p 1 , K 1 ) trigger provided
by the K6兾SKS system alone.
Energy calibration of the Ge detectors was made in
the energy range of 0.1–1.8 MeV using a standard mixed
source containing 241 Am, 109 Cd, 57 Co, 139 Ce, 51 Cr, 113 Sn,
85
Sr, 137 Cs, 60 Co, and 88 Y. The in-beam performance ofeach Ge detector was monitored simultaneously with thedata taking through the whole beam time by using triggered g rays from a weak (1 kBq) 60 Co source embedded in a plastic scintillator and installed behind each Ge
detector. With the beam on, single counting rates of the
14 Ge detectors were 20–60 kHz, and their energy resolutions were 3–4 keV FWHM at 1.33 MeV. The elec5964

26 JUNE 2000

tronics dead time due to pileup and preamplifier reset
was 共46.6 6 0.5兲% on average. The energy scale of thein-beam g-ray spectrum was checked with beam-induced
g-ray peaks from surrounding materials in a high-statistics
g-ray spectrum before the (p 1 , K 1 ) event selection. It
was found to be correct within 0.6 keV for an energy region less than 2 MeV. The absolute efficiency of all the
Ge detectors was measured with the standard mixed source
and corrected for the reaction point distribution and absorption effects in the target. The inefficiency due to accidental
suppression by the BGO counters was 共6 6 2兲%. The target was irradiated with 1.0 3 1012 pions in total.
Figure 1 shows the excitation spectrum of 7L Li plottedversus the L binding energy, BL . The analysis procedure
to obtain the excitation spectrum is almost identical to thatused in previous (p 1 , K 1 ) experiments with the SKS [12].
In a recent experiment for 7 Li共p 1 , K 1 兲 (KEK E336), performed under the same condition but with a thin 7 Li target(2.2 MeV FWHM resolution), the bound state region of theobserved 7L Li spectrum was decomposed into four peaks[13], the energies and relative intensities of which agreed
fairly well with theoretically calculated level energies andcross sections [14,15] of the four most strongly populated
states, namely, 12 1 共T 苷 0兲, 52 1 共T 苷 0兲, 12 1 共T 苷 1兲, and51
2 共T 苷 1兲 (see Fig. 3). [The notation “(T 苷 0)” is omitted hereafter.] In the present spectrum, the energy resolution is worse due to energy loss effects in the thick target.
By folding these four peaks with 4.2 MeV FWHM resolution, the present spectrum was reproduced well, as shownin the Fig. 1 inset. The absolute energy scale was adjusted
using the known BL of the 7L Li ground state. We set thegate for the “bound region” at 210 , 2BL , 2 MeV, as
shown in the figure.
Figure 2 shows the g-ray energy spectra. Figure 2(a) is
the spectrum when the unbound region (2BL . 2 MeV)
of 7L Li is gated. We notice peaks from Bi x rays, e1annihilation, and several g rays from normal nuclei suchas 6 Li, 7 Li, 27 Al, 74 Ge, etc. Figure 2(b) shows the g-ray
spectrum for the bound region, where more peaks appear
at 429, 692, and at 2050 keV, and the peak at 478 keV
is more prominent than in 2(a). The 429 and 478 keV

FIG. 1. Hypernuclear mass spectrum of 7L Li (plotted versus
the L binding energy, BL ) taken in the (p 1 , K 1 ) reaction witha 25 cm thick 7 Li target. The “bound region” is defined asshown. The inset shows decomposition of states (see text).

VOLUME 84, NUMBER 26

PHYSICAL REVIEW LETTERS

26 JUNE 2000

FIG. 2. g-ray spectra measured in the 7 Li共p 1 , K 1 兲 reaction for (a) the unbound region (2BL . 2 MeV) and for (b) the boundregion (210 , 2BL , 2 MeV) of 7L Li. The two peaks at 692 and 2050 keV in (b) are assigned as M1共 32 1 ! 12 1 兲 and E2共 52 1 !
11兲 transitions of 7L Li, respectively. The curve in the left inset in (b) shows a Doppler-broadened peak shape simulated for g-ray2
emission before the 7L Li slows down. (c) is the same spectrum as (b) but the event-by-event Doppler-shift correction was applied.
In the right insets of (b) and (c), the bound region condition is changed to 25.7 , 2BL , 2.3 MeV. Two peaks observed in theright inset of (c) are assigned as M1共共 12 1 共T 苷 1兲 ! 32 1 , 12 1 兲 transitions.

peaks are interpreted as transitions in the daughter nuclei
resulting from 7L Li weak decay, 7L Li ! p 2 1 7 Beⴱ 共429兲
and 7L Li ! p 0 1 7 Liⴱ 共478兲.
The peak at 2050 keV is attributed to the E2共 52 1 !
711
2 兲 transition in L Li. As shown in the level scheme in
Fig. 3, it is essentially the E2共31 ! 11 兲 transition of thecore nucleus 6 Li. This transition was previously observed
at 2.034 6 0.023 MeV using NaI counters at Brookhaven
National Laboratory (BNL) [2]. In our spectrum the peak
was revealed to have a sharp part and broad tails due topartial Doppler broadening. This fact indicates that the
lifetime of the g-emitting excited state is of the same orderof the stopping time of a recoil hypernucleus in the target(⬃10 ps), which is consistent with a transition rate for E2[16]. From the sharp peak the g-ray energy was derived as2050.1 6 0.4共stat兲 6 0.7共syst兲 keV, which corresponds to
a 52 1 excitation energy of 2050.4 6 0.4 6 0.7 keV.
As shown in the left inset in Fig. 2(b), the shape of the
692 keV peak is well reproduced by a simulation (solid
curve) in which the Doppler shift is calculated assumingthat g rays are emitted from 7L Li promptly before slowing
down. Therefore, this transition is faster than 2 ps andassigned as an M1 transition in 7L Li. After the event-byevent Doppler-shift correction, the broad peak at 692 keV
becomes sharp as shown in Fig. 2(c). It evidently showsthat the 692 keV g ray is emitted from 7L Li, and excludes
the possibility that the peak is due to a 692 keV 72 Geline induced by the 共n, n0 兲 reaction in the Ge detectors.
From the Doppler-corrected spectrum, the g-ray energy
was obtained as 691.7 6 0.6共stat兲 6 1.0共syst兲 keV. Herethe systematic error stems mainly from ambiguity in the
Doppler-shift correction.
We also observed two peaks at 3877 6 5 6 7 keV andat 3186 6 4 6 5 keV with a statistical significance about

FIG. 3. Level scheme and g transitions of 7L Li. Thick arrows
show transitions observed, and “present” shows level energies
measured in the present experiment. Transitions from 12 1 共T 苷
1兲 to the ground-state doublet were observed with less statisticalsignificance. s共u 苷 0± 15±兲 shows calculated production crosssections by (p 1 , K 1 ) reaction at 1.05 GeV兾c and integrated for0± – 15± (⬃SKS acceptance) [15].

5965VOLUME 84, NUMBER 26

PHYSICAL REVIEW LETTERS

3s as shown in the right inset in Fig. 2(c). Here the boundregion gate was changed to 25.7 , 2BL , 2.3 MeV to
tag the 12 1 共T 苷 1兲 state selectively. They are M1 transitions because they appeared in the Doppler-shift-corrected
spectrum 2(c). From their energies, they are assigned as
M1共共 12 1 共T 苷 1兲 ! 32 1 and 12 1 兲 (see Fig. 3). Their relative yields (⬃1:1) also support this assignment. Their energy difference (691 6 6 keV) coincides with the 692 keVpeak energy. Therefore, the 692 keV transition is assigned
as M1共 32 1 ! 12 1 兲, but not as M1共 72 1 ! 52 1 兲. This assignment is further supported by g-ray yields. Using theratios of the theoretical cross sections for the 72 1 and 32 1
states to that of the 12 1 state in Fig. 3, the g-ray yields
were estimated from the numbers of events for the 12 1 ,51
11
2 , and 2 共T 苷 1兲 states derived from the peak decomposition in the Fig. 1 inset and the absolute efficiency
of Hyperball [共1.20 6 0.04兲 3 1022 at 692 keV, including the electronics dead time]. The M1共 72 1 ! 52 1 兲 yieldis estimated to be 24 counts, while the M1共 32 1 ! 12 1 兲yield is estimated to be 102 6 17 counts including g-ray
cascades after the 12 1 共T 苷 1兲 ! 32 1 and the 52 1 ! 32 1
transitions. The observed yield of the 692 keV peak,
128 6 17 counts, agrees with the latter estimation. This
fact also indicates that the 7L Li ground state is 12 1 but not
31[17]. It is to be noted that the estimated E2共 52 1 !
211
2 兲 yield (186 counts) agrees well with the observed one(188 6 17 counts), which provides confidence that the estimated Hyperball efficiency is correct. The transition51! 32 1 which should have 1358 keV energy was not2
observed. It is consistent with a calculated branching ratioof 3.4% for the core E2 transition without M1 mixing.
Our result of 692 keV for the ground-state doublet spacing provides a stringent constraint to the strength of the
LN spin-spin interaction. In the phenomenological approach [9], our result gives D ⬃ 0.5 MeV, since the 7L Li
doublet spacing is given by 32 D in the LS coupling limit
with L 苷 0. The shell-model calculation of Ref. [9] gave
a 7L Li doublet spacing of 610 keV for a parameter set withD 苷 0.5 MeV. On the other hand, another shell-model
calculation by Fetisov et al. [18], in which the parameter set (with D 苷 0.3 MeV) was selected to explain the10
L B result rather than the A 苷 4 doublet spacing, gave
440 keV. Recently, another theoretical approach was introduced by Hiyama et al. [19]. They carried out a fourbody (a 1 p 1 n 1 L) cluster-model calculation using
the Nijmegen model F interaction, in which the SN channel contribution is renormalized into LN interactions andthe strength of the LN spin-spin interaction was adjusted
to reproduce the A 苷 4 doublet spacing. They predicted a
7
L Li doublet spacing of 650 keV without an LS interaction.
The present result is thus consistently explained with the10
A 苷 4 data but not with the 10
L B result, implying that L B
should be further investigated. In addition, the contributionof the LNN effective interaction to the doublet spacing
should be studied theoretically for both of the A 苷 4 and
5966

26 JUNE 2000

A 苷 7 cases. Finally, the excitation energies of the 52 1 and11
2 共T 苷 1兲 states are sensitive to both D and SN and areconsistent with D ⬃ 0.5 MeV and SN ⬃ 20.4 MeV [20].
In summary, we have observed well-identified hypernuclear g transitions with Ge detectors for the first time.
Using Hyperball, we observed g transitions in 7L Li,
the spin-flip M1共 32 1 ! 12 1 兲 at 691.7 6 0.6共stat兲 6
1.0共syst兲 keV, as well as the E2共 52 1 ! 12 1 兲 andM1共共 12 1 共T 苷 1兲 ! 32 1 , 12 1 兲 transitions, by gating thebound region in the 7 Li共p 1 , K 1 兲7L Li spectrum. The
measured energy of the ground-state doublet ( 32 1 , 12 1 )spacing provides crucial information on the LN spinspin interaction. In order to obtain information on theother spin-dependent interactions, more hypernuclear
g-spectroscopy experiments with Hyperball are in
progress at BNL (E930).
The authors thank K. Nakamura and the KEK-PS staff.
They are also grateful to T. Motoba, E. Hiyama, and D. J.
Millener for theoretical estimates in the design stage of theexperiment and discussion of the results. This work was
supported by the Grant-In-Aid for Scientific Research on
the Priority Area “Strangeness Nuclear Physics” from The
Ministry of Education of Japan, No. 08239102.

[1] M. Bedjidian et al., Phys. Lett. 62B, 467 (1976); M. Bedjidian et al., Phys. Lett. 83B, 252 (1979).
[2] M. May et al., Phys. Rev. Lett. 51, 2085 (1983).
[3] M. May et al., Phys. Rev. Lett. 78, 4343 (1997).
[4] R. E. Chrien et al., Phys. Rev. C 41, 1062 (1990).
[5] H. Tamura, Nucl. Phys. A639, 83c (1998).
[6] R. H. Dalitz and A. Gal, Ann. Phys. (N.Y.) 116, 167 (1978).
[7] R. H. Dalitz and A. Gal, J. Phys. G 4, 889 (1978).
[8] T. Motoba, H. Bandō, and K. Ikeda, Prog. Theor. Phys. 70,
189 (1983).
[9] D. J. Millener et al., Phys. Rev. C 31, 499 (1985).
[10] B. F. Gibson and D. R. Lehman, Phys. Rev. C 37, 679(1988); Y. Akaishi et al., Phys. Rev. Lett. 84, 3539 (2000).
[11] T. Fukuda et al., Nucl. Instrum. Methods Phys. Res.,
Sect. A 361, 485 (1995).
[12] T. Hasegawa et al., Phys. Rev. C 53, 1210 (1996).
[13] O. Hashimoto et al., Nucl. Phys. A639, 93c (1998).
[14] O. Richter, M. Sotona, and J. Žofka, Phys. Rev. C 43, 2753
(1991).
[15] E. Hiyama et al., Phys. Rev. C 59, 2351 (1999).
[16] We have extracted B共E2兲 of this transition using the
Doppler-shift attenuation method [K. Tanida, Ph.D. thesis, University of Tokyo, 2000; K. Tanida et al. (to be
published)].
[17] The 7L Li ground-state spin has been independently determined to be 12 from the yield of the 429 keV 7 Be g ray
after weak decay [J. Sasao et al. (to be published)].
[18] V. N. Fetisov et al., Z. Phys. A 339, 399 (1991).
[19] E. Hiyama et al., Nucl. Phys. A639, 173c (1998).
[20] D. J. Millener, in “Strangeness Nuclear Physics,” edited by
I. T. Cheon, S. W. Hong, and T. Motoba (World Scientific,
Singapore, to be published).

VOLUME 86, NUMBER 10

PHYSICAL REVIEW LETTERS

5 MARCH 2001

7
Measurement of the B共共E2兲兲 of L
Li and Shrinkage of the Hypernuclear Size

K. Tanida,1 H. Tamura,2 D. Abe,2 H. Akikawa,3 K. Araki,2 H. Bhang,4 T. Endo,2 Y. Fujii,2 T. Fukuda,5 O. Hashimoto,2K. Imai,3 H. Hotchi,1 Y. Kakiguchi,5 J. H. Kim,4 Y. D. Kim,6 T. Miyoshi,2 T. Murakami,3 T. Nagae,5 H. Noumi,5H. Outa,5 K. Ozawa,2 T. Saito,7 J. Sasao,2 Y. Sato,2 S. Satoh,2 R. I. Sawafta,8 M. Sekimoto,5 T. Takahashi,2 L. Tang,9H. H. Xia,10 S. H. Zhou,10 and L. H. Zhu3,10
1

Department of Physics, University of Tokyo, Tokyo 113-0033, Japan
Department of Physics, Tohoku University, Sendai 980-8578, Japan3
Department of Physics, Kyoto University, Kyoto 606-8502, Japan
4
Department of Physics, Seoul National University, Seoul 151-742, Korea
5
Institute of Particle and Nuclear Studies, KEK, Tsukuba 305-0801, Japan
6
Department of Physics, Sejong University, Seoul 143-747, Korea
7
Laboratory of Nuclear Science, Tohoku University, Sendai 980-0826, Japan8
Physics Department, North Carolina A&T State University, Greensboro, North Carolina 27411
9Department of Physics, Hampton University, Hampton, Virginia 23668
10
Department of Nuclear Physics, CIAE, P.O.Box 275(80), Beijing 102413, China(Received 28 August 2000)
2

We report on the first measurement of a hypernuclear g-transition probability. g rays emitted in the
E2共5兾21 ! 1兾21 兲 transition of 7L Li were detected by a large-acceptance germanium detector array(Hyperball), and the lifetime of the parent state (5兾21 ) was determined by the Doppler shift attenuation
method. The obtained result, 5.810.9
20.7 6 0.7 ps, was then converted into the reduced transition probability
10.5 2[B共E2兲] to be B共E2; 5兾21 ! 1兾21 兲 苷 3.6 6 0.520.4
e fm4 . Compared with the B共E2兲 of the corre1
1
6
sponding E2共3 ! 1 兲 transition in the Li nucleus, our result gives evidence that the size of the 6 Li corein 7L Li is smaller than the 6 Li nucleus in the free space.
DOI: 10.1103/PhysRevLett.86.1982

PACS numbers: 21.80.+a, 21.10.Tg, 23.20.– g, 27.20.+nChange of matter properties under the presence of impurities is an important subject in condensed matter physics.
Similarly, bulk properties of nuclei, such as size, shape,collective motion, and so on might be changed by the presence of hyperons as impurities, although no experimental
evidence has been found yet. In particular, significant reduction of nuclear size could be expected when a L hyperon is added to loosely bound light nuclei such as 6 Li
[1–3]. Since a L particle does not suffer from Pauli blocking, it can locate at the center of a nucleus; then the L attracts surrounding nucleons and makes the nucleus shrink.
In order to obtain information on such a possible size contraction experimentally, we used the E2共5兾21 ! 1兾21 兲transition in 7L Li (see Fig. 1). The reduced transition probability [B共E2兲] is very sensitive to size contraction as it isapproximately proportional to fourth power of the nuclear
size. In the weak coupling limit, since the E2共5兾21 !
1兾21 兲 transition in 7L Li is due to the E2共31 ! 11 兲 transition in 6 Li core, we introduce a factor S which naively
represents degree of size change of 6 Li core [3] by∏∑
9 B共E2;7L Li5兾21 ! 1兾21 兲 1兾4
,
(1)
S苷
7
B共E2;6 Li31 ! 11 兲
where the factor 9兾7 comes from the fact that the B共E2兲 ofthe core transition (31 ! 11 ) is distributed to the two E2
transitions in 7L Li as B共E2; 7L Li5兾21 ! 1兾21 兲 : B共E2;71
1
6
L Li5兾2 ! 3兾2 兲 苷 7 : 2 in this limit [4]. If the Li core
7
6
in L Li is the same as the Li nucleus in the free space, thissize factor equals unity. As we know the B共E2;6 Li31 !

11 兲 to be 10.9 6 0.9 e2 fm4 [5] or 9.3 6 2.1 e2 fm4 [6],we can extract hypernuclear size information from
B共E2; 5兾21 ! 1兾21 兲.
The experiment (E419) was performed at the K6 beamline of the 12 GeV Proton Synchrotron (PS) in the HighEnergy Accelerator Research Organization (KEK) as thefirst experiment using the germanium (Ge) detector array
recently constructed for hypernuclear g-ray spectroscopy(Hyperball). We have already reported in Ref. [7] on theenergies and assignments of the four g transitions in 7L Li
observed in the experiment. In this paper, we report on
the measurement of the B共E2兲 value of the E2共5兾21 !
1兾21 兲 transition. Taking advantage of high resolution of
Ge detectors, we successfully measured the lifetime of the
5兾21 state by the Doppler shift attenuation method andthen converted it into the B共E2; 5兾21 ! 1兾21 兲 value.

1982© 2001 The American Physical Society

0031-9007兾01兾86(10)兾1982(4)$15.00

FIG. 1. Low-lying states of 7L Li. Excitation energies are takenfrom Ref. [7]. Corresponding levels of 6 Li are also shown.

VOLUME 86, NUMBER 10

PHYSICAL REVIEW LETTERS

In E419, we used the 7 Li共p 1 , K 1 兲 reaction at incident
pion momentum of 1.05 GeV兾c to produce 7L Li, and detected g rays in coincidence. The large momentum transfer of the 共p 1 , K 1 兲 reaction (⬃350 MeV兾c) enables us toutilize the Doppler shift attenuation method, as the stopping time of the recoiling 7L Li (⬃13 ps) is of the same order as the expected lifetime (3–10 ps) [1,2,4] of the 5兾21
state. Momenta and trajectories of beam pions and outgoing kaons were measured by the beam line spectrometerand the Superconducting Kaon Spectrometer (SKS). Fromthe measured momenta, we reconstructed missing mass of
7
L Li and selected the production events of the bound statesof 7L Li. More descriptions of the K6 beam line and SKS arefound in Refs. [8,9]. A typical intensity of the pion beam
was 1.8 3 106 per 3 second cycle. We irradiated a 98%
enriched 7 Li target of 25 cm long with about 1.0 3 1012
pions in the beam time of about 25 days.
For g-ray detection, we used Hyperball, which consistedof fourteen coaxial type Ge detectors having a crystal sizeof about 70 mmf 3 70 mm. The Ge detectors were placed
15 cm away from the beam line and the total solid angle
and the photopeak efficiency were about 15% 3 4p sr and2.5% for 1 MeV g rays, respectively. Each Ge detector
was surrounded by six bismuth germanate (BGO) counters,
which provided veto signals for Compton scattering andelectromagnetic shower of high-energy g rays from p 0
decay. It is noted that we did not use the Ge detectors
or the BGO counters for triggering, so that the inclusive
7
Li共p 1 , K 1 兲 spectrum could be taken.
Energy calibration of the Ge detectors was performed in
the energy range of 0.1–1.8 MeV using a standard mixed
source containing 241 Am, 109 Cd, 57 Co, 139 Ce, 51 Cr, 113 Sn,
85
Sr, 137 Cs, 60 Co, and 88 Y. Those calibration g rays werealso used to obtain response functions of the Ge detectors,
which play an important role in the lifetime analysis described below. It is noted that slight deterioration of the response functions, especially in their tail parts, induced byradiation damage was observed during the beam time. The
in-beam performance of each Ge detector was monitored
simultaneously with the data taking through the wholebeam time by using triggered g rays from a weak (1 kBq)
60
Co source embedded in a plastic scintillator and installed behind each Ge detector. With the beam on, slightbroadening (typically 10%– 20%) of peak width due to thehigh-counting rate of the Ge detectors (20– 60 kHz) was
observed, while the tail shape was found to be the same.
More experimental details are given in Ref. [7].
Figure 2 shows the excitation spectrum of 7L Li plottedin the scale of L binding energy (BL ). See Ref. [9,10] forthe analysis procedure to obtain the excitation spectrum.
We decomposed the bound region of the observed spectrum into four Gaussian peaks, as described in Ref. [7].
The absolute energy scale was adjusted to reproduce theknown BL of the 7L Li ground state. We set the gate for the“bound region” at 210 , 2BL , 2 MeV, as shown in
the figure.

5 MARCH 2001

FIG. 2. Hypernuclear mass spectrum of 7L Li (plotted versus
the L binding energy, BL ) taken in the 共p 1 , K 1 兲 reaction witha 25 cm thick 7 Li target. See Ref. [7] for the decomposition andassignments of states. The “bound region” is defined as shown.

Figure 3 shows the g-ray energy spectra summed up for
all the fourteen Ge detectors after the energy calibration.
Figure 3(a) is the spectrum for the bound region of 7L Li,
and Fig. 3(b) is for the unbound region (2BL . 2 MeV).
See Ref. [7] for the assignment of the peaks observed in the
spectra. The peak due to the E2共5兾21 ! 1兾21 兲 transitionis the one at 2050 keV in spectrum 3(a).
Figure 4 is a magnification of Fig. 3(a) around theE2共5兾21 ! 1兾21 兲 peak. The high resolution of Ge detectors reveals that the peak has two components, namely,
the sharp part and the broad tails; this fact indicates thatthe g-ray energy is partly Doppler broadened. The g-ray
energy was determined to be 2050.1 6 0.4 6 0.7 keVfrom the position of the narrow peak as described inRef. [7].
The lifetime of the parent state (5兾21 ) was examinedby analyzing the peak shape. First, in order to obtainthe lifetime dependence of the peak shape, we performed
Monte Carlo simulations. Then the lifetime was derived

FIG. 3. g-ray energy spectra measured in coincidence with the7
Li共p 1 , K 1 兲 reaction. (a) For the bound region, and (b) for
the unbound region (2BL . 2 MeV). See Ref. [7] for the assignment of the peaks.

1983

VOLUME 86, NUMBER 10

PHYSICAL REVIEW LETTERS

FIG. 4. g-ray energy spectrum around the E2 peak. The resultof the fitting to simulated spectra (see text) is shown with thesolid line.

by fitting the simulated spectra with a background to theobserved spectrum. Here we assumed a linear shape forthe background, and there was no significant change of the
lifetime when we used quadratic or exponential shapes.
The Monte Carlo simulations were performed in thefollowing way. First, for each of the measured 共p 1 , K 1 兲events, we generated g rays at the reaction point, which
was determined by taking the vertex of the measured
trajectories of beam p 1 and outgoing K 1 projected onto
the horizontal (xz) plane. The rms spatial resolutionsof the reaction point thus determined were 1, 5, and1.2兾juhorizontal j mm, for the x, y, and z coordinates,
respectively, where uhorizontal is the horizontal scattering
angle measured in radian. In order to obtain good z-vertex
resolution, we rejected events with juhorizontal j , 10 mrad.
The effect of this finite resolution together with that of asystematic position uncertainty of 5 mm was consideredin estimating the systematic error of the lifetime and wasfound to be negligible. We also performed a simulation
where we used a vertex position uniformly distributedin the target volume instead of the measured one, andobtained the same lifetime.
The energy of the generated g ray was fixed to be
2050.1 keV in the rest frame of 7L Li; even if we changed
the g-ray energy by 60.4 keV, no significant change ofobtained lifetime was found. The angular distribution of
the g rays was assumed to be uniform in the rest frame of7
L Li, since we could not obtain it reliably due to the limited statistics. Because of a possible spin alignment effectof 7L Li, this assumption may not be correct. In order toestimate the systematic error due to this assumption, we
also performed a simulation in which the spin of 7L Li wasaligned to jjz 0 j 苷 5兾2 where the z 0 direction is normal to
the reaction plane.
Second, for a given lifetime, the g-decay timings
were determined to follow exponential distributions. The
Doppler shift of the g-ray energy was calculated from thevelocity of 7L Li at the time of the decay, which was derived
from the recoil momentum of 7L Li by taking into accountthe slowing down process in the target. The initial recoil
1984

5 MARCH 2001

momentum was calculated by subtracting the measured
K 1 momentum vector from that of p 1 with a correction
for the momentum loss in the target. The resolution of therecoil momentum was estimated to be 6 MeV兾c (FWHM)
from the 7L Li mass resolution. The absolute value wascalibrated by using the position of the ground state in the7
L Li mass spectrum (see Fig. 2) with a systematic error of
61 MeV兾c coming from the uncertainty of central beam
momentum (64 MeV兾c). The systematic error and theresolution of the recoil momentum were found to havenegligible effects on the lifetime analysis.
Slowing down of 7L Li in the lithium target was simulated by the SRIM code [11] which is based on experimental data and is commonly used for calculating the stopping
powers of ions in matter. We estimated the overall uncertainty of the stopping power given by the code to be 65%
from the errors of the calculated stopping powers of various ions in helium and beryllium [12] and that of protons
in lithium [13].
Last, when a g ray hits a Ge detector, the g-ray energy,
after convolution with the response function of the Ge detectors, was booked into a histogram. The response function
was derived from the calibration data of the 1.836 MeV grays from 88 Y, averaged over all the Ge detectors and thecalibration runs throughout the beam time. Here the peakwidth and tail width were extrapolatedpto 2.05 MeV by assuming that they were proportional to Eg and Eg , respectively. We also used response functions at the beginningand at the end of the beam time to estimate the systematic
error. It is noted that the in-beam peak broadening of about
20% was taken into account in the simulations and wasfound to have negligible effect on the obtained lifetime.
The results of the simulations are shown in Fig. 5 for the
lifetimes of 5.8 ps (best fit value), 4.4 ps (best fit 22s),
and 7.6 ps (best fit 12s). It is noted that the simulation
well reproduced the peak shape of the M1共3兾21 ! 1兾21 兲transition at 692 keV by assuming a very short lifetime(,1 ps) for the 3兾21 state [7].
The fitting result is shown with the solid line in Fig. 4.
The reduced x 2 of the fit was 0.78 at the best-fit point given

FIG. 5. Simulated g-ray energy spectra of the E2共5兾21 !
1兾21 兲 g rays for 5兾21 lifetimes of 4.4 ps (dashed line), 5.8 ps(solid line), and 7.6 ps (dotted line). Peak heights are normalized.

VOLUME 86, NUMBER 10

PHYSICAL REVIEW LETTERS

by the maximum likelihood method. The lifetime of the
5兾21 state obtained by the fitting is 5.810.9
20.7 6 0.7 ps. Herethe main sources of the systematic errors are uncertainties
in the distribution of g-ray direction (60.4 ps), in the Ge
response function (60.4 ps), and in the stopping power(60.3 ps).
We then converted the lifetime thus obtained into the
B共E2兲 value. To do this, we need the branching ratio of
the E2共5兾21 ! 1兾21 兲 transition, which was derived bysubtracting estimated branching ratios of the other possible
decay modes, namely, the 5兾21 ! 3兾21 transition and theweak decay. We took the weak coupling limit value (3.6%)
as the branching ratio of the 5兾21 ! 3兾21 transition; thisvalue is consistent with the calculation of Hiyama et al.
(3.8%) [3] and our nonobservation of the transition, whosebranching ratio was found to be smaller than 4.3% at 68%
confidence level. The branching ratio of the weak decay
was estimated to be 2.6 6 0.4%, assuming the partial decay rate to be 共230 6 40 ps兲21 from the lifetime data ofA 苷 4, 5, 11, 12 hypernuclei [14–17]. Thus the branchingratio of the E2共5兾21 ! 1兾21 兲 transition was obtained to
13.6
be 93.820.8%.
Using this branching ratio, the B共E2; 5兾21 ! 1兾21 兲 was
10.5 2
e fm4 . Then the size faccalculated to be 3.6 6 0.520.4
tor in Eq. (1) is S 苷 0.81 6 0.04 [for B共E2; 6 Li兲 苷 10.9 6
0.9 e2 fm4 : use of B共E2;6 Li兲 苷 9.3 6 2.1 e2 fm4 gives
0.84 6 0.06], and is significantly smaller than unity expected if the 6 Li core in 7L Li is the same as the 6 Li in thefree space. This fact gives strong evidence for the shrinkage of 6 Li core in 7L Li.
In cluster models, the reduction of the B共E2兲 value canbe interpreted as due to shrinkages of intercluster dis5
tances. For example, in a simple a共L
He兲-d cluster model
7
6
for Li共L Li兲, S 苷 0.81 means the rms distance between
5
7
L He and d in L Li is by 19% shorter than that of a and d
6in Li. This interpretation remains valid in a more elaborate
5He兲-p-n model calculation [2,3] where the contraca共L
tion occurs along the distance between the 共np兲 center of5He兲 cluster with the n-p internal momass and the a共L
tion hardly changed. The B共E2兲 values themselves arecalculated by Motoba et al. [a-d共-L兲 model] [1] and by5
Hiyama et al. [a共LHe兲-p-n model] [3], and they predicted S 苷 0.84 and S 苷 0.75, respectively. As for thelatter calculation, Hiyama et al. recently updated it byusing a four-body a-p-n-L model for 7L Li [18] and gave
S 苷 0.78. These calculated values are consistent with thepresent result. Thus the present result can be understoodwithin the frameworks of those cluster models.

5 MARCH 2001

In summary, we obtained the reduced transition probability of the E2共5兾21 ! 1兾21 兲 transition in 7L Li by measuring the lifetime of the 5兾21 state using the Doppler
shift attenuation method. This is the first determination ofa g-transition probability in hypernuclei. The obtained re10.5 2sult, 3.6 6 0.520.4
e fm4 , is about one-third of the B共E2兲
of the corresponding E2共31 ! 11 兲 transition in 6 Li, andis evidence for the shrinkage of the 6 Li core in the 7L Li
hypernucleus.
The authors thank K. Nakamura and the KEK-PS staff
for support of the experiment. They are grateful to J. F.
Ziegler for providing them with valuable information on
the stopping of ions in matter. They also thank E. Hiyama
and T. Motoba for the basic idea of the experiment andtheoretical calculations. This work was supported by the
Grant-In-Aid for Scientific Research on the Priority Area
“Strangeness Nuclear Physics” from The Ministry of Education of Japan, No. 08239102.

[1] T. Motoba, H. Bandō, and K. Ikeda, Prog. Theor. Phys. 70,
189 (1983).
[2] E. Hiyama et al., Phys. Rev. C 53, 2075 (1996).
[3] E. Hiyama, M. Kamimura, K. Miyazaki, and T. Motoba,
Phys. Rev. C 59, 2351 (1999).
[4] R. H. Dalitz and A. Gal, Ann. Phys. (N.Y.) 116, 167 (1978);J. Phys. G 4, 889 (1978).
[5] F. Eigenbrod, Z. Phys. 228, 337 (1969).
[6] R. Yen et al., Nucl. Phys. A235, 135 (1974).
[7] H. Tamura et al., Phys. Rev. Lett. 84, 5963 (2000).
[8] T. Fukuda et al., Nucl. Instrum. Methods Phys. Res.,
Sect. A 361, 485 (1995).
[9] T. Hasegawa et al., Phys. Rev. C 53, 1210 (1996).
[10] O. Hashimoto et al., Nucl. Phys. A639, 93c (1998).
[11] J. F. Ziegler, The Stopping and Range of Ions in Matter,
available via www at the URL http://www.research.ibm.
com/ionbeams/home.htm#SRIM.
[12] J. F. Ziegler, J. P. Biersack, and U. Littermark, The Stopping
and Range of Ions in Solids (Pergamon Press, New York,
1985).
[13] Ch. Eppacher et al., Nucl. Instrum. Methods Phys. Res.,
Sect. B 96, 639 (1995).
[14] H. Park et al., Phys. Rev. C 61, 054004 (2000).
[15] H. Outa et al., Nucl. Phys. A547, 109c (1992).
[16] R. Grace et al., Phys. Rev. Lett. 55, 1055 (1985).
[17] J. J. Szymanski et al., Phys. Rev. C 43, 849 (1991).
[18] E. Hiyama et al., in Proceedings of the 16th International
Conference on Few-Body Problems in Physics [Nucl.
Phys. A (to be published)].

1985

Physics Letters B 579 (2004) 258–264
www.elsevier.com/locate/physletb

7
Λ Li ground-state spin determined by the yield of γ -rays

subsequent to weak decay
J. Sasao a , D. Abe a , H. Akikawa b,1 , K. Araki a , H. Bhang c , T. Endo a , Y. Fujii a ,T. Fukuda d,2 , O. Hashimoto a , K. Imai b , H. Hotchi e,1 , Y. Kakiguchi d , J.H. Kim c ,Y.D. Kim f , T. Miyoshi a,3 , T. Murakami b , T. Nagae d , H. Noumi d , H. Outa d,4 ,K. Ozawa a , T. Saito g , Y. Sato a,5 , S. Satoh a , R. Sawafta h , M. Sekimoto d , T. Takahashi a ,H. Tamura a , L. Tang i , K. Tanida e,4 , H.H. Xia j , S.H. Zhou j , L.H. Zhu b,ja Department of Physics, Tohoku University, Sendai 980-8578, Japanb Department of Physics, Kyoto University, Kyoto 606-8502, Japanc Department of Physics, Seoul National University, Seoul 151-742, South Korea
d Institute of Particle and Nuclear Studies, KEK, Tsukuba 305-0801, Japane Department of Physics, University of Tokyo, Tokyo 113-0033, Japanf Department of Physics, Sejong University, Seoul 143-747, South Korea
g Laboratory of Nuclear Science, Tohoku University, Sendai 980-0826, Japanh Physics Department, North Carolina A&T State University, Greensboro, NC 27411, USAi Department of Physics, Hampton University, Hampton, VA 23668, USAj Department of Nuclear Physics, China Institute of Atomic Energy, P.O. Box 275(80), Beijing 102413, China

Received 13 October 2003; received in revised form 10 November 2003; accepted 10 November 2003
Editor: J.P. Schiffer

Abstract
Using a large-acceptance germanium detector array (Hyperball), we measured a γ -ray spectrum in coincidence withproduction of 7Λ Li bound states in the 7 Li(π + , K + ) reaction. We observed a γ -ray peak at 429 keV due to the transition
from the first excited state of 7 Be produced by the weak decay of 7Λ Li, 7Λ Li → 7 Be∗ (1/2− , 429 keV) + π − . From the yield−2of this γ -ray, the branching ratio of the 7Λ Li weak decay to 7 Be∗ (429) was obtained to be (6.0+1.3−1.6 ) × 10 . Compared withtheoretically calculated values of the partial decay rates, the measurement indicates that the ground-state spin of 7Λ Li is 1/2 but
not 3/2.
 2003 Elsevier B.V. All rights reserved.

E-mail address: tamura@lambda.phys.tohoku.ac.jp (H. Tamura).
1 Present address: Center for Proton Accelerator Facilities, Japan Atomic Energy Research Institute, Tokai 319-1195, Japan.
2 Present address: Laboratory of Physics, Osaka Electro-Communication University, Neyagawa 572-8530, Japan.
3 Present address: Department of Physics, University of Houston, Houston, TX 77204-5506, USA.
4 Present address: Institute of Physical and Chemical Research (RIKEN), Wako 351-0198, Japan.
5 Present address: Institute of Particle and Nuclear Studies, KEK, Tsukuba 305-0801, Japan.

0370-2693/$ – see front matter  2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.physletb.2003.11.022

J. Sasao et al. / Physics Letters B 579 (2004) 258–264

259

PACS: 21.80.+a; 13.30.Eg; 13.75.Ev; 21.30.Fe; 23.20.Lv
Keywords: Λ hypernuclei; Mesonic decay; γ spectroscopy; ΛN interaction

1. Introduction
Ground-state spins have particular importance for
hypernuclei, because they reflect strengths of spindependent interactions between a hyperon and a nucleon. Low-lying level structure of Λ hypernuclei canbe understood from a weak coupling of a Λ particle in the s-orbit and a core nucleus; each level ofthe core nucleus (spin J (=0)) is split into two levels (J + 1/2 and J − 1/2). The order and the spacingof these two levels are determined by the ΛN spindependent (spin–spin, spin–orbit, and tensor) interactions [1,2]. Experimental determinations of not only
the energy spacing of the doublet but also their spinsare necessary to obtain information on the ΛN spindependent interactions.
Recently, we have developed a technique of highresolution hypernuclear γ spectroscopy using a germanium detector array, Hyperball, and measured theenergy spacing of the ground-state doublet of 7Λ Li(3/2+ , 1/2+ ) by observing the spin–flip M1 transition
between the members of the doublet [3]. The yield of
this γ -ray suggested that the lower state of the doublet (the ground state) is 1/2+ , and the strength of theΛN spin–spin interaction (the parameter ∆) was derived to be ∆ = +0.5 MeV based on this spin assignment. Since the other assignment gives the opposite
sign and a different magnitude of the spin–spin interaction strength, it is desirable to determine the spinwith another independent method.
The ground-state spins of hypernuclei can be experimentally determined using the property of π -mesonicweak decay of Λ that the s-wave (spin–non-flip) amplitude is three times larger than the p-wave (spin–flip) amplitude [4,5]. It has been used to determine the12ground-state spins of 3Λ H, 4Λ H, 4Λ He, 8Λ Li, 11
Λ B, and Λ B[4,6–12]. The branching ratios of the mesonic decays,
A Z → A [Z + 1]∗ + π − or A Z ∗ + π 0 , going to someΛ
specific states of the daughter nucleus, A [Z + 1]∗ orA Z ∗ , are sensitive to the spin of the parent hypernucleus, AΛ Z, if the specific decays are caused selectively
by the spin–flip amplitude or by the spin–non-flip amplitude.

In this Letter, we present the first result on determination of the ground-state spin of a hypernucleus
using a new method, namely, measurement of a weakdecay branching ratio to a specific excited state of thedaughter nucleus using γ -ray spectroscopy technique.
We detected the 429 keV γ -ray which is emitted fromthe excited daughter nucleus 7 Be∗ (1/2− , 429 keV) of
the 7Λ Li mesonic decay, 7Λ Li → 7 Be∗ (429) + π − , andmeasured its weak decay branching ratio from the γ ray yield.

2. Experiment
The experiment (KEK E419) was performed at the
12 GeV proton synchrotron (PS) in the High Energy
Accelerator Research Organization (KEK). It aimed
at high-resolution γ -ray spectroscopy of 7Λ Li usinga large-acceptance germanium (Ge) detector arraycalled Hyperball. The details of the experiment andthe results on hypernuclear γ transitions in 7Λ Li havealready been reported in Refs. [3,13].
Employing the K6 beam line and the Superconducting Kaon Spectrometer (SKS) [14,15], we produced bound states of 7Λ Li by the 7 Li(π + , K + ) reaction with 1.05 GeV/c pions. Each incident pion was
momentum-analyzed with the K6 beam spectrometerand hit a 7 Li target (25 cm thick, 98% enriched), andthe outgoing K + meson was identified and analyzed
by the SKS. The target was irradiated with 1.0 × 1012
pions in total.
γ -rays emitted from the target were detected with
Hyperball installed around the target. Hyperball consisted of fourteen N -type coaxial Ge detectors and had
a photo-peak efficiency of about 2.5% at 1 MeV. Each
Ge detector was surrounded by bismuth germanate(BGO) counters which provide veto signals to suppress backgrounds for the Ge detector due to Compton scattering, high-energy γ -rays from π 0 , and highenergy charged particles. Data from all the detectors
in Hyperball and the K6/SKS system were recordedwith a (π + , K + ) trigger provided by the K6/SKS system alone. Energy calibration of the Ge detectors was

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J. Sasao et al. / Physics Letters B 579 (2004) 258–264

made in the energy range of 0.1–1.8 MeV using a standard mixed source containing 241Am, 109Cd, 57 Co,
139 Ce, 51 Cr, 113 Sn, 85 Sr, 137 Cs, 60 Co and 88 Y.
The in-beam performance of each Ge detector wascontinuously monitored through the whole beam time,by using triggered γ -rays from a weak (1 kBq) 60 Co
source embedded in a plastic scintillator and installedbehind each Ge detector. The most important roleof this monitoring system is to measure the in-beamdead time caused by pile-up and preamplifier reset.
The dead time was obtained by comparing 60 Co peakcounts measured in the beam-on time gate and in the
beam-off time gate for every synchrotron cycle. Sincea peak shift of 1–2 keV was observed between the
beam-on and beam-off periods, the energy scale of thein-beam γ -ray spectrum was corrected, using beaminduced γ -ray peaks from surrounding materials, inan accuracy less than 0.6 keV.

3. Derivation of branching ratio
With the K6/SKS spectrometer system, the excitation spectrum of 7Λ Li hypernucleus in 7Λ Li(π + , K + )reaction was obtained in the same manner as in previous SKS experiments [15–17]. Fig. 1 shows the excitation spectrum of 7Λ Li plotted in the Λ binding energy,
BΛ , where the absolute energy scale was adjusted using the known BΛ of the 7Λ Li ground state (5.58 MeV[18]). The same 7Λ Li spectrum but with a thinner target
was previously measured with 2.2 MeV FWHM resolution (KEK E336) [16]. In this thin-target spectrum,
the bound state region of the observed 7Λ Li spectrum
was decomposed into four Gaussian peaks, a quasifree continuum, and a constant background, according
to theoretical predictions [19,20] that the four states(1/2+ (T = 0), 5/2+ (T = 0), 1/2+ (T = 1), and5/2+ (T = 1)) of 7Λ Li (see Fig. 3) are strongly populated. (Hereafter, the notation “(T = 0)” is omitted.)
The energies and relative intensities for the fitted fourpeaks agreed fairly well with the predictions. In thepresent spectrum, the energy resolution is worse dueto the energy loss in the thick target. By folding thesefour peaks of the same energies and relative intensities
with 4.2 MeV FWHM resolution, the present spectrum
was reproduced well, as shown in Fig. 1. We set thegate for the “bound region” at −10 < −BΛ < 2 MeV,
as shown in the figure.

Fig. 1. Hypernuclear mass spectrum of 7Λ Li, in the scale ofΛ binding energy BΛ , taken by the (π + , K + ) reaction with a
25 cm thick 7 Li target. The “bound region” of 7Λ Li is defined as−10 < −BΛ < 2 MeV. The spectrum was decomposed into four
Gaussians, a quasi-free continuum, and a constant background (seetext).

Fig. 2(a) is the γ -ray energy spectrum when thebound region (−10 < −BΛ < 2 MeV) of 7Λ Li isgated, while Fig. 2(b) is the spectrum when the unbound region (−BΛ > 2 MeV) is gated. The peaks
at 429, 692 and 2050 keV were observed only in thebound region spectrum, and the peak at 478 keV ismore prominent in Fig. 2(a) than in Fig. 2(b). As described in Ref. [3], the peaks at 692 and 2050 keV
are attributed to the M1 (3/2+ → 1/2+ ) and E2(5/2+ → 1/2+ ) transitions in 7Λ Li, respectively. Theenergies of the 429 keV peak (429.7 ± 0.8 keV)and the 478 keV peak (478.0 ± 0.8 keV) agreewith those of 7 Be(1/2− → 3/2− , 429.1 keV) and7 Li(1/2− → 3/2− , 478.6 keV) transitions. Since the429 and 478 keV γ -rays prominently appear in coincidence with 7Λ Li bound states, they are interpreted
as transitions in the daughter nuclei resulting from the
7
7
7
∗
−
−
Λ Li weak decays, Λ Li → Be (1/2 , 429 keV) + π
∗
7
7
−
0
and Λ Li → Li (1/2 , 478 keV) + π , as shown inthe decay scheme in Fig. 3. Since each of 7 Be and 7 Li
has only one particle-bound excited state, the 429 and478 keV γ -ray events correspond to the direct productions of these first excited states of 7 Be and 7 Li, respectively, in the mesonic weak decay of 7Λ Li.
The γ -ray spectrum around the 429 keV peak was
fitted with three Gaussians and a linear background as
shown in the inset in Fig. 2(a), and the numbers ofevents in the 429 keV peak and the 478 keV peak were

J. Sasao et al. / Physics Letters B 579 (2004) 258–264

261

Fig. 2. γ -ray spectra measured in the 7 Li(π + , K + ) reaction (a) for the bound region (−10 < −BΛ < 2 MeV) and (b) for the unbound region(−BΛ > 2 MeV) of 7Λ Li. The insets show the spectra expanded around 400–500 keV. The two peaks at 429 and 478 keV observed in (a) are
attributed to the transitions from the first excited states of 7 Be and 7 Li, respectively. In (b), only the 478 keV peak was observed. The region
around these peaks were fitted with three Gaussians and a linear background.

obtained as:Nγ (429) = 78 ± 16,
Nγ (478) = 121 ± 19.
In the unbound-region spectrum (Fig. 2(b)), the 7 Be∗(429) peak was not observed at all but the 7 Li∗ (478)
peak was seen with a smaller S/N ratio. It is understood from the fact that the 7 Li∗ (478) state canbe also excited through the (n, n ) reaction by secondary neutrons produced in the (π + , K + ) reaction, while the production rate of 7 Be∗ (429) throughthe 7 Li(p, n)7 Be∗ (429) reaction by secondary protons is expected to be much smaller than that of

7 Li∗ (478). This isbecause the 7 Li(p, n)7 Be∗ (429)cross section is by a factor of 3–5 smaller than the
7 Li(n, n )7 Li∗ (478) (and 7 Li(p, p )7 Li∗ (478)) crosssections [21,22], and the effective 7 Li target thickness
for most protons is smaller than that for neutrons dueto the proton ranges (< 2 cm for < 30 MeV protons)
shorter than the target size (5.6 × 4 × 25 cm).
Here we estimate possible contamination of the
7 Li(p, n)7 Be∗ (429) events in N (429). Since 7 Be andγ
7 Li are mirror nuclei, the branching ratio of 7 Li →
Λ
7 Li∗ (478) + π 0 to 7 Li → 7 Be∗ (429) + π − shouldΛ
be almost 1 to 2 due to the ∆I = 1/2 rule, while
the observed ratio of Nγ (478) to Nγ (429) is 1.6
to 1. This difference is ascribed to a contribution

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J. Sasao et al. / Physics Letters B 579 (2004) 258–264

Fig. 3. Scheme of γ transitions and mesonic weak decays of 7Λ Li.
The ground state of 7Λ Li decays into 7 Be or 7 Li through the π −or π 0 mesonic weak decays, respectively. When 7Λ Li decays into
the first excited states of 7 Be or 7 Li, γ -rays of 429 or 478 keV areemitted, respectively.

of the 7 Li(n, n )7 Li∗ (478) reaction in the 7 Li∗ (478)
γ -ray yield in the bound region spectrum. If all the
7 Be∗ (429) γ -rays stem from the 7 Li weak decay, 85 ±
Λ
20 counts out of Nγ (478) = 121 ± 19 counts are attributed to the 7 Li(n, n ) reaction, considering the energy dependence of the Hyperball efficiency. From thesimilar fit for the unbound region spectrum (Fig. 2(b)inset), the upper limit of the 429 keV γ -ray yield,
which could be attributed to the 7 Li(p, n)7 Be∗ (429)reaction, was obtained to be 12% of the 478 keV γ ray yield by the 7 Li(n, n )7 Li∗ (478) reaction. Therefore, by applying the same upper limit to the boundregion spectrum, the upper limit of the contaminationof the 7 Li(p, n)7 Be∗ (429) reaction events in Nγ (429)
was estimated to be (85 + 20) × 0.12 = 13 counts.
Thus, the yield of the 429 keV γ -rays due to the 7Λ Li
weak decay is:
Nγweak (429) = 78 ± 16(stat)+0−13 (contamination).
It is to be mentioned that the 429 keV peak has
a width of σ = 5.1 ± 1.2 keV in the Gaussian
fit, which is consistent with the expected Doppler
broadening width of ±7 keV (σ = 4 keV) for therecoil velocity of 7 Be∗ in the 7Λ Li → π − + 7 Be∗ (429)decay and the prompt γ -ray emission (the half lifeof 7 Be∗ (429) is 0.13 ps) before slowing-down of therecoil hypernucleus (stopping time is about 5 ps). The
observed larger width of the 7 Li∗ (478) γ -ray peak(σ = 6.6 ± 1.0 keV) is understood by the larger recoil
motion in the (n, n ) reaction.
In order to extract the branching ratio of 7Λ Li →7 Be∗ (429) + π − decay, we need to know the total
number of events of all the populated 7Λ Li bound states which undergo weak decay from the 7Λ Li groundstate. As described in Ref. [3], we also observed two
γ -ray peaks at 3877 ± 7 keV and at 3186 ± 6 keVin the Doppler-shift corrected spectrum. Since theirenergy difference (691±6 keV) coincides with theenergy (692 keV) of the M1 transition between the
ground-state doublet members (3/2+ , 1/2+ ), they are
assigned as M1 transitions from the 1/2+ (T = 1) state
to the ground-state doublet members. The observationof these transitions, as well as the 3877 ± 7 keVexcitation energy of the 1/2+ (T = 1) state slightly
lower than the 5Λ He + d decay threshold (3.94 ±0.04 MeV), indicates that the 1/2+ (T = 1) state isbound and contributes to the weak decay of the 7Λ Li
ground state. On the other hand, the 5/2+ (T = 1)
state, of which energy is expected to be much higher
than the 5Λ He + d threshold [20], does not contributeto the 7Λ Li weak decay. The total number of eventsof all the 7Λ Li bound states, Nbs , was thus obtained tobe:Nbs = (8.3 ± 0.3) × 104 ,as the sum of the peak counts for the 1/2+ , 5/2+ , and1/2+ (T = 1) states integrated in the bound region gate
in Fig. 1.
In order to obtain the branching ratio from theγ -ray yield, the absolute photo-peak efficiency of
all the Ge detectors should be precisely determined.
When the beam was off, we carried out calibration
measurements of the photo-peak efficiency with thestandard mixed source, which was located at several
points in the target region along the beam axis.
A simulation with the GEANT code was also carriedout in the same setup as the calibration measurements,
and the calculated efficiency agreed with the measured
one. Then the simulation was made again with thetarget material and with the realistic source point
distribution taken into account. Then the off-beamabsolute efficiency was obtained as (3.27 ± 0.11) ×
10−2 at 429 keV.

J. Sasao et al. / Physics Letters B 579 (2004) 258–264

Using the 60 Co monitoring system, the dead timesof the Ge detectors were continuously measured. They
ranged from 30 to 50%, being small for the detectors
located upstream of the target and large for thoselocated downstream. They were almost stable throughthe beam time of one month. The dead time averaged
for all the Ge detectors and through the whole beamtime was 46.6 ± 0.5%. Namely, the in-beam/off-beamefficiency ratio was 0.534 ± 0.005. The inefficiency
due to accidental veto by the BGO counters was
6 ± 2%, and the inefficiency due to a timing cut with
the Ge detector TDC’s was 5 ± 3%. Thus, the γ -raydetection efficiency at 429 keV was obtained to be:εγ (429) = (1.56 ± 0.08) × 10−2 ,as a product of the off-beam absolute efficiency at
429 keV, the in-beam/off-beam efficiency ratio, andthe analysis efficiency for the BGO veto and the Getiming cut.
Then the branching ratio of the 7Λ Li weak decay
going to the first excited state of 7 Be was obtained as:
BR 7Λ Li → 7 Be∗ (429) + π −=Nγweak
Nbs γ (429)−2= 6.0+1.3−1.6 × 10 ,where the error is given from a quadratic sum of all theerrors.

4. Determination of the spin
The mesonic weak-decay rates of p-shell Λ hypernuclei were theoretically calculated by Motoba et al.
with a shell model and with pion distortion taken into
account [23]. For each case of the 7Λ Li spin of 1/2+
and 3/2+ , the calculated decay rate of 7Λ Li to the firstexcited state of 7 Be isΓ 7Λ Li(1/2+ ) → 7 Be∗ + π − = 0.070ΓΛ (0.052 ΓΛ ),Γ 7Λ Li(3/2+ ) → 7 Be∗ + π − = 0.007ΓΛ,
where ΓΛ denotes the total decay rate of a free Λ.
The value in the square parenthesis is the result calculated with a cluster model. The 1/2+ and 3/2+ members of the ground-state doublet of 7Λ Li dominantly
have L = 0 with S = 1/2 and S = 3/2, respectively,
while the first excited state 7 Be∗ (1/2− ) has L = 1

263

and S = 1/2. Therefore, the weak decay of 7Λ Li(3/2+ )to 7 Be∗ (1/2− ) + π − requires spin–flip and is consequently an order of magnitude weaker than that of
7 Li(1/2+ ) due to the property of the π -mesonic weakΛ
decay of Λ.
In order to compare these theoretical decay rates
with the measured branching ratio, the decay rates
were converted to branching ratios by dividing by
an expected total decay rate, Γtot( 7Λ Li) ≈ (1.2 ±0.4)ΓΛ . Here, as there are neither measurements nor
calculations of Γtot( 7Λ Li), it was assumed to be within
the range of measured total decay rates of A = 4–4
12 hypernuclei, namely, 4Λ H (1.36+0.21−0.15 [24]), Λ He
+0.12
(1.03−0.10 [25], 1.07 ± 0.11 [26]), 5Λ He (1.03 ± 0.08[27]), 11Λ B (1.37 ± 0.16 [28], 1.25 ± 0.08 [29]), and12 C (1.25 ± 0.18 [28], 1.14 ± 0.07 [30]) in the unit
Λof ΓΛ . The theoretical branching ratios to the firstexcited state of 7 Be are derived to be (5.8 ± 1.9) ×
10−2 (or (4.3 ± 1.4) × 10−2 for the cluster model)
for the 1/2+ case, and (0.6 ± 0.2) × 10−2 for the
3/2+ case. The measured branching ratio agrees withthe value for the 1/2+ case, but obviously disagrees
with the value for 3/2+ . Therefore, the ground statespin of 7Λ Li was determined to be 1/2. It is notedthat the calculation of Ref. [23] well reproduces
experimental π − and π 0 mesonic weak decay rates12of 11
Λ B and Λ C [31,32]. Furthermore, it is to be
stressed that the present determination of the spin isbased on the large difference of the spin–flip/spin–
non-flip amplitudes and not affected by details of thetheoretical calculations.
As mentioned in Ref. [3], the measured yields ofhypernuclear γ -rays of 7Λ Li suggested the ground-statespin of 1/2, and the effective interaction parameterof the ΛN spin–spin force was derived to be ∆ =+0.5 MeV based on this assignment. The present
result independently supports this spin assignment.

5. Summary
In summary, we observed a 429 keV γ -ray peakfrom 7 Be∗ (1/2− , 429 keV) produced by the weak
decay of 7Λ Li, 7Λ Li → 7 Be∗ (429) + π − . From the yieldof this γ -ray, the branching ratio of the 7Λ Li weak−2
decay to 7 Be∗ (429) was obtained as (6.0+1.3−1.6 ) × 10 .
By comparing this value with theoretically calculated

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J. Sasao et al. / Physics Letters B 579 (2004) 258–264

ones, the 7Λ Li ground-state spin was determined to
be 1/2. This is the first experiment in which γ -rays
emitted from excited daughter nuclei subsequently
to hypernuclear weak decay are identified and used
to determine the ground-state spin of the parent
hypernucleus.

Acknowledgements
The authors thank K. Nakamura and the KEK-PS
staff for support of the experiment. They are also
grateful to T. Motoba for discussion of the result.
This work is supported by Grant-In-Aid for Scientific
Research from The Ministry of Education of Japan,
Nos. 08239102 and 11440070.

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E438 H. Noumi
Study of Σ-Nucleus Potential by the (π-, K+) Reaction on Heavy Nuclei
H. Noumi et al.
[attached]
Sigma-Nucleus Potential in A=28
Phys. Rev. Lett. 89 (2002) 072301; Phys. Rev. Lett. 90 (2003)049902[E].
★ P. K. Saha
Study of Σ-Nucleus Potential by the (!-, K+) Reaction, Soken-dai, 2001
KEK Report 2001-17.
◇ H. Noumi for the KEK-PS E438 collaboration
S-Nucleus Interactions in Medium to Heavy Nuclei
16th International Conference on Particles and Nuclei (PANIC 02), Osaka, Japan, 30 Sep - 4 Oct
2002, published in Nucl.Phys.A721:995c-998c,2003.
◇ P. K. Saha
Study of S-nucleus potential by the (π-, K+) reaction (brief summary of KEK-PS-E438)
Recent Progress of Strangeness Nuclear Physics, Tsukuba, Japan, 15-17 Mar 2002, published in
KEK Proceedings 2002-29, p68-79.
◇ H. Noumi et al.
First Results on Quasifree S Production in the (π-, K+) Reactions
HYP2000: 7th International Conference on Hypernuclear and Strange Particle Physics, Torino, Italy,
23-27 Oct 2000, published in Nucl.Phys.A691 (2001) 29c-35c.
＊ H. Noumi
Study of the interaction between aΣ- hyperon and a nucleus,
KEK NEWS, May, 2002 (in Japanese).
●________________________________________________________________________● Physics papers published in refereed journal.
○★
◇＊

Technical papers.
PhD theses.
Conference and Symposium.
Internal Report and others.

VOLUME 89, NUMBER 7

PHYSICAL REVIEW LETTERS

12 AUGUST 2002

Sigma-Nucleus Potential in A  28
H. Noumi,1 P. K. Saha,1,* D. Abe,2 S. Ajimura,3 K. Aoki,1 H. C. Bhang,4 T. Endo,2 Y. Fujii,2 T. Fukuda,1,* H. C. Guo,5K. Imai,7 O. Hashimoto,2 H. Hotchi,6,† E. H. Kim,4 J. H. Kim,4 T. Kishimoto,3 A. Krutenkova,8 K. Maeda,2 T. Nagae,1
M. Nakamura,6 H. Outa,1 M. Sekimoto,1 T. Saito,2,‡ A. Sakaguchi,3 Y. Sato,1,2 R. Sawafta,9 Y. Shimizu,3,* T. Takahashi,2
L. Tang,10 H. Tamura,2 K. Tanida,6 T. Watanabe,2 H. H. Xia,5 S. H. Zhou,5 L. H. Zhu,7 and X. F. Zhu5
1

High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan2
Department of Physics, Tohoku University, Sendai 980-8578, Japan3
Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan
4
Department of Physics, Seoul National University, Seoul 151-742, Korea
5
Department of Nuclear Physics, CIAE, P.O. Box 275(80), Beijing 102413, China
6
Graduate School of Science, University of Tokyo, Tokyo 113-003, Japan
7
Department of Physics, Kyoto University, Sakyo, Kyoto 606-8502, Japan
8
Institute of Theoretical and Experimental Physics, Moscow 117218, Russia
9
Physics Department, North Carolina A&T State University, Greensboro, North Carolina 27411
10Department of Physics, Hampton University, Hampton, Virginia 23668(Received 16 December 2001; published 30 July 2002)
We have studied the  ; K   reaction on a silicon target to investigate the sigma-nucleus potential.
The inclusive spectrum was measured at a beam momentum of 1:2 GeV=c with an energy resolution of
3.3 MeV (FWHM) by employing the superconducting kaon spectrometer system. The spectrum wascompared with theoretical calculations within the framework of the distorted-wave impulse approximation, which demonstrates that a strongly repulsive sigma-nucleus potential with a nonzero size of theimaginary part reproduces the observed spectrum.
DOI: 10.1103/PhysRevLett.89.072301

PACS numbers: 21.80.+a, 13.75.Ev, 25.80.Hp, 25.80.Nv

The sigma()-nucleus potential describes the behaviorof a  hyperon in the nuclear medium. However, the-nucleus potential is still unclear because experimental
information is limited, as described below. Hyperonnucleon potentials based on a one-boson exchange modelallow several sets of parameters, since hyperon-nucleon
scattering data are poor. The energy shifts and widths of the -atomic x-ray transition carry information on the interaction of a  to a core nucleus [1,2]. The current precisionof the x-ray data allows several possible potentials of
various shapes inside a nucleus because the x-ray data
are mainly sensitive to the potential shape outside of thenucleus. Although past measurements claimed observationof narrow peak structures in -hypernuclear excitation
spectra, they were excluded in later experiments [3]. The
existence of a 4 He bound state was claimed at KEK [4] andestablished at BNL [5]. A 4 He bound state has been
predicted theoretically [6]. The theory explains that acancellation of the repulsive central part with the attractive
isospin-dependent part (so-called Lane term) in the
-nucleus folding potential implies a potential shapewith a repulsive core and an attractive pocket at the nuclear
surface, and results in a narrow bound state. The existenceof the Lane term was suggested in light nuclei, where
systematic differences between the inclusive K  ;  
spectra were observed, although no narrow states were
found [3]. Many other attempts to find narrow-hypernuclear states, via the K  ;   reactions on lightnuclei of the mass number A  16, have failed [7]. No
data are available in heavier nuclei of A > 16. An analysisof the spectrum shape in the (stopped K  ;   reaction on
12 C was made, where theoretical calculations based on the
distorted-wave impulse approximation (DWIA) developedby Morimatsu and Yazaki were employed [8]. This analysis gave a limit on the real part V0 > 12 MeV and theimaginary part W0 < 7 MeV, assuming the WoodsSaxon type of the -nucleus potential, expressed as V0 
iW0 fr, fr being the Fermi function [9].
Recently, the role of strangeness in a neutron star has
been intensively discussed. The behavior of hyperons inhigh-density nuclear matter determines the abundance ofhyperons in a neutron star, which is related to its thermal
evolution, the maximal mass of a neutron star, and aformation scenario of a neutron star or a black hole
[10,11].  is a hyperon expected to appear first in dense
nuclear matter due to its negative charge [12], for which
the interaction of  in nuclear matter, and hence the-nucleus potential, is of particular importance. New experimental information is therefore awaited in order toimprove the ambiguous situation concerning the -nucleus potential in medium heavy nuclei.
Since the Morimatsu-Yazaki formalism has been successfully applied to the analysis of the K  ;   spectra inhelium [13] and carbon [9], the formalism was employed
for the  ; K   reaction, and the inclusive spectrum wasfound to be sensitive to the -nucleus potential [14]. We
carried out an experiment to measure the inclusive ; K   reaction on several targets (CH2 , Si, Ni, In, andBi) of natural isotopic composition in order to extract
information on the -nucleus potential [15,16]. In this072301-1

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Letter, we first report on a measurement of the inclusive ; K   spectrum on Si, and discuss the spectrum shape
to derive the -nucleus potential. Specifically, we obtained
the information on the central part of the -nucleus potential in Si since the Lane term is expected to be reduced due
to its A1 dependence. In heavier systems, such as In andBi, the isospin dependence may be investigated because of
their large neutron to proton ratios. We will report on theother targets elsewhere [17].
The present experiment was performed at the K6 beamline of the KEK 12-GeV Proton Synchrotron. It is known
that the cross section of the elementary p ; K  
reaction decreases with an increase in the incident beam
momentum at forward scattering angles. On the other
hand, the momentum transfer (p) increases rapidly below
1:2 GeV=c. The rapid change of p with beam momentum
complicates the spectrum analysis and a larger p requiresa large scattered kaon acceptance. The beam momentum
was thus chosen at 1:2 GeV=c. Spectroscopic studies of 
hypernuclei have been conducted with the superconducting
kaon spectrometer (SKS) system at K6. It has been demonstrated that SKS has a momentum resolution of 0.1%
(FWHM) and a linearity of within 0:1 MeV=c in its momentum acceptance of 10% [18]. SKS is suitable tomeasure the inclusive spectrum over a wide energy range.
Its resolving power is helpful to maintain the sensitivity to
W0 , since W0 makes the spectrum spread like a resolutionfunction [19].
A natural Si target of 6:53 g=cm2 was used. A CH2target of 1:00 g=cm2 was placed 250 mm upstream fromthe Si target in tandem to monitor the energy scale, theenergy resolution, and the cross section. The reaction target
was identified by cutting on the reconstructed vertex position, as superimposed at the upper right corner of Fig. 1.
Because a small fraction of the CH2 contamination into the
Si window was found, the measured CH2 spectrum, properly scaled, was subtracted from the Si spectrum.
Figure 1 shows the measured missing mass spectrum on
CH2 in proton-target kinematics at a kaon scattering angleof K  6  2 , where the spectrum is plotted as a function of M  Mp MeV=c2 . Here M and Mp represent
the reconstructed missing ( ) mass and the target (proton) mass, respectively. The spectrum comprises a prominent peak and a broad bump, which correspond to theproton and carbon contributions, respectively. The energy
losses of a pion and a kaon in the target were taken into
consideration, and the reconstructed momenta were calibrated so that the proton peak is found at the known
position (259:177  0:029 MeV=c2 ) [20]. The detector
and analysis efficiencies and the SKS acceptance werecorrected to obtain the cross section. The acceptance wasestimated by a Monte Carlo simulation, taking into accountthe detector geometry, the magnetic field distribution, andthe cut conditions in the analysis.
The spectrum was fitted with two Gaussians, as plottedwith the solid curve in Fig. 1. One Gaussian corresponds tothe proton peak, and the other represents the carbon con072301-2

12 AUGUST 2002

FIG. 1 (color online). Missing-mass spectrum in the protontarget kinematics of the  ; K   reaction on CH2 . The vertex
distribution along the beam line is superimposed at the upper
right corner. See text.

tribution. From the proton peak, the energy resolution was
found to be 3:3  0:3 MeV (FWHM). The proton-peakyield was extracted by subtracting the carbon contribution.
The elementary cross sections were obtained for the otherK ’s to see the angular distribution, as shown by the closed
circles in Fig. 2. The angular distribution of thep ; K   reaction has been measured at a beam momentum of 1:225 GeV=c [21]. The crosses and the shaded
area in Fig. 2 are a part of the measured points and the
deduced angular distribution in the Lab frame, and compared to the previous experiment, which was originallyreported in the center-of-mass frame. The shaded areashows the fitting error boundary. The present measurementagrees well with the previous one.
Figure 3 shows the inclusive  ; K   spectrum on Si asa function of the  binding energy (  B ) at K 
6  2 . This spectrum was made by combining the datataken in three different field settings of SKS (nominal 1.9,

FIG. 2 (color online). Measured angular distribution (closed
circles) of the elementary reaction in the Lab frame. The
previous data are also shown (crosses and shaded area) [21].

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2.2, and 2.4 T) in order to cover a wider kaon momentum range. The spectrum in one setting could be connected
to another smoothly, since the acceptance regions partly
overlap.
We found two notable features in the Si spectrum. First,
the spectrum shows that the cross section gradually increases with an increase of B . The maximum of thespectrum is achieved at B > 120 MeV. The maximum
occurs at an energy larger than that predicted by the Fermigas model. Second, we observed significant yields in thebound state region. These yields are beyond those expectedfrom the combination of the Coulomb and nuclear potentials and suggest a strong  -nucleus interaction.
In order to extract further information, we calculated theinclusive spectrum, according to the Morimatsu-Yazaki
formalism [8]. The cross section of the inclusive reaction
can be written as


d2 
d

SE;
(1)
ddE
del
where  is the kinematical factor for a coordinate transferfrom a two-body system to a many-body system [22], andd=del represents the averaged differential cross sectionof the elementary reaction. Here the elementary crosssection [d=ds; K ] is averaged over the momentumof a proton (k) moving in the target nucleus with a weightof the momentum distribution k, as written asRds; K k  Pdkk d
d
R
E 
;
(2)
del
kk  Pdk
P  kK  k  k :(3)

The delta function is required for energy-momentum conservation in the elementary reaction. Relevant particles aretreated on mass-shell, and the possible k is given by k

12 AUGUST 2002

and kK at EB . As a result, d=del depends on E.
In this calculation, d=ds; K  was taken from
Refs. [21,23,24], where s is the Mandelstam variable.
k was given by the wave function of the initial state.
The strength function SE is characterized by distorted
waves of the incident pion and scattered kaon and thenuclear response function. The response function can be
described by means of the Green’s function [cf. Eq. (2.2) in
Ref. [8] ]. Here we assume the Woods-Saxon–type onebody potential for the -nucleus potential U r. The
Green’s function (G) is a solution ofE  h 2 =2%  U r GE; r0 ; r  r0  r; (4)where % is the reduced mass.
Distorted waves of the pion and kaon were calculated in
the eikonal approximation, employing the meson-nucleon
total cross sections of  N  35 mb and K N  14 mb[25]. The distortion effect reduces the magnitude of the
inclusive spectrum but does not affect the spectrum shapevery much. Observed  ; K   spectra on different targets
show similar shapes with an increase of the magnitude as
an increase of the mass number [16]. However, more careful and systematic analysis on the different targets would
be needed to state the absolute strength of the distortion.
The spectrum shape is directly related to the response
function since the -nucleus potential appears explicitlyin Eq. (4). Therefore, we present the results of the shapeanalysis, where the measured spectrum was fitted by thecalculated ones for various U rs. Two of the calculated
spectra (solid and dashed curves) are shown in Fig. 3, forexample. The potential parameters used in the calculations
are listed in Table I. The solid and dashed curves are forthe cases of a repulsive and a shallow -nucleus potential,
respectively. In the fitting, we introduced a free parameter(scaling factor) ' to adjust the magnitude of the spectrum.
The scaling factor ' and (2 per degree of freedom (number
in parentheses) are shown in Fig. 3. A shallow potential
represents a weakly attractive potential, which has notbeen excluded by the analysis in the (stopped K  ;  )TABLE I. Potential parameters used for the present calculations. The Woods-Saxon type potential is assumed to be Ur 
2 1V0  iW0 fr  VSO h=m c r df=dr‘  VCoulomb r,fr  f1  exp r  c=z g1 .

V0 (MeV)W0 (MeV)VSO (MeV)c (fm)z (fm)
FIG. 3. Inclusive  ; K   spectrum on Si at K  6  2 .
The curves are the calculated spectra for the repulsive (solid) andshallow (dashed) -nucleus potentials, fitted to the measured
spectrum. A value of the scaling factor ' and (2 per degree of
freedom are shown for each fitting. See text for details.

072301-3

-nucleus pot.
US a
UR a

Initial Si
UT b

+150
-15
0
3.3c
0.67

-49.6d
0
7
4.09d
0.536

-10
-10
0
3.3c
0.67

a

UR (US ) denotes a repulsive (shallow) -nucleus potential.
Proton single-particle potential of the initial nucleus (Si).
cc  1:1  A  11=3 .
dThese are adjusted to reproduce the proton separation energy(11.585 MeV) [26] and nuclear radius r21=2  3:15 fm [27].
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experiment [9]. The dashed curve is greatly shifted to thelower B side, and does not reproduce the spectrum inshape. On the other hand, the solid curve gives the best (2 .
We made calculations for various values of the real (V0 )and imaginary (W0 ) parts of the -nucleus potential. It canbe demonstrated that the calculated spectrum becomes
lower in height and shifts toward a higher B in the
distribution as a result of suppression in the lower B
region along with an increase of V0 . When jW0 j increases,
the calculated spectrum becomes spread out, with a worse
fitting. An increase of W0 from 10 MeV makes thefitting slightly worse, although the sensitivity becomes
lower for W0 * 5 MeV. The present fitting shows thatthe cases of V0 & 70 MeV give very poor (2 s with theconfidence level less than 0.1% for any W0 . In addition,
the cases of W0 & 30 MeV, independent of V0 , have theconfidence level less than 0.3%. Here the confidence level
just represents the consistency of the model, and the above
regions are far from consistency. The solid curve gives thebest fitting among the variation of V0 and W0 for thepresent choices of the other potential parameters.
The spectrum shape depends little on the choice of the
diffuseness parameter (z). The spin-orbit term VSO in the
-nucleus potential may be as large as that in an ordinary
nucleus. In the case of the same order of magnitude, thecalculated spectrum slightly changes in the highly excited
region, which does not affect the present argument.
However, the spectrum is sensitive to the choice of theradius parameter (c). When c of the initial nuclear potential
decreases by 10% with keeping c of the -nucleus potential, the optimum V0 decreases by approximately
20%–30%, for example. In this case, the wave functionof the initial state is located at the inner side of the nucleus,
and thus the overlap of the initial wave to the final one
becomes worse, resulting in the spectrum being pushed to ahigher B region.
The present result shows that a strongly repulsive
-nucleus potential having a nonzero size of the imaginary
part is required to reproduce the observed spectrum within
the framework of DWIA. The repulsive -nucleus potential should be taken into account for discussing the hyperon
constituents of neutron stars [10]. A repulsive -nucleus
potential has been claimed from an analysis of the -atomic x-ray data [1,2]. Further studies on the
-nucleus potential, particularly for details of its shapefrom the central nuclear region to the outer atomic orbit
region, would be required to explain the present result andthe x-ray data.
The authors express their deep gratitude to Professor
Y. Akaishi, Professor T. Harada, and Dr. T. Koike forinvaluable discussions on the DWIA calculations. They
are sincerely grateful to Dr. R. E. Chrien for the proofreading of this Letter. They thank Professor K. Nakamura,the head of Physics Division III, and Professor

072301-4

12 AUGUST 2002

Y. Yoshimura of the Experiment Planning and Program
Coordination Office in KEK for their support and encouragement to perform this work. The efforts of the KEK-PS
staff members and the beam channel group members todeliver a stable proton beam are greatly appreciated. They
are grateful to indispensable support from the CounterExperimental Hall, Online/Electronics, and Cryogenics
Groups.

*Present address: Laboratory of Physics, Osaka ElectroCommunication University, Neyagawa, Osaka 572-8530,
Japan.
†
Present address: Brookhaven National Laboratory, Upton,
New York 11973.
‡
Present address: Tohoku Gakuin University, Tagajo,
Miyagi 985-8537, Japan.
[1] C. J. Batty, E. Friedman, and A. Gal, Prog. Theor. Phys.
Suppl. 117, 227 (1994).
[2] J. Mareš, E. Friedman, A. Gal, and B. K. Jennings, Nucl.
Phys. A594, 311 (1995).
[3] S. Bart et al., Phys. Rev. Lett. 83, 5238 (1999), andreferences therein.
[4] H. Outa et al., Prog. Theor. Phys. Suppl. 117, 177 (1994).
[5] T. Nagae et al., Phys. Rev. Lett. 80, 1605 (1998).
[6] T. Harada, S. Shinmura, Y. Akaishi, and H. Tanaka, Nucl.
Phys. A507, 715 (1990).
[7] A review of past measurements can be found in Ref. [3].
[8] O. Morimatsu and K. Yazaki, Nucl. Phys. A483, 493(1988).
[9] M. Iwasaki, Dr. thesis, University of Tokyo, 1987.
[10] S. Balberg and A. Gal, Nucl. Phys. A625, 435 (1997).
[11] T. Takatsuka et al., Int. J. Mod. Phys. B 15, 1609 (2001),
and references therein.
[12] M. Baldo et al., Phys. Rev. C 58, 3688 (1998).
[13] T. Harada, Nucl. Phys. A672, 181 (2000).
[14] T. Koike et al. (private communication).
[15] H. Noumi et al., KEK-PS Proposal No. E438.
[16] P. K. Saha, Dr. thesis, Sokendai (KEK Report No. 200117).
[17] P. K. Saha et al. (to be published).
[18] T. Fukuda et al., Nucl. Instrum. Methods Phys. Res., Sect.
A 361, 485 (1995).
[19] C. B. Dover, D. J. Millener, and A. Gal, Phys. Rep. 184, 1(1989).
[20] D. E. Groom et al., Eur. Phys. J. C 15, 1 (2000).
[21] M. L. Good et al., Phys. Rev. 183, 1142 (1969).
[22] S. Tadokoro, H. Kobayashi, and Y. Akaishi, Phys. Rev. C
51, 2656 (1995).
[23] J. C. Doyle et al., Phys. Rev. 165, 1483 (1968).
[24] O. I. Dahl et al., Phys. Rev. 163, 1430 (1967).
[25] C. B. Dover, L. Ludeking, and G. E. Walker, Phys. Rev. C
22, 2073 (1980).
[26] Table of Isotopes, Eighth Edition, edited by R. B.
Firestone et al. (John Wiley & Sons, New York, 1996).
[27] H. de Vries et al., At. Data Nucl. Data Tables 36, 495(1987).

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week ending
31 JANUARY 2003

Erratum: Sigma-Nucleus Potential in A  28
[Phys. Rev. Lett. 89, 072301 (2002)]
H. Noumi, P. K. Saha, D. Abe, S. Ajimura, K. Aoki, H. C. Bhang, T. Endo, Y. Fujii, T. Fukuda, H. C. Guo, K. Imai,
O. Hashimoto, H. Hotchi, E. H. Kim, J. H. Kim, T. Kishimoto, A. Krutenkova, K. Maeda, T. Nagae, M. Nakamura,
H. Outa, M. Sekimoto, T. Saito, A. Sakaguchi, Y. Sato, R. Sawafta, Y. Shimizu, T. Takahashi, L. Tang, H. Tamura,
K. Tanida, T. Watanabe, H. H. Xia, S. H. Zhou, L. H. Zhu, and X. F. Zhu(Received 12 December 2002; published 28 January 2003)
DOI: 10.1103/PhysRevLett.90.049902

PACS numbers: 21.80.+a, 13.75.Ev, 25.80.Hp, 25.80.Nv, 99.10.Cd

In Fig. 3 and its inset the vertical scales should be reduced by a factor of 4. This plotting error affects only the figure.
All relevant quantities in the text and in the table are correct as published. We regret the oversight.
The corrected version of Fig. 3 is reproduced here. This correction does not affect any results or conclusions of the
published paper.

FIG. 3. Inclusive  ; K  spectrum on Si at K  6  2 . The curves are the calculated spectra for the repulsive (solid) andshallow (dashed) -nucleus potentials, fitted to the measured spectrum. A value of the scaling factor  and 2 per degree of
freedom are shown for each fitting. See text for details.

049902-1

0031-9007=03=90(4)=049902(1)$20.00

 2003 The American Physical Society

049902-1

E443 N. Shigyo
Neutron-Production Differential Cross Sections in the (p,nx) Reaction at Forward
Angle
D. Satoh et al.
[attached]
Neutron-Production Double-Differential Cross Sections of Iron and Lead by 0.8 and 1.5 GeV
Protons in t he Most-Forward Direction
J. Nucl. Sci. Technol., 40 (2003) 283.
○ D. Satoh et al.
[attached]
Study of neutron detection efficiencies for liquid organic scintillator up to 3 GeVIEEE Trans. Nucl. Sci., 48 (2001) 1165.
★
D. Satoh
Measurement of neutron-production double-differential cross sections for high energy proton
incidence at most-forward direction
Kyushu University, 2003.
◇ N. Shigyo et al.
Measurement of Neutron-Production Double-Differential Cross Sections for 0.8 and 1.5 GeV
Proton Incidence in the Most-Forward directions
International Conference on Nuclear Data for Science and Technology, Tsukuba, October 7-12,
2001.
◇
D. Satoh et al.
Development of SCINFUL-QMD code to calculate the neutron detection efficiencies for liquidorganic scintillator up to 3GeVInternational Conference on Nuclear Data for Science and Technology, Tsukuba, October 7-12,
2001.
◇ D. Satoh et al.
Neutron Production by 0.8 and 1.5 GeV Protons on Fe and Pb Targets at the Most-Forward Region
The 2002 Symposium on Nuclear Data, Tokai-mura, November 21-22, 2002.
◇ N. Shigyo et al.
Measurement of 0.8 and 1.5 GeV Proton Induced Neutron Production Cross Section at 0°
The Second iTRS International Symposium On Radiation Safety and Detection Technology(ISORD-2), Sendai, July 24-25, 2003.
●________________________________________________________________________● Physics papers published in refereed journal.
○★
◇＊

Technical papers.
PhD theses.
Conference and Symposium.
Internal Report and others.

Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 40, No. 5, p. 283–290 (May 2003)

Neutron-Production Double-Differential Cross Sections of Iron and Leadby 0.8 and 1.5 GeV Protons in the Most-Forward Direction
Daiki SATOH1 , Nobuhiro SHIGYO1 , Kenji ISHIBASHI1,* , Yasushi TAKAYAMA1 ,Shunsuke ISHIMOTO1 , Yosuke IWAMOTO1 , Hideki TENZOU1 , Tatsushi NAKAMOTO2 ,Masaharu NUMAJIRI2 and Shin-ichiro MEIGO3
1

Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University,
6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581
2High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba-shi, Ibaraki 305-0801
3
Japan Atomic Energy Research Institute, Tokai-mura, Naka-gun, Ibaraki 319-1195(Received December 24, 2002 and accepted in revised form February 4, 2003)
Neutron-production double-differential cross sections of iron and lead for 0.8 and 1.5 GeV protons incidences were
measured in the most-forward direction. Neutrons were measured by the time-of-flight (TOF) method. An NE213
liquid organic scintillator was set at 0◦ as a neutron detector. Neutron detection efficiencies were obtained by calculations with a Monte Carlo simulation code SCINFUL-QMD. The present experimental data were compared with otherreported experimental data and the results of calculation codes based on Intranuclear-Cascade-Evaporation (INC/E) and
Quantum Molecular Dynamics (QMD) models. For neutrons associated with  formation, the present data deviatedfrom predictions of these codes, and gave magnitudes between them in both 0.8 and 1.5 GeV incidences.
KEYWORDS: neutron-production double-differential cross sections, most-forward direction, proton incidence,
iron, lead, GeV, SCINFUL-QMD, high-resolution time-of-flight method, INC/E, QMD

I. Introduction
Studies on the spallation reaction have recently been madefor various applications, such as a spallation neutron source
and an Accelerator-Driven-System (ADS) for transmutationof nuclear waste or energy production. For example, projects
for intense neutron sources based on the proton-incident spallation reaction have been proposed in Japan (Japan Proton Accelerator Research Complex: J-PARC1) ), the USA (National
Spallation Neutron Source: SNS2) ) and Europe (European
Spallation Source: ESS3) ).
For the design of these facilities, evaluated nuclear data inthe energy region up to a few GeV are required. Compilationof high energy nuclear data files is being carried out in many
countries worldwide, for instance, JENDL High Energy File4)(JENDL-HE, Japan), LANL High Energy File5) (LA-150,
USA) and Medium Energy Nuclear Data Library6) (MENDL2, Russia). Nuclear data evaluation is generally performed on
the basis of experimental data and theoretical model calculations. However, when experimental data are sparse, theoretical model calculations play an important role. IntranuclearCascade-Evaporation (INC/E) model7) and Quantum Molecular Dynamics (QMD) model8) have often been utilized in theenergy region above a few hundred MeV. Although improvement of the calculation codes has been performed, some discrepancies still remain between experimental data and model
predictions. In general, calculation codes tend to fail to reproduce the neutron-production double-differential cross sections in the most-forward direction, due to ambiguity in themodeling of Pauli blocking, isospin flip and collective excitation.9) Experimental data are required to improve the model
precision in this direction.
∗

Corresponding author, Tel. +81-92-642-3765, Fax. +81-92-6423769, E-mail: kishibashi@kune2a.nucl.kyushu-u.ac.jp

Measurement of neutron-production double-differential
cross sections at the most-forward angle was planned atthe π 2 beam line of 12-GeV proton synchrotron (12-GeV
PS) in the High Energy Accelerator Research Organization (KEK). At KEK, we have already measured neutronproduction double-differential cross sections by the time-offlight (TOF) method, except for 0◦ measurement. Details ofthese experiments were given in our previous papers.10, 11)
The recoil proton method combined with a magnetic spectrometer has often been utilized for high-energy 0◦ -neutron
spectrum measurement,12, 13) because the TOF method has apoor energy resolution for high-energy neutrons. The TOF
method, however, provides a higher detection efficiency anduses a simpler data analysis than the recoil proton method.
Improvement of energy resolution in the TOF measurement
is essential for obtaining meaningful experimental data in the
most-forward direction. A high-resolution TOF method wasdevised to meet this requirement: three fast-rise-time photomultipliers were connected with a neutron detector.
In this work, we report the results of 0◦ measurementby the high-resolution TOF method. Cross sections wereobtained on targets of iron and lead at incident proton energies of 0.8 and 1.5 GeV. We aimed at measuring the emitted neutrons above a few hundred MeV because of an experimental constraint explained later. Comparisons are madebetween the experimental data and results of the calculation codes, NMTC/JAM14) based on the INC/E model and
JQMD15) based on the QMD model.

II. Experimental Method
The experiment was carried out using the π 2 beam line of
the 12-GeV PS at KEK. A schematic view of the experimental
arrangement is shown in Fig. 1. Incident protons were gener-

283

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D. SATOH et al.
M01(Bending Magnet)

S01

P01

P1

20 m

Veto detector (NE102A)
Target+n0Flight path = 3.5 m at 0.8 GeV
NE102A

Pilot-U

S02

P02

P2

= 5.0 m at 1.5 GeVCharged particle

NE213( 12.7x12.7 cm )H2431 x 3
Beam Scintillators

Beam dump(

5.1 cm )Fig. 1 Illustration of experimental arrangement

ated as secondary particles from an internal target which wasplaced in the 12-GeV PS main ring. After passing the bending and focussing magnets, they were supplied to the π 2 beamline. Therefore, incident protons were accompanied by pions
which have the same electric charge polarity and momentum.
A pair of Pilot-U scintillators located at a separation distanceof 20 m were employed for TOF measurement for discriminating incident protons from pions. NE102A plastic scintillators (P1 and P2) were set in front of the target to define theincident proton beam area. The coincidence of signals fromthese scintillators was counted to determine the number ofincident particles. On passing through the target, the proton
beam was deflected as it moved downstream from the target
by an electric bending magnet (M01), so that the deflection
prevented the beam from being incident on the 0◦ detector.
Since the vertical gap between the magnetic poles was narrow(30 cm), secondary scattering of emitted particles took placeon the magnet pole pieces. It was difficult to set shielding between the electric bending magnet and the neutron detector,
without influencing the neutrons of interest.10) Therefore, themeasurement was made without the shielding.
The iron and lead targets were cylindrical disks with a diameter of 50 mm. Thicknesses of iron and lead targets were
30 and 20 mm, respectively. An NE213 liquid organic scintillator was employed as a neutron detector. The shape wascylindrical with a 12.7 cm diameter and 12.7 cm thickness.
The neutron detector was placed at 0◦ . For improvementof the energy resolution, photomultipliers with a fast-risetime (Hamamatsu H2431; rise time, 0.7 ns, diameter, 5.1 cm)
were adopted. Three H2431s were connected with the NE213
scintillator. The neutron arrival time was obtained by timeaverage of the three photomultipliers to give the best time
resolution. Flight path lengths were 3.5 and 5.0 m for 0.8 and
1.5 GeV proton incidences, respectively. Although the path
length of 5.0 m was insufficient to identify the quasi-elastic
events at 1.5 GeV, the special constraint of the π 2 area andthe low incident beam intensity forced us to choose this path
length. In front of the neutron detector, an NE102A scintillator was mounted as a veto detector to reject the charged
particle events. Background measurements were performed
without the target to subtract the neutrons produced from the
upstream devices such as the beam scintillators.

III. Electronic Circuit
Data were obtained for three parameters, i.e. TOF, pulse
height and pulse shape event by event using CAMAC mod-ules. A block diagram of the electronic circuit is shownin Fig. 2. The anode signal of each photomultiplier combined with the NE213 scintillator was divided into threepulses. One pulse was fed into a constant fraction discriminator (CFD) and then sent to COIN.4 through a fan-in module (FAN IN/OUT). If this pulse reached COIN.4 during
the gate width of 100 ns, start pulses were generated to actuate CAMAC modules such as analog-to-digital converters(ADCs) and time-to-digital converters (TDCs). The other two
pulses were fed into two charge sensitive ADCs (fast and slow
gates). TDCs were used for measuring the TOF that corresponded to the time difference between the start pulse fromthe neutron detector and the stop pulse from the incident beam
detectors (P01 and P02). A signal from the veto detector was
fed into one of the ADCs which had a gate width of 45 ns.
When the coincidence of the signals from all beam scintillators occurred at coincidence module (COIN.3), incident particles hit the target. Then, a pulse with a typical time durationof 100 ns was sent to the next coincidence module of COIN.4.
All experimental data were acquired via the CAMAC
modules which were connected to a personal computer andrecorded on a compact disk. Then, they were analyzed by an
off-line method.

IV. Data Analysis
1. Discrimination of Incident Particles
As mentioned in Chap. II, the incident beam at the π 2beam line includes protons and pions. Discrimination between protons and pions was carried out using the incident
particle TOF measurement. TOF difference between S0 andP0 at the incident proton energy of 0.8 GeV is shown in Fig. 3.
Protons and pions were well separated. The events in the region from 6.4 to 7.4 ns on the horizontal axis were extracted
and analyzed as proton incidence.
2. TOF Analysis
Neutron spectra were obtained by subtracting the resultsof the target-out measurement from those of the target-in, after normalization by the number of incident protons. Example TOF spectra of the target-in and -out measurements for
0.8 GeV protons on iron target are shown in Fig. 4, where
the spectra included events of both neutrons and gamma-rays.
The horizontal axis of the TOF spectra was reversed, because
the start signal for the TOF measurement was taken from theneutron detector. A sharp peak due to prompt gamma-rays
was utilized as the basis of the TOF spectrum for converting
JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

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Neutron-Production Double-Differential Cross Sections of Iron and Lead

S01
S02
P01

TDC
STOP

DL

CFD
CFD
CFD

P02

CFD

P1

CFD

DL

TDC
STOP

DL

TDC
STOP

DL

TDC
STOP

Inhibit

FAN IN/OUT

COIN.
1
15 ns

COIN.
2

Camac
Scaler

COIN.
3

G.G
(Latch mode)

Inhibit

ADC (Fast) Gate

G.G 350 ns ADC (Slow) Gate
100 nsADC VETO Gate
COIN. 4CFD

TDC (Start)

VETO

ADC VETO

NE-2130 -1

OUTPUT. REG

Delay : 50ns

DL 2 ns

P2

Stop

Start

NIM
Scaler

15 ns

Camac
ScalerFAN IN/OUTDL

TDC STOPDL

COIN. REG 0 -2

CFD
0 -3

NIM
Scaler

45 ns

ADC (Fast)

COIN. REG Gate

ADC (Slow)

Fig. 2 Block diagram of measurement circuit
2.0 x 10-5

1.5 x 104
Incident beam momentum = 1.46 GeV/c

Ep = 0.8 GeV

PionsCountsCounts

1.0 x 104In
Out
In - Out

1.0 x 10-5

Protons

5.0 x 1030-10Prompt gamma

Fe target

0

-5

0

5

10

15

Time (ns)Fig. 3 Time-distribution of incident particles
Energy of incident protons was 0.8 GeV.

TOF spectra to the emitted neutron energy. A prompt gamma
peak was also seen in the target-out results. These prompt
gamma-rays came mainly from the beam scintillators locatedupstream from the target, and were generated slightly prior tothose from the target.
In usual TOF experiments using a high-intensity pulsed
beam, the time resolution is determined by the pulse widthof the beam. On the other hand, the beam intensity at theπ 2 beam line was very weak and about 105 particles/2.5 s, so
incident protons were individually counted in this measurement. Hence, the uncertainty of TOF came from the resolution of the detectors. The time resolution was obtained fromelastic scattering events of protons which were measured bycoincidence of the veto detector and the NE213 scintillator.
The value was 0.82 ns as shown in Fig. 5, and this gave theenergy resolution of 150 MeV for 800 MeV protons. The pro-VOL. 40, NO. 5, MAY 2003

80

85

90

95

100

105

TDC (ns)Fig. 4 Time-of-flight (TOF) spectra for neutrons and gamma-rays
obtained by 0.8 GeV protons on iron target
Long and short dashed lines indicate the results with and without target, respectively, and the solid line shows the difference
between target-in and target-out events.

ton events in Fig. 5 were not observed at all in the case of pion
incidence. Time resolution was also evaluated for the prompt
gamma-ray peak. The result was 0.53 ns in FWHM which
was better than that for larger photomultipliers, e.g. 55% better than for the 12.7 cm diameter R1250. The time resolutionof 0.82 ns for protons was appreciably worse than 0.53 ns for
gamma-rays. We thought that this difference was attributable
to the following. Gamma-rays produce fluorescent photons ina rather local region in the scintillator. On the contrary, highenergy protons create them along their paths, and some photons reach the photoelectric surface of the photomultiplier indirectly through reflections. This space-distributed photon behavior may deteriorate the rise time of the photomultiplier sig-

286

D. SATOH et al.
2.5 x 103Fe(p, p)Counts

2.0 x 103

Ep = 0.8 GeV

1.5 x 1030.82 ns at FWHM

1.0 x 103

5.0 x 102

0
92

93

94

95

96

97

TDC (ns)Fig. 5 TOF spectrum for elastic scattering protons obtained by
0.8 GeV protons on iron target
These events were extracted from the coincidence of the vetodetector and NE213 scintillator.

nal, resulting in the poorer time resolution. For high-energy
neutron incidence, recoil protons are generated and the situation is considered to be more similar to that of the proton
incidence. In fact, we deduced the time resolution from quasielastic scattering events of neutrons. The values were practically the same as those of protons. Unfortunately, we could
not utilize those data because of the large statistical error. For
this reason, the time resolution was estimated to be 0.82 ns forlater analysis.
3. Neutron-Gamma Discrimination
Pulse shape discrimination (PSD) has often been used for
eliminating gamma-ray events from neutron ones. Figure 6
shows the results of PSD by the two-gate integration method.
The widths of fast and slow gates were 45 and 350 ns, respectively. The slow gate was delayed by 50 ns from the fast gate.
The neutron events observed as recoil protons had a longer
fluorescence tail in time than the gamma-ray events. Therefore, the neutron events appeared at higher channels on thevertical axis than the gamma ones. In this figure, neutrons(proton-like events) and gamma-rays (electron-like events)
were well separated.
When the neutron energy is high, for instance 500 MeV,
most recoil protons escape from the detector and pions are often generated. This situation deteriorates the PSD characteristics, and neutron events tend to overlap with gamma ones forhigh energy neutron incidence.16) Therefore, the PSD method
was unusable for neutrons of interest in this measurement.
Fortunately, most gamma-rays have energies below several
MeV, and we could reduce the effect of gamma-rays without
PSD by employing higher bias level. The bias level was basically twice the Compton edge position at half-maximum forthe gamma-rays from Am-Be source (2×4.33 MeVee ). In thehigh-energy region, however, the light outputs decreased with
the incident neutron energy, since high-energy recoil protons
readily escaped from the scintillator and deposited only a partof their energy. In fact, the energy deposit of 0.8 GeV protons
in the NE213 scintillator was estimated to be ∼7.5 MeVee .
For this reason, the Am-Be bias (4.33 MeVee ) was utilized
for neutrons with energies above 600 MeV.
4. Detection Efficiency
The neutron detection efficiencies were obtained from calculation results of SCINFUL-QMD.17, 18) The code was developed to calculate the detection efficiency of the NE213
liquid organic scintillator for incident neutron energies up to
3 GeV. Detailed description of the computational scheme ofthis code was made in our previous papers.17, 18) To validate
the accuracy of the code, the results of SCINFUL-QMD werecompared with previous experimental data19–22) and resultsof another computer code CECIL23) for both response function and detection efficiency. In calculations with both codes,
1×105 initial collision histories were accumulated.
Figures 7 and 8 indicate the response functions of theNE213 scintillator of 12.7 cm in diameter and 12.7 cm thick,for incident neutron energies of 180–260 and 400–600 MeV,
respectively. In Fig. 7, SCINFUL-QMD gave good agreement with the experimental data measured by Sasaki et al.20)
below about 40 MeVee . In Fig. 8, SCINFUL-QMD reproduced the shape of the response function appreciably better

Response [(counts MeVee-1)/(n cm-2)]

100
En = 180-260 MeV
10-1

10-2

Sasaki et al.20)
10-3

10-4

Nakao et al.19)
SCINFUL-QMD
CECIL
0

20

40

60

80

100

120

140

160

Light output (MeVee)

Fig. 6 Results of pulse shape discrimination (PSD) by the two-gate
integration method

Fig. 7 Response function of NE213 scintillator at incident neutronenergies of 180–260 MeV

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Neutron-Production Double-Differential Cross Sections of Iron and Lead

Response [(counts MeVee-1)/(n cm-2)]

En = 400-600 MeV

Sasaki et al.20)
SCINFUL-QMD at 600 MeVSCINFUL-QMD at 400 MeV
CECIL at 500 MeV

10-1

10-2

10-3

10-4

0

50

100

150

200

250

Light output (MeVee)

Fig. 8 Response function of NE213 scintillator at incident neutronenergies of 400–600 MeV

than CECIL.
The neutron detection efficiencies calculated by SCINFULQMD at the Am-Be bias of 4.33 MeVee are shown in Fig. 9
together with experimental data and the results of CECIL. Results of SCINFUL-QMD generally agreed with the experimental data. SCINFUL-QMD gave increased neutron detection efficiency at neutron energies above 300 MeV due to inelastic pion generation.
In the past experiments, CECIL has been used to determine neutron detection efficiencies at incident neutron energies above 100 MeV.10, 11) Results in Figs. 7 and 8 led us toconsider that SCINFUL-QMD was more capable of reproducing the response functions than CECIL was. We employed theresults of SCINFUL-QMD to deduce the double-differential
cross sections. The uncertainty of the neutron detection efficiency was decided11) as 15% at energies above 100 MeV,
consulting the results in Figs. 7 and 8.

V. Results and Discussion
Neutron-production double-differential cross sections ofiron and lead by 0.8 and 1.5 GeV protons are presented in

Figs. 10 to 13. The bias level was set basically at twice theAm-Be threshold, 8.66 MeVee , because the main purpose of
the experiment was to measure neutrons in the high-energy
region. In addition, the Am-Be bias (4.33 MeVee ) was also
used at neutron energies above 600 MeV for reducing ambiguity due to threshold determination of the cross section. Error bars included both statistical uncertainties and ambiguityof neutron detection efficiency (15%) in a form of one sigma.
Experimental data of iron and lead for 0.8 and 1.5 GeV protons were compared with other experimental data12, 13) measured by recoil proton spectrometers and the results of model
calculation codes. The results of other experiments and calculation codes were broadened by Gaussian-type function totake account of the energy resolution. In Figs. 10(a) to 13(a),the solid and dashed lines indicate the original values and thebroadened results, respectively. Our data are compared withthe broadened results in Figs. 10(b) to 13(b). The experimen-

101
Fe(p, xn) at 0-deg(a)Incident proton energy = 0.8 GeV
100
Leray et al.13)
10-1
NMTC/JAM x 10-1
10-2
JQMD x 10-2
Double-differential cross section (mb/sr/MeV)

100

10-3

10-4

10-5
(b)

100

20

Neutron detection efficiency (%)

Am-Be bias
Present work
Leray et al.13)
NMTC/JAM
JQMD

15

10-1
100

10

Verbinski et al. 21)
5

Meigo

22)

Nakao et al.19)
SCINFUL-QMD
CECIL
0

101

102

103

Neutron energy (MeV)

Fig. 9 Neutron detection efficiency of NE213 scintillator at
Am-Be bias

VOL. 40, NO. 5, MAY 2003

1000
Neutron energy (MeV)

Fig. 10 Neutron-production double-differential cross sections ofiron for 0.8 GeV protons
Figure (a) shows original values (solid lines) and broadened results (dashed lines) adjusted to fit the present experimental energy
resolution. Comparison between our experimental data and thebroadened results is shown in (b). Open circles indicate the possibility of the containing effect of scattering at bending magnet polepieces.

288

D. SATOH et al.
102

101
Pb(p, xn) at 0-deg

101

(a)

(a)

Fe(p, xn) at 0-deg

Incident proton energy = 0.8 GeV

Incident proton energy = 1.5 GeVLeray et al.13)
010Bonner et al.12) x 10-1

100

-1

10

NMTC/JAM x 10

NMTC/JAM-2JQMD x 10-3
10-3

Double-differential cross section (mb/sr/MeV)

Double-differential cross section (mb/sr/MeV)

10-2

-4

10

10-5
10-6
(b)

10-1
JQMD x 10-1

10-2
(b)

100

100

Present work

10-1

Leray et al.13)
Bonner et al.12)

Present work

NMTC/JAM
JQMD
10-1
100

NMTC/JAM
JQMD
1000

Neutron energy (MeV)

10-2
400

500

600

700 800 9001000

1500

Neutron energy (MeV)

Fig. 11 Same as for Fig. 10, except for use of a lead target

tal data indicated with open circles possibly contain the effectcaused by the scattering at the bending magnet pole pieces.10)
Solid lines show the calculation results of NMTC/JAM.14)
The code adopts the in-medium nucleon-nucleon elastic scattering cross sections of Cugnon et al.24, 25) Dashed lines indicate the calculation results of JQMD,15) which incorporates QMD with Statistical Decay Model (SDM) model. The
experimental data obtained by Leray et al.13) are plotted in
Figs. 10(b) and 11(b) with chain lines. Experimental data by
Bonner et al.12) are also shown in Fig. 11(b) with the dottedline. The cross sections consist of two main components. The
peak close to the energy of the incident protons corresponds toquasi-elastic (charge exchange: CEX) nucleon-nucleon collisions. The lower broad peak positioned around 3/4 of the incident energy is ascribed to pion-associated neutrons via the resonance excitation in inelastic nucleon-nucleon collisions.
In Figs. 10(b) and 11(b) for the 0.8 GeV proton incidence, the experimental data exhibit both the CEX and pionassociated peaks. For the iron target, our data are in good
agreement with the experimental results of Leray et al. in both
the CEX and pion-associated regions. For the lead target, theexperimental data of Bonner et al. are the highest among thethree experiments in the CEX and pion-associated regions.
Our experimental data in the CEX region are slightly smaller

Fig. 12 Neutron-production double-differential cross sections ofiron for 1.5 GeV protons
Open circles indicate the possibility of containing the effect of
scattering at bending magnet pole pieces.

than those of other experiments and larger than the predictions of NMTC/JAM. At the pion-associated peak, our data
are 30% lower than those of Leray et al. Below 250 MeV,
the experimental data tend to include the neutrons scatteredfrom the bending magnet pole pieces. The effect is clearer foriron which has a relatively lower cross section in this energy
region than lead.
In Figs. 12(b) and 13(b) for the 1.5 GeV proton incidence,the experimental data exhibit the pion-associated peak, but
the CEX peak does not appear due to the deterioration of theenergy resolution. The absolute values in the pion-associatedregion are larger than those of JQMD and smaller than thoseof NMTC/JAM. Below 500 MeV, the experimental data tend
to give larger values than code predictions. NMTC/JAM underestimates the cross section at the CEX peak. The INC
model code does not take into account such effects as smeared
Fermi surface, isospin flip and isospin-related collective excitation.26)
For both the 0.8 and 1.5 GeV proton incidences,
NMTC/JAM overestimates the pion-associated peak. Thisoverestimation may be explained by an excessive amount of
JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGYNeutron-Production Double-Differential Cross Sections of Iron and Lead
101(a)Pb(p, xn) at 0-deg
Incident proton energy = 1.5 GeV
NMTC/JAM
0

Double-differential cross section (mb/sr/MeV)10

JQMD x 10-1

289

results on iron agreed with experimental data of Leray et al.
which were measured by the recoil proton method in both the
CEX and pion-associated regions. For the lead target, ourcross sections were typically 30% lower at the pion associated peak than the data of Leray et al. For the 1.5 GeV proton incidence, our results gave intermediate values between
NMTC/JAM and JQMD predictions in the pion-associated region. These codes have some weaknesses concerning nucleon
emission at the most-forward direction.

-1

10

Acknowledgment
We express our gratitude to Prof. Y. Yoshimura, Mr. M.
Taino and the beam channel staff of KEK for their continuous
encouragement and generous support of the experiment.

10-2
(b)

References

100

10-1
Present work
NMTC/JAM
JQMD
10-2
400

500

600

700 800 9001000

1500

Neutron energy (MeV)

Fig. 13 Same as for Fig. 12, except for use of a lead target

forward angular distribution of -particles adopted in thecode. In contrast, JQMD gives an appropriate height of thepion-associated peak, but forms it at energies lower than theexperimental data by about a hundred MeV. The code does
not exhibit the CEX peak at all. As the angular distribution
for the neutron-proton scattering, the old parametrization of
Cugnon et al.27) was adopted in the JQMD code. The rapid
rise term of the cross section at very backward angles, due tothe CEX process, is absent in the old parametrization. This
lack was corrected by a new parametrization.26)

VI. Conclusion
The neutron-production double-differential cross sectionsof iron and lead were measured for 0.8 and 1.5 GeV protons
incidences. The high-resolution time-of-flight (TOF) method
was adopted to determine the neutron energy. Three fastrise-time photomultipliers were connected with an NE213
scintillator to improve energy resolution in the measurement.
The energy resolution derived from the elastic proton scattering was 0.82 ns corresponding to 150 MeV for 0.8 GeV neutrons. The neutron detection efficiencies were calculated by
the SCINFUL-QMD. For 0.8 GeV proton incidence, the CEX
peak was identified by the high-resolution TOF method. Our
VOL. 40, NO. 5, MAY 2003

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Spin-dependent Interactions in Σ+-p Scattering
T. Kadowaki et al.
[attached]
Asymmetry Measurement of the Polarized Σ+p Elastic Scattering and the Hyperon Nucleon Spin-Orbit
Interaction.
Eur. Phys. J. vol. A15 (2002) pp.295-298.
● J. Asai et al.
[attached]
Hyperon Scattering Experiments with a new Tracking Detector 'SCITIC'.
Jpn. J. Appl. Phys. vol.43 (2004), pp.1586-1592.
○ A. Sato et al.
[attached]
Scintillating Track Image - SCITIC.
Jpn. J. Appl. Phys. vol.43 (2004) pp.1593-1601.
★ Akira Sato
Development of a New Tracking Detector for Hyperon-Nucleon Scattering Experiments
Tokyo University of Science 2001.
★ T. Kadowaki
Asymmetry Measurement of the Polarized Hyperon-Nucleon Elastic Scattering
Tokyo University of Science 2002.
★ J. Asai
Measurements of asymmetry and depolarization parameters in the polarized Σ+p elastic scattering
Tokyo University of Science 2004.
◇ K. Nakai
Studies of Spin-Dependent Hyperon Nucleon interactions
KEK-Seminar on “Recent Progress of Strangeness Nuclear Physics” (Tsukuba, March 2002).
◇ K. Nakai
Spin-Dependent Interactions in Hyperon-Nucleon Scattering
PANIC02: XVI International Conference on Particles and Nuclei (Osaka, September , 2002).
◇ J. AsaiSCITIC for Hyperon-Scattering Experiments
PANIC02: XVI International Conference on Particles and Nuclei (Osaka, September , 2002).
◇ K. Nakai
Spin –Dependent Interactions in Hyperon Nucleon Scattering
Nucl.. Phys. vol. A721 (2003) pp.673c-678c.
◇ J. Asai
Asymmetry measurement of the polarized Σ+p elastic scattering and the hyperon-nucleon spin-orbit interaction
SENDAI 2003: the international symposium on "Electrophoto-production of Strangeness on Nucleons andNuclei" hosted by the department of physics” (Tohoku University, June 2003).
◇ M. Kurosawa
The study of spin-dependent interaction between hyperon and nucleon
SENDAI 2003: the international symposium on "Electrophoto-production of Strangeness on Nucleons andNuclei" hosted by the department of physics, Tohoku University, June 2003.
●________________________________________________________________________● Physics papers published in refereed journal.
○★
◇＊

Technical papers.
PhD theses.
Conference and Symposium.
Internal Report and others.

Eur. Phys. J. A 15, 295–298 (2002)
DOI 10.1140/epja/i2002-10054-4

THE EUROPEAN
PHYSICAL JOURNAL A

Short Note
Asymmetry measurement of the polarized Σ+p elastic scatteringand the hyperon-nucleon spin-orbit interaction
T. Kadowaki1 , J. Asai1 , W. Imoto1 , S. Iwata1 , M. Kurosawa1 , K. Nakai1,a , A. Sato2 , K. Imai3 , H. Takahashi3 ,C.J. Yoon3 , T. Maruta4 , M. Ieiri5 , T. Nagae5 , H. Noumi5 , T. Fukuda6 , P.K. Saha6 , and B. Bassalleck7
1

Department of Physics, Tokyo University of Science, Noda, Chiba, Japan
Department of Physics, Osaka University, Toyonaka, Osaka, Japan3
Department of Physics, Kyoto University, Kyoto, Japan
4
Department of Physics, University of Tokyo, Tokyo, Japan
5
High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki, Japan
6
Laboratory of Physics, Osaka Electro-Communication University, Osaka, Japan
7
Department of Physics, University of New Mexico, Albquerque, NM, USA2

Received: 27 May 2002 /c Società Italiana di Fisica / Springer-Verlag 2002
Published online: 19 November 2002 – Communicated by Th. Walcher
Abstract. In order to study the hyperon-nucleon spin-orbit interaction, asymmetries of the polarized Σ +elastic scattering on protons have been measured. The polarized hyperons were produced through thep(π + , K+ )Σ + reaction in a liquid-scintillator active target, and tracks of Σ + p scattering events wererecorded with a newly developed track detector named SCITIC (scintillating track image camera). Although analyses of 10 times more data with more sophisticated data-taking systems and more elaborated
analyzing tools are under way, data from an early stage of this experiment indicate a large Σ + p spin-orbit
interaction in contrast to the case of the small ΛN spin-orbit interaction.
PACS. 13.75.Ev Hyperon-nucleon interactions – 25.80.Pw Hyperon-induced reactions

One of the most interesting results from hypernuclear
experiments was the indication of the small spin-orbit(LS) interaction in the old Λ-hypernuclear experimentby the Heidelberg-Saclay collaboration at CERN in
1978 [1]. The smallness of the LS splitting was alsosuggested in other hypernuclei at BNL [2]. This result
has recently been conﬁrmed by experiments at KEK
and BNL with better precision. Two KEK-PS experiments [3,4] reported small but ﬁnite values of the LS
splitting, and two BNL-AGS experiments [5,6] measuringhypernuclear gamma-rays determined the small splittings
quantitatively with high precision.
Just after the CERN experiment in 1978, H. Pirner [7]pointed out that, in contrast to the ΛN system, the LS
ΛNinteraction must be large in the ΣN system (VLS

4 NN
ΣN
0, VLS = 3 VLS ) based on a naive quark model.
K. Yazaki [8] has also indicated a possible large LS interaction for the ΣN system, while the ΛN LS interaction
is small due to cancellation between anti-symmetric andae-mail: nakai@post.kek.jp

symmetric LS terms. Following these suggestions, eﬀorts
were made to observe the LS eﬀect in Σ-hypernuclear experiments at CERN and KEK. However, it was shownthat the Σ-hypernuclei were observable only in very limited cases [9,10].
Since that time, the possibility has been exploredat KEK-PS to study the ΣN interaction through thehyperon-scattering experiments using active targets ofscintillating track detectors. A scintillation-ﬁber detector,
named SCIFI [11] was successfully used for the scattering experiments, E251 [12] and E289 [13]. For the present
experiment, however, we developed another type of track
detector named SCITIC (scintillating track image camera) [14,15]. The track image in the liquid scintillator
was focussed on a photocathode of an image intensiﬁer
tube (IIT) through an optical-lens system, and after multiplications of the photoelectrons in 4-stages of IIT, thepicture was recorded with a CCD camera. These track
detectors were used for observation of hyperon events selected with electronic trigger signals from beam and kaon
spectrometers.

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The European Physical Journal A

Fig. 1. (a) Schematic illustration of SCITIC. (b) Typical pictures of hyperon scattering seen by two sets of SCITIC arranged
as shown in (c).

We started an experiment (KEK-PS E452) to measure asymmetries of the polarized hyperon-nucleon elastic
YN
scattering for the determination of VLS. The experimentconsisted of three stages, E452A, B and C. This paper isreporting a result from the early-stage experiment E452A
ΣN
which has already indicated a large VLS.
The experiments were carried out at KEK-PS. The polarized hyperons were produced through the p(π + , K+ )Σ +reaction. Pion beams of 1.6 GeV/c from the K2 beam line
were used on the active target of SCITIC with a liquid
scintillator (Bicron BC-517S with H/C = 1.7). The experimental setup was essentially the same as that for theprevious experiment E289 [12] except for the active target (SCITIC) and the angular coverage of the kaon spectrometer. The (π, K) events were selected with trigger signals indicating kaon production from the kaon spectromLabeter whose coverage of kaon angle was θK= 20–30 de+
gree. Large polarizations of the Σ in this angular range
have been reported in previous experiments [16,17] (seeﬁg. 2(a) below).
Scintillating track images of the selected events wererecorded with two sets of SCITIC at diﬀerent azimuthal
angles, from which we deduced three-dimensional picturesfrom oﬀ-line analyses. Shown in ﬁg. 1(a) is a schematic
illustration of the SCITIC. A conventional camera lens(Canon F = (f /D) = 1.2, f = 85 mm) was used for
the SCITIC in the early phase, E452A. In order to obtaina large acceptance for photons, an electrostatic-type IIT(Hamamatsu V4440PX) with 10 cm diameter was used

for the ﬁrst stage of the IIT cascade while the diameter ofthe second to fourth stage IIT were 2.5 cm. The ﬁrst- andsecond-level trigger signals from the beam and kaon spectrometers were applied to the third and fourth IIT whichwere of microchannel-plate type. The images on the ﬂuorescent output screen of the last stage IIT were recorded
by a CCD camera (Kodak ES310).
The pictorial data from the CCD camera were stored
after compression so as to speed up the data recording.
Using data-compression up to 25 events per beam spill of
2 s duration could be recorded. A typical picture of a Σ + p
scattering event is shown in ﬁg. 1(b).
About 60000 pictures of (π, K) events were taken in
E452A. These contain pictures of Σ + or Λ productions,and their scatterings on a proton or a carbon nucleus in
the active target, as well as background nuclear reactions.
Scanning of the pictorial data was assisted with a computer, which indicated vertex points from pion and kaon
tracks and possible directions of the produced hyperons
with use of the beam and particle spectrometer data. All
events were scanned by two or three scanners, and only
those events selected by at least two scanners were taken
as the candidates.
Out of the 60000 pictures about 500 candidates forthe Σ + p scattering were selected through the eye scanning. These candidates, however, include also the quasifree
scattering with a proton in carbon nuclei. Therefore, in order to distinguish pure hyperon-proton elastic scatteringfrom the quasifree-scattering or background due to mul-

T. Kadowaki et al.: Asymmetry measurement of the polarized Σ + p elastic scattering...

297

Fig. 2. Polarizations of hyperons produced through the (π + , K+ ) reaction, and left/right asymmetries of the polarized hyperon
elastic scattering on protons: (a), (b) for Σ + and (c), (d) for Λ. Data on the Σ + polarization in (a) are from refs. [16, 17].

tiple tracks possibly from nuclear reactions, kinematicalcuts were applied. Those were 1) missing-mass cut to select the Σ + hyperon, 2) co-planarity cut on the Σ + production angle relative to the (π, K)-plane in the hyperonproduction stage, and 3) co-planarity cut on the Σ + scattering angle relative to the plane of the incident Σ + anda recoil proton in the scattering stage. The cuts 1) and 2)
were for the Σ + production process. They were necessary
to assure that our analysis was consistent with the existing data on the Σ + polarization from refs. [16] and [17],which were taken with hydrogen targets. The cut 3) was
for rejection of the hyperon scattering on a proton in carbon nucleus. After these cuts, 31 candidates for the Σ + p
scattering survived.
Since Σ + hyperons are polarized perpendicular to thereaction plane, the eﬀect of the LS interaction on the scattering can be observed as a left/right asymmetry, which
can be determined simply by identifying the left or right
direction of the scattered hyperons. In the Σ + p scattering,
the scattered Σ + was identiﬁed against the recoil proton
by its decay kink.
The left/right asymmetry A was determined from thenumbers of events of left-scattering (NL ) and of rightscattering (NR ) asA=

N L − NR.
NL + N R

Shown in ﬁg. 2 is a summary of the data. The dataon polarization of the Σ + produced through the (π + , K+ )reaction (ﬁg. 2(a)) are from previous experiments [16,17].
The polarization was determined also from the analysis of
the present data, which was consistent with the previous

result. The data in ﬁg. 2(a) were used to determine thepolarization to be P = 0.8, averaging over the kaon-angle
Lab= 20–38 degree.
range θK
Results from the present experiment E452A on theasymmetry of the Σ + p scattering are shown in ﬁg. 2(b)together with data from an analysis of the E289 data [15].
Signiﬁcantly large left/right asymmetries are observed forLab
the range of θK= 20–30 degree where the polarization+of Σ was large (P = 0.8) as seen in ﬁg. 2(a).
Although the motivation for the present experiment
was to study the Σ + interactions, the data include comparable number of Λ-associated events. The asymmetry ofΛ is also interesting to compare the LS eﬀect in the ΛN
two-body interaction with the weak LS eﬀect in hypernuclei. Since the Λ hyperons are produced only throughinteraction of a pion with a neutron in a carbon nucleus,
the polarization axis is not unique because of the Fermi
motion. The normal vector of a plane determined by tracksof the incident pion and the produced Λ was selected as
the polarization axis. The E452A data on Λ hyperon polarization formed through the (π, K) reaction are plotted
in ﬁg. 2(c). The asymmetries of the Λp scattering shownin ﬁg. 2(d) are results from the present experiment E452A
and from a new analysis of the E289 data. While the polarizations of Λ were large, the Λp scattering asymmetry
was consistent with zero, indicating the weak LS interaction in the ΛN system.
Although these data suﬀer from poor statistics, we
may already conclude experimentally that the asymmetry of the Σ + p scattering is about as large as predictedby the quark model [7,8].

298

The European Physical Journal A

been carried out, and further analyses are under way withdata containing 10 times more pictures taken with moresophisticated detector and trigger system.

Fig. 3. Comparison of the experimental result on Σ + p scattering with the theoretical calculation [18]. The solid and dashedcurves are by the RGM and FSS calculations, respectively. The
asymmetry parameters were calculated versus scattering angle
θcm of Σ + . The experimental asymmetry aexp is an averagevalue over the scattering angles.

The experimental result is compared with a theoretical calculation in ﬁg. 3. Taking the average of 3 points
in ﬁg. 2(b) for the range of pΣ = 800 ± 200 MeV/cLab(θK= 18–32 degree), we obtain the left/right asymmetry A = 0.35 ± 0.15. Theoretical curves drawn inthe ﬁgure are for quark models developed by the KyotoNiigata group (RGM and FSS) [19]. Calculated asymmetry parameters a(θ) are drawn versus the scatteringc.m.
angle θ of Σ + hyperon, θΣ[18]. Experimentally, theexpasymmetry parameter a
was determined dividing theleft/right asymmetry A by the incident hyperon polarization PΣ . The experimental asymmetry aexp = A/PΣ =(0.35 ± 0.15)/0.8 = 0.44 ± 0.2 is shown in the ﬁgure. Direct comparison with theories is not possible, since the experimental number is an average value integrated over theangle θΣ with weights determined by angular dependencesof cross-sections, scanning eﬃciency, etc. Nevertheless, thetheoretical calculation by the Kyoto-Niigata group [18] forpΣ = 800 MeV seems to be in accord with the data. So
far, this quark model predicted the largest asymmetry.
Detailed arguements are, however, meaningless under
the present statistics. The experiments E452B and C have

We are deeply indebted to the KEK-PS operation crew andthe experimental supporting group for their continuous eﬀorts
to provide excellent experimental conditions. The present work
was partly supported by the SUT Presidential Fund for Promotion of Research in Speciﬁed Field, the Grants-in-Aid for Scientiﬁc Research by MONBUSHO and Research Grants by US
National Science Foundation. We acknowledge that the present
subject to study the LS eﬀect in the Σ + p interaction was motivated by stimulative discussions with Professor B. Povh andhis colleagues in the Heidelberg group.

References
1. W. Bruckner et al., Phys. Lett. B 79, 157 (1978).
2. M. May et al., Phys. Rev. Lett. 47, 1106 (1981); 51, 2085
(1983).
3. S. Ajimura et al., Nucl. Phys. A 639, 93c (1998).
4. H. Hotchi et al., Phys. Rev. C 64, 044302 (2002).
5. S. Ajimura et al., Phys. Rev. Lett. 86, 4255 (2001).
6. H. Akikawa et al., Phys. Rev. Lett. 88, 082501 (2002)
7. H.J. Pirner, Phys. Lett. B 85, 190 (1979).
8. K. Yazaki, Nucl. Phys. A 479, 217c (1988).
9. R.S. Hayano et al., Nucl. Phys. A 478, 113c (1988).
10. T. Nagae et al., Phys. Rev. Lett. 80, 2605 (1998); S. Bart
et al., Phys. Rev. Lett. 83, 5238 (1999).
11. J.K. Ahn et al., Nucl. Instrum. Methods A 457, 137(2001).
12. J.K. Ahn et al., Nucl. Phys. A 648, 263 (1999).
13. Y. Kondo et al., Nucl. Phys. A 676, 371 (2000).
14. A. Sato, et al., TUS-Report; TUS-PN-0011, submitted toJpn. J. Appl. Phys.
15. A. Sato, PhD Thesis (Tokyo Science University, 2001)
TUS-Report, TUS-PN-0010.
16. D.J. Candlin et al., Nucl. Phys. B 226, 1 (1983).
17. J. Haba et al., Nucl. Phys. B 299, 627 (1988).
18. T. Fujita, PhD Thesis (Kyoto University, 1999).
19. Y. Fujiwara et al., Phys. Rev. C 54, 2180 (1996); T. Fujita
et al., Prog. Theor. Phys. 100, 931 (1998).

Japanese Journal of Applied Physics
Vol. 43, No. 4A, 2004, pp. 1586–1592#2004 The Japan Society of Applied Physics

Hyperon Scattering Experiments with a New Tracking Detector ‘‘SCITIC’’
Jyunkichi A SAI, Tomokazu FUKUDA2 , Masaharu I EIRI1 , Kenichi I MAI3 , Wataru I MOTO2 , Soma I WATA,
Tetsuhito K ADOWAKI, Maki K UROSAWA, Tomofumi M ARUTA4 , Tomofumi N AGAE1 , Hiroyuki NOUMI1 ,Planab Kumar S AHA2 , Akira SATO5 , Hitoshi T AKAHASHI1 , Choong Jae Y OON3 and Kozi N AKAI
Physics Department, Tokyo University of Science, Noda-shi, Chiba 278-8501, Japan
1
High Energy Accelerator Research Organization (KEK), Tsukuba-shi, Ibaraki 305-0801, Japan
2
Laboratory of Physics, Osaka Electro-Communication University, Osaka 572-8530, Japan3
Department of Physics, Kyoto University, Kyoto 606-8501, Japan
4
Department of Physics, University of Tokyo, Tokyo 113-0033, Japan
5
Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan(Received October 21, 2003; accepted January 6, 2004; published April 9, 2004)

A new tracking detector, scintillating track image camera (SCITIC) was used for hyperon-scattering experiments. Since thehyperon lifetimes are short, the low-energy hyperon-nucleon scattering can be studied only with a track detector used as anactive target. The present experiment has shown that the SCITIC is a promising detector for the hyperon scatteringexperiments. Polarized hyperons þ were produced through pðþ ; Kþ Þþ reactions with a 1.6 GeV/c pion beam on a liquid
scintillator of the active target. Three sets of SCITIC were used to record the pictures of þ production and scattering. A kaon
spectrometer was used to trigger the SCITIC with a signal of kaon from the reaction. Left/right asymmetries of the þ pscattering were determined through analyses of the pictorial data, and the results were in accordance with a quark-model
prediction that the asymmetry was large in the þ p scattering while it was small in the N case.
[DOI: 10.1143/JJAP.43.1586]
KEYWORDS: tracking detector, SCITIC, active target, hyperon-nucleon scattering

1.

Introduction

2.

A new tracking detector, scintillating track image camera(SCITIC) was developed for use as an active target/detector
for observation of hyperon-nucleon scattering. The principle
and performance of the SCITIC have been reported in theprevious paper.1) The SCITIC consists of a scintillator, an
optical lens, a cascade of image intensiﬁer tubes (IIT) and acharge coupled device (CCD) camera. An image of particle
tracks passing through the scintillator is focused on thephotocathode of the ﬁrst image intensiﬁer tube with an
optical lens system, and after amplication via three or fourstages of image intensiﬁer tubes, the track image is recordedby the CCD camera.
Since the lifetimes of hyperons are so short (1010 s)that the ﬂight paths of low-momentum hyperons are notmore than a few cm, the hyperon interactions can be studied
only with a tracking detector used as an active target.
However, the classical tracking detectors such as the cloud
chamber, the bubble chamber or nuclear emulsion are not
eﬃcient because thet are not capable of obtaining data with
event selections by a trigger system. Recently, new track
detectors, scintillating ﬁber (SCIFI) detector2) and SCITIC
were developed for a series of hyperon experiments at KEKPS. The SCITIC was chosen for the present hyperon-nucleon
scattering experiment.
In this paper, we discuss a series of experiments at KEK tostudy spin-dependent hyperon interactions. The contents are;. Motivation and principle of the experiments. Experimental setup for the KEK-PS E452
. SCITIC used as an active target/detector. Analysis of the SCITIC data
. Experimental resultE-mail address: nakai@post.kek.jp

Motivation and Principle of the Experiments

In order to study the spin-dependent hyperon-nucleon
interactions, a series of experiments E452 A, B and C havebeen carried out at the KEK 12-GeV proton synchrotron(KEK-PS). The experiment was motivated by a -hypernuclear experiment3) at CERN which indicated that theeﬀect of spin-orbit (LS) interactions is very small. A
theoretical interpretation for the small N LS-interaction
was given based on a quark model,4,5) and the model
predicted that the LS eﬀect must be large for þ N
interaction.
The theoretical prediction was conﬁrmed by the E452A
experiment6,7) that was carried out with a prototype SCITIC(Fig. 1). Experimental results are summarized in Fig. 2.
The principle of the experiment was to use polarized þhyperons produced through the p(þ , Kþ )þ reactions toobserve left-right asymmetry of elastic scattering. Pictures ofthe ðþ ; Kþ Þ reaction followed by þ p scattering were takenby shooting 1.6-GeV/c pions on the liquid scintillator(H=C ¼ 1:7) of the active target/detector, SCITIC.
Out of the pictorial data by SCITIC, events were selected
so that the production and scattering were on protons(hydrogen atoms) in the liquid scintillator, through on-line
trigger and oﬀ-line analyses. The left/right asymmetry wasdetermined as A ¼ ðNL  NR Þ=ðNL þ NR Þ, where NL and NR
are the numbers of events for hyperons to get scattered to theleft and right, respectively.
From previous experiments,8,9) the polarization of þ
produced through the pðþ ; Kþ Þþ reactions with 1.6 GeV/
c pion beam was known to be large in the range ofLab
K ¼ 20 {40 as shown in Fig. 2(a). With use of the same
pictorial data, analyses were also carried out to measure theasymmetry of N scattering.
The large asymmetries for þ hyperon scattering shownin Fig. 2(b) indicated that the LS eﬀect is large in contrast to
the case of  hyperons [Fig. 2(d)] where the asymmetry was

1586

Jpn. J. Appl. Phys., Vol. 43, No. 4A (2004)

J. ASAI et al.

1587

Fig. 1. SCITIC for E452A.6,7) (a) Schematic illustration of SCITIC. (b) Typical pictures of hyperon scattering seen by two sets of
SCITIC arranged as shown in (c).

Fig. 2. Summary of the ﬁrst-phase experiment E452A.6,7) Polarization of hyperons produced through the (þ ; Kþ ) reaction, and left/right asymmetries of the polarized hyperon elastic scattering on protons: (a), (b) for þ and (c), (d) for . Data on the þ polarizationin (a) are from refs. 8 and 9.

consistent with zero.
Although the data in Fig. 2 were suﬃcient to conclude thediﬀerence between the þ N and N interactions, theexperiment was continued as E452B and C to obtain 10times better statistics with a more sophisticated experimental
setup.

3.

Experimental Setup

The experimental setup for the hyperon scattering isshown in Fig. 3(b). The description of each component isgiven in the table attached to the ﬁgure.
The setup used in the previous experiment E28910) for the1588

Jpn. J. Appl. Phys., Vol. 43, No. 4A (2004)

J. ASAI et al.

Fig. 3. Experimental setup for E452: (a) Active target/detector conﬁguration with three SCITICs seen downstream of the beam, and(b) Beam and Kaon spectrometers.

measurement of þ N crossections was modiﬁed. The main
modiﬁcatications were; (1) change of the active target fromSCIFI to SCITIC, (2) change of acceptance angle of the kaon
spectrometer (to Lab
K ¼ 20 {40 ) to obtain the large hyperon polarization, and (3) change of the trigger scheme.
The polarized hyperons were produced through thepðþ ; Kþ Þþ reaction. Pion beams of 1.6 GeV/c from theK2 beam line of KEK-PS were guided onto the active target
SCITIC made of liquid scintillator. Kaons and othersecondary particles from the reaction were detected using
the kaon spectrometer.

The (þ ,Kþ ) events were selected with trigger signals
indicating kaon production from the kaon spectrometer. The
coverage of kaon angle was selected to be Lab
K ¼ 20{30 deg, because large polarization of the þ in this angularrange had been reported in previous experiments8,9) [see Fig.
2(a)].
The trigger signals consisted of two levels. The ﬁrst-level
trigger was for identiﬁcation of charge and momentum of
beam particles, and for brief selection of positive kaons insecondary particles. A matrix coincidence of CH and FTOF
hodoscopes was used to determine brief momentum andJpn. J. Appl. Phys., Vol. 43, No. 4A (2004)

J. ASAI et al.

Standard Trigger

1589

New Trigger

E452A, B

E452C

K
P

π
K

π
Mass

P

Mass

Fig. 4. Mass spectra of trigger particles to select kaon-emitting events. The proton-emitting events were greatly suppressed by the new
trigger scheme for E452C. The hatched zone indicates the selected events through the oﬀ-line analysis.

charge of the secondary particle from the active target. The
trigger logic was: ‘‘1st-level trigger’’ = ‘‘beam’’‘‘Kþ ’’,
where ‘‘beam’’ = T1T2BGCBAC and ‘‘Kþ ’’ =BV1  BV2  FACYH(CH-FTOFmatrix).
In addition, the second-level trigger (called ‘‘mass
trigger’’) was applied to reduce ‘‘proton associated events’’by adding the time-of-ﬂight information between T2 andFTOF to the momentum information by the CH-FTOF
matrix coincidence.
The scheme mentioned above was the standard trigger
used in E452A and B. Although the selection of ‘‘Kþ
associated events’’ could be carried out almost perfectly byfurther oﬀ-line analysis, more eﬃcient selection in the ﬁrstlevel trigger was required to speed up the data-taking rate. In
the ﬁnal phase of the experiment, E452C, a new trigger
scheme was introduced to improve the data-taking rate.
While the combination of CH and FTOF counters was used
for brief determination of the momentum of particles in thestandard trigger scheme for E452A and B, in the new trigger
scheme for E452C, the resolution of momentum measurement was improved by adding the drift chamber (DC0)
information to the CH and FTOF matrix coincidence. Thenthe /K/p separation in the ﬁrst-level trigger was greatly
improved. The ratio of ‘‘proton associated events’’ leaked
through the trigger was reduced to approximately 30% by
the new trigger scheme from 70% in the standard trigger (seeFig. 4). Hence the data-taking rate of relevant events was
doubled by the new trigger scheme. The data-taking rates arecompared in the summary table of E452A, B and C (Table I).
4.

Active Target/Detector: SCITIC

The active target/detector system shown in Fig. 3(a) wasused for E452B and C. The target was a liquid scintillator
BC-517S which is made of 60% C6 H3 (CH)3 , 30% Cn H2nþ2

Table I. Summary of data taking.

Data-taking rate
Total eventsþ p candidates

E452A

E452B

2.5

1.5

E452C
9

600,000

3,000,000

8,000,000

Triggered events

500

2,200

6,500

Candidate events

Kþ events/spill

and a wavelength shifter. The light output is 66% of theanthracene. This scintillator was chosen to maximize the H/
C atomic ratio which was 1.7, since it was essential for thepresent experiment to maximize the hyperon production andscattering with free protons. The H/C atomic ratio is about 1
in a conventional plastic scintillator.
The SCITIC used for the ﬁrst stage of the experiment
E452A consisted of a conventional camera lens (Canon
F ¼ ð f =DÞ ¼ 1:2, f ¼ 85 mm) installed on an IIT-CCD
cascade. The cascade consisted of IIT1,2,3,4, relay lensand CCD: (Fig. 1(a)). In order to obtain a large acceptancefor photons, an electrostatic-type IIT (Hamamatsu
V4440PX) with 10 cm diameter was used for the ﬁrst stageof the IIT cascade while the diameter of the second to fourthstage IIT was 2.5 cm. The ﬁrst and second level trigger
signals from the beam- and kaon-spectrometers were applied
to the third and fourth IIT which were microchannel-platetype. The images on the ﬂuorescent output screen of the laststage IIT were transported via the relay lens to the CCD
camera (Kodak ES310) for digital recording. Two sets of
SCITIC were used as shown in Fig. 1(c), and a threedimensional pattern was reproduced from the pictures fromthe top and the 45-deg direction in the oﬀ-line analyses.
For the E452B and C, however, three sets of new SCITIC
with new lenses were installed [Fig. 3(a)]. The lens withlarger aperture (F ¼ ð f =DÞ ¼ 0:8, f ¼ 125 mm) was specially designed to obtain a larger area of view. The lenssystem was installed on the IIT-CCD cascade with the same
conﬁguration as that for E452A described above. The
magniﬁcation of the optical lens system was set to 0.5 sothat tracks in a disk of 20 cm diameter on the target werefocused on the ﬁrst photocathode of 10 cm diameter of the
IIT-CCD cascade. Hence, the area of view, which was 10 cm
diameter in the E452A, was expanded to 20 cm diameter.
The larger diameter of active area helped not only toincrease the event rate but also to observe the scattering anddecay vertexes more clearly.
Typical SCITIC pictures are shown in Fig. 5. Clear
pictures of þ and  production and þ p scattering wereobtained. In the ﬁgure, however, background noise spots arealso seen. The whole SCITIC and target system was installed
in a dark room to form a perfect shield against enternal light.

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Jpn. J. Appl. Phys., Vol. 43, No. 4A (2004)

Fig. 5. Typical pictures of hyperon production and scattering seen by the SCITIC.

Then, the source of background photons must be thereﬂection of the scintillation light from the target scintillator.
Indeed the background spots increased with the brightness ofthe track pattern. All components inside the dark room were
black painted or covered with black sheet or tape. The target
chamber fabricated of aluminum was coated with black
aluminum oxide to suppress reﬂection inside the target
chamber. However, it was not possible to shield thereﬂection of scintillation light on the inner surface of theglass window. Hence the windows were arranged so that thereﬂected light would not enter other SCITICs in a single step
reﬂection. The background noise spots were reduced
markedly after those eﬀorts, but the reduction was not
perfect.
A number of corrections and calibrations were carried out
on the pictorial data before performing analyses. The beam
direction in the SCITIC pictures was calibrated by a straightbeam without a magnetic ﬁeld. Final checks of position
calibration of spectrometer chambers were carried out withbeam or secondary particle tracks.
Since the SCITIC is installed so close to the large magnet[KURAMA: Fig. 3(b)], the SCITIC pictures were swept
away due to the stray ﬁeld of the magnet. The IIT-CCD
cascade was covered by a cylinder of heavy magnetic shieldof 20-mm-thick pure iron and 1-mm-thick mu-metal tosuppress the eﬀects of the magnetic ﬁeld. Between the endguard of magnet and SCITIC, additional iron plates were
mounted by trial and error. Finally, the eﬀect of themagnetic ﬁeld was suppressed suﬃciently to observe the
SCITIC pattern, but still the pictures were distorted. The
distortion was corrected empirically by observing a matrixof light spot which was made artiﬁcially by placing anelectro luminescence plate at the focus position. The plate
was covered with a black sheet on which tiny holes of
0.5 mm diameter were formed at every 5 mm distance in X
and Y directions. The picture of the light spot matrix underthe magnetic ﬁeld was used for the correction.
5.

Analysis of the SCITIC data

Nearly seven million Kþ -triggered events were accumulated in E452B and C. The data included both þ and 
production events as well as background events. Then, thegoal of the oﬀ-line analysis is to select events of þ and 
production and scattering on free proton in the liquid

Table II. A number of steps for the oﬀ-line data analysis.
(1) The total events recorded by the ‘‘Kþ -trigger’’.

7,000,000(2) (,k) missing mass, momentum, and vertex cuts

700,000

to select þ and  production events.
(3) Eye scanning to identify and classify events:

37,000‘‘þ production’’ events,

9,600‘‘þ scattering’’ events,

2,400‘‘’’ events.
(4) Pointing to recognize tracks.

13,50027,000(5) 3D reconstruction to reproduce track pattern.

27,000(6) Kinematical selection to select scattering
events on free proton.

111

scintillator. The oﬀ-line data analysis was carried out
through a number of steps summarized in Table II.
The number at the end of each line is the number of events
selected. By the process of hardware cuts for (2), one tenthof the total pictures (700,000 events) were selected for eye
scanning.
No attempt was made to develop automatic scanning of
the pictorial data because the quality of the SCITIC data was
not suﬃcient. Instead, for computer-assisted scanning andpointing, tools were provided such as to show the possible
direction of the hyperon track calculated from the on-line
information on momentum vectors of beam pion andsecondary kaon [Fig. 6(a)]. A tool to obtain a least squares
ﬁt for a train of light spots between two points was useful forthe pointing job [Fig. 6(b)].
Since three pictures were taken for each event with threesets of SCITIC, the data are redundant to perform the 3D
reconstruction. Pictures from two SCITICs out of three are
suﬃcient to obtain the 3D information. The redundancy
could be used in two diﬀerent ways.
First, since the production process is coplanar in thehorizontal (; K) plane, and the polarization axis is perpendicular to the plane, the picture-0 taken by SCITIC-0 fromthe top of the (; K) reaction plane must have a greater
weightage than others. A 3D-pattern from pictures by
SCITIC-0 and SCITIC-1 was compared with that by
SCITIC-0 and SCITIC-2, and the better combination withsmaller 2 was chosen.
The second method is to use all three combinations

Jpn. J. Appl. Phys., Vol. 43, No. 4A (2004)

J. ASAI et al.

(a)

1591

(b)
K

K

Σ

π
Fig. 6. Typical example of þ production pictures with predicted tracks by on-line data from the beam- and kaon-spectrometers.
(b) Least squares ﬁt to a Kþ track drawn from points given by the scanner/pointer.

Fig. 7. The left/right asymmetries determined from E452B and C are plotted against the production kaon angles which correspond tothe momentum of þ . The present data from E452B&C conﬁrmed the previous data from E28910) and E452A6,7) shown in Fig. 2(b).

SCITIC-0 and SCITIC-1 (0-1), SCITIC-1 and 2 (1-2) and
SCITIC-2 and 0 (2-0) with equal weightage. Averages ofevery three tracks were used as the ‘‘best ﬁt’’ track pattern.
Since the second method has no access to human judgment,
it is ideal when the pictorial data was clean and only
statistics were the source of error. However, since thepresent pictures suﬀer other uncertainties such as the
background noise spots, the eﬀect of defocusing etc., theanalysis of tracks always required on observer’s judgment.
Because of this, the ﬁrst method was chosen after a numberof trials.
After the selection and the 3D-reconstruction of the(þ ; Kþ ) events, the kinematical selection was applied to
select the þ p scattering events on free proton (hydrogenatom). Since the liquid scintillator target contains carbon andoxygen atoms besides the hydrogen atom, further analyses

were carried out to select events in which the hyperon was
produced and scattered with free protons. The following
kinematical cuts were applied for the selection of such
events;1. Coplanarity cut in the þ production stage. (Coplanarity of þ -beam, Kþ and þ tracks)
2. Coplanarity cut in the þ scattering stage. (Coplanarityof þ , recoil proton and scattered þ tracks)
3. Cut on deviation of þ track from the predicteddirection by the spectrometer.
This selection was so severe that the ﬁnal number of eventsused for physical analyses was about 100.
6.

Experimental Results

As mentioned in §2, we have previously reported on theasymmetry measurement of polarized þ p and p elastic

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scattering.6,7) The report was on the experiment E452A inwhich we observed large asymmetries in the polarized þ pscattering in contrast to the p case. We concluded that theresult indicated a large þ p spin-orbit interaction in contrast
to the case of the small N as predicted theoretically.4,5)
Although it was suﬃcient to make a qualitative comparison, the accuracy of the E452A data was rather limited. Inorder to obtain more accurate information with better
statistics, the experiment was continued as E452B and C
with the new SCITIC conﬁguration and the new trigger
scheme. Tenfold more data have been accumulated, and theanalysis is going on. Figure 7 shows the latest result of theanalysis. The left/right asymmetries from E452B&C withthreefold more statistics are plotted together with those from
E452A and E289. The data conﬁrmed the previous result.6,7)
The SCITIC was used for a hyperon scattering experiment, and the promising feature of the new tracking detector
has been shown. It was like an experiment with triggerable
bubble chamber. The other attractive possibility is to use the
SCITIC as a triggerable nuclear emulsion to take a picture of
tracks of a few 100-mm with a larger magniﬁcation such as afactor of 100 or even more.

J. ASAI et al.

Acknowledgment
We are indebted to the KEK-PS operation crew and theexperimental supporting group for their continuous eﬀorts toprovide excellent experimental conditions. The present work
was partly supported by the Tokyo University of Science
Presidential Fund for Promotion of Research in Speciﬁed
Field, and the Grants-in-Aid for Scientiﬁc Research from theJapan Ministry of Education, Culture, Sports, Science andTechnology.

1) A. Sato, J. Asai, M. Ieiri, S. Iwata, T. Kadowaki, M. Kurosawa, T.
Nagae and K. Nakai: Jpn. J. Appl. Phys. 43 (2004) 1593.
2) J. K. Ahn et al.: Nucl. Instr. Methods A 457 (2001) 137.
3) W. Bruckner et al.: Phys. Lett. B 79 (1978) 157.
4) H. J. Pirner: Phys. Lett. B 85 (1979) 190.
5) H. J. Pirner and B. Povh: Phys. Lett. B 114 (1982) 308.
6) T. Kadowaki et al.: Eur. Phys. J. A 15 (2002) No. 3.
7) K. Nakai: Nucl. Phys. A 721 (2003) 673c.
8) D. J. Candlin et al.: Nucl. Phys. B 226 (1983) 1.
9) J. Haba, T. Homma, H. Kawai, M. Kobayashi, K. Miyake, T. S.
Nakamura, N. Sasao and Y. Sugimoto: Nucl. Phys. B 299 (1988) 627.
10) Y. Kondo et al.: Nucl. Phys. A 676 (2000) 371.

Japanese Journal of Applied Physics
Vol. 43, No. 4A, 2004, pp. 1593–1601#2004 The Japan Society of Applied Physics

Scintillating Track Image Camera-SCITIC
Akira S ATO, Jyunkichi A SAI, Masaharu I EIRI1 , Soma I WATA, Tetsuhito K ADOWAKI,
Maki K UROSAWA, Tomohumi N AGAE1 and Kozi N AKAI
Physics Department, Tokyo University of Science, Noda-shi, Chiba 278-8501, Japan
1
High Energy Accelerator Research Organization (KEK), Tsukuba-shi, Ibaraki 305-0801, Japan(Received October 21, 2003; accepted January 6, 2004; published April 9, 2004)

A new type of track detector, scintillating track image camera (SCITIC) has been developed. Scintillating track images ofparticles in a scintillator are focused by an optical lens system on a photocathode on image intesiﬁer tube (IIT). The image
signals are ampliﬁed by an IIT-cascade and stored by a CCD camera. The performance of the detector has been tested withcosmic-ray muons and with pion- and proton-beams from the KEK 12-GeV proton synchrotron. Data of the test experiments
have shown promising features of SCITIC as a triggerable track detector with a variety of possibilities.
[DOI: 10.1143/JJAP.43.1593]
KEYWORDS: scintillation track image, IIT-CCD Camera, active scintillation target, hyperon-nucleon scattering

1.

Introduction

In the history of nuclear and particle physics, tracking
detectors such as the cloud chamber, the bubble chamber andthe nuclear emulsion have played important roles in the
discovery of new particles and new phenomena. Althoughthese detectors have been replaced by the modern large-scale
multi-element detectors, the direct observations of tracks
with relatively simple detectors are still indispensable. We
are developing a new type of tracking detector, named
scintillating track image camera (SCITIC). The recent
progress of the image intensiﬁre tube (IIT) and charge
coupled device (CCD) camera technology has provided us
with a variety of possibilities for observing images ofparticle tracks passing through a scintillator. The new
tracking detector is attractive because of its triggerable
data-acquisition function.
The possibility of observing tracks in a NaI scintillator
was shown in 1961 by Perl1,2) using image intensiﬁers. The
track detector was named ‘‘Luminescent Chamber’’. This
concept was reconsidered thirty years later, when the INS
group3) reported the observation of cosmic muon tracks in abulk scintillator. We have further tested the possibilities andlimitations of using the SCITIC for practical experimentswith cosmic muons and beam particles from the KEK 12GeV proton synchrotron.
One of the motivating factors in our developmental eﬀorts
was to use this track detector for studies of hyperon-nucleon
interactions by taking image data of hyperon scatterings
which were produced in the detector through (þ , Kþ )reactions.4) So far, the scintillation ﬁber detector (SCIFIdetector) has been successfully used for the hyperon
experiments at KEK.5) The present development, however,will open up new possibilities for the use of various
scintillating materials under various conditions. We can use
a liquid scintillator to obtain a H/C atomic ratio nearly
twofold that of plastic scintillators for the N scatteringexperiment so as to increase the ratio of free processes toquasifree processes. For future experiments, we are interested in polarizing protons in the scintillator active target tomeasure the eﬀects of the antisymmetric LS interaction inthe N system. While testing our new ideas, we studied avariety of possibilities with the SCITIC depending upon thedesign of the optical system.
In this paper, we discuss general aspects of the SCITIC,
and more speciﬁc details of a SCITIC used for the Nscattering experiment are discussed in a separate paper.6)2.

Principle; based on the Thin-lens Concept

The composition of the SCITIC is illustrated in Fig. 1. It is
composed of an active target of a scintillator, an optical lenssystem to focus the track image on the photocathode of an
IIT, a three- or four-stage IIT for signal ampliﬁcation, and a
CCD camera connected to a fast data-acquisition system.
The particle track is recorded as a train of pixel clusters
corresponding to the photoelectrons.
In the following subsections, the principle of the method
is discussed based on a picture on the basis of the thin-lensconcept. It provides a good guide line for designing
experiments using the SCITIC. In practice, however, moreelaborate considerations are required for comparision withexperimental data regarding the performance.
2.1 Photon yield
When a charged particle passes through a scintillationmaterial, it deposits energy to excite electronic states along
its path, and eventually, those states emit photons in the deexcitation process. The number of emitted photons is
proportional to the energy loss of the charged particle anddepends upon the scintillation material. Average energies
required for a single photon production by an electron are
known to be 60, 25, 100 (eV/photon) for anthracene,
NaI(Tl) and plastic scintillators, respectively. The photon
yield of anthracene is often used as a standard for the light
output, and compared with those of other scintillators.
Examples of data for typical scintillators are listed in Table I.
2.2 Photon-collection eﬃciency
The number of photons per unit length observed in the
SCITIC picture depends upon the energy loss of the particle,
the scintillation eﬃciency, the solid angle of the optical lenssystem, the quantum eﬃciency of the IIT photocathode andthe eﬃciency of the IIT-CCD camera including the readout
system.
The photon-collection eﬃciency L is given as,E-mail address: nakai@post.kek.jp
1593

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A. SATO et al.

Charged Particleabf
Scintillator

Optical Lens

IIT & CCD camera

Fig. 1. Composition of the scintillating track image camera (SCITIC).

Table I. Examples of scintillator data.
Density

Light output

Wavelength (g/cm3 ) (% of Anthr.)

peak (nm)Anthracene

1.25

100

447

0.8

1.62

NaI(Tl)

3.67

230

413

—

1.78

CsI(Tl)

4.51

95

580

—

1.79

Plastic Sci.

1.03

68

406

1.10

1.58

0.85

66

423

1.70

1.49

Scintillator

Refractive

H/C ratio

index nNE104
Liquid Sci.
BC-517S

L¼

dE
Q Rdx

R (%)

ðclusters/cmÞ:ð1Þ

Since a photon is observed as a ‘cluster’ of pixels of the
CCD the unit of L is given as (clusters/cm) instead of(photons/cm). Each factor in the equation for L is given as
follows based on the thin-lens concept.
dE
Energy deposition rate of particle indx (MeV/cm)
unit length
dE
3 dE
2
dx ¼  (g/cm ) d (MeV/(g/cm ))3
 (g/cm ): Density of the scintillator (photons/MeV)
Number of photons per unit energy
deposition in the scintillator ¼ ð1="Anth. ðeVÞÞ  r¼ ð1=60Þ  r ðphotons/eVÞ  106"Anth. : Average energy for single photon production in anthracener (%): Light output relative to anthraceneGeometrical eﬃciency of the lenssystem (solid angle/4) ¼ ðDeff =2Þ2 =4a2
Deff ¼ D=n (cm): Eﬀective diameterof the lens (note; Fig. 2)
D ¼ f =F (cm): Diameter of the lensn: refractive index, f : focal length, F:
F number of the lensa (cm): Distance between object andlens (Fig. 2)
Q (%)
Quantum eﬃciency of the ﬁrst IIT:
(photoel./photon)

Eﬃciency to detect a photon as acluster in the IIT-CCD camera
picture: (clusters/photoel.)With these parameters, the photon-collection eﬃciency L
is given as,

2
Deff



 


dE
106
r
Q
R
2
ð2Þ
L¼




d
60 1004a2
100100ðclusters/cmÞ:The geometrical eﬃciency  ¼ ðDeff =2Þ2 =4a2 is determined through design of the optical lens system which
depends upon the experimental requirements. By using theeﬀective diameter Deff ¼ D=n, the eﬀect of refraction of thelight emitted in the scintillator of index n on the solid angle
was included. There remains, however, a correction to aCharged Particle

Deff

D

Scintillator
Optical Lens

Fig. 2. The eﬀect of refraction of the light emitted in a scintillator ofindex n on the solid angle can be included by using the eﬀective diameter
Deff ¼ D=n.

Jpn. J. Appl. Phys., Vol. 43, No. 4A (2004)

A. S ATO et al.

The value for  can be chosen to be comparable to theresolution of the IIT-CCD camera. The focusing
depth spﬃﬃﬃ
becomes shorter inversely proportional to L. This meansthat the requirement for larger focusing depth contradicts
that for larger photon yield. The focusing depth of the lenssystem, therefore, has to be determined by compromising thephoton-collection eﬃciency.

Source Point
Designed Source Point

D

∆

x

a

b

Scintillator

Optical Lens

IIT photocathodeFig. 3. Focusing depth s  2x.

which was small and thus neglected. The photon collection
eﬃciency is inversely proportional to the square of the F
number (F ¼ f =D) and also depends upon the magniﬁcation
factor M ¼ ðb=aÞ:
 2
D
M2
L/
¼
:
ð3Þ
a
ð1 þ MÞ2 F 2
2.3 Focusing depth
Another important factor to be considered in designing thelens system is the focusing depth. Although a larger solid
angle  is preferable for the collection of photons, itsacriﬁces the depth of focusing.
When a light source deviates by x from a designed source
point, the image of a point source on the IIT photocathode(focal plane) has a ﬁnite size  given as (see Fig. 3),
¼

xM 2
:
ð1 þ MÞF

ð4Þ

The focusing depth s  2x at the scintillator is deﬁned when
the tolerable size for  was determined;
s

IIT-2

IIT-3

IIT-4

ð5Þ

Relay Lens CCD

φ100

IIT-1

2ð1 þ MÞF2
¼ pﬃﬃﬃ :M2
M L

640 mmFig. 4. Composition of the IIT-CCD camera.

2.4 IIT-CCD camera
The composition of the IIT-CCD camera used for thehyperon-scattering experiment is shown in Fig. 4 andTable II. The scintillation track image focused on thephotocathode of IIT was ampliﬁed by a 3- or 4-stage IIT andstored by the CCD camera.
The electrostatic-type IIT is used for the ﬁrst stage tocollect as many photoelectrons as possible. Photons from thescintillator are converted to photoelectrons with the quantum
eﬃciency Q (%), which is typically about 20%. In this typeof IIT, electrons emitted from the photocathode areaccelerated before striking against a ﬂuorescence screen to
amplify the number of photons. which are converted again toelectrons on the photocathode of the second IIT. The photon
numbers are further multiplied in the second IIT which isalso of the electrostatic type. A single photoelectron
generates about 10 photons in the ﬁrst IIT. After themultiplication of photon number in the ﬁrst-stage IIT, thequantum eﬃciencies of the second-, third- and fourth-stage
IITs minimally aﬀect the photon-collection eﬃciency. A
number of factors, however, could reduce the photoncollection eﬃciency in the stages from the 2nd IIT to CCD
and in the readout electronics. The overall eﬃciency of the
IIT-CCD camera system after the photocathode of the ﬁrst
IIT is represented as R (%).
One of the important factors which determines the R is
ﬂuorescence decay time of the IITs. Although the decay timeof the ﬂuorescence material is required to be short for a highrate operation mode, it should not be so short as to losephotons. One must choose the material carefully compromising the two requirements. We chose the ﬂuorescencematerials with decay times of 1.3 ms for the 1st IIT and 0.3 ms
for the 2nd. The decay time of 1.3 ms was chosen to hold thescintillating track-image to tolerate a delay of the fast trigger
which is about 0.5 ms. The third and fourth IITs are themultichannel-plate (MCP) type. The signals are multiplied
further, but a more important function for them is triggering
or gating. In the hyperon-scattering experiments, we usedthe ﬁrst and second trigger scheme. The camera was
triggered ﬁrst by a beam-particle condition and then by aparticle identiﬁcation signal after a logical judgement on-

Table II. Elements of the IIT-CCD camera.
Element

Model (Maker)

IIT
IIT-1
IIT-2

VP4440PX (HAMAMATSU)
PP0030X (Delft)

1595

Characteristics
Type

Window

Q

Phospher

StaticStatic

100 mm
25 mm

19%
14%

PS-5;  ¼ 1:3 ms
P46;  ¼ 0:3 ms

IIT-3

BV2563MG (PROXITRONIC)

MCP

25 mm

10%

P20:  ¼ 20 ms

IIT-4

BV2563MG (PROXITRONIC)

MCP

25 mm

10%

P20:  ¼ 20 ms

Relay lensH1212B (PENTAX)

1/2’’CCTV camera lens

CCD

ES310 (Kodak)

640  480 pixelJpn. J. Appl. Phys., Vol. 43, No. 4A (2004)

A. SATO et al.

3.

Performance Test of the SCITIC

3.1 Test with the cosmic-ray muonsShown in Fig. 5 is a setup for the test of SCITIC withcosmic-ray muons. The SCITIC was triggered by a muon
signal generated by the triple coincidence of signals from thetwo scintillaters located above and below the SCITIC
scintillator and its own light output. The scintillating track
image was focused by a camera lens (NIKON: Nikkor lenswith f ¼ 105 mm and F2.5) on the photocathode of a statictype IIT of 80-cm diameter. The image data was ampliﬁedby two MCP-type IITs in cascade, and stored by a CCD
camera on a video tape.
A CsI(Tl) of 60  60  10 mm3 size or a plastic scintillator of the same size was used as the SCITIC scintillator. All
the facets of the scintillator except for the one facing the lens
were covered by black tape for ensuing antireﬂection.
Distances from the scintillator to the lens (a) and the lens to
IIT photocathode (b) were both 21 cm so that magniﬁcation
determined by the ratio (b=a) was unity.
Examples of the track image data are shown in Fig. 6. The
muon tracks are shown as a train of pixel clusters whose
density is determined by the photon-collection eﬃciency.
Compared to the plastic scintillator, the cluster density is

incident particle
PMT
IIT

Lens

Scintillator

CCD

PMT
a

b

PMT

HV

Discri.
Discri.
Discri.

Gate gene.

PCI-1408
TTL
PC

Fig. 5. Setup for the test with cosmic-ray muons.

Muon

Muon
cm

line in less than 14 ms. The readout gate width was typically
100 ms.
The picture of the track image which appeared on the
ﬂuorescence plate at the fourth IIT exit is then transmitted to
the CCD camera through a relay lens. We used the CCDwith 640  480 pixels of 9 mm square area. The data size ofeach picture could become over 300 kBytes.
A conventional CCD and readout unit based on the
NTSC-system was used for taking the cosmic-ray muon
pictures. The data-taking rate was, however, limited to lessthan 10 pictures/s. Since this speed is not suﬃcient for highrate operations in accelerator experiments, we developed a
new data-taking system using the Kodak CCD ES310 with aprogressive scanning system. The data from the fast CCD
are compressed with a real-time picture editing module
MVC150/40 by Imaging Technology Co. With the new
system, a three or four times faster data rate was achieved.
The details will be published separately.

cm

1596

5

5

4

4

3

3

2

2

1

1

0

0

4

2

(a)

6 cm

0
0

2

4

(b)

6 cm

Fig. 6. Cosmic-ray muon tracks in (a) CsI(Tl) and (b) plastic scintillators.

higher with the CsI(Tl) because of its high density as well as
its high luminosity.
The clusters outside the track are because of backgroundnoise due to the reﬂection on the scintillator surface. The
signal-to-noise ratios were not satisfactory, and more eﬀorts
were made for improvement in subsequent experiments.
3.2

Three-dimensional data obtained with the use of
mirrors
We have tested a unique method to obtain a threedimensional view of a track with the use of mirrors. Figure7(a) shows the arrangement of mirrors surrounding ascintillator cube to obtain (b) pictures of a track from ﬁve
diﬀerent viewpoints. With the setup shown in (c), thecosmic-ray muon data was obtained as shown in Fig. 7(d).
A CsI(Tl) scintillator of 50  50  50 cm3 size wasviewed by a lens system with the magniﬁcation M ¼ ð1=3Þ
(a ¼ 420 mm, b ¼ 140 mm). The photon-collection eﬃciency determined from ﬁve pictures with diﬀerent views agreed
with the calculations. The method using mirrors is very
eﬃcient because the three-dimensional picture can be takenwith a single SCITIC. However, it has disadvantages, too.
Since the antireﬂection treatments are diﬃcult in this case,
we see more noise spots in Fig. 7(c) due to reﬂections on theﬁve surfaces. We also noted a diﬃculty in obtaining a large
focusing depth. Although this arrangement using ﬁve mirrors
is limited, a single-mirror setup was used successfully as
shown in Fig. 11.
3.3 Test with pion and proton beams from the KEK-PS
The SCITIC was also tested with pion and proton beams
from the KEK-PS, the 12-GeV proton synchrotron. Figure 8
shows the experimental setup. The secondary beam from aninternal target in the KEK-PS was delivered by a test beam
channel T1. The momentum range of the beam was from
0.45 GeV/c to 1.65 GeV/c, and the beam-particles were
mostly pions or protons. The time of ﬂight of the beam
particle was measured with counters T1 and T2 to identify
pions or protons. The CsI(Tl) or the plastic scintillator was
mounted at the beam focusing point inside a dark box inwhich the SCITIC was set. Small-sized scintillators S3 andS4 were used for the beam deﬁning.
The track-image data obtained using the CsI(Tl) and theplastic scintillators are shown in Figs. 9(a) and 9(b),respectively. The four pictures for each scintillator are
tracks of:

Jpn. J. Appl. Phys., Vol. 43, No. 4A (2004)

A. S ATO et al.

1597

Setup

Fig. 7. (a) Arrangement of mirrors surrounding a scintillator cube to obtain (b) pictures of a track from 5 diﬀerent viewpoints. (d) A
typical example of cosmic-ray muon data from 5 diﬀerent viewpoints taken with the setup shown in (c).

Concrete Block
Dark Box

Beam Slit

Optical Lens
T2

D3

T1

(a)
5cm

Beam

GC

S1

S3

1.0GeV/c beam ( π )1.0GeV/c beam ( p )Shieldbeam

IIT

ShieldS4
GeV/c beam ( p )1.65GeV/c beam ( p or π )1.0GeV/c beam ( p )1.0GeV/c beam ( π )Scintillator (Target)

Fig. 8. Experimental setup to test the SCITIC with pion and proton beamfrom the KEK-PS.

3.4 Particle identiﬁcation
The particle-identiﬁcation capability using the linear
density of clusters along the track was tested. The punchthrough tracks of 1 GeV/c proton- and pion-beams such as
those in Figs. 9(a) and 9(b) were analyzed. While in the caseof CsI(Tl), the cluster density distributions of the proton andpion tracks were barely separated, in the case of the plastic
scintillator the separation was poor. The distributions areshown in Fig. 10, and reproduced with Poisson functions asshown in the ﬁgure by solid curves. Average values for thecluster densities determined from the Poisson functions are

beam

5cm

. Upper-left: a proton punched through the scintillator.
. Upper-right: a pion punched through the scintillator.
. Lower-left: a proton stopped in the scintillator.
. Lower-right: a nuclear reaction by a pion.

(b)

0.45GeV/c beam ( p )1.65GeV/c beam ( p or π )Fig. 9. Typical track-images in (a) CsI(Tl) and (b) plastic scintillators.

1598

Jpn. J. Appl. Phys., Vol. 43, No. 4A (2004)

A. SATO et al.

(b)countscounts

(a)

π

π

p

p

clusters

clusters

Fig. 10. Distributions of the cluster numbers per 5 cm along particle tracks in (a) CsI(Tl) and (b) plastic scintillators.

Table III. Cluster densities of pion and proton tracks determined from
Fig. 10.
CsI(Tl)Plastic

data

calc.

data

calc.



7:3  1:7

(6.7)

2:8  1:4

(2.2)

p

9:6  1:8

(10.0)

3:7  2:0

(3.2)

(cluster/cm)

given in Table III, and compared with calculated values. The
clusters were counted per 5 cm from edge to edge of thescintillators. Since the separating capability is simply
determined by statistical ﬂuctuation of the cluster number,separation must become possible by improving the lightcollection eﬃciency and/or increasing the track length.

3.5 Resolution
The resolution of track reconstruction is governed bythree factors; statistical ﬂuctuations, the optical focusing size
and the instrumental resolution of the IIT-CCD camera. The
last one is determined by the pixel number density of the
CCD.
With the particle beams from the KEK-PS, the track
resolution of the SCITIC was tested. The proton or pion
beams of 1.0 GeV/c were incident on a plastic scintillator
perpendicular to the optical axis of SCITIC at diﬀerent
focusing depths x. A single mirror arrangement shown in
Fig. 11(b) was used to obtain the data in Y–Z and X–Z
planes [Fig. 11(a)]. The depth of beam position x wasdetermined from the mirror image of the X–Z plane. For
each data at diﬀerent x, the most probable line of the track
and the standard deviations  of clusters from the track were
determined. Plotted in Fig. 11(c) against the depth of beam

Fig. 11. (a) An example of a beam particle track observed by the single-mirror setup shown in (b). (c) The deviation  vs the depth x of
the beam track.

Jpn. J. Appl. Phys., Vol. 43, No. 4A (2004)

A. S ATO et al.

1599(a) Plastic scintillator without coating.
Signal S =1.0
Noise N1= 0.79
N2=1.55

S

N1
N2(b) Plastic scintillator coated with black tape.
Signal S =1.0
Noise N1= 0.79
N2=1.0S

N1
N2(c) Liquid scintillator in a black alumite container
Signal S =1.0
Noise N1= 0.37
N2=0.35

S

N1

N2

Fig. 12. Distribution of distances of clusters from the particle track. Solid curves are best ﬁts which consist of three Gaussiancomponents. The curves S are the particle track, and others are noise due to reﬂection.

position are the deviations .
We deﬁned the standard deviation  as the resolution for
tracking. As mentioned above, the observed  is composedof two factors, opt. and IIT-CCD , and is given as,22þ IIT-CCD
 2 ¼ opt.

ð6Þ

The dashed curve in Fig. 11(a) was calculated with
2
opt.
¼

jxjDeff1
pﬃﬃﬃﬃﬃx þ ð1 þ 1=MÞf 12

ð7Þ

where D and M are the eﬀective diameter of the lens and themagniﬁcation respectively, as deﬁned in §2.2. A point
source located at x from the focusing position causes thedefocusing to from a distributed light spot with a mean
diameter 2opt. at the focal plane which is the photocathodeof the ﬁrst IIT. The instrumental resolution IIT-CCD isdetermined by the size of CCD elements which is 9 mm
square. This number has to be converted to the resolution at
the photocathode of the ﬁrst IIT with a multiplication factor
through the four IITs to obtain, IIT-CCD ¼ 300 mm.

3.6 Noise ﬁgure
Background photons were observed in almost all pictures.
Clearly those were reduced by light shielding and antireﬂection eﬀorts carried our with extreme care. We noted,
however, that the background noise level is large when abright particle track or many tracks appeared. This suggested
that the background was due to reﬂections of real scintillation light by the scintillator surface or the window. An
example of distribution of deviations of clusters from thebest ﬁt of a track is shown in Fig. 12. Shown in the ﬁgure are
ﬁts with three Gaussian functions to decompose the signal
and the noise. There seem to exist two noise components.
The noise component could be reduced by coating the wallswith a black tape, sheet, or paint [Fig. 12(a)]. The black
alumite coating was found to be eﬀective and convenient foran aluminum container of the liquid scintillator [Fig. 12(b)].
In addition, antireﬂection coating on a glass window facing
to the lens was eﬀective.

1600

Jpn. J. Appl. Phys., Vol. 43, No. 4A (2004)

A. SATO et al.

3.8 Examples of the hyperon track-image data
Typical examples of the SCITIC data are shown in Fig. 14.
Data are tracks of (a) 0 production, (b) þ production and(c) þ scattering. Those hyperons were produced throughthe (þ , K þ ) reaction in the SCITIC scintillator with pion
beams at the K2 beam-line of the KEK 12 GeV proton
synchrotron. Data were taken with trigger signals from aspectrometer indicating the Kþ production. The SCITIC
comprised a setup for magniﬁcation M ¼ 1=2 using a
CANON F1.2 lens ( f ¼ 85 mm), and the IIT-CCD camera
in Fig. 4 was used.

(a)

(b)
Cluster density (relative)

4.

Distance from center of view [mm]Fig. 13. (a) Track image of cosmic-ray muon. (b) Comparison of thecluster-density distribution with calculated results.

3.7 Viewing ﬁeld
The viewing ﬁeld is also one of the most importantcharacteristics of a tracking detector. We have measured thesize of the viewing ﬁeld experimentally, for comparision
with calculations. Figure 13(a) is a picture of a cosmic-ray
muon track observed with an optical system using a lens of
F1.0 ( f ¼ 50 mm), 25 cm from the scintillator. The viewing
ﬁeld was studied with the use of an electroluminescenceplate (EL-plate) which emits single photons in a twodimensional plane. Data in Fig. 13(b) shows cluster
distribution along a line, and the solid curve indicates thecalculated result. The thin-lens approximation, however,does not work and we used thick-lens parameters.

[pixel](a)

Summary of the Test and Future Possibilities

The test of the SCITIC has successfully indicated its
promising features as well as its limitations. The contradiction between requirements for acceptance and for focusing depth is a severe limitation for applications. It is,however, solvable by a careful design, depending upon theexperimental purpose and conditions.
The resolution is a subject of design. Since statisticalﬂuctuation of the cluster distribution is a governing factor of
the resolution, a large acceptance of the lens is preferable but
the focusing depth becomes small which is another factor.
It was shown that the particle identiﬁcation would bepossible when a suﬃcient number of clusters along the track
were collected since the capability is determined simply by
the statistical ﬂuctuation. A large acceptance and large
viewing ﬁeld to obtain long particle tracks would make it
useful.
The viewing ﬁeld was limited in the test experiments
applying conventional camera lenses even though we used
the best available lens with F1.0. The camera lenses were not
the best choice because those were designed to focus images
on a ﬁlm with a small size. We have designed a large
aperture lens with F0.8 (150 mm) for the hyperon scatteringexperiment whose performance will be reported separately
when completed. Since the order-made lens is 10 times more
expensive, the test experiment has been carried out withcamera lenses. Although camera lenses were designed toassure ﬂexibility, it had to be given up for the order-madelens.
There are a variety of possible applications which are
subjects for further investigations.

(c)

(b)

450

450

400

400

400

350

350

350

450

300

300

300

250

250

250

200

200

200

150

150

150

100

100

100

50

50

50

0

0
0

50

100 150 200

250 300 350 400 450[pixel]0050

100 150 200

250 300 350 400 450

050

100 150 200

250 300 350 400 450[pixel][pixel]( 0.5 mm / pixel )Fig. 14. Track image data of (a) 0 production, (b) þ production and (c) þ scattering.

Jpn. J. Appl. Phys., Vol. 43, No. 4A (2004)

One of the merits of the SCITIC as an active target is thatany scintillation material can be used under any environmental condition. As mentioned previously, if we were able
to polarize protons in the scintillator, we would be able tostudy the antisymmetric LS-interaction from observations ofpolarized hyperon scattering by a polarized proton. This was
the original motivation for the SCITIC development.
So far, the hyperon-scattering experiment was the goal ofour development in which the optical system was optimized
with the magniﬁcation M ¼ 1=3{1. The SCITIC was used
like a ‘triggerable bubble chamber’. It would be possible touse the SCITIC with M ¼ 10{100 to take microscopic
pictures such as of emulsion data. Then the SCITIC would
act like a ‘triggerable emulsion chamber’. Decay patterns ofD mesons, B mesons, charmed hadrons,  and neutrino could
be observed under a trigger condition. Contrarily, a large
object could be observed by setting M ¼ 1=10{1=100. Thereis a plan to observe a pion or a proton from a neutrino
reaction in the ﬁrst detector of the K2K neutrino long
baseline experiment.7)

A. S ATO et al.

1601

Acknowledgment
We are deeply indebted to Professors K. Nakamura, Y.
Yoshimura and the KEK-PS operation crew for their support
in the present experiment. We thank Professors S. Yamada,K. Imai and K. Niwa for their stimulation and continuous
encouragement. The present work was partly supported by
the SUT Presidential Fund for Promotion of a Speciﬁc Fieldin the Graduate Course, and Grants-in-Aid for ScientiﬁcResearch from the Japan Ministry of Education, Culture,
Sports, Science and Technology.
1) M. L. Perl: SLAC-PUB-7049 (1996).
2) K. W. Lai, L. W. Jones and M. L. Perl: Proc. 1960 Int. Conf.
Instrumentation in High Energy Physics (Interscience, New York,1961) p. 186.
3) M. Hazumi, T. Hasegawa, S. Kasai, K. Tokushuku and S. Yamada:
Nucl. Instr. & Methods A 317 (1992) 515.
4) K. Nakai: Nucl. Phys. A 721 (2003) 673c.
5) J. K. Ahn et al.: Nucl. Instr. & Methods A 457 (2001) 137.
6) J. Asai et al.: Jpn. J. Appl. Phys. 43 (2004) 1586.
7) T. Kobayashi: private communication.

E462/E508 H. Outa

Exclusive Measurement of the Non-Mesonic Weak Decay of 5ΛHe / Coincidence
Measurement of the Weak Decay of 12ΛC
S. Okada et al.
[attached]
Neutron and Proton Energy Spectra from the Non-Mesonic Weak Decays of 5ΛHe and 12ΛC
Submitted to Physics Letter B.
● B.H. Kang et al.
[attached]
Coincidence Measurement of the Non-Mesonic Weak Decay from 5ΛHe
Submitted to Physical Review Letters.
★ S. Okada
Non-mesonic Weak Decays of 5ΛHe and 12ΛC Hypernuclei in (π+, K+) Reactions
Tokyo Institute of Technology, 2004.
◇ S. Kameoka et al.
Measurement of the π- Decay Width of 5ΛHe
HYP2003, Newport News, Virginia, 14-18 Oct 2003, nucl-ex/0402023 : to appear in Nucl. Phys.A.
◇ S. Okada et al.
π0 Decay Branching Ratios of 5ΛHe and 12ΛC Hypernuclei
HYP2003, Newport News, Virginia, 14-18 Oct 2003, nucl-ex/0402022 : to appear in Nucl. Phys.A.
◇ T. Maruta et al.
Proton Asymmetry in Non-mesonic Weak Decay of Light Hypernuclei
HYP2003, Newport News, Virginia, 14-18 Oct 2003, nucl-ex/0402017 : to appear in Nucl. Phys.A.
◇ H.C. Bhang et al.
Hypernuclear Weak Decay Experiments at KEK: Nucleon Inclusive Measurements
HYP2003, Newport News, Virginia, 14-18 Oct 2003, to appear in Nucl. Phys. A.
◇ H. Outa et al.
Hypernuclear Weak Decay Experiments at KEK: n-n and n-p Coincidence Measurements
HYP2003, Newport News, Virginia, 14-18 Oct 2003, to appear in Nucl. Phys. A.
◇ H. Outa for KEK-PS E167/E462 collaboration
Weak Decay of Light s-shell Hypernuclei 4ΛH, 4ΛHe and 5ΛHeInternational Symposium on Electrophoto-production of Strangeness on Nucleons and Nuclei (Sendai03)
World Scientific, in press.
◇ S. Okada et al.
Neutron Energy Spectra from Non-Mesonic Weak Decay of 5ΛHe and 12ΛC Hypernuclei
International Symposium on Electrophoto-production of Strangeness on Nucleons and Nuclei (Sendai03)
World Scientific, in press.
◇ S. Kameoka for KEK-PS E462/E508 collaboration
Measurement of the π- Decay Width of 5ΛHeInternational Symposium on Electrophoto-production of Strangeness on Nucleons and Nuclei (Sendai03)
World Scientific, in press.
◇ B.H. Kang for KEK-PS E462/E508 collaboration
The n+p and n+n Coincidence Measurement of the Non-Mesonic Weak Decay of 5ΛHeInternational Symposium on Electrophoto-production of Strangeness on Nucleons and Nuclei (Sendai03)
World Scientific, in press.
◇ T. Maruta for KEK-PS E462/E508 collaboration
Asymmetry in Non-Mesonic Weak Decay of Light Hypernuclei
International Symposium on Electrophoto-production of Strangeness on Nucleons and Nuclei (Sendai03)
World Scientific, in press.
◇ H. Yim for KEK-PS E462/E508 collaboration
Measurement of π0 Mesonic Decay Widths(Γπ0) of Λ Hypernuclei, 5ΛHe and 12ΛCInternational Symposium on Electrophoto-production of Strangeness on Nucleons and Nuclei (Sendai03)
World Scientific, in press.
●________________________________________________________________________● Physics papers published in refereed journal.
○★
◇＊

Technical papers.
PhD theses.
Conference and Symposium.
Internal Report and others.

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  %     * $%  & $$    %&  #  ! & #&%      &&  &, /01 2.  
3%$ * ( &  $ %&     
"- +   %  + % (   )
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$% %& %&     %   #)
 %  %     %&   & (&( $    %   &   5 95  $ ::&  %    $  (&  *    ;    * 3$    </=>?

  3 %    *  $ % ) %& ,7 .  % "- % $ <6? % &&  % % &        % %  * + $ &%   # % %  *          *  "-    *  +   +$ $ ,5 5 .  % #  &  %$% #  3$ & (&)    %& %&% &  $% &&  $  3% &,!.  ( (  && 5 5 )  * 88@)878 #%  *    % $
   * $(   3$  $    %&     & <@ 1?   $$  %$)
%  #  *  68    A$ ::&
* 5 5  B    & %  %& # !  *   %& %&% &   "- ( # #   # 3%
, . &   % (     3% CD/97  &  $ <0=E?  %&  (# $ (&$     %& %&% &)   %   F%& %  (&  *     % G  %  & %&% &  $ %
   *   $  808 </8?   3 ((  3% & %&% &  *    &  %&  $  3% & *  
& &  #&  $ %  % (&  *   %   8@0 <E //? H & %&  &&&&  %&% &  $ %  $ % * $) (   )  % "-  & & 
&&  3$ &  &  %   *  &  # ##& % #    % *   ) %& $ (&$  & ( #  * 3$ &       * 3$      % * + $ %&   %%  )   * $ ,>80.    $% ,>1E.
* "- *  ;  $ </> /6?   )!>1E 3$ </6?   $% *  % *  ;      % $(  )
 % #  788 %    &  #%     $(     5 5    ( % )$     $%  %&   * $
,* >80.  8@/8/@,.     F 3$ )&  &        F%&  &&        I% *  F&  )%  ,.    & 3 %%&& #   %% $  (   && %    %      I%   $ #& &  * 7
 % "- $%   $(  3$ & %% %  
% #(#& * "-   $ F    &  $ %&  &      I%      %&  &  %
  * % % %& 3%&  (&     # )        * 3$& %   F%  *+   +  %&    #  )%& ,$  $ .  % % % %  * "* + $ %&   %  %  *    %&* "-   #  % 3$%   
 %& ( & #%))#% %&    
$  &        J)(&  /@>2    &  %  *   7 $)% # G    %  %     &9
%&      F%  * % % %)& % #    %  %  #   "  %&  %    % &  %    #%)   I% *  F&  %     %&  % "- I% (& 3$   %     1 # & *  /7)K2 !   /8@ K29%  # (  L ,  . %    *  %)     %% &      /8 &       $% %   $% ,.</@?  % %  $ %&  $%)  3$ &  $ * % % %   * % $ % $ %& *   % *      /  %   % % % %  &%  $  # *   $ : *#%))#% % % %       &$&%      % %%$% % % )% % %   %  * *   %  ,7.
 * % # ,!-;. (  $   %  ,>.

Neutron Counter (T4)

Veto Counter (T3)

T2
T1

m
+
π bea

Target+ SKS
K to

Drift camber (PDC)

Y

40 cm

Z

X

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 787'9* 8: %   %  ,6.  %%$% *  * % *    $ M  M    )  7118NO  /888>O #  ;&  )&   %  &  #  >8 2 *$    Æ% %    %&% & #  ;&  &    F -  </1? %
 %  $$& %#&   & )&    %
  #   $ % (   3$ & </6?
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  >  -;  * G  && F % )  $$3 &   $$ &  
& , . !  $   %&& )
 F   $  %     $ $%
 &  /O  $  &    )  #  66 2 & $ %&  Q         %     >  ( %   & $ %&  F%       $%)  * /    %   & (   *
&& @6   %        $ $  Q*  ,  $R. %   $ %& &   *  % #   *  &   *   $ &%  / D /  / &   *  $ *    867  "  % %$   &   * // 2, . * 0@ 2  
- & *  % % % %    % # 6(a) InclusiveLi(π+,K+)Λ6 Li

10000

g.s gate

YNN(cosθ)0
20 (b) np coincidence
100
15 (c) nn coincidence
10
5
0
150

30

(a) Tn+Tp

4

10

2

0

0

100

200

300

180

0

100

200

300

190

200

210

MeV
20

(d) nn-pair

40
10
20
0
0.2

0
0.2
a

a

0.1

0.1

-0.5

0

0.5

1

cosθnp1700(c) np-pair

0
-1

160

(b) Tn+Tn

6

20

MeV

NNN(cosθ)

counts / MeV

counts / 10MeV

20000

0
-1

-0.5

0

0.5

1

cosθnn220
5-6%     
 #  0,      %

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 %&  (  L  $%      7 ,.   7,#.  7,%.   $%  % % %   %    $  $% (&  %    *  L  #(   T  &    
$ $& %          $ <@? ( %& &    $$&   *   "(  #(   $  % % % (     %  & *  $  % % %
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E471 M. Iwasaki
Search for Strongly Bound Kaonic Nuclear State
M. Iwasaki et al.
Evidence for a Strongly Bound Kaonic System K-ppn in the 4He(Stopped K-, n) Reaction.
e-Print Archive: nucl-ex/0310018.
○ M. Iwasaki et al.
Feasibility Study on Deeply Bound Kaonic State
Nucl. Instr. Meth. A473 (2001) 286-301.
◇ K. Itahashi, M. Iwasaki, T. Suzuki
Experimental search for kaonic nuclear bound states in the 4He(stopped K-, n) reaction
Prog. Theor. Phys. Suppl. 149 (2003) 233-239.
●________________________________________________________________________● Physics papers published in refereed journal.
○★
◇＊

Technical papers.
PhD theses.
Conference and Symposium.
Internal Report and others.

[attached][attached]arXiv:nucl-ex/0310018 v1 17 Oct 2003

Evidence for a strongly bound kaonic system
K− ppn in the 4He(stopped K− ,n) reaction

M. Iwasaki a,b, T. Suzuki c, H. Bhang d, G. Franklin e,
K. Gomikawa c, R.S. Hayano c, T. Hayashi b,∗, K. Ishikawa b,
S. Ishimoto f , K. Itahashi a, T. Katayama b, Y. Kondo b,
Y. Matsuda a, T. Nakamura b, S. Okada b, H. Outa f,∗∗,B. Quinn e, M. Sato b, M. Shindo c, H. So d, T. Sugimoto b,
P. Strasser a, K. Suzuki c,∗ ∗ ∗ , S. Suzuki f , D. Tomono b,
A.M. Vinodkumar b, E. Widmann c, T. Yamazaki a,T. Yoneyama ba RIKEN Wako Institue, RIKEN (The Institute of Physical and Chemical

Research), Wako-shi, Saitama, 351-0198, Japanb Department of Physics, Tokyo Institute of Technology, Ookayama, Meguro-ku,

Tokyo 152-8551, Japanc Department of Physics, University of Tokyo, Hongo, Bunko-ku, Tokyo 113-0033,

Japan
d Department of Physics, Seoul National University, Shikkim-dong, Kwanak-gu,

151-742, Korea
e Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213, USA
f IPNS, KEK (High Energy Acceralator Research Organization), Ooho,

Tsukuba-shi, Ibaraki 305-0801, Japan

Abstract
We measured a neutron energy spectrum by means of time-of-flight (TOF) from the
4 He(K −stopped , n) reaction by stopping negative kaons in a super-fluid helium target.
A distinct peak was observed in the neutron spectrum, indicating the formationof a strongly bound kaonic system, K − ppn, with the binding energy and width,
BKppn = 173 ± 4 MeV and ΓKppn . 25 MeV, respectively.
Key words:
PACS: 11.30.Rd 13.75.Jz 21.10.Dr 25.75.Nq 26.60.+cPreprint submitted to Elsevier Science

17 October 2003

Since the so-called “kaonic hydrogen puzzle” was solved by the kaonic hydrogen (KpX) experiment at KEK [1], the KN interaction was established to bestrongly attractive in the isospin (I) = 0 channel, so that Λ(1405) can beinterpreted as a bound state between K − and proton. Recently, Akaishi and
Yamazaki performed a coupled-channel calculation to empirically determine
the KN interaction strengths and their range using the kaonic hydrogen and
the KN scattering data, as well as the energy and width of Λ(1405). With
their KN interaction parameters, they predicted that strongly bound kaonicstates with narrow widths can be formed in light nuclei [2]. Especially, the0
T = 0 K − ppn (or K pnn) system is predicted to be a strongly bound statewith a total binding energy BK = 115 MeV (measured from K − + p + p + n;
M = 3309.8 MeV) and a width ΓK = ΓN + ΓN N = 20 + 12 = 32 MeV,
where ΓN and ΓN N are for the one-nucleon (πΛNN) and two-nucleon (ΛNNor ΣNN) decay channels respectively.
According to their prediction the K− -p interaction is so strong as to shrink thenuclei and to form extraordinarily dense systems with a central nucleon densityof ∼5 ρ0 . A further study using the anti-symmetrized molecular dynamics
method (AMD) by Dote et al. predicted even higher central density values of8∼10 ρ0 [3]. If this is the case, and such bound states are found, we may be able
to study dense and cold nuclear systems in which chiral symmetry is expected
to be restored, and quarks may be deconfined [4]. Hence, experimental studies
of such states are of great importance, and may provide rich information tounderstand the dynamics of the neutron and/or strange star [5].
−In the present experiment we adopted the (Kstopped, n) reaction on 4 He. If the−K ppn state is formed, a mono-energetic “Auger” neutron should be produced
via theK − +4 He → K − ppn + n(1)

reaction. The mass of K − ppn system can be determined from the kinetic
energy of the emitted neutron [6].
The experiment was carried out at the KEK 12 GeV proton synchrotron. We
stopped negative kaons in a super-fluid helium target, and applied the TOF
method to measure the energy of the neutron, by using plastic scintillation
counter (NC) matrices of about 2 m3 , located 2 m from the helium target(Fig. 1). We adopted a target cell of 1.87 g/cm2 thick with window thicknessof 75 µm, operated at 20 torr. The target cell was covered by a 0.2 mm-thickaluminum heat shield and a 0.9 mm-thick CFRP vacuum jacket. The highthermal conductivity of super-fluid helium allowed us to keep major cryogenic∗ Present address: Dept. of Legal Med., Osaka Univ.
∗∗Present address: Wako Institue, RIKEN∗ ∗Present∗
address: GSI
Email address: muon@riken.go.jp (M. Iwasaki).

2yTC topz
VDC top+π-

He target

BDC
K

NC
2m apartfrom center

B1
LC

VDC veto

DegraderVDC bottom

beam π vetographite

TC bottom

B2 and T00

Beam veto

50 cmFig. 1. A schematic cross-sectional view of the setup, showing Lucite Čerenkov
counters LC, a beam counters B1, B2 and T0, beam drift chamber BDC, a helium
target, neutron counters NC (placed 2 m behind the target), drift chambers VDCup
and VDCdown and two layers of TC counters.

elements away from the target cell.
The incoming kaon was registered by beam line counters (B1, B2 and T0)
and tracked by high-rate drift chambers (BDC). As shown in Fig. 1, we also
required that a charged particle is emitted from the target region, which we
tracked by large-area drift chambers (VDC) placed above and below the helium target. Behind the VDCs, we placed two slabs of plastic counters (TC’s)of the thickness of 6 and 30 mm. With these counters, we could distinguish
π ± ’s from protons, and could also roughly measure their momenta.
The reaction (or neutron-production) point was determined by the vertex
between the trajectory of an incoming kaon and that of an outgoing charged
particle. The resolution of the reaction points/vertex was ∼5 mm (σ), which
is predominantly determined by kaon multiple scattering in the target helium.
The kaon reaction at-rest in the target volume was selected (purity > 95%) byusing the correlation between the pulse height of T0 and the vertex position(the range in the helium target). The reaction (neutron TOF start) timing
was calculated from the T0 timing and the kaon range in the target. Absolute
time calibration of NC was performed by electromagnetic showers producedin iron-plate converters, placed between NC veto counters and NC itself. The
overall TOF resolution of ∆(1/β) = 0.04(σ) was achieved.
3In addition to the “Auger” neutron, many other processes such as hyperon
decays, kaon two-nucleon absorption processes, etc., contribute to the neutron
spectrum. Let us first demonstrate that we can obtain a rather simple spectrum by selecting slow (. 100 MeV/c) pions in coincidence with a neutron(see Fig. 1).
Fig. 2 (top) shows the obtained neutron momentum spectrum. The biggest
structure exists from ∼200 to ∼400 MeV/c. This comes mainly from theπ − two-nucleon absorption reaction (occurring in or around of the target)
π − NN → NN (denoted as nπN N ) [6]. The lower momentum cut-off at ∼200 MeV/c is due to the energy threshold of NC (threshold =10 MeV electron equivalent (MeVee)). The spike at ∼ 100 MeV/c is due to accidental
background.
The spectrum has a “plateau” structure beyond ∼500 MeV/c. This comesfrom the kaon two-nucleon absorption and its chain;
K − NN → ΛnΛNand Λ → π − p,(2)where nΛN is the neutron from two-nucleon kaon absorption reaction. Notethat pions from Σ decays do not contribute when we select low-momentum
pions [7]. We can also exclude contributions from other reaction channels suchas hyperon quasi-free (QF: K − N → πY ), because there is no energy left toproduce a neutron in this momentum region.
The spectrum shape of nΛN is dominantly determined by the three-body phasespace and the nuclear form factor, and is very broad as shown in the figure.
It has the maximum momentum of ∼700 MeV/c, and forms a flat-“plateau”
below ∼600 MeV/c.
The lower momentum background in “slow-pi” is due to a QF-Σ production
followed by a Σ − Λ conversion inside the nucleus;
K − N → Σ ± π ∓ , Σ ± N → ΛnΣΛ and Λ → π 0 nΛD ,(3)where nΣΛ and nΛD respectively denote the neutron from Σ − Λ conversion
and that from the Λ decay.
Thus, the high-momentum neutrons observed in coincidence with slow pionsin Fig. 2 are due to the reaction (2). Then, there must be a clear back-to-backcorrelation between the neutron and the Λ directions. In order to quantify
this, we introduce a vector-at-closest-approach (vCA ), which is the vector fromthe kaon trajectory to that of the charged pion at the minimum distance. In
addition, we define a scalar product vCA ·v̂n , where v̂n is the normalized vector
of detected neutron motion (see Fig. 3).
The back-to-back correlation should be reflected in the negative value of vCA ·4

800“slow-pi” triggered event

counts (per 10 MeV/c)

accidental

pπ~< 100 MeV/c

600

nπNN
400

nΛN

.

| vCA v n |negative200

tails fromn ΣΛ, nΛD.

~0
| vCA v n | ~0300

forward and backward
comparison
backward (scale: right)
forward (scale: left)

counts (per 10 MeV/c)

300

200

.

5 < | vCA v n | < 60 mm
pπ~< 100 MeV/c

200

nΛN

100

.

| vCA v n |negative

0100

n ΣΛ, nΛD.

~0
| vCA v n | ~

0

0

200

400

600

800

neutron momentum (MeV/c)

Fig. 2. The neutron momentum spectrum measured in coincidence with a low
momentum pion pπ . 100 MeV/c (top). The “backward” events (filled circles) (−60mm < vCA · v̂n < −5mm) and “forward” events (open circles)(5mm < vCA · v̂n < 60mm ) are selected and compared (bottom). The roughshape of the spectrum is shown in curve. The curve is to indicate rough spectrum
shape, which is obtained simple spline fit and displaced vertically.

v̂n (backward). As shown in Fig. 2 (bottom), this is indeed the case. The“plateau” appears only in the backward spectrum as expected. We have thus
clearly understood the “slow-pi” event set.
What then happens, if we set “fast-pi” event window to select Σ decay (1355

VDCtop tracking region

Charged Veto

Charged Vetoy

NC L-armxvCA v^ n

n ΛN

Λ

p

Residualrejected byveto

NC R-armπ-vertex resolution

Charged Veto

VDCbott. tracking region

Charged Veto

Charged Veto

VDCtop tracking region

Charged Veto

y

x

nΛD

Residual

v^ n

Λ

π0
Charged Veto

v^ n

nΣΛ

NC R-arm

NC L-arm~ 0)π-+ ( vCA . v n ~π- (vCA . v n ~~ symmetric))

VDCbott. tracking region

Charged Veto

Fig. 3. Top: the definition of vCA . As an example, a KN N → ΛN event topology
is shown. A kaon comes from behind the paper, and a pion is tracked by the VDC.
A vector from the kaon track to the pion track at their closest approach is vCA .
When we require a high momentum neutron hit in the NC, then the vCA should bea good approximation of the hyperon motion, and thus vCA · v̂n gives the hyperon
momentum vector and its distance relative to the neutron momentum vector. A
scalar product vCA ·v̂n should be negative due to the topology. Bottom: An exampleof the event topology of reaction (3) is shown. The event is mainly triggered by thepion from the QF-Σ reaction (|vCA · v̂n | ≈ 0), while a chance to be triggered by thepion from the Λ decay (vCA · v̂n ∼ symmetric) should be smaller.

. pπ . 200 MeV/c)? Fig. 4 (top) shows the obtained neutron momentum
spectrum without vCA · v̂n selection. As shown, the higher momentum part
looks more complicated.
Naturally, the dominant process is the kaon two-nucleon absorption followedby the hyperon decay:
K − NN → Σ ± nΣN (or p) and Σ ± → π ± nΣD ,(4)where nΣN is the neutron at the primary reaction and nΣD is that after Σ ±decay. This is the event depicted in Fig.5. The nΣN should have a nature very6

counts (per 10 MeV/c)

1250

accidental“fast-pi” triggered event135 ~< pπ <~ 250 MeV/c

1000

750

nΣN / n ΣDshape issymmetric to

500.

| vCA v n |nΛNtails from250n ΣΛ, nΛD.

~0
| vCA v n | ~0150

forward and backward
comparison
counts (per 10 MeV/c)backward (scale: right)

100

forward (scale: left).

5 < | vCA vn | < 10 mm

150

135 <
~ pπ <~ 250 MeV/c

50

100
0
50

tails fromn ΣΛ, nΛD.

~0
| vCA v n | ~
0
0

200

400

600

800

neutron momentum (MeV/c)

Fig. 4. Top: The neutron momentum spectrum measured in coincidence with ahigher-momentum pion (135 . pπ . 250 MeV/c). Bottom: “backward” events(filled circles) (−10mm < vCA · v̂n < −5mm) and “forward” events (open circles)( 5mm < vCA · v̂n < 10mm ) are selected and compared.

similar to the nΛN in reaction (2), but shifted by ∼100 MeV/c to the lower
side. Therefore, a “plateau” at ∼500 MeV/c is expected, and vCA · v̂n shouldbe negative. The shape of nΣD should be nearly identical to that of nΣN , since
the momenta of Σ± and nΣN are almost equal and we detect decay pions in the90◦ direction, which constrains the momentum change to be small. However,its vCA · v̂n should be positive.
7

Charged Veto

VDCtop tracking region

Charged Veto

y

x

nΣD

v^ n

vCA v^ n+n ΣNΣResidual

Charged Veto

VDCbott. tracking region

NC R-arm

NC L-armπ-+Charged Veto

Fig. 5. An example of the event topology of the reaction 3 is shown. A scalar
product vCA · v̂n is negative if nΣN is detected by the neutron counter, while it ispositive if nΣD is detected.

Thus, if we were to apply the same “forward-backward” decomposition to theneutron spectrum measured in coincidence with fast pions, we expect nΣD toappear in the “forward” spectrum, nΣN to appear in the “backward” spectrum, both having the same shape. The results are shown in Fig. 4 (bottom).
Here we applied a relatively narrow window of 5 < |vCA · vn | < 10 mm. Thisis because the Σ dominantly decays in short distance so that the maximum
asymmetry should be observed in a short range. The comparison of the two
spectra reveals a peak-like enhancement at ∼ 470 MeV/c in the “backward”
spectrum. We assumed that the background shape of the “backward” spectrum can be inferred from that of the “forward” spectrum (as discussed above),and fitted the latter with a smooth function (shown by the grey curve). We
then overlaid the same curve on top of the “backward” spectrum (also shownby the grey curve), which fairly well represents the data, except for the ∼ 470
MeV/c region.
In order to test its statistical significance, we fitted the “peak” region using
a Gaussian function. Although the actual functional form of the peak should
be quite involved, we adopted the Gaussian form because the statistics of thepeak structure does not stand for sophisticated fitting function. The excess
is evaluated to be 3.6 σ, and the binding energy and the width are obtained
to be BKppn = 173 ± 4 MeV (measured from K − ppn = 3309.8 MeV) andΓKppn < 25 MeV, obtained by observed width of Γobs = 14 ± 4 MeV and theexperimental resolution of 6 MeV (σ @ B = 170 MeV).
Lets discuss what we can expect if the peak at ∼470 MeV/c is the signal for abound state. If the intermediate state K − ppn is really formed with such a largebinding energy, then it should be boosted strongly opposite to the “Auger”
neutron. It should then decay dominantly to ΣNN rather than ΛNN, since weobserve the enhancement only in Fig. 4. The calculated momentum of decay8Σ ± is distributed around 450 MeV/c in the backward direction and around
80 MeV/c in forward. Thus, vCA · v̂n for such events should be dominantlyin the negative side. If there is a strongly bound K − ppn state which decays
dominantly to ΣNN, we expect that: i) the signal is enhanced by the vCA ·v̂n selection, and ii) the enhancement is in the negative vCA · v̂n side. The
enhancement observed in the “backward” spectrum of Fig. 4 does satisfy thesecriteria, and is statistically significant (3.6σ). It is not an artifact of reduced
statistics, introduced by the rather tight cut on vCA · v̂n , since the structureis already visible in the high-statistics “fast-pi” spectrum before applying thevCA · v̂n cut.
In summary, we have found an evidence for a strongly bound kaonic system
K− ppn, whose major decay mode is K − ppn → ΣNN.
The present finding shows the presence of strongly bound systems with K−
and provides important information on the KN interaction, nuclear compressibility, high-density nuclear structure and chiral symmetry restoration, which
are more or less unknown as of today.
We are grateful to Y. Akaishi and R. Seki for useful discussion. We owe muchfor M. Iwai, K. Taniguchi, O. Sasaki, N. Kimura, M. Sekimoto and all themember of KEK-PS for their substantial cooperation. We thank IHI for collaborative development for CFRP vacuum jacket. This work is supported by
MEXT, KEK, Riken, KOSEF, NSF, DOE and KRF.

References[1] M. Iwasaki et al., Phys. Rev. Lett., 78 (1997) 3067.
T.M. Ito et al., Phys. Rev. C58 (1998) 2366.
[2] Y. Akaishi and T. Yamazaki, Phys. Rev., C65 (2002) 044005.
[3] A. Dote, Y. Akaishi and T. Yamazaki, Prog. Theor. Phys. Suppl., 146 (2003)
508.
[4] T. Hatsuda and T. Kunihiro, Prog. Theor. Phys., 74 (1985) 765.
[5] B. Freedman, L.D. McLerran, Phys. Rev., D17 (1978) 1109; ; G.E. Brown etal.. Nucl. Phys. A567, 937 (1994)..
[6] M. Iwasaki et al., Nucl. Instrum. Meth., A473 (2001) 286.
[7] H. Outa et al., Prog. Theor. Phys. Suppl., 117 (1994) 177.

9

Nuclear Instruments and Methods in Physics Research A 473 (2001) 286–301

4

He(stopped K, n) spectroscopy for deeply boundkaonic nuclear states

M. Iwasakia,*, K. Itahashia, A. Miyajimaa, H. Outab, Y. Akaishib, T. YamazakicaDepartment of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japanb
High Energy Accelerator Research Organization (KEK), Ibaraki 305-0801, Japanc
The Institute of Physical and Chemical Research (RIKEN), Saitama 351-0198, Japan
Received 10 October 2000; received in revised form 12 January 2001; accepted 10 February 2001

Abstract
A feasibility study has been made on a new type of nuclear spectroscopy}namely the (stopped K , n) reaction, to% bound states. Particular attention was focussed on the process 4 He(stopped K , n) to form asearch for nuclear K%
deeply bound K state in 3 He, which is predicted to be a narrow discrete state with a binding energy of 108 MeV and awidth of 20 MeV. A monoenergetic neutron peak resulting from this reaction is expected to be seen on a continuous
background arising from the successive reactions and decays produced in K absorption by 4 He. # 2001 Elsevier
Science B.V. All rights reserved.
PACS: 13.75.Jz; 25.80.Nv; 26.60.þc; 14.40.Aq

1. Introduction‘‘How are the hadron masses modiﬁed in the
nuclear medium?’’ is an important but yet
unsolved question in hadron physics today. Oneof the ways to determine the in-medium hadron
masses is to study nuclear bound states of hadrons,
when the scalar part of the strong-interaction
potential can be separated from the vector part.
Very recently, deeply bound p states in 207 Pb
were identiﬁed in the 208 Pb(d,3 He) reaction at GSI[1–4] and a pion mass excess of about 20 MeV in
the nuclear medium has been deduced [2,5].
*Corresponding author. Tel.: +81-35734-2080; fax: +8135734-2742.
E-mail address: iwasaki@nucl.phys.titech.ac.jp(M. Iwasaki).

Similarly a search for Z and o bound states has
been proposed at GSI [6]. The in-medium Kmeson mass is another extremely interesting
subject related to the issue of possible strangenesscondensation [7–9,5], but no experimental information has yet been obtained.
A recent KEK experiment on kaonic hydrogenatoms [10] revealed a ‘‘repulsive-type’’ stronginteraction shift in the atomic ground state, and% –N
thus settled a long-standing ambiguity in the K
interaction. This observed negative shift of the 1s
atomic state of kaonic hydrogen is now in good
agreement with the scattering length analysis of% –N interactions. Very recently, Akaishi andthe K
Yamazaki [11] obtained a set of two-body% N; K% N  Sp andcoupled-channel potentials (K%KN  Lp), which have simultaneously accounted% –N scattering data andfor the K -p data, the K

0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 8 6 2 - 2

M. Iwasaki et al. / Nuclear Instruments and Methods in Physics Research A 473 (2001) 286–301

the energy and width of the L(1405) by assertingthat the L(1405) is an isospin I ¼ 0 bound state of% –N. Furthermore, they have calculated g-maK% –N interactions andtrices for the in-medium K%
obtained K-nucleus potentials. The calculated realand imaginary parts of the optical potential
for describing shallow kaonic atoms (V0atom ¼ 134 MeV and W0atom ¼ 65 MeV) are in good
agreement with the phenomenological parameters
deduced by Batty et al. [12] to ﬁt the existingatomic shift data. However, this optical potential
for the shallow atomic states is not applicable forthe deeper ‘‘nuclear states’’ in which all of the
wave function is located inside the nuclear medium.
Therefore, the interaction depends strongly on theisospin, energy and nuclear density so that the gmatrices should be determined self-consistently.
The given potential is characterized by a verystrong attraction of the I ¼ 0 channel, and thus
% states are accommodated. The
deeply bound K
typical binding energy can be as large as 100 MeV
for several light nuclei. If the state is located below
the Sp threshold, then the I ¼ 0 single-nucleon
absorption channel is energetically closed. Thus,% states are expected to havesuch deeply bound K
narrower decay widths compared with the widthof the Lð1405Þð40 MeV). In a sense, the L(1405) inthe two-body system is ‘‘melted’’ in many-body% nuclear states. This is asystems to form K
hitherto unknown domain of strangeness nuclear
systems.
Particular attention has been paid to the three% 0 3body system, 3K% HT¼0;1 ¼ ðK 3 He þ KT¼0;1HÞ: According to the calculation, the total
isospin T ¼ 1 member of 3K% H is shallow and broad,whereas the T ¼ 0 member has a binding energyof B 3K% HT¼0 ¼ 108 MeV and a width of G 3K% HT¼0 =20 MeV: Such a large binding energy arises
primarily from the strong attraction in the I ¼ 0% –N interaction, which shrinks the distanceK% and the nucleons. Thus the strong
between the K
attraction increases the nuclear density andenhances the binding energy. Because this state is
located below the Sp threshold, the natural width
was simply evaluated by the absorption throughthe I ¼ 1 single-nucleon reaction channel. It
should be noted that the decay to Lp0 d isforbidden by the total isospin selection rule.

287

Therefore the evaluated width of 20 MeV can betreated as an upper limit. Fig. 1 shows thepredicted states of 3K% HT¼0;1 :Whether or not such deeply bound narrowstates exist is really an exciting question. The
predicted states are located around 220 MeV above
the bottom of the S ¼ 1 sector (L-hypernucleus),
yet with a narrow width. The known hadron–nucleus bound states (nucleons, hyperons, pions)are rather shallow; their binding energy scales are
small compared with the rest masses. In the present
case, the predicted potential depth is comparable to
the kaon rest mass and the binding energy is 20%of the kaon rest mass. The predicted binding is far
deeper than in any other hadron–nucleus boundsystems. This raises a question as to how well the
kaon keeps its identity in the nuclear medium. It% -nucleus systemshould also be noted that the K
gives information on the nature of the sub-threshold resonance of L(1405), the eﬀective kaon mass inthe nuclear medium in relation to chiral symmetry
breaking, kaon condensation, existence of stable
multi-strangeness nuclear matter (strangelet) andvery light neutron stars, etc.
How can we populate and identify such a
nuclear bound state? In the present paper we

Fig. 1. Energy level diagram of the K þ3 He system, calculated
by Akaishi and Yamazaki. The energies of relevant decay
channels are also shown.

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M. Iwasaki et al. / Nuclear Instruments and Methods in Physics Research A 473 (2001) 286–301

discuss a new type of nuclear spectroscopy for% states, namely, (stopped K ; n)deeply bound K
reactions. The characteristic features of this% bound state isprocess are as follows: (i) a K
formed in a nucleus with one neutron less than thetarget nucleus (thus, enhancing the attractive I ¼ 0
component), (ii) the K in an atomic stateproceeds to a deeply bound nuclear state, while
emitting a neutron by the nuclear Auger process
and (iii) the emitted neutron serves as a spectatorof the state formed. In the present case, the K –3
He state can be formed in a 4 He(stopped K ; n)reaction and identiﬁed by observing a discrete
neutron component. If the branching ratio for thisprocess is large enough, the (stopped K ; n)reaction may open a new type of hadron–nucleus
spectroscopy. We mention that Kishimoto [13]proposed making use of the in-ﬂight ‘‘knock-out’’%reaction (K ; N) to produce deeply bound K
nuclear states in medium nuclei which are expectedfrom the phenomenological potential [12].
For the case of an absorption reaction on lightnuclei, particularly 4 He, the nuclear Auger process
takes on a very important role in the formation.
This is because of the strong attractive nature of
the K –3 He system and the fact that the centerof mass of 3 He-n system is distant from 3 He. The
K -n single nucleon reaction makes a relatively
small contribution. Therefore, the branch is
expected to be large.
The branching ratio B can be calculated as aratio of the partial and total absorption width of
the atomic states of kaonic helium as
G3 HT¼0
%
B¼ K
:
ð1Þ
Gtot

the experimental resolution. It is also true that thegiven widths are inconsistent with each other.
In contrast, almost all the theoretical widths arearound Gtheo
tot ¼ 2–4 eV [17] without assuming theexistence of a deeply bound kaonic state. Even if
one takes into account its existence, one needs to% N coupling to have a
invoke extremely strong K
20 eV width [18]. Therefore, purely theoretically,
the lowest branching ratio one can have is
G3 HT¼0%
B ¼ Ktheo: > 6%:
ð3Þ
Gtot

Akaishi and Yamazaki estimated G3 HT¼0 ¼ 1:2 eV:

In this section we simulate inclusive neutron
spectra taking into account various backgrounds
as well as the signal.

%
K

On the other hand, Gtot is not well known. There
are three experiments which observed kaonic
helium atomic transitions from 3d to 2p [14–16].
The latter two experiments are known to show
evidence for an anomalous energy shift and widthof the 2p state. If Gtot is replaced by the experimental average of Gexp:
tot ¼ 55  34 eV; we obtain
þ3:5%:
B ¼ 2:20:9ð2ÞThe experimental absorption width could have alarge ambiguity, because it is signiﬁcantly less thanThe deﬁnitive limitation on the upper limit of the
branch, coming from helium bubble chamber data,
gives the non-mesic reaction to be 17  2% of thekaon absorption reaction. It is normally regarded
as a kaon two-nucleon absorption reaction. The
deeply bound kaonic state formation is also a nonmesic reaction, hence the branching ratio cannot
exceed 17%.
The purpose of the present paper, however, is to
examine whether the monoenergetic neutrons
corresponding to the bound kaonic state can berevealed as a peak in the neutron energy spectrum.
Thus, we assume a conservative value ( 2%) as
the branching ratio. From the experimental pointof view, the other theoretical uncertainty is in thenatural width of the 3K% HT¼0 state. If the widthis smaller, it is much easier to identify the peak,
but we use 20 MeV in the present paper.
For convenience, we denote 3K% HT¼0 ; B3 HT¼0 and%K
G3 HT¼0 as 3K% H; BK and GK ; respectively.
%
K

2. Inclusive neutron spectrum

2.1. Monoenergetic neutrons fromHe ðstopped K ; nÞ 3K% H

4In the 4 He(stopped K ; n)3K% H reaction, theQ-value is given by
Q ¼ ðMK þ M4 He Þ  ðM3% H þ Mn ÞK

¼ BK  Bn ¼ BK  21 MeV

ð4ÞM. Iwasaki et al. / Nuclear Instruments and Methods in Physics Research A 473 (2001) 286–301

289

where M3% H represents the mass of the kaonic nuKcleus, Bn is the neutron binding energy in 4 He, and
BK is the kaon binding energy
M3% H ¼ MK þ M3 He  BK :

ð5ÞKThe T ¼ 0 state is formed with a large Q valueof 87 MeV and a width of 20 MeV: We
can determine the binding energy BK fromthe neutron energy, which can be measured by
the time-of-ﬂight (TOF) method. The total energy
En and momentum pn of the neutron in thelaboratory frame in K absorption at rest are
given by

En ¼

ðMK þ M4 He Þ2  M32 H þ Mn2%
K2ðMK þ M4 He Þ

ð6ÞFig. 2. Simulated neutron momentum spectrum and its decomposition. The signal was generated by a Lorentz function with
108 MeV binding energy, 20 MeV in width, assuming abranching ratio of 2%. The detector resolution is not folded in.

qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃððMK þ M4 He Þ2  ðM3% H þ Mn Þ2 ÞððMK þ M4 He Þ2  ðM3% H  Mn Þ2 Þ
K
K
:
pn ¼
2ðMK þ M4 He Þ
The binding energy is, in turn, obtained from theneutron energy and momentum as

ð7Þ

2.2. Background neutrons from the K 4 He
reaction at rest

simulation and experimental data was checkedusing data from 4 He(stopped K ; p Þð4=SÞ He [19]and the helium bubble cham-ber [20]. The comparison is given in Appendix A.
Fig. 3 shows inclusive (a) TOF and (b) binding
energy spectra. Neither detection eﬃciency nor
detector solid angle was considered. As seen fromthe ﬁgure, it is crucial to improve the signal-tonoise ratio dramatically, if the branching ratio is as
low as 2%. Each neutron production is given in
the following sections.

In the 4 He(stopped K ; n) reaction, there aremany background neutrons. To estimate therealistic neutron spectrum, we developed a dedicated simulation program which traces all the
successive reactions starting from the kaon
absorption by 4 He at rest. In the simulation, weused multi-step PWIA calculations based on thephase space volume and the nuclear form factor
for simplicity.
The estimated inclusive neutron momentum
spectrum is shown in Fig. 2. As shown in theﬁgure, the signal of 3K% H is very weak compared tothe background. The consistency between the present

2.2.1. Quasi-free hyperon production(a) Quasi-free hyperon production: This process,K N ! Yp; is the major branch ( 80%), which
produces S’s with Q  250 MeV and Ls with Q 330 MeV: It is known that the primary process
is well reproduced in PWIA three-body pion
production. The Q values are relatively large but
most of the energy is carried out by the pion, andthus the hyperon kinetic energy is not very high(560 MeV). However, these hyperons decay
dominantly into pN: If the pion boosts the neutron
towards the motion of the hyperon, then theBK ¼ M3% H  M3 He  MK

ð8Þ

K

qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ¼ ððM4 He þ MK Þ  En Þ2  p2n
 M3 He  MK :

ð9Þ

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M. Iwasaki et al. / Nuclear Instruments and Methods in Physics Research A 473 (2001) 286–301

neutron energy is boosted into the energy region ofinterest.
(b) SN ! LN conversion: A large fraction( 50%) of the S hyperons successively reactwith the daughter nucleus and are converted to Ls
in the process SN ! LN with Q  210 MeV: In
this reaction, Ls can also be produced at highenergy so that decay neutrons can be in the energy
region of interest.
(c) Pion, hyperon and nucleon cascade reactions:
Helium bubble chamber data shows that a large
fraction of the reactions result in small fragments
which cannot be represented by a single step
reaction and the subsequent S  L conversion. We
invoked a cascade reaction which makes theneutron tail extend even further into the energy
region of interest. The virtual pion absorption
process is treated in a four-body PWIA calculation. The reaction products from these processes
were summed up and denoted as fragment formation in Fig. 2.
2.2.2. Kaon two-nucleon absorption
The dominant component of the non-mesic
process is the two-nucleon absorption reaction;K ‘‘NN’’ ! LN (or SN), which has a large Q
value  320 MeV: Because there is no associatedpion, the kinetic energy of these neutrons is very
high and reaches about 220 MeV; but the nucleon
Fermi-motion and successive nuclear breakup
processes (cascade reactions) slow down thoseneutrons to the energy region of interest. The
measured branching ratio for this process is about
15%. However, it is an open question as to why
this process is such a large branch, even when the
single-nucleon reaction channel (quasi-free hyperon production: K ‘‘N’’ ! Yp) is open. As described in the previous section, the non-mesic
reaction fraction may come from the production
and decay of the deeply bound kaonic states. If
one assumes that a large fraction of the reactions
come from the deeply bound kaonic states, thenthese high-energy neutron components are substantially suppressed.
2.2.3. Hyperon decay
When a hyperon is produced in the processdescribed above, a neutron can be produced fromits decay, S ! np : The following rather trivial
processes are also taken into account:(a)S energy loss in the target;(b)The stopped S can be re-absorbed or form an
atomic metastable state.

2.3.4. Negative-pion absorption
Negative-pion absorption is one of the crucial
backgrounds to the present experiment. As a resultof quasi-free hyperon production and hyperon
decay, large numbers of low energy p areproduced as shown in Appendix A. These canstop quite easily even in a low-density target. The
pions of 50 MeV=c stop in a few cm in the liquid
helium target. Pions at rest react with nuclei in a
two-nucleon absorption process which dominantly
yields two neutrons in the ﬁnal state p NN ! nn:
Unfortunately, the Q value is 140 MeV and theprocess produces neutrons of the energy up to
70 MeV which are exactly in the area of interest.
The yield of these background neutrons, which
depends on the target thickness and the material
around the target, can only be suppressed by
reducing the thickness of the target and vacuum
vessel. The yield given in Fig. 2 is based on theexperimental setup described in Section 4.
2.2.5. Contribution from other small branches
There are small contributions from (i) hypernuclear formation and its decay and (ii) decay of
deeply bound kaonic nuclei. The yields are too
small to show on Fig. 2.
2.3. Beam-originated background sources
There are two other beam-related neutron
sources. One is neutrons scattered along the beamline and the other is neutron production by pions.
The kaon beam is contaminated with a number ofpions which produce neutrons by way of p‘‘NN’’:
Both are forward-peaked along the beam axis, sothey can be safely avoided by placing the neutron
detector at a large angle.
Also g-rays can produce fake neutron signals.
The TOF of a g is 10 ns whereas the neutron signal
starts after 15 ns: Kaons react instantly when they

M. Iwasaki et al. / Nuclear Instruments and Methods in Physics Research A 473 (2001) 286–301

291

Fig. 3. Simulated neutron spectra of (a) TOF and (b) binding energy BK with LTOF ¼ 2:50  0:05 m; folded by a time resolution of
Dt ¼ 0:7 ns: A branching ratio of 2% to the kaonic nuclear state is assumed along with a 100% eﬃcient 4p neutron counter.

stop in the target and the delayed g-ray production
from L decay occurs in  0:26 ns; so that it iseasily discriminated. These signals can be used forthe time zero adjustment.
There is also a small delayed production of grays,  10 ns; due to atomic metastable state formation with a branching ratio  3% per stopped
kaon. The formation probability is small, thus it
does not produce a peak in the TOF spectrum.

3. Neutron spectra and tagging method
The signal is located on the edge of a hugeneutron signal from events which come from SN! LN conversion, hyperon decay and negative
pion absorption as shown in Fig. 2. When the
branching ratio is small ( 2%), the identiﬁcationof a monoenergetic neutron peak becomes extremely diﬃcult. Thus, we studied how to improve
the signal-to-background ratio by event selection.
There are several possibilities. The simplest
background suppression would be to reject hyperon decay by using a kinematic constraint on the
two-body decay. However, we found that there isno signiﬁcant improvement in the signal-to-background ratio ðS=NÞ without drastically reducing
the signal in a simple experimental setup as
described in the present paper. In general, we needto know the decay branch of the 3K% H state to tag
the signal event. The Lp0 d channel is isospin
forbidden so that the possible decay channels are(i) Lp0 pn; (ii) Sþ nn; S0 np and S pp; (iii) Lpn; andpossibly (iv) Ld; excluding unlikely channels suchas L hypernuclear production. Unfortunately,
there is no reliable evaluation of the branchingratio, at present. However, channel (i) is possible
only via a ﬁnal state interaction and the phasespace is very small. Channel (iv) requires acoherent reaction. Therefore, the branch to those
two channels should be small. Channels (ii) and(iii) are two-nucleon absorption processes and, if
we assume that the ratio of the two is similar tothat of kaon two-nucleon absorption in the
primary reaction of K 4 He, then channel (iii) isthe dominant process. Therefore, we can eﬃciently
select the signal by tagging the L in the ﬁnal state.
Moreover, S0 ’s decay instantly to Ls (100%) sothat this component also contributes to the tagging
eﬃciency. To be cautious, we assume the L
production fraction to be 50%. On the other
hand, if we require L decay via L ! pp ; then theneutrons associated with S quasi-free production
and hyperon decays L ! np0 ; S ! np arenaturally omitted.
To tag L ! pp ; one straightforward procedure
is to detect the decay vertex. This is only feasible if
the target is extremely thin, which can only be292

M. Iwasaki et al. / Nuclear Instruments and Methods in Physics Research A 473 (2001) 286–301

possible with low and monoenergetic kaon source
like in a f-factory. In this case, one can use thefact that the path lengths of the L’s are relatively
long. The distance from reaction point to L decay
can be roughly determined by the diﬀerence inclosest approach (DCA ) of the incoming kaon andoutgoing pion trajectories, provided the L moves
perpendicular to the direction toward the pion
tracking detector.
Fig. 4a shows the comparison of the DCA
spectra between inclusive and 3K% H formation
events (left scale). The ratio of the two is also
shown (right scale). Most of the inclusive events
have a small DCA ; which is mainly due to quasifree pion production and decay of the S which has
a short track. On the other hand, the L is longlived resulting in a longer DCA distribution with along tail. In this ﬁgure, multiple scattering of thepion is taken into account based on the experimental setup as described in the following section.
The DCA distribution of signal events is inset inthe ﬁgure. It is clear that one can enhance S=N
drastically if we select the events which have large
DCA ; i.e. trajectories having an inconsistent vertex.
The upper limit for the DCA comes from therequirement for the accuracy of TOF distance
LTOF : Therefore, we selected events with DCA in
the range 8–40 mm as shown in Fig. 4b. The
S=N ratio is clearly improved. Note that the

redundancy of the trajectory measurement is the
key for this tagging, otherwise ghost trajectories
would easily form larger DCA in the analysis.

4. Experimental setup and feasibility study
From the point of view of the experimental
setup, the key points can be summarized as; (i)accurate start timing of neutron TOF, (ii) reduced
material around the target including the target
itself, (iii) DCA determination}especially redundancy of the charged trajectory measurement, and(iv) high neutron detection eﬃciency. A f-factoryhas many experimental advantages as a kaonsource for the present experiment over hadronmachines. However, in this case, one needs tooperate a liquid helium target in the beam pipe
which seems unrealistic. Therefore, we focused on
hadron machines, in particular KEK K5.
4.1. Experimental setup
Figs. 5 and 6 present a possible schematic setup
for the hadron machine, and the neutron spectrum
was estimated under the following conditions. The
incident beam is identiﬁed at the entrance by asimple plastic scintillation counter used as a beamdeﬁning counter. Then, the contaminating pions

Fig. 4. (a) The DCA distribution and (b) the neutron spectrum selected by 85DCA 540 mm: The formation event can be selected
eﬃciently because the formation reaction does not produce a pion at the interaction point. Other conditions are the same as Fig. 3.

M. Iwasaki et al. / Nuclear Instruments and Methods in Physics Research A 473 (2001) 286–301

Fig. 5. Schematic ﬁgure of the experimental setup (side view).

$ herenkov
should be rejected by the Lucite C
counters (LC) using hardware logic. The incident
kaons with momentum of 600 MeV/c are thendegraded by a carbon block of about 60 g/cm2 :Next comes the TOF start counter, a 3 cm thickplastic scintillation counter. The track is measured
by 12 layers of drift chambers to give redundancy.
The slowed kaons drift into the target vessel

293

through a thin window, and are trapped by the4
He forming 3K% H nuclei.
The charged pions emitted from the K 4 He
reaction are measured by the two planar drift
chambers of 12 layers each (100 100 cm2 ineﬀective area, 24 cm thick). The chambers, located
40 cm above and below the beam center line, coverabout 34% (Ap ) of the total solid angle. To reducethe material around the target, the pion trigger
scintillation counters are located 160 cm behindthe chamber. These pion trigger counters are a
combination of thin ð5 mmÞ and thick (30 mmÞ
counters to identify pions and protons by means of
dE=E and E=TOFp :The most challenging device in the present
experiment is the liquid helium target. One of themost serious backgrounds comes from negatively
charged pion absorption at rest. To minimize thenumber of pions absorbed, we need to reduce thematerial surrounding the target. A detailed simulation indicated that the target should be as thin as
3 cm along the beam direction, otherwise theneutron background caused by pion absorption
becomes much larger than the signal. Anotherconstraint comes from the kaon reaction time
determination which requires that the target shapeshould be ﬂat as described in the following section.
Therefore, we designed the target to be operated at
the low pressure of 30 Torr at around 1 K;
having plastic windows of 40 mm; covered by1 mm thick vacuum vessel made of CFRP.

Fig. 6. Schematic ﬁgure of the experimental setup (downstream view).

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M. Iwasaki et al. / Nuclear Instruments and Methods in Physics Research A 473 (2001) 286–301

The emitted neutrons are observed by a wall ofneutron detector ð2:1 2:1 m2 ; 40 cm thick) placedon both sides. With a detection eﬃciency of Deffðpn Þ (36% at 360 MeV/c) and located 2:5 m distantfrom the target vessel (see Appendix A), they cover
8% of the total solid angle (An ). To improve time
resolution, we need to divide the counter into 7
sets, each of 30 cm width. One set consists of eightlayers of 5 cm thick plastic-scintillation counter as
described in Appendix A: The neutron detector
should be symmetrically centered on the target. In
this conﬁguration, we can measure the contribution and the shape of the spectrum of neutrons
coming from pion absorption at rest}the highenergy neutrons produced back-to-back. Using
their correlation between energy and opening
angle, this component can be studied.
4.2. Kaon reaction time determination
As described above, we need to trace the kaon
trajectory after the kaon momentum degrader withhigh redundancy, which requires more than 6 cm
of free space between the TOF start counter andthe target. We should also take into account thedead space due to the vacuum vessel of the target.
For this condition, the kaon reaction time (T0 )cannot be determined directly by the kaon injection time (Tinc ). This is because the target isextremely thin ( 0:38 g=cm2 ), and the velocitiesof the kaons which stop in the target are low. We
studied the optimum distance between the TOF
start counter and the target (L0 ), to obtain thesmallest dependence of travel time (T0  Tinc ) on
the momentum of the kaon. Fig. 7 shows the travel
time distribution for several L0 values. The
distribution becomes narrowest when we assume
12 cm for L0 : The width is about 20 ps FWHM,
however, the spectral shape is not symmetricbecause of extremely slow kaons.
4.3. Tagged spectrum and required beam estimation
Fig. 8 shows the simulated semi-inclusive andtagged neutron spectra assuming the branchingratio to be B ¼ 2; 7 and 12%. We denoted these
spectra as semi-inclusive because we required thecharged pion trajectory to be reconstructed by thepion arm. The DCA tagging method is also
applicable to protons, though the number oftagged events is much smaller than that for pions
and the S=N improvement is not as great asfor pions. Therefore, they were omitted fromthe spectrum. In these spectra, we generated
3:3 108 kaon absorption reactions at rest in theliquid helium target.
Let us estimate the expected ‘‘at rest’’ kaon yield(YK ) per day. We take KEK-K5 as the kaon

Fig. 7. Simulated travel time spectra with several drift space values (L0 ) as labeled. The distribution becomes narrowest when L0 is
12 cm:

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295

Fig. 8. Simulated neutron spectra triggered by charged particles identiﬁed as pion. Left column shows semi-inclusive (DCA 540 mmÞ
and right shows tagged ð85DCA 540 mmÞ spectra for branching ratios B of 2, 7 and 12%. The 3:3 108 kaon absorption reactions at
rest were generated in the present setup. All the experimental conditions were taken into account. We deﬁned the neutron signal
threshold to be 10 MeV electron equivalent.

source. The kaon stopping rate in a liquid helium
target was measured in a similar beam line (KEKK3) to be 400 stopped K /spill in cylindrical liquid
helium target of 2:8 g=cm2 for 1 TP ðTP ¼ 1012
protons) on the primary target per spill. The K5line is shorter and if we scale the length (factor two
in yield), the resulting number of kaons at rest is
100 K (Kinc ) in a 3 cm thick liquid helium target

per 1 TP. If we apply the wedge degrader at
the momentum dispersive point in the beam line,
we can improve the kaon stopping density byanother factor two [21]. The KEK 12 GeV-PS is
operated nominally at 2 TP with a cycle time of 3 s
which means 5:8 104 TP per day. Thus, YK ¼
Kinc 5:8 104 ¼ 1:2 107 kaon stops per day inthe target. This means that we need 83 shifts

296

M. Iwasaki et al. / Nuclear Instruments and Methods in Physics Research A 473 (2001) 286–301(28 days) to obtain these spectra without taking
into account the computer dead time, analysis
eﬃciency, etc.
To examine the detection capability, we performed a series of ﬁts assuming two natural widthsof GK (10 and 20 MeV), and several branchingratios B: It is diﬃcult to deﬁne the conﬁdence levelof the peak identiﬁcation because the signal islocated at the edge of the broad structure. For thiscase, if the branching ratio is small, the systematic
error becomes dominant and the error largely
depends on the ﬁtting function. Moreover, theshape of the spectrum also depends on thethreshold energy for the neutron detector. The
most important point at present is to deduce themaximum systematic error rather than obtaining
the best ﬁt. Therefore, we intentionally refrained
from applying sophisticated functions. Background was simply two Gaussian functions peakedat around BK ¼ 185 and 60 MeV: The signal
was ﬁtted by a Lorentz function folded with thedetector resolution. The ﬁt was performed without
using the background information given by thesimulation. A ﬁt result is shown in Fig. 9 for B ¼
2%; BK ¼ 108 MeV and GK ¼ 20 MeV: Fig. 10shows the ﬁt result for the tagged spectra. Error
bars indicate the statistical error only. The top
ﬁgure shows the ﬁtted area S (yield) of the boundstate divided by the actual simulated number S0 :Fig. 9. A typical ﬁt result for B ¼ 2%; BK ¼ 108 MeV and
GK ¼ 20 MeV: The simulated background was not used in theﬁtting procedure and is shown only for comparison.

Fig. 10. The results of the ﬁts: (a) the ﬁtted areas S divided by
the number of generated signal S0 ; (b) binding energy and (c)natural width. The deviation from S=S0 ¼ 1:0; BK ¼ 108 MeV
and GK ¼ 10 or 20 MeV indicates the systematic error for thesimplest background.

The center and bottom ﬁgures show the ﬁtted
value of the binding energy and the width,
respectively. The w2 values were of around 65 for
NDOF ¼ 41: This is acceptable for such an extremely simpliﬁed background.
It is clearly seen that the ﬁtted values of yieldand binding energy are diverging from the ideal
value for smaller branching ratio and larger width.
This deviation, which occurs simply because weused Gaussian functions to represent the background shape, represents the magnitude of the
maximum systematic error one can have. The mostdiﬃcult background component to be represented
by such a simple background shape is pion
absorption, as shown in Fig. 8. However, thecontribution and the shape of this component
can be measured with the present experimental
setup as described in Section 4. The background
spectrum can be studied by using several targets,
such as carbon.

M. Iwasaki et al. / Nuclear Instruments and Methods in Physics Research A 473 (2001) 286–301

5. Conclusion
We found that an experimental search fordeeply bound kaonic states looks promising. The
experimental setup was designed to study theenergy region BK > 100 MeV (Sp threshold), GK
520 MeV with adequate sensitivity for a branching ratio bigger than 2%. To better understand the% N interaction, several light nuclei must beK
examined including the deuteron and beryllium
as a second step of the experiment.

Appendix A
A.1. Monte Carlo simulation
The most precise experimental data used inestimation of the neutron spectrum is from the S
hypernuclear production by kaons at rest in aliquid helium target [19] at KEK. In this experiment, charged pion momentum spectra wereprecisely measured as shown in Fig. 11. The shapeof the quasi-free hyperon production component
shows that the dominant kaon reaction withhelium nuclei is three body. Therefore, one canevaluate the hyperon momentum spectra using
three-body phase space and the nuclear form
factor. These hyperons then decay into neutrons.
This data, however, is only applicable for the
primary reaction and not for the cascade of297

reactions, e.g. SN ! LN conversion, followed byhyperon scattering with residual nuclei, etc.,
because the KEK data is in the form of spectraof the charged pions which come from the primary
reaction and the hyperon decay. It gives tightconstraints for higher-energy neutron components,
but it is not adequate for obtaining the lowerenergy region which comes from cascade reactions.
To determine the strength of the successive
cascade reactions, one must refer to the helium
bubble chamber experimental data [20]. Theenergy resolution and the statistics are not
suﬃcient for the estimation of neutron energy
spectra, but all the charged particles produced in
the ﬁnal state are detected. The simulation
program should reproduce the branching ratio tothe ﬁnal states which deﬁne the strength of thesuccessive reactions in the nucleus which lead tolower-energy neutrons. However, we found that it
is very diﬃcult to be consistent with both data. To
be consistent with the bubble chamber data, which
gives relatively high multiplicity in the ﬁnal state,
we need very strong cascade reaction or ﬁnite
strength in four-body primary reaction. In themesic reaction, to be consistent with the S hypernuclear experimental data, a very narrow window
is open for the four-body reaction (K 3 He !
pþ S nd etc.) and the branch was negligible.
Though in the non-mesic reaction, there is nospectroscopic data available. In this process, one
can have a large branch to the four-body primaryFig. 11. Comparison of the present simulation (histogram) and the KEK data (point with error bar) for: (a) pþ momentum and (b) p
momentum spectrum.

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M. Iwasaki et al. / Nuclear Instruments and Methods in Physics Research A 473 (2001) 286–301

reaction and give small successive reaction
strengths to the hyperon/nucleon scattering withthe residual nucleus, or one can have the strength
the opposite way. In both cases, the resultant
neutron spectrum is similar. In the present paper,
analogous to quasi-free hyperon production, we
assumed that the four-body reaction is a relatively
small branch of 20%, excluding several channels
which must be four body, e.g., K 4He ! Sþ nnn:
The comparison of the charged pion momentum
spectra with KEK data is given in Fig. 11 and thecomparison of the branch to the ﬁnal state is given
in Table 1. As shown in the ﬁgure and the table,
the present simulation reproduces both data sets
relatively well.
We could not consistently reproduce the branchboth for the KEK data and the bubble chamberdata. In general, the multi-fragment branch of thesimulation is less than that of bubble chamberdata. This is because cascade reactions (breakup
Table 1Comparison of the branching ratio to the ﬁnal states
Final state

Simulation

Bubble chamber data

S p t
S+pdnS+ppnn
S+p0nnn
S+nnn
S+ total

9.55
0.85
0.79
1.81
1.31
14.31

9.3  2.3
1.9  0.7
1.6  0.6
3.2  1.0
1.0  0.4
17.0  2.7

Sp+t
Sp+dnSp+pnnSp0 3HeSp0dpSp0ppn
Sdp
Sppn
S total

4.51
0.99
0.39
1.38
0.68
0.25
1.78
1.17
11.15

4.2  1.2
1.6  0.6
1.4  0.5
1.0  0.5
1.0  0.5
1.0  0.4
1.6  0.6
2.0  0.7
13.8  1.8

Lp 3He
Lpdp
LpppnS0p 3He
S0pdp (ppn)
L(S0)p0dn (pnn)
L(S0)dn (pnn)
L(S0)p+ nnn

11.48
18.99
4.98
2.15
0.94
23.24
10.56
2.20

11.2  2.7
10.9  2.6
9.5  2.4
0.9  0.6
0.3  0.3
22.5  4.2
11.7  2.4
2.1  0.7

L(S0) total

74.55

69.2  6.6+ 

processes) reduce the kinetic energy of the particles
in the ﬁnal state, resulting in the enhancement ofpion yields of both the monochromatic peak due
to the hyperon decay at rest and very low
momentum p s from L decay. If we fully
reproduce the bubble chamber data, the simulated
pion spectra are unreasonably inconsistent with
the KEK data. This means that our proposed
experiment is also able to give information on
multi-fragment and two-nucleon absorption reactions from the spectrum shape of the neutrons.
A.2. Neutron detector and TOF
In the present experiment, neutron energy ismeasured by the TOF method. Let us discuss the
optimization of the TOF distance (LTOF ). To
observe the natural width of the 3K% H state, theexperimental resolution should be well below thenatural width. This is because, if the energy
resolution is larger than the width, we cannot
resolve the width and, moreover, the signal-tonoise ratio ðS=NÞ becomes worse linearly with theresolution. For better energy resolution by TOF,
we need a longer distance. On the other hand, thespatial acceptance of the neutron detector is
proportional to L2
TOF ; therefore we need to compromise the distance without spoiling the width
determination capability. Here, we take half of theestimated natural width (10 MeV) as our design
value for the TOF resolution around the estimatedbinding energy (15 MeV), to keep a safety margin
in the energy resolution. There are two points to be
discussed in terms of the energy resolution measured by the TOF method, namely time resolutionof the system and spatial resolution of the neutron
source point, where the reaction occurs. The time
resolution is predominantly deﬁned by the startand stop counter. In the present experiment, thestart and stop signals are given by the incoming
kaon trigger counter and the neutron counter,
respectively. One realistic counter for detecting
neutrons is a plastic scintillation counter. Because
the interaction length of neutrons in the energy
region of interest is about 50 g=cm2 ; we need very
thick plastic to detect neutrons eﬃciently. Plastic
scintillator has the best performance in time resolution among the bulk detectors}typically 0:7 ns

M. Iwasaki et al. / Nuclear Instruments and Methods in Physics Research A 473 (2001) 286–301

pﬃﬃﬃ(FWHM) including a factor 2 to account forstart and stop uncertainty.
The experimental energy resolution DBL;t contributed from the errors in TOF distance (DL) andin time (Dt) can be expressed in the simple formula
!
 2
pn
Mn
DBL;t ¼ ðEn þ E3% H Þ
K
Mn
M3% H
K
sﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
2ððpn =En ÞcDtÞ þ ððEn =Mn ÞDLÞ2
ðA:1Þ
L2TOF

pn ¼

qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
QðQ þ 2Mn ÞðQ þ 2M3% H ÞðQ þ 2ðMn þ M3% H ÞÞ
KK2ðQ þ Mn þ M3% H Þ

:

K

ðA:2Þ
The experimentally observable width DBobs is thenwritten as
qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
ðA:3Þ
DBobs ¼ G2K þ DB2L;t :Using this relation, we plotted several ﬁgures todetermine the basic parameter, namely TOF
distance, time and spatial resolution as shown in
Figs. 12a and 13. Fig. 12a shows the LTOF
contribution for given Dt; Fig. 12b shows the Dt
contribution for given LTOF ; and Fig. 13 shows the
DL contribution for given LTOF and Dt: These299

ﬁgures show that the distance from the target to
the neutron counter should be longer than 2:5 m:
The requirement for the kaon reaction time
analysis depends largely on the target and thekaon source. To stop kaons fast and eﬃciently, thetarget should be liquid helium, which has thehighest ð0:125 g=cm3 ) density one can easily
obtain. In the kaon-at-rest experiment in a hadronmachine, kaons are transported to the experimental area at a high momentum of about 600–900
MeV=c; because of their short life time. The target
thickness, to stop kaons in the target eﬃciently, isdeﬁned by the momentum bite of the beamline rather than the range straggling. Typically
about 12 g=cm2 (dp=p ¼  2.5% (full-width) at
700 MeV=c) of the target material is needed in the
direction of the beam axis. Such a large target is
not suitable for the present experiment, not only
from the ‘‘pion absorption at rest’’ requirement
but also in terms of the resolution. Because thetarget density is relatively low, the time diﬀerence
between the kaon injection and the reaction is a
few ns, as shown in Fig. 13.
A.3. Structure and the eﬃciency of the TOF counterFig. 14 shows a schematic top view of the TOF
counter. The simplest way to detect neutrons ofFig. 12. Energy resolution as a function of the binding energy of 3K% H state for: (a) TOF distance of 1.5, 2, 2.5 and 3 m at Dt ¼ 0:7 ns;(b) Dt = 0.5, 0.7, 0.9 and 1:1 ns at distance of 2:5 m: Spatial resolution of 4 cm is assumed.

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M. Iwasaki et al. / Nuclear Instruments and Methods in Physics Research A 473 (2001) 286–301

in the present experiment, though to suppress thetrigger rate of the hardware logic it should not bethicker than one radiation length. Therefore, wedeﬁned the total thickness of the neutron counter
layers to be 40 cm: The neutron detection eﬃciency and its energy dependence were evaluatedusing the most precise neutron detection simulator
SINFUL [22]. In this thickness, the neutron
detection eﬃciency is estimated to be about 36%
at the threshold energy of 5 MeVee (MeV electron
equivalent) for momentum of around 70 MeV:
Because we cannot specify the neutron reaction
point to better than the thickness of the detector,
this deﬁnes the error of TOF length determination.
Therefore, the thickness of one layer should be lessthan 5 cm as shown in Fig. 14.
Fig. 13. Eﬀects of the spatial resolution of the kaon reaction
point, which deﬁnes the TOF distance and the energy resolution
directly.

Acknowledgements
We are very grateful for fruitful discussions with
S. Ishimoto, S. Meigo, G. Beer, S. Okada and
A. Sanderson.
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Theo. Suppl. 117 (1994) 177.

301

[20] P.A. Katz, K. Bunnell, M. Derricl, et al., Phys. Rev. D 1(1970) 1267.
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Instr. and Meth. A 408 (1998) 438.
[22] S. Meigo, Nucl. Instr. and Meth. A 401 (1997) 365.

E509 K. Tanida
γ-ray Spectroscopy of Hyperfragmentas using stopped K- method
K. Imai and L. Zhu
Hypernuclear gamma ray spectroscopy by stopping KProc. Int. Conf. on Hypernuclear and Strange Particle Physics, (HYP2000), Torino, October 2000, Nucl. Phys.
A691 (2001) 127.
◇ K. Tanida et al.
[attached]
High-resolution gamma-ray spectroscopy of hyperfragments produced by stopped K- reactions
16th International Conference on Particles and Nuclei (PANIC 02), Osaka, Japan, Sep 30 - Oct 4,
2002, Nucl. Phys. A721 (2003) 999c.
◇ K. Miwa et al.
High-resolution γ-ray spectroscopy of hyperfragments produced by stopped K- reactions
Int. Symp. on Electrophoto-production of Strangeness on Nucleons and Nuclei (SENDAI03),
Sendai, Japan, Jun 15-18, 2003, in press.
◇ H. Tamura et al.
Gamma-Ray Spectroscopy of Hypernuclei
Int. Symp. on Electrophoto-production of Strangeness on Nucleons and Nuclei (SENDAI03), Sendai, Japan,
Jun 15-18, 2003, in press.
◇ K. Miwa et al.
[attached]
High-resolution γ -ray spectroscopy of hyperfragments produced by stopped K- reactions
VIII Int. Conf. on Hypernuclear and Strange Particle Physics (HYP2003), Newport News, USA, Oct 14-18,
2003, in press.
◇ H. Tamura et al.
Gamma Spectroscopy in Λ Hypernuclei
VIII Int. Conf. on Hypernuclear and Strange Particle Physics (HYP2003), Newport News, USA, Oct 14-18,
2003, in press.
＊ K. Tanida et al.
γ -ray spectroscopy of hyperfragments with stopped KRIKEN Acc. Prog. Rep. 36 (2003) 92.
◇________________________________________________________________________● Physics papers published in refereed journal.
○★
◇＊

Technical papers.
PhD theses.
Conference and Symposium.
Internal Report and others.

ELSEVIER

Nuclear Physics A721 (2003) 999c-1002~
www.elsevier.com/locate/npe

High-resolution y-ray spectroscopy of hyperfragments
stopped K- reactions

produced by

K. Tanidaa, H. Akikawab, Y. Fukaob, H. Hotchi”, K. Imaib, Y. Miurad K. Miwab,
K. Mizunumad, S. N. Nakamurad, M. Niiyamab, S. Ohtab, P. K. Saha”, H. Takahashib,
T. Takah&hid, H. Tamurad, S. Terashimab, M. Togawab, and M. Ukaid(KEK-PS E509 collaboration)“RIKEN(The Instituteof Physical and ChemicalbDepartmentof Physics, Kyoto

University,“Brookhaven

NationaldDepartmentof Physics, Tohoku“Laboratory
Japanof Physics, Osaka Electra-communication

Laboratory,

Research), Saitama 351-0198, Japan

Kyoto 606-8502, Japan

NY 11973, USA
University,

Sendai 980-8578, Japan
University,

Osaka 572-8530,

We measured y rays of hyperfragments produced by stopped K- reactions on light (A 512) targets (KEK-PS E509). By preliminary analysis, 2050 keV 7 ray from LLi(E2:5/2+-+
l/2+) is seen with l”B and C targets. Also, two candidates for new hypernuclear y rays
are observed.

1. INTRODUCTION
The success of high-resolution
y-ray spectroscopy of hypernuclei with germanium detectors [l-3] opened a new era of hypernuclear physics. These experiments revealed finestructures of light hypernuclei and gave valuable informationon underlying AN spindependent interaction.
However, in these experiments, where direct reactions such as(x+, I(+) and (I(-,n-)were used to produce hypernuclei, beam time of more than amonth was necessary. Therefore, systemactic study of hypernuclei was very difficult.
On the other hand, it is known that two orders of magnitude more hypernuclei canbe produced as hyperfragments
by stopped li’- reactions than by direct reactions [4].
Furthermore, various species of hypernuclides lighter than target nucleus, including neutron (proton) rich ones, are available in stopped K- reactions and thus systematic study
becomes possible.
Such studies are expected to extend capabilities of hypernuclear physics. For example, if data from many hypernuclei are available, we can discuss not only two-body ANinteraction, but also three-body ANN interaction, which may play an important role instructures of hypernuclei because of strong AN-EN coupling [5,6]. Also, charge symmetry breaking in AN and ANN interaction can be investigated by comparing structures of0375-9474/03/$ - see front matter 0 2003 Elsevier Science B.V
doi:lO.l016/S0375-9474(03)01271-5

All rights reserved.

1ooocmirrorK. Tanida et al. /Nuclear Physics A721 (2003) PPPc-1002~
hypernuclei.

2. EXPERIMENT
Motivated by above discussions, we performed an experiment to measure 7 rays fromhyperfragments produced by stopped h’- reactions in Apr. 2002 at KEK-PS K5 beamline(KEK-PS E509). W e used h’- beam extracted at 650 MeV/c. Three cherenkov counters(LCl-3) and 3 time-of-flightcounters (Bl-3) were used to identify kaons, which weredegraded by carbon blocks of N 90 g/ cm2 thick in total, and stopped in the targets. Nonstopping kaons were rejected by a veto counter (FV) pl aced downstream of the targets.
We used five targets (Li, Be, “B, ilB, and C) m
. order to see the target dependence of
y-ray yield. This enables us to eliminate background 7 rays coming from outside of thetargets, because yields of these 7 rays are expected to have no target dependence.
We used Hyperball [7] for y-ray detection.
Hyperball consists of 14 germanium detectors (Ge), each surrounded by 6 bismuth germanate (BGO) counters for suppressing
backgrounds such as Compton scattering and electro-magnetic
shower produced by highenergy 7 rays from X”S. Each Ge has an efficiency of 60% relative to an NaI crystal of
3”r$x3”. Ge’s are placed 15 cm away from the beam axis and total photo-peak efficiency
of Hyperball was about 1% at 1 MeV including the effect of deadtime due to the highcounting rate of Ge.
We collected data for about 10’ stopped kaons in the beam time of two weeks. This isroughly half of the planned beam time and number of stopped kaons. In addition, BGO
veto did not work very well due to high-counting rates of BGO counters, we further lost
data quality. We are planning to fix this problem in future experiments.

3. PRELIMINARYRESULTSANDDISCUSSIONSPreliminary analysis shows that 7 ray from LLi(E2:5/2++ l/2+) is seen at 2050 keV(see Figure 1). This 7 ray is observed with l”B and C targets, but not from the othertargets, especially, “B . This result cannot be explained by statistical decay of compound
hypernucleus.
Furthermore, since “B has J” = 3/2- and 12C has J” = O+, high-spinstates such as 5/2+ of :Li are expected to be more favorably produced from “B than“C from naive discussions. Thus, the present result is interesting with respect to the
production mechanism of hyperfragments.
In this point of view, y-ray intensity is valuable information.
Rough estimation gives
1.5 x 10m3 7 rays per stopped kaon on l”B target. Further analysis to elaborate the result
is in progress.
We also observed two more candidates for hypernuclear 7 rays. One is at 1303 keVwith ‘Be and l”B targets, and the other is at 1400 keV with C target (see Figure 2). Since
there are no known 7 rays for light (A < 12) nuclei at these energies, they are most likely
from unknown hypernuclear 7 transitions, possibly in :Li and ;‘B, respectively.

4. SUMMARYAND

FUTUREPROSPECTS

We performed an experiment to measure hypernuclear 7 rays using stopped I<-. We
suceeded to observe 7 rays from :Li, and found two more candidates for new hypernuclear

1OOlcK. Tanida et al. /Nuclear Physics A721 (2003) 999c-1002~
E509

nreliminarv

2000 1: Li
1500 _w

E509 preliminary

,

7

,

,

’

’

’

,

’

’,

,

,

,j

lOOOr
600

1000
1000 u500 1,
2000

2100

E, 04

Figure 1. Preliminary y-ray energy spectrum around 2 MeV for Li, Be, *‘B, llB,
and C targets. The 2050 keV y ray from
LLi(E2:5/2+-+ l/2+) is seen for l”B andC targets. Two known y rays from normal nuclei are also seen at 2154 keV (lrB,
l/2- + 3/2-) and 2124 keV (“B, 1: -+3+) in several targets. The peak at 2115
keV is not identified, and considered to be
background y ray coming from outside of
the targets.

c

,

1400

1300
E, (kev)

Figure 2. Preliminary -y-ray energy spectrum between 1.25 MeV and 1.45 MeV for
Li, Be, *‘B, rlB, and C targets. Amongthe many peaks seen in the figure, only
two peaks at 1303 keV and 1400 keV havetarget dependence of the yields; 1303 keVpeak is seen with Be and l”B targets, andthe other is seen with C target only. Since
no y rays are known at these energies inlight (A 5 12) normal nuclei, they are candidates for hypernuclear y rays.

1002cK. Tanida et al./Nuclear Physics A721 (2003) 999c-1002~

y transitions. The target dependence of the LLi y-ray yield casts a problem on our naive
understandings for the production mechanism of hyperfragments from stopped K-. These
preliminary results show that y-ray spectroscopy of hyperfragments with stopped li’- isa useful tool for hypernuclear study.
Currently, we are refining our analysis to finalize y-ray spectra and to obtain y-ray
intensities. y-y coincidence analysis will also be performed. We are planning to improvethe experimental setups and acquire four times more statistics than the present result.
For future, further experiments with heavier targets are being considered.

REFERENCES
1. H. Tamura
2.
3.
4.
5.
6.
7.

et al., Phys. Rev. Lett. 84 (2000) 5963.
K. Tanida et al., Phys. Rev. Lett. 86 (2001) 1982.
H. Akikawa et al., Phys. Rev. Lett. 88 082501 (2002).
D. H. Davis et al., Nuovo Cim. 22 (1961) 275.
B. F. Gibson, I. R. Afnan, J. A. Carlson, and D. R. Lehman, Prog. Theor. Phys.
Suppl. 117 (1994) 339.
Y. Akaishi, T. Harada, S. Shinmura, and Khin Swe Myint, Phys. Rev. Lett. 84 (2000)
3539.
H. Tamura, Nucl. Phys. A 639 (1998) 83~.

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B2 ADC vs LC1 ADC
LC1 ADC (channel)

E509 Setup

LC1,2,3
B2

BGO

350
300

FV

50

CarbonDegrader00

Pedestal

B1(3) K + π

150
100

B3

(2) π

250
200

t  t

K-

400

(1) K
Cut region forpure K- event
100

200

300

400

500
600
B2 ADC (channel)

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E518 H. Tamura:High-Precision γ Spectroscopy of 11ΛB
H. Tamura et al.
Recent Progress in γ -Ray Spectroscopy of Hypernuclei
Int. Workshop on “Electrophotoproduction of Strangeness on Nucleons and Nuclei” (SENDAI03),
Sendai, June 16-18, 2003, Ed. K. Maeda et al., World Scientific (2004) in press.
◇ Y. Miura et al.
[attached]A Recent Experiment with Hyperball,
28th Mazurian Lakes Conference On Physics, Krzyze, August 31-September 7, 2003, Acta Physica
Polonica B 35 (2004) 1019-1031.
◇ Y. Miura et al.
γ Spectorscopy of 11ΛB,
8th Int. Conf. on Hypernuclear and Strange Particle Physics (HYP2003), Jefferson Lab., October
14-18, 2003, Nucl. Phys. A in press.
◇ H. Tamura et al.
γ -Ray Spectorscopy in Λ Hypernuclei,
8th Int. Conf. on Hypernuclear and Strange Particle Physics (HYP2003), Jefferson Lab., October
14-18, 2003, Nucl. Phys. A in press.
◇ H. Tamura
Experimental Progress in Hypernuclear Physics,
18th Nishinomiya Yukawa Memorial Symposium on “Strangeness in Nuclear Matter”, Nishinomiya,
December 4-5, 2003, Prog. Theor. Phys. in press.
◇ Y. Miura et al.
g-Factor ofΛin the Hypernuclei Studied from B(M1) Measurement,
YITP Workshop on "Nuclear Matter under Extreme Conditions” (Matter03), Kyoto, December 1-3,
2004, Prog. Theor. Phys. in press.
＊ H. Tamura
Future Plans of γ Spectorscopy --Spectorscopy of 11ΛB and B(M1) Measurement,
Workshop on Recent Progress of Strangeness Nucler Physics, KEK, March 15-17, 2002, KEK
Proceedings 2002-29 (2003) p.46-57 (in Japanese).
＊ Y. Miura et al.
γ Spectorscopy of 11ΛB,
Workshop on Prospect of Strangenss Nuclear Physics at J-PARC, KEK, July 29-31, 2003, KEK
Proceedings 2003-18 (2004) p.51-57 (in Japanese).
◇________________________________________________________________________● Physics papers published in refereed journal.
○★
◇＊

Technical papers.
PhD theses.
Conference and Symposium.
Internal Report and others.

Vol. 35 (2004)

ACTA PHYSICA POLONICA B

No 3

A RECENT EXPERIMENT WITH HYPERBALL∗
Y. Miuraa , S. Ajimurab , Y. Fujiia , T. Fukudac , O. Hashimotoa
H. Hotchic , K. Imaid , W. Imotoc , Y. Kakiguchie , S. Kameokaa
A. Krutenkovaf , T. Marutae , A. Matsumuraa , K. Miwad
T. Miyoshia , K. Mizunumaa , S.N. Nakamuraa , T. Nagaee
H. Nomuraa , H. Noumie , Y. Okayasua , T. Otakic , H. Outae
P.K. Sahac , T. Saitohg , Y. Satoe , M. Sekimotoe , T. Takahashia
H. Tamuraa , K. Tanidah , A. Toyodae , M. Ukaia
and H. YamauchiaaDepartment of Physics Tohoku University, Sendai 980-8578, Japan
Department of Physics Osaka University, Toyonaka 560-0043, Japanc
Osaka Electro-Communication University, Neyagawa 572-8530, Japand
Department of Physics Kyoto University, Kyoto 606-8502, Japane
Institute of Particle and Nuclear Studies, KEK, Tsukuba 305-0801, Japanf
Institute of Theoretical and Experimental Physics, Moscow 117218, Russiag
GSI, Darmstadt D-64291, Germanyh
RIKEN, Wako 351-0198, Japanb(KEK-PS E518 Collaboration)(Received February 10, 2004)
Using a germanium detector array, Hyperball, we have measured γ raysfrom several Λ hypernuclei. In the previous experiments, spin-dependenceof ΛN eﬀective interaction was investigated. In the latest experiment, wehave studied 11
Λ B in which six γ transitions have been observed. The resultof this experiment suggests that the level structure is quite diﬀerent from atheoretical prediction based on the ΛN spin-dependent eﬀective interaction
parameters determined in the previous experiments.
PACS numbers: 21.80.+a, 21.10.–k, 23.20.–g, 27.20.+n

1. Physics motivation
Detailed level structure of hypernuclei enables us to investigate several
interesting subjects, such as Y N interactions, hypernuclear shrinking eﬀect,
and nuclear medium eﬀect of baryons.
∗Presented at the XXVIII Mazurian Lakes School of Physics, Krzyże, Poland,
August 31–September 7, 2003.

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1020

Y. Miura et al.

In recent years, we have developed γ spectroscopy of hypernuclei. By using a large germanium (Ge) detector array, Hyperball, the energy resolutionof hypernuclear spectroscopy has been improved from 1–2 MeV to a few keV,and a great progress has been made in studying structure of hypernuclei.
The structure of hypernuclei can be understood by combining a Λ and theother part called “core”. If jcore = 0, one state of the core nucleus splits into
two states of the hypernucleus according to the spin direction of a Λ as shownin Fig. 1. Since the energy spacing between these two states is typically ofthe order of 100 keV, it is necessary for us to use Ge detectors to observe
such a ﬁne structure. Such spin doublets are often called “hypernuclear ﬁne
structure”.
From precise measurements of γ rays from hypernuclei, the following
three subjects can be studied.

Fig. 1. Hypernuclear ﬁne structure. By observing γ rays from hypernuclei, spindependent forces of ΛN interaction can be investigated. See text for details.

1.1. ΛN eﬀective interaction
By observing γ rays from p-shell hypernuclei, spin dependence of the ΛNinteraction can be investigated. Theoretical estimations which are based on
ΛN potential models and available experimental data such as level structureand Λ binding energies of several hypernuclei have been performed to determine pN sΛ two-body eﬀective interactions [1,2]. The two-body ΛN eﬀective
interaction can be expressed in terms of ﬁve radial integrals associated witheach term inVΛN (r) = V0 (r)+ Vσ (r)sN ·sΛ + VΛ (r)lNΛ ·sΛ + VN (r)lNΛ ·sN + VT (r)S 12 ,(1)
where lNΛ is the relative orbital angular momentum andS 12 = 3(sN · r̂)(sΛ · r̂) − sN · sΛwith r = |r N −r Λ |. These ﬁve integrals denoted by V , ∆, SΛ , SN , and T aretaken to be constant throughout the shell. To obtain explicit expressions forthe radial forms appearing in Eq. (1), several versions of Nijmegen models
are used as the ΛN potential.

A Recent Experiment with Hyperball1021

The average central interaction (V ) is determined to reproduce the measured Λ binding energies (BΛ ) for several p-shell hypernuclei. The otherfour parameters express spin-dependent forces. The spin-spin force (∆), theΛ-spin-induced spin-orbit force (SΛ ), and the tensor force (T ) are obtained
from hypernuclear ﬁne structure. On the other hand, the nucleon-spininduced spin-orbit force (SN ) causes the diﬀerence of nucleon excitationenergy between a hypernucleus and its core nucleus. Fig. 1 shows typical
low-lying levels of a hypernucleus. Using the shell model calculations, level
energies of hypernuclei are described with these parameters and comparedwith their experimental data to improve the parameters.
The basic philosophy behind this phenomenological approach was proposed by Gal, Soper, and Dalitz [1, 2]. The calculations presented in [1, 2]
were improved by Millener [3]. In this calculation, the shell model with{s4 pA−5 sΛ } and {s4 pA−5 pΛ } conﬁgurations were used, and the eﬀect of
three-body ΛN N interaction was also parameterized.
1.2. Impurity eﬀect
Inside a nucleus, a Λ in the 0s orbit attracts nucleons around it andcauses its shrinkage. This eﬀect is called “glue-like role” of Λ in the nucleus
and experimentally conﬁrmed in the case of 7Λ Li by the KEK-PS E419 experiment. By comparing B(E2) of 7Λ Li (5/2+ → 1/2+ ) with that of the coretransition 6 Li (3+ → 1+ ), it was found that the size of 7Λ Li is 19± 4% smaller
than that of 6 Li [4]. Such a nuclear shrinking eﬀect is a general property oflight hypernuclei, which was pointed out by Motoba et al. [5] for the ﬁrst
time and precisely calculated by Hiyama et al. [6]The measurement of B(E2) was carried out by Doppler shift attenuation
method (DSAM). If γ ray emission is fast enough after the production of thehypernucleus, the width of the observed γ ray peak is broadened by Doppler
eﬀect. The lifetime of the excited state is obtained from the observed peak
shape compared with that from a Monte Carlo simulation. This method
can be used when the lifetime of the excited state is comparable with the
stopping time of the hypernucleus.
1.3. Medium eﬀect of baryons
The properties of baryons in a nucleus may change due to the medium
eﬀect, namely, it is expected that the mass of baryons is aﬀected by partial
restoration of chiral symmetry. Since a Λ is free from Pauli blocking eﬀect, it
can probe the inside of the nucleus. Therefore, the medium eﬀect of baryons
can be measured by comparing the g-factor of a Λ in the nucleus with thatin the free space. Because of its short lifetime, it is diﬃcult to measureg-factors of hypernuclei directly. However, they can be measured from the1022

Y. Miura et al.

B(M1)’s of Λ-spin-ﬂip M1 transitions. If we assume a weak coupling betweena Λ and the core nucleus, the B(M1) can be expressed as
B(M 1) ∝ |φf | µz |φi |2∝ |φ↓Λ φcore | gcore j zcore + gΛ j zΛ |φ↑Λ φcore |2(2)2∝ (gΛ − gcore ) .
This equation shows that the g-factor of a Λ in the nucleus (gΛ ) can bederived by measuring the B(M1) of the Λ spin-ﬂip M1 transition and the gfactor of the core nucleus (gcore ). The B(M1) of the Λ spin-ﬂip M1 transition
can be measured with Doppler shift attenuation method.
2. Hyperball and γ spectroscopy of hypernuclei
Since the construction of the Hyperball in 1998, γ spectroscopy of hypernuclei has made a great progress. Five experiments were performed using
the (π + , K + γ), (K − , π − γ), and (stopped K − , γ) reactions. In these experiments, germanium detectors were located around π + or K − beams available
at KEK or BNL. Before the Hyperball was built, using germanium detectors
with these meson beams was almost impossible.
In the previous γ ray measurements of hypernuclei, ΛN eﬀective interactions and impurity eﬀect were studied.
2.1. Hyperball
The Hyperball, which consists of 14 germanium detectors, covers 15 %of a solid angle for a point γ-ray source. The photo-peak eﬃciency of germanium detectors is about 2 for the γ ray at 1 MeV. Although there areserious background-induced problems in using germanium detectors for γ
spectroscopy of hypernuclei, the Hyperball overcomes such diﬃculties withspecial readout electronics and BGO counters.
The BGO scintillation counters are used to suppress backgrounds fromhigh energy γ rays from π 0 mesons and Compton scattering. Modiﬁcationsof readout electronics are also necessary to use germanium detectors under
the condition of high intensity beam. When a high energy beam particle isscattered on the target and hits the germanium detectors, it gives an energy
deposit of the order of 50 MeV. Since the energy deposit rate as well as thecounting rate is extremely high, we need to use reset-type preampliﬁers andfast shaping ampliﬁers with gated integrators.

A Recent Experiment with Hyperball1023

Fig. 2. Hyperball consists of 14 Ge detectors and BGO scintillation counters.

2.2. Present status of the experiments with the Hyperball
The ﬁrst experiment with the Hyperball was performed at KEK-PS K6
beamline in 1998 (KEK-PS E419). Four γ transitions from 7Λ Li including M1 (3/2+ → 1/2+ ) and E2 (5/2+ → 1/2+ ), were observed from the
7 Li(π + , K + γ) reaction in this experiment. The spin-spin force and the nucleon induced spin-orbit force of ΛN eﬀective interactions were investigatedfrom these measured energy spacings [7]. The shrinkage of 7Λ Li was also
conﬁrmed [4].
Using the (K − , π − γ) reaction, we carried out two experiments at BNLAGS D6 beamline. In 1998, γ rays from 9Λ Be were observed. The energy
spacing of the Λ spin doublet (3/2+ , 5/2+ ) was measured and the Λ-spininduced spin-orbit force was investigated [8]. Three years later, in order todetermine the strength of the tensor force, we measured γ rays from 16
Λ O [9].
With this result, we completed to determine all the parameters for the spindependent terms of ΛN eﬀective interactions. On the other hand, we alsoacquired the data with 10 B target in this experiment, but did not observe
γ rays from 10
Λ B. This result is inconsistent with the spin-dependent force
parameters which were determined from the other hypernuclear data.

1024

Y. Miura et al.

Fig. 3. 7Λ Li (E2 ; 5/2+ → 1/2+ ): Result from KEK-PS E419 [4]. B(E2) was
measured from simulated peak shape (solid line) using Doppler shift attenuation
method.

Although no γ rays were observed from 10
Λ B, we observed some γ transitions from hyperfragments with 10 B target. Motivated by this result, we
carried out an experiment to measure γ rays from hyperfragments using
the stopped K − method with several light targets (7 Li, 9 Be, 10 B, 11 B, and12 C) at the KEK-PS K5 beamline [10]. From 10 B, 11 B, and 12 C targets, the
7 Li (5/2+ → 1/2+ ) transition was observed again. Based on the measuredΛproduction rate, the stopped K − method seems to be an useful tool for γ
spectroscopy of hypernuclei, particularly by using γ-γ coincidence techniquein the future.
3. A recent experiment — γ spectroscopy of 11
ΛB
The latest experiment using the Hyperball (KEK-PS E518) was performed with the 11 B(π + , K + γ) reaction at KEK in the autumn of 2002.
The purpose of this experiment consists of two subjects.
The ﬁrst subject is to investigate the spin dependent forces of ΛN effective interactions. Before this experiment, the strengths of the individual
four terms have been already determined from experimental results with theHyperball. However, cross check of these values using other hypernuclear
data is still necessary.
Another subject is to measure the g-factor of Λ in a nucleus. At present,
11 B (3/2+ → 1/2+ ) is considered to be the only Λ-spin-ﬂip M1 transitionΛin which we can measure the B(M1) with Doppler shift attenuation method
within a reasonable beamtime.

A Recent Experiment with Hyperball1025

3.1. Structure of 11
ΛB
The structure of 11
Λ B is complicated. There are many bound states below the particle decay threshold into 10
Λ Be and a proton. Figure 4 shows
the level scheme of 11
B
calculatedbyMillener[11]. In his calculation, exΛ
citation energies, branching ratios, and lifetimes of 11Λ B bound states are
estimated. Then, nine γ transitions are expected to be observed within agiven beamtime.

Fig. 4. Expected level scheme of 11Λ B calculated by Millener [11]. Expected excitation energy, cross section, and lifetime of each state are listed on the right side. In
total, nine γ transitions are expected to be observed in E518.

1026

Y. Miura et al.

3.2. Experimental setup
Figure 5 shows the experimental setup of KEK-PS E518. The experiment
was performed at the K6 beamline. At this beamline, high intensity π +beam is available. The momentum of π + was 1.04 GeV/c, and a typicalbeam intensity was 4×106 π + per 1.2 second spill. In this experiment,
11 B(π + , K + ) reaction was used in producing 11 B. Considering the γ rayΛyield, we used two kinds of thick targets whose thickness was 6 cm and 10
cm. The total amount of irradiation in one month was 0.85×1012 π + for the
10 cm target and 0.75×1012 π + for the 6 cm target.

Fig. 5. Experimental setup of KEK-PS E518

The momenta of incident π + and scattered K + were measured with magnetic spectrometers. In particular, we used Superconducting Kaon Spectrometer (SKS) in measuring the momentum of the scattered K + . This
spectrometer was constructed to realize both high resolution and large acceptance [12]. The trajectory of each particle was reconstructed with drift
chambers and used for calculation of the excitation energy and the momentum vector of the produced 11
Λ B. This information was used to choose theevents in which bound states of 11
Λ B were produced, as well as to correct
γ-ray energies for Doppler shift.
3.3. Results
From the analysis of the magnetic spectrometers, the excitation energy
spectrum of 11
Λ B was obtained. In order to investigate the precise structureof this hypernucleus, a measurement with a thin (2 g/cm2 ) target was also
performed. The thin-target spectrum is shown in Fig. 6 (top). The horizontal axis of this spectrum has the scale of the binding energy of Λ (−BΛ )deﬁned as(3)−BΛ = Mhyp − MΛ − Mcore .

A Recent Experiment with Hyperball1027

Fig. 6. Exciting energy spectra of 11Λ B. Upper spectrum is measured with a thintarget. Lower spectrum is the theoretically calculated one by Motoba [13].

Fig. 7. Excitation energy spectrum with the thick targets. The region A is selecled
for bound states, and B is selected for highly unbound states.

Here, Mhyp is the mass of the hypernucleus, MΛ is the mass of Λ, andMcore is the mass of the core nucleus. Comparing this spectrum with atheoretical calculation by Motoba [13], which is shown in the lower spectrumof Fig. 6, we can see that the spectra resemble each other.
Figure 7 shows the mass spectrum obtained with the thick targets. In thismass spectrum, the energy resolution is worse than that with the thin target.
To observe γ rays, the region “A” (−20 MeV < −BΛ < −2 MeV) is selected
as the bound state region, and the region “B” (20 MeV < −BΛ < 40 MeV)
is selected as the highly unbound region.

Y. Miura et al.

1028

10
10

+

10

Fig. 8. γ ray spectra for 11Λ B. By comparing spectra for two regions, namely A(−20 MeV < −BΛ < −2 MeV) and B (20 MeV < −BΛ < 40 MeV), six γ
transitions from 11
Λ B are observed. The 2478 keV peak is observed after Doppler
shift correction.

Figure 8 shows preliminary results of the observed γ ray spectra fromthese regions. By comparing the γ ray spectra for the A and B regions,
six γ ray peaks at 262 keV, 454 keV, 500 keV, 564 keV, 1482 keV, and2479 keV are identiﬁed as the γ transitions in 11Λ B, because they are observed
only in the bound state region. Among these γ rays, the 2479 keV peak isobserved after Doppler shift correction. The observed number of events andthe relative intensity for each γ transition are listed in Table I. In the relative
intensity, energy dependence of the germanium detector eﬃciency is taken
into account, and then it is normalized with the number of events for the1482 keV γ ray.

A Recent Experiment with Hyperball1029

TABLE I
Number of events and relative intensities for observed γ transitions. Relative intensities are normalized by the number of events for the 1482 keV γ ray.
Eγ (keV)
262
454
500
564
1482
2479

Number of event
68
49
77
61
225
20

Relative intensity
0.09
0.10
0.16
0.17
1.00
0.13

3.4. Identiﬁcation of observed γ rays
Identiﬁcation of the observed γ transitions is not easy because there aremany possible candidates for level assignment. However, the 1482 keV γtransition can be identiﬁed from the following three points.
At ﬁrst, estimated from the peak width, this γ ray peak is not broadened
by Doppler eﬀect. Therefore, the lifetime of this transition should be longer
than 10 psec. If this transition is M1, B(M1) is smaller than 10−3 µ2N . As it
is too small, this γ ray should be identiﬁed as E2.
Next, this γ transition has the largest yield. According to the Millener’s
prediction [11], the E2 (1/2+ → 5/2+ ) transition is expected to have thelargest yield.
In addition, from the excitation energy spectrum of 11Λ B plotted by choosing those events which have γ rays at 1482 keV, we found that this γ ray isemitted not only from the region near the ground state, but also from thehigher excited bound states. This situation is consistent with the property of
the E2 (1/2+ → 5/2+ ) transition that the 5/2+ state is populated not only
directly by the reaction but through higher excited states by γ ray cascade.
++
Therefore, this 1482 keV γ ray is identiﬁed as 11
Λ B (1/2 → 5/2 ) transition. To identify other transitions, γ-γ coincidence is necessary. Due tolow statistics, however, this method seems almost impossible. We need morebeam time to identify all the observed transitions.
3.5. Comparison with theory
Using the results of the previous experiments, the eﬀective interaction
parameters are determined as follows:∆ = 0.5 MeV [10], SN = −0.4 MeV [10],SΛ = −0.01 MeV [8], T = 0.03 MeV [9] .

(4)

Y. Miura et al.

1030

For the identiﬁed E2 transition of 11Λ B, the energy is given by the following
equation from a shell model calculation by Millener [11].

∆E

5+
1+
→
21
21


= 0.243∆ − 1.090SN + 1.234SΛ − 1.627T + ΛΣ + 718 keV= 1020 keV(5)Here, ΛΣ means the three-body force eﬀect which is related to two-pionexchange with a Σ in intermediate state. The three-body force is calculated
in the case of s-shell hypernuclei by Akaishi et al. [15], and applied to p-shellby Millener [3]. If we substitute spin-dependent parameters in Eq. (5) by
Eq. (4), this energy becomes 1020 keV.
In Table II, the measured and expected energies are compared for thetransitions observed in previous experiments. The energy spacings shownin the ﬁrst four lines of this table are used to determine the spin-dependent
parameters of ΛN interaction shown in the last column. For the next threetransition, the expected energy seems consistent with the measured one++
within 100 keV. In case of 11Λ B (E2; 1/2 → 5/2 ) , however the diﬀerence
is more than 400 keV.
TABLE II
Comparison of measured and expected energy spacings of p-shell hypernuclei (seetext). The last column shows the spin-dependent force parameter which has thelargest contribution to the energy spacing.
Levels7++
Λ Li (3/2 , 1/2 )
7
+
+
Li
(5/2
,
1/2
)
Λ
+
+
9
Be
(3/2
,
5/2
)
Λ
16
− −
O
(0
,
1
)
1
Λ
+
+
7
Λ Li (1/22 , 1/21 )++13
Λ C (3/2 , 1/2 )− −16
Λ O (12 , 11 )
+
11
B
(1/2+
1 , 5/21 )Λ

Measuredenergy spacing
692 keV [4]2050 keV [4]48 keV [8]26 keV [9]3877 keV [4]4880 keV [14]6534 keV [9]1482 keV

Expectedenergy spacing3779 keV [11]4831 keV [11]6435 keV [11]1020 keV [11]Spin-dependent
term
∆
SN
SΛ
T
SN
SN
SN
SN

The energies for the last four transitions in Table II are mainly determined by SN . If we determine SN from the measured E2 energy of 11Λ B,
this parameter becomes −0.9 MeV. It is much diﬀerent from the value of−0.4 MeV obtained from 7Λ Li. From the other transitions, −0.9 MeV is also
unacceptable. Therefore, only 11
Λ B seems inconsistent with other results. It
suggests that the present experimental data are not enough to conﬁrm ΛNspin-dependent interaction.

A Recent Experiment with Hyperball1031

4. Summary
An experiment for γ spectroscopy of 11Λ B (KEK-PS E518) was performed
to conﬁrm ΛN eﬀective interaction parameters and to measure the magnetic
moment of Λ in a nucleus.
Using the (π + , K + ) reaction, six γ transitions from 11
Λ B were observed.
Among the observed γ transitions, the 1482 keV γ ray was identiﬁed as 11
ΛB(E2; 1/2+ → 5/2+ ), but its energy is diﬀerent from a theoretical prediction.
It seems interesting to investigate structure of several more hypernuclei to
conﬁrm ΛN interaction.
In order to identify all of the γ transitions observed in this experiment,
γ-γ coincidence method is necessary. Due to low statistics, it is diﬃcult to
identify them with the present data, but it will be possible in near future byusing an upgraded Hyperball.
REFERENCES[1] R.H. Dalitz, A. Gal, Ann. Phys. (N.Y.) 116, 167 (1978).
[2] D.J. Millener, A. Gal, C.B. Dover, R.H. Dalitz, Phys. Rev. C31, 499 (1985).
[3] D.J. Millener, Nucl. Phys. A691, 93c (2001).
[4] K. Tanida et al., Phys. Rev. Lett. 86, 1982 (2001).
[5] T. Motoba, H. Bandō, K. Ikeda, Prog. Theor. Phys. 80, 189 (1983).
[6] E. Hiyama, M. Kaminuma, K. Miyazaki, T. Motoba, Phys. Rev. Lett. 86, 1982
(2001).
[7] H. Tamura et al., Phys. Rev. Lett. 84, 5963 (2000).
[8] H. Akikawa et al., Phys. Rev. Lett. 88, 082501 (2002).
[9] H. Tamura et al., Proc. Int. Symp. on Electrophoto-production of Strangenesson Nucleons and Nuclei (SENDAI03), June 2003, World Scientiﬁc, Ed. K.
Maeda et al., to be published.
[10] K. Tanida et al., Nucl. Phys. A721, 999c (2003).
[11] D.J. Millener, private communication.
[12] T. Fukuda et al., Nucl. Instrum. Methods A361, 485 (1995).
[13] T. Motoba, private communication.
[14] H. Kohri et al., Phys. Rev. C65, 034607 (2002).
[15] Y. Akaishi, T. Harada, S. Shinmura, K.S. Myint, Phys. Rev. Lett. 84, 3539
(2000).

E521 T. Fukuda
Production of Neutron-rich Λ Hypernuclei by the the (π-, K+) Double-charge
Exchange Reaction
P. K. Saha
[attached]
Production of Neutron-Rich Lambda Hypernuclei by the (!-, K+) Double Charge-Exchange
ReactionInternational Conference on Hypernulear and Strange Particle Physics, JLab, Oct. 14-18, 2003.
＊ W. Imoto
Neutron-Rich Lambda Hypernuclear Production by the (π-, K+) Double Charge-Exchange Reaction
Poster presentation at the International Conference on Hypernulear and Strange Particle Physics,
JLab, Oct. 14-18, 2003.
◇________________________________________________________________________● Physics papers published in refereed journal.
○★
◇＊

Technical papers.
PhD theses.
Conference and Symposium.
Internal Report and others.

                  

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E522 K. Imai
Search for H-dibaryon Resonance via 12C(K-, K+Λ Λ) and Study of Ξ-NInteractions
C.J.YoonInvarinat-Mass Spectrum of ΛΛ
KEK Strangeness Workshop,KEK, Japan (To be printed KEK-PS Annual Review '03).
◇ C.J.Yoon[attached]
Search for H-dibaryon Resonsnce
Yukawa Institute for the Theoretical Physics at Kyoto Univ.,Japan, Feb.,2004
YITP workshop on YITP-W-03-21(To be Printed in 'Soryuushi-ron Kenkyu'; Summer 2004).
◇ J.K.Ahn
Multiquark states from (π-,K-) (K-,K+) reactions
Workshop on Quark Nuclear Physics 2004(QNP'04), Busan, Korea, Feb., 2004.
◇________________________________________________________________________● Physics papers published in refereed journal.
○★
◇＊

Technical papers.
PhD theses.
Conference and Symposium.
Internal Report and others.



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10τΛPDG=2.632±0.020×10-10s

1
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02∆MΛ=70MeV/c

-100
-100

0

100
MΛ1-1115.7(MeV/c2)

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QRB+KdQ"kJKdVJLWEJ'+ ´W6BZg=?. pJLk6=f.MJLW"^`JKQRQ !FW67+ *,JK.)V¶VmZW6Z W6=f+p.
Z  =fZ5. tr`JLQJ I. W6=?+/.Ä+ $W6B;M
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100

2



Efficiency (Arbitrary Unit)

MΛ2-1115.7(MeV/c2)

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0.2

20
15
10
5
0
2220 2240 2260 2280 2300 2320 2340 2360
2ΛΛ Invariant Mass (MeV/c )2220 2240 2260 2280 2300 2320 2340 2360
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2Phase Space for-1210+
K C→ Be(g.s.)ΛΛK

1.5

INC Calculation
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2

"*

1

Preliminary
0.5

02220 2240 2260 2280 2300 2320 2340 2360
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