POSITRON-PRODUCTION EXPERIMENT BY 8-GeV CHANNELING
ELECTRONS IN CRYSTAL TUNGSTEN AT THE KEKB INJECTOR LINAC
T.Suwada* , S.Anami, A.Enomoto, K.Furukawa, K.Kakihara, T.Kamitani,Y.Ogawa, S.Ohsawa,and T.Oogoe
Accelerator Laboratory, KEK, Tsukuba, Japan
H.OkunoInstitute of Particle and Nuclear Studies, KEK, Tsukuba, Japan
T.Fujita, K.Umemori, and K.Yoshida
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima, Japan
R.Hamatsu and K.Sasahara
Department of Physics, Tokyo Metropolitan University, Hachioji, Japan
V.Ababiy, A.P.Potylitsyn, and I.E.Vnukov
Nuclear Physics Institute, Tomsk Polytechnic University, Tomsk, Russia
Abstract
An experiment to study the positron production
efficiency by 8-GeV electrons hitting an axially oriented
tungsten monocrystalline target was carried out by using asingle-bunch beam (10 ps width) from the KEKB injector
linac. In this experiment a 2.2-mm-thick crystal tungsten
is tested either alone or in combination with anamorphous tungsten (5.0 and 10.0 mm thick). Positrons
emitted from the target in the forward direction are
momentum-analyzed by a magnetic spectrometer in the
momentum range 10-30 MeV/c and are detected with a 3mm-thick acrylic Cherenkov counter and a lead-glass
calorimeter. Since produced positrons are shortly bunched,
the number of positrons per bunch are measured as a pulse
charge from each detector. The positron yields have beenmeasured for each target as a function of the rotational
angles of a goniometer. For the 2.2-mm-thick crystal
target, a five-times enhancement of the positron yield wasobserved when the crystal axis, <111>, was oriented tothe incident electron beam. For the combined-type target,
the positron yield enhancement decreased depending on thethickness of the amorphous part. This paper describes thefirst result of this series of experiments.

1 INTRODUCTION+ -For future e e linear colliders and high-luminosity Bfactories, it is critically important to develop a highintensity positron source. In a conventional method using
an amorphous heavy-metal target, the target thickness is
optimized by taking into account the electromagnetic
shower process and the positron capture efficiency in thesucceeding acceleration section. The optimum thickness is
4-5 radiation length for the 4-8 GeV electron beam. The
only possibility to increase the positron intensity is toincrease the incident electron intensity. However, theelectron intensity is limited due to a heat load on the*target. One promising method utilizing a crystal target
has been proposed by Chehab et al.[1] in 1994. The
benefit of this method is a high positron-production
efficiency due to channeling radiation (CR) and coherent
bremsstrahlung (CB), since CR and CB increase lowenergy photons in the radiation process. This results in athinner target compared with the conventional method,which is due to high-intensity low-energy photons
originating from the CR and CB. It is also expected thatthe thin target relaxes its heat load, and the spatial spreadof positrons due to multiple scattering in the target issuppressed. Yoshida et al. demonstrated a clear
enhancement of the positron yield in a crystal tungsten
target using a 1.2-GeV electron beam[2]. This new
scheme was tested at the positron station of the KEKB
injector linac[3,4]. The result indicates that, when ahybrid target made of 1.7-mm-thick crystal tangsten and
7.0-mm-thick amorphous one is used and the crystal axis,<111>, is oriented to the 3-GeV electron beam, thepositron yield is enhanced by 40% compared with that forthe disoriented case. Chehab et al. have also studied thepositron yield from the crystal target for 5-40 GeVelectrons at CERN-SPS[5]. Although the positronenhancement is expected, there have been so far only a
few experimental researches over a wide energy range ofprimary electron beams and, thus, theoretically unifiedtreatments taking into account both processes of the CRand CB have not yet been established. More precise
experimental data will help us to understand thecomplicated mechanism of these elementary radiation
processes and to realize the high-intensity positron source.
A series of experiments to investigate the positronyields under various crystal target conditions are planned
for an incident electron energy below 8 GeV.

Corresponding author. Tel.: +81-298-64-5684; fax: +81-298-64-7529. E-mail address: tsuyoshi.suwada@kek.jp (T. Suwada).

2 EXPERIMENTAL SETUP
2.1 Beam Line
Our experiment was performed at the energy analyzer
line in the beam switchyard of the KEKB 8-GeV injector
linac. The 8-GeV electron beam with a pulse width of 10
ps impinged on a tungsten target at a repetition rate of 2Hz. The beam intensity in this experiment was 0.2
nC/bunch, which was limited mainly by the radiation
safety condition. The electron beam charge was measured
by a wall-current monitor pulse-by-pulse. The transverse
beam sizes and the positions of the electron beam at thetarget were monitored by a screen monitor during theexperiment. The transverse beam size was about 3 mm(FWHM) in diameter. The normalized transverse
emittance was about 120 πmm•mrad, and the angularspread of the electron beam was expected to be less than 5µrad. Since the electron beam impinged on the target after
passing through a vacuum window made of 100-µm-thickstainless steel (SUS304), it was expected that the angularspread of the electron beam would be 0.1 mrad in total,taking into account multiple scattering. This angularspread was less than the critical angle (0.43 mrad) of thechanneling condition at 8 GeV.

which deflects positrons at 60o from the beam axis in a
momentum range less than 30 MeV/c. The positron
trajectory was determined by the collimators installed
before and behind the magnetic spectrometer. The
momentum-analyzed positrons were detected with a 3mm-thick acrylic Cherenkov counter and a lead-glass
calorimeter well shielded by lead blocks. The lead blocks
suppressed any background caused by electromagnetic
showers generated upstream of the beam line due to theoff-momentum electron beam, and caused byelectromagnetic showers generated at the collimators.
Since the emitted positrons were also shortly bunched, thenumber of positrons per bunch was measured as a pulse
charge from each detector. Signals from the positrondetectors and the signal of the electron beam charge weresent to a data-acquisition system using a PC-based
CAMAC/ADC system, where all signal charges weresimultaneously digitized. The goniometer could rotate thecrystal target around two axes (the horizontal(H) andvertical(V) axes) by a pulse motor. The angularresolutions of the goniometer were 10.5 and 34.9µrad/pulse in the H and V axes, respectively. The crystalaxis, <111>, was searched by changing the relativerotational angles around the two axes with a step of 2mrad. The positron yields were measured for each target as
a function of the rotational angle of the goniometer and as
a function of the positron momentum.

Beam

Rotational Axes ofthe Goniometer

Lead-Glass Calorimeter
Acrylic Cherenkov Counter

V Axis
H AxisLead-Block Shields

Collimators(φ50mm)

e+Target/Goniometere+e-60o

8-GeV e- BeameVacuum Window
Screen Monitor
Wall-Current Monitor

Figure 1: Photograph of the beam line and theexperimental setup viewed from the electron beam.

2.2 Experimental Setup
Figure 2 shows a schematic drawing of theexperimental setup. This comprises a positron-production
target mounted on a precise goniometer, a magnetic
spectrometer, collimators, and two kinds of positrondetectors. In this experiment, a 2.2-mm-thick crystal
tungsten (W c) was tested either alone or in combination
with an amorphous tungsten (Wa) (5.0 and 10.0 mmthick). Positrons emitted from the target in the forward
direction were momentum-analyzed by the magnetic field,

Collimators(φ20mm)Magnetic Spectrometer

Figure 2: Schematic drawing of the experimental setup.

3 EXPERIMENTAL RESULTS
3.1 Result for the Tungsten Crystal
After a linearity check for two positron detectors, theenhancement of the positron yield was measured bychanging the V and H angles of the goniometer for the2.2-mm-thick crystal tungsten target (W c). Figure 3(a)shows the result of the relative positron yield (rockingcurve) measured by the lead-glass calorimeter at amomentum of 20 MeV/c. Each data point shows theaverage value and the standard deviation measured twentytimes at the same rotational angle after correcting the85.0 ± 0.1
1.9 ± 0.1
1.2 ± 0.1

3Off-Axis
21

FWHM(mrad)
7.6
10.0
15.5

4 CONCLUSIONS
Positron-production experiments by 8-GeV electrons
hitting an axially-oriented tungsten monocrystalline target
were carried out using single-bunch beams at the KEKB
injector linac. The enhancements of the positron yields
were 5.0 ± 0.1, 1.9 ± 0.1, and 1.2 ± 0.1 for targets with
2.2-mm-thick crystal tungsten, two combinations with 5mm-thick and 10-mm-thick amorphous tungsten at a
momentum of 20 MeV/c, respectively, when the crystal
axis, <111>, was oriented to the incident electron beam.
The angular width of the rocking curve varied, dependingon the total thickness of the target. The results indicatethat the angular width is mainly due to the contribution
from the coherent bremsstrahlung and multiple scattering
at the target since the critical angle of the channelingradiation is much less than it.

-80-60-40-20020020020

Rotational Angle [mrad]
8(b)Wc + 5-mm-thick Wa data

65Off-Axis
4On-Axis3210-100-80-60-40-20

Rotational Angle [mrad]
87

Relative Positron Yield[arbitrary unit](a) 2.2-mm-thick Wc(b) Wc+5-mm-thick Wa(c) Wc+10-mm-thick Wa

4-100

Table 1: Measured enhancements of the positron yields forthree target combinations.
Enhancement

On-Axis50

Relative Positron Yield[arbitrary unit]

Enhancement measurements were also performed for
combined targets, that is, the same W c with a 5.0-mmthick amorphous tungsten (Wa) and W c with a 10.0-mmthick W a . The W a’s were mounted just behind the W ctarget. The results at a positron momentum of 20 MeV/c
are shown in Figs.3 (b) and 3 (c). It is noted that thevertical axis of the figures indicates a relative positronyield in arbitrary unit and only the enhancement factors
are meaningful in the results. Three experimental data aresummarized in Table 1.

2.2-mm-thick Wc data

67

3.2 Results for the Combined TargetsTarget type(a)7Relative Positron Yield[arbitrary unit]

pedestals of the detectors, subtracting the backgrounds,
and normalizing by the electron-beam intensity. The
pedestal data of the detection system were measured under
the conditions: (1) without an electron beam, (2) without
the following RF power of the end klystron, and (3)without a magnetic field. The beam-associated backgrounddata were also measured under the condition without amagnetic field. The enhancement of the relative positronyield is defined by the ratio of the peak yield (on-axis) tothe yield at the base region 50 mrad apart from the crystal
axis in the H scan. The observed enhancement was 5.0
and the FWHM of the rocking curve was 7.6 mrad. The
momentum dependence of the positron enhancement wasalso measured in the same way in the 10-30 MeV/cregion. No appreciable dependence was observed.

(c)

Wc + 10-mm-thick Wa data

6On-Axis
Off-Axis543
210-100-80-60-40-20

Rotational Angle [mrad]
Figure 3: Relative positron yields as a function of thegoniometer rotational angle around the H axis for threetarget combinations. The angle around the V axis is fixed
to the position giving the yield peak. The positron
momentum: 20 MeV/c, and the positron emission angle:
0 degree.

REFERENCES[1] R.Chehab, Orsay Report No. LAL/RT94-09, 1994.
[2] K.Yoshida, et al., Phys. Rev. Lett. 80, 1437 (1998).
[3] Design Report on PF Injector Linac Upgrade for
KEKB, KEK Report 95-18, 1996.
[4] M.Inoue, et al., NIM B 173 (2000) p.104.
[5] R.Chehab, et al., LINAC2000, Monterey, CA,USA, August 2000, p.143.