Research Report of Laboratory of Nuclear Science, Tohoku University Vol. 41 (2008)

1(LNS Experiment : #2534, #2576)

Performance of the Tagging System for GeV-γ Beamline
STB-Tagger II
T. Ishikawa1 , T. Fujibayashi2 , H. Fujimura1 , H. Fukasawa1 , R. Hashimoto1 ,T. Iwata3 , H. Kanda2 , J. Kasagi1 , H. Kato3 , T. Kinoshita1 , M. Itoh4 ,K. Maeda2 , M. Matsuzawa2 , T. Michigami3 , F. Miyahara1 , K. Mochizuki1 ,K. Murakami3 , T. Nakabayashi1 , K. Nawa1 , T. Nomura3 , K. Okamura1 ,H. Okamura5 , H. Okuyama3 , Y. Onodera1 , Y. Saitoh1 , T. Sasaki3 , M. Sato1 ,H. Shimizu1 , T. Shishido3 , S. Suzuki1 , K. Suzuki1 , Y. Tajima3 , T. Takahashi6 ,S. Takita3 , H. Ueno3 , H.Y. Yoshida3 , S. Yamaguchi3 , and H. Yamazaki1
1Laboratory of Nuclear Science, Tohoku University, Sendai 982-0826
23456

Department of Physics, Tohoku University, Sendai 980-8578

Department of Physics, Yamagata University, Yamagata 990-8560

Cyclotron and Radioisotope Center, Tohoku University, Sendai 980-8578

Research Center for Nuclear Physics, Osaka University, Ibaraki 567-0047

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

High energy photons shed light on nucleon resonances by measuring produced mesons on the nucleon. In the GeV-γ beamline [1], bremsstrahlung photons are used as a beam, which are generated
by inserting a carbon ﬁber (radiator) into circulating electrons on the STretcher Booster (STB) ring. A
tagging system has been constructed to determine the energy of the photon associating with the meson
produced event measured by a detector system on the beamline.
The STB-Tagger II consists of 116 telescopes of two scintillating ﬁbers, and each telescope (tagger)
determines the trajectories of momentum-analyzed electrons which lose energy due to bremsstrahlungat the radiator. The photon energy is determined from the difference of energies of the circulating andrecoil electrons. Details of the STB-Tagger II are described elsewhere [2]. The tagging energy of eachtagger is reported in the internal notes [3]. A part of the tagging energy calibration is also presented ina research report of LNS [4]. In this report, the transmission rate of the photon, a low energy componentof the tagged photon, and a fraction of tagged photons to all the incident ones will be described.

§ 1. Transmission Rate
Some electrons which do not generate a photon caused by
1. Møller scattering,
2. Coulomb multiple scattering on the radiator or the gas particle in the vacuum chamber, and3. unstability of the circulating beam in the STB ringcome into the STB-Tagger II. Even if photons are generated, they may not come to the target position
due to2
1. large difference in direction between the generated photons and the beamline, and
2. conversion to positron and electron pairs with materials on the beamline.
The transmission rate is deﬁned as a ratio of the number of associating photons coming to the target
position to the number of electrons that each tagger responds to. It is experimentally measured since itis difﬁcult to incorporate all the status of circulating electrons in the calculation.
The energy of incident photons is measured with an SF-5 lead glass Čerenkov counter with a size of150 mm (W) × 150 mm (H) × 300 mm [5]. Undesirable charged particles in the beam are swept out witha dipole magnet RTAGX [6]. The trigger for the data taking system is an OR signal of the STB-Tagger II.
The transmission rate for the i-th tagger Ti is obtained asTi =Niγ,Ni(1)where Ni is the number of signals from the i-th tagger and Niγ is the number of coincidences between
the i-th tagger and the SF-5 counter with the corresponding photon energy. Figure 1 shows a sampleof the measured transmission rate. The details of determining the transmission rate are described
elsewhere [7].

Transmission Rate

0.6
0.5
0.4
0.3
0.2
0.1
0020406080100Tagger Channel
Fig.1. Measured transmission rate. The circles show the transmission rate for 1200 MeV circulating electron beam, and the triangles shows that for 920 MeV one.
The transmission rate as a function of the tagger channel shows two dips. These dips correspond tothe taggers to which supporting rods lie on the central orbit of ongoing electrons. The vacuum chamberof the STB ring in the BM5 has a Ti window with a thickness of 50 μm to introduce post bremsstrahlungelectrons into taggers. Two rods made of SUS support the window frame. The post bremsstrahlungelectrons impacting against these rods are not detected with the STB-Tagger II.
The circulating electrons have various momentum vectors at the radiator point. Some electrons
generating bremsstrahlung photons take different courses and are detected with the STB-Tagger II. The
corresponding photons are blocked with a lead aperture. These events deteriorate the transmission rate,and make two dips in that as a function of the tagger channel. Figure 2 shows how the transmission

3rate is deteriorated for the taggers which are shaded with the supporting rods.

TaggerLead Aperture
Recoil ElectronRadiator

Gamma RayFig.2. How the transmission rate is deteriorated for the taggers which are shaded with thesupporting rods. The photons generated by inclined electrons do not come to the target
point, while the post bremsstrahlung electrons do not hit the rods and are detected withthe STB-Tagger II.
The transmission rate is estimated with a Monte Carlo calculation, and the measured one is qualitatively reproduced. Figure 3 shows the transmission rate estimated with the calculation. Its shape in
the lower channel region is determined by an effect of Møller scattering electrons. Most of the Møller
scattered electrons are blocked with an iron fence (ﬁeld cramp) [8] for the reducing leakage magneticTransmission Rateﬂux in the region where photo-multiplier tubes of the STB-Tagger II are placed.

0.7

0.7

0.6

0.6

0.5

0.5

0.4

0.4

0.3

0.3

0.2

Moeller Scattering

0.1
000.2

Moeller Scattering

1200 MeV 0.125507510000920 MeV
25Tagger Channel

5075100Fig.3. Transmission rate estimated with a Monte Carlo calculation. The left and right panels
show the transmission rates for 1200 and 920 MeV circulating electron beams, respectively. The upper points show the transmission rate and the lower show the Møller scattering effects. The transmission rate obtained by the calculation qualitatively reproduces
the measured one.

§ 2. Low Energy Component of Tagged Photons
The measured energy of incident photons shows a low energy component for the tagging channels
larger than 102 [9]. This low energy component has been a long standing problem on the STB-Tagger II,

Counts

4400300

105200Recoil Electron

1000x0500
1000
1500
Lead Glass Cherenkov Gainzγ RayFig.4. Events with post bremsstrahlung electrons impacting against the frame of the Ti window. The energy of the generating photons is lower than that of the nominal tagged
photons. The left panel shows the ADC distribution for the 105-th tagging channel, andthe right one shows the trajectories of generated particles in such a event.
and a Monte-Carlo simulation has revealed its source. Some post bremsstrahlung electrons impacting
against the frame of the Ti window are detected with the STB-Tagger II. These electrons are mainly
detected with the tagging channel larger than 102, and the energy of the corresponding photons is lower
than that of the nominal tagged photons. Figure 4 shows the events with post bremsstrahlung electrons
impacting against the frame of the Ti window.

§ 3. Fraction of Tagged Photons
A fraction of tagged photons to all the incident ones was measured with the SF-5 counter locatedon the GeV-γ beamline. In the ﬁrst step, the energy of the counter was measured where the trigger forthe data taking system was the OR signal of the STB-Tagger II. Energy calibration was made by using
these data so that the measured energy should be the incident photon one corresponding to the tagging
channel. Only the tagged photons without the low energy component were used in the calibration.
Figure 5 shows the mean of the ADC distribution as a function of the tagging energy. The photon energy
was obtained from the ADC value A as
Eγ =A − 78.87 ± 0.01.
1.3202 ± 0.0001(2)

In the next step, the energy was measured with the SF-5 counter where the trigger was the signalof the counter itself. Figure 6a) shows the energy distribution measured with the SF-5 counter together
with that in coincidence with STB-Tagger II, and Figure 6b) shows a fraction of tagged photons as afunction of the energy measured with the SF-5 counter. Fractions of tagged photons in the photon
energy raging from 800 to 1100 MeV are about 65%. A fraction of all the tagged photons to all thephotons detected with the SF-5 counter were 7.22%.
Number of incident photons measured with the SF-5 counter depends on the threshold value of its
discriminator. The energy distribution was ﬁtted from 0 to 1000 MeV with a function as
x − 33.12 ± 0.04
Eγ−1.149f (Eγ ) = (5.91 ± 0.02) freq
9.23 ± 0.03

(3)where freq stands for the Gaussian convoluted step function [11]. The threshold values were found to becorresponding to 33.12 ± 0.04 MeV. Number of incident photons with the energy less than discriminator518001600
1400

ADC

1200
1000
800
600
40020000200400600
800
Incident γ Energy (MeV)

1000

Fig.5. Mean of the SF-5 ADC distribution as a function of the tagging energy. The solid curveshows the function (2).

10 5Counts/ 10 MeV

10 410 310 210
10200 400 600 800 1000 1200 1400
Energy (MeV)

Fig.6. a) Energy distribution of the SF-5 counter. The upper histogram shows the energy distribution and the lower shows that coincidence with STB-Tagger II. The solid curve shows
the ﬁtted function (3). b) Fraction of tagged photons as a function of the energy measuredwith the SF-5 counter.

6threshold were deduced as an integration of a function
f (Eγ ) = (5.91 ± 0.02) Eγ−1.149 ,(4)from 2me to the threshold value where me denotes the mass of the electron. The number of all theincident photons were given by summing the numbers of photons detected with the SF-5 counter andphotons with the energy less than the threshold. A fraction of all the tagged photons to all the incident
photons was obtained to be 2.78%. The details of the analysis are described elsewhere [12].

References[1]

H. Shimizu et al.: Research Report of LNS, Tohoku University 37 (2004) 13.

[2]

T. Nakabayashi et al.: Research Report of LNS, Tohoku University 37 (2004) 17.

[3]

T. Ishikawa: Internal GeV-γ Analysis Note No. 15 (2006);
T. Ishikawa: Internal GeV-γ Analysis Note No. 23 (2006);
T. Ishikawa: Internal GeV-γ Analysis Note No. 24 (2006);
T. Ishikawa: Internal GeV-γ Analysis Note No. 25 (2006);
K. Suzuki: Internal GeV-γ Analysis Note No. 35 (2006–2007);
T. Ishikawa: Internal GeV-γ Analysis Note No. 55 (2007);
T. Ishikawa: Internal GeV-γ Analysis Note No. 57 (2007);
K. Suzuki: Internal GeV-γ Analysis Note No. 59 (2007);
T. Ishikawa: Internal GeV-γ Analysis Note No. 60 (2007).

[4]

K. Suzuki et al.: Research Report of LNS, Tohoku University 40 (2007) 10.

[5]

M. Sato and T. Ishikawa et al.: in this Research Report of LNS, 41 (2008) 22;
M. Sato: Internal GeV-γ Analysis Note No. 86 (2008).

[6]

T. Ishikawa et al.: Research Report of LNS, Tohoku University 39 (2006) 39.

[7]

T. Nakabayashi: Internal GeV-γ Analysis Note No. 29 (2006);
T. Ishikawa: Internal GeV-γ Analysis Note No. 31 (2006);
F. Miyahara: Internal GeV-γ Analysis Note No. 88 (2008).

[8]

T. Nakabayashi et al.: Research Report of LNS, Tohoku University 37 (2004) 17;
T. Ishikawa: Internal GeV-γ Analysis Note No. 24 (2006);
T. Ishikawa: Internal GeV-γ Analysis Note No. 25 (2006).

[9]

T. Nakabayashi: Internal GeV-γ Analysis Note No. 14 (2006);
T. Ishikawa: Internal GeV-γ Analysis Note No. 15 (2006).

[10] T. Ishikawa: Internal GeV-γ Analysis Note No. 61 (2007).
[11] Computing and Networks Division, CERN Program Library Ofﬁce: CERNLIB — CERN Program
Library Short Writeups (1996).
[12] T. Ishikawa: Internal GeV-γ Analysis Note HD No. 125T (2009).