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First beam test on a BSO electromagnetic calorimeter
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 550 (2005) 258–266 www.elsevier.com/locate/nima
First beam test on a BSO electromagnetic calorimeter H. Shimizua,, F. Miyaharaa, H. Hariub, T. Hayakawaa, T. Ishikawaa, M. Itayab, T. Iwatab, T. Kinoshitaa, M. Moriyab, T. Nakabayashia, T. Sasakib, Y. Tajimab, S. Takitab, M. Yamamotob, H. Yamazakia, H.Y. Yoshidab, Y. Yoshidab a
Laboratory of Nuclear Science, Tohoku University, Sendai 982-0826, Japan b Department of Physics, Yamagata University, Yamagata 990-8560, Japan Received 11 May 2005; accepted 7 June 2005 Available online 6 July 2005
Abstract A beam test has been performed on a prototype electromagnetic calorimeter of bismuth silicate BSO crystals for the first time using electrons of 0.5–3.0 GeV. The calorimeter consists of nine BSO crystals, 22 mm 22 mm 180 mm (15.6 radiation lengths), arranged in a 3 3 array. Each crystal is coupled to a small photomultiplier tube of 20 mm in pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi diameter. The obtained energy resolution is sE =E ¼ ð0:023 0:003Þ= EðGeVÞ ð0:017 0:002Þ for incident electrons of the energy E in GeV. The position information for an electron injected onto the center crystal of the calorimeter array is given withpenergy deposit signals from nine BSO crystals. The RMS position resolution sx is parameterized as ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi sx ¼ ½ð3:2 0:1Þ= EðGeVÞ ð1:3 0:1Þ mm at the center of the crystal. Misidentification probability of pions to electrons is found to be 10 3 for the pion momentum ranging from 1.0 to 3.0 GeV/c. r 2005 Elsevier B.V. All rights reserved. PACS: 29.40.Vj; 25.20.Lj; 29.40.Mc Keywords: Bismuth silicate; BSO
1. Introduction Different types of electromagnetic (EM) calorimeters have been utilized in Nuclear and Particle Physics, depending on the energy of photons to be Corresponding author. Tel.: +81 22 743 3423;
fax: +81 22 743 3401. E-mail address: [email protected] (H. Shimizu).
detected and on the event rate, etc. We are especially interested in a 4p EM calorimeter in the energy region of several hundred MeV for Quark Nuclear Physics experiments to be conducted at SPring-8/LEPS and Laboratory of Nuclear Science, Tohoku University. In this energy region, GSO, BGO and CeF3 are excellent materials for the 4p calorimeter in principle among the available crystals. They have good
0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.06.040
ARTICLE IN PRESS H. Shimizu et al. / Nuclear Instruments and Methods in Physics Research A 550 (2005) 258–266
characteristics such as a good energy resolution, fast response (a little longer decay time for BGO, though), a short radiation length, and no hygroscopicity. However, GSO and BGO are rather expensive for a 4p detector. Presently, it is difficult to grow a single long CeF3 crystal. Bismuth silicate, Bi4 Si3 O12 (BSO) [1], produced by replacing Ge in BGO with Si has been developed as an alternative to BGO. The material cost for BSO is expected to be significantly reduced, compared with that for BGO since the main part of the cost for BGO production comes from high-purity powder of GeO2 which is expensive. BSO resembles BGO in physical and optical properties but has a little different scintillation characteristics. The decay time of BSO is faster than that of BGO, which is welcome for calorimeters, while the light output is 25% [3] of BGO, which is, however, still quite acceptable for detecting several hundred MeV photons. Table 1 shows the characteristics of BSO together with some available crystals for EM calorimeters just for comparison. BSO crystals have not been used up to now notwithstanding their good features as a scintillation material for EM calorimeters because it was difficult to grow a large single crystal before. Employing the vertical Bridgman method, Ishii and his colleagues recently succeeded in growing up single crystals of BSO [2,20,21] with a size of 35 mm in diameter and about 200 mm in length. Subsequently, some additional techniques for mass production of large BSO crystals were put into operation. Now clear single crystals of BSO as large as 70 mm in diameter have become available.
259
2. Experiment 2.1. BSO calorimeter The calorimeter used in the present study comprises nine identical BSO crystals arranged in a 3 3 array, as shown in Fig. 1. The size of the crystals is 22 mm 22 mm in cross-section and 180 mm long, which corresponds to 15.6 radiation lengths along the direction of incident photons/ electrons. These single crystals were produced at Futek Furnace Co., Yokohama, where three large BSO ingots of 70 mm in diameter were grown for this purpose. The crystals, all the faces of which were optically polished, were wrapped with 15 mm thick aluminum foil. There are several other wrapping materials such as Teflon, Millipore foil, Tyvek paper and so on. We used aluminum foil
Fig. 1. Schematic view of the prototype BSO calorimeter.
Table 1 Comparison of properties of scintillating crystals
Density ðg=cm3 Þ Radiation length (mm) Decay time (ns) Peak emission (nm) Relative light outputa Hygroscopicity
BSO [1–3]
BGO [4]
GSO(Ce) [5–8]
CeF3 [9–11]
CsI (pure) [12–14]
PWO [15–19]
6.80 11.5 100 480 0.04 No
7.13 11.2 300 480 0.1 No
6.71 13.8 30–60 430 0.2 No
6.16 16.8 10–30 310–340 0.07 No
4.53 18.5 30f , 680s 300f , 480s 0:08f , 0:02s Some
8.28 8.9 10 410–500 0.01 No
f ¼ fast component, s ¼ slow component. a Approximate light yield at room temperature, 1 for NaI(Tl).
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simply because it would be one of the thinnest reflector giving good optical isolation. One end of each crystal was left unwrapped to give an optical contact with a 32 mm long light guide connected to a photomultiplier tube, Hamamatsu R4125GMOD. The optical contact was made by using OKEN6262A optical compound, the refractive index of which was 1.42 at l 450 nm. The decay time was measured by utilizing the conventional single photoelectron method in a separate-scintillator arrangement [22] for a small block of BSO which was cut with a size of 22 mm 22 mm 20 mm from one of the ingots providing the nine crystals used in the beam test. The setup for the measurement of the scintillation decay time is shown in Fig. 2. A 60Co radioactive source, emitting two cascade g-rays (1.17 and 1.33 MeV), was employed in the decay time measurement. A 30 mm thick plastic scintillator was mounted on a 2 in. photomultiplier tube, HAMAMATSU H2431 to detect g-rays from the 60 Co source and to produce the start pulse. The distance between the photocathode of the stop counter, HAMAMATSU H6410, and the BSO block was adjusted so as to give single photoelectrons. The overall time resolution of the system was 0.7 ns for this measurement. The obtained 60Co
Collimater Start counter
BSO
200mm Plastic scintillator
Lead block
Number of Events
260
0
50
100 150 200 250 300 350 Decay time [ns]
Fig. 3. Decay time spectrum of BSO scintillation. The solid curve gives a fit with a sum of two exponential functions and a constant background.
decay time spectrum is shown in Fig. 3. The data were fitted with two exponential functions with a constant background, giving the decay times, t1 ¼ 30 13 ns with an intensity of 43 4% and t2 ¼ 106 6 ns with 57 4%. The very fast component of the decay time of about 2 ns reported by Kobayashi et al. [3] was not observed in the present measurement in which no correction was made for spectral response of the phototubes to different excitation–emission wavelengths. The decay times of scintillation in PWO and NaI(Tl) were also measured as points of reference in the same system. The obtained values for PWO were t1 ¼ 4:8 0:7 ns (95 1%) and t2 ¼ 25 3 ns (5 1%) and that for NaI(Tl) was t ¼ 242 5 ns. These are consistent with known values for PWO and NaI(Tl) scintillators. 2.2. Experimental setup for the beam test
Stop counter Fig. 2. Setup for the measurement of the BSO scintillation decay time.
A beam experiment on the BSO calorimeter was performed at the p2 beam line of the proton synchrotron facility in KEK. The beam momenta used in the experiment were 0.5, 1.0, 1.5, 2.0 and 3.0 GeV/c. The experimental setup is illustrated in
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Plastic greenhouse
MWPC S1
261
S2
S3
S4
BSO crystals CO2 Gas Cherenkov Counter SF6 Gas Cherenkov Counter 11.0 m Fig. 4. Plan view of experimental setup.
5000 π+ Number of Events
Fig. 4. Three plastic scintillation-counters, S1, S2 and S3, were used as a set of trigger counters. Particle identification was made with time of flight (TOF) signals between S1 and S3. Three gas Cherenkov counters (GCC1-3) were installed in the p2 beam line to distinguish electrons/positrons from the other charged particles. The GCC1 was filled with 1.8 atm CO2 gas while 1 atm SF6 gas was used for GCC2 and GCC3. Electrons/positrons were identified by requiring signals of GCC1 \ GCC2 \ GCC3. Four sets of multiwire proportional chambers (MWPC) were employed. Each MWPC has two wire planes, x and y. The wire spacing for all these planes is 2 mm. The trajectory of a charged particle injected onto the calorimeter was given by three sets of MWPCs placed downstream. The temperature dependence of the light output of BSO is as large as 2%= C at room temperature [1]. Therefore, the calorimeter was placed in a thermally controlled plastic greenhouse located at the end of the beam line so as to keep the temperature of BSO crystals unchanged. The temperature in the greenhouse was monitored during the experiment and was kept to be 17 1 C. The momentum spread of the beam in the p2 beam line has been evaluated with TOF spectra given by the time difference between signals from S1 and S4 plastic counters, the thickness of which is 10 and 30 mm, respectively. The distance between S1 and S4 is just 11.0 m. In this measurement the calorimeter was moved down so that the beam was able to pass through the S4
4000
3000 µ+
2000
1000
0
e+
30
32
34
36 38 40 S1-S4 TOF [ns]
42
44
Fig. 5. TOF spectrum for 0.5 GeV/c particles.
counter. The time resolution in the TOF measurement is 0.18 ns which is deduced from the TOF spectra of b ’ 1 pions. Fig. 5 shows one of the TOF spectra for 0.5 GeV/c particles. The momentum 0.5 GeV/c corresponds to a rather low kinetic energy for protons to pass through all the materials such as the air, Cherenkov counters, MWPCs and plastic scintillators on the p2 beam line. Therefore, the data for protons with a momentum of 1 GeV/c ðb ¼ 0:73Þ have been used to estimate the momentum spread of the beam. The TOF spectrum for 1 GeV/c protons shows a beautiful Gaussian shape instead and gives a fitted
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value of sTOF ¼ 0:25 ns, where the broadening effect due to the energy loss of the beam is negligible. The relation between the momentum spread sp =p of the beam and the spread of the TOF spectrum sTOF =T is given by sp =p ¼ g2 sTOF =T, where T denotes TOF and g is the Lorentz factor of the beam. Thus, the momentum spread of the incident beam sp =p is found to be 0.7%, taking the time resolution into account. It is known that the momentum spread of the beam in the p2 beam line is roughly independent of the beam momentum. 2.3. Energy resolution
Number of Events
The obtained energy spectra are shown in Fig. 6 for 0.5, 1.0, 1.5, 2.0 and 3.0 GeV/c electrons incident on the 4 mm 4 mm area of the central crystal. The deposited energy was given by summing up all the signals of the nine crystals. The energy calibration for the crystal detectors was made by using 3.0 GeV/c electrons injected on to the center of each crystal one by one in such a way that the peak corresponding to the deposited energy in one crystal was equivalent to 2157 MeV
which was given by an EGS4 simulation. Every spectrum in Fig. 6 shows a tail on the low energy side, which is mainly due to longitudinal and also transverse EM-shower leakage. In this case the sum of a Gaussian and an additional broader Gaussian function can fit the spectrum empirically. There exist some unknown components making a tail on the high energy side as far as the 3.0 GeV spectrum is concerned. In any case each spectrum is fitted with two Gaussian functions to get the energy resolution. The energy resolution sE =E is deduced by subtracting the effect of the beam-momentum spread from the observed value sob =E ob which may be written as sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi s 2 s 2 sob E b . (1) ¼ þ E ob E Eb Here E b represents the beam energy and sb denotes the energy spread of the beam which is given from the momentum spread sp =p ¼ 0:007 of the p2 beam line. The data are summarized in Table 2, showing a good linearity between the incident energy and the energy deposit in the
150
0.5 GeV 40
1.0 GeV
100
1.5 GeV
80 60 40
20
50 20
0 200
400
0
600
800
[MeV]
0
1000
1250
[MeV]
1500
[MeV]
Number of Events
200
2.0 GeV
3.0 GeV
200
150 150 100
100
50 0 1500
50 1750
2000
[MeV]
0
2500
3000
[MeV]
Fig. 6. Response of the BSO calorimeter for 0.5, 1.0, 1.5, 2.0 and 3.0 GeV/c electrons. The solid curve gives a fit with a sum of two Gaussian functions represented with a dashed and a dotted curve.
ARTICLE IN PRESS H. Shimizu et al. / Nuclear Instruments and Methods in Physics Research A 550 (2005) 258–266 Table 2 Summary of the data obtained in the present beam test Beam momentum E b (GeV/c)
Energy deposit
Energy resolution
E ob (GeV)
sob =E ob
s=E
0.5 1.0 1.5 2.0 3.0
0.470 0.913 1.413 1.859 2.751
0:038 0:010 0:029 0:002 0:027 0:002 0:024 0:001 0:023 0:001
0:038 0:010 0:028 0:002 0:026 0:002 0:023 0:001 0:022 0:001
0.07 Experiment
Energy Resolution σE/E
0.06
Simulation
0.05 0.04 0.03
263
where denotes a quadratic sum, a means the statistical fluctuation of the detected energy E given in GeV and b is a constant. By fitting the data of sE =E with function (2), the coefficients are found to be a ¼ 0:023 0:003 and b ¼ 0:017 0:002, while a ¼ 0:019 0:001 and b ¼ 0:014 0:001 are given by the EGS4 simulation. There is a discrepancy between the data and the simulation result. The difference on the parameter a comes from the statistics of scintillation photons which are not taken into account in the simulation, since EGS4 can only handle the transport of electrons and photons in materials. However, the disagreement for the constant term b is not well understood. In general, shower leakage out of the calorimeter mostly accounts for the constant term. The rate of the shower leakage can be estimated to be about 7% from the observed energy deposits given in Table 2. The constant term itself can be reduced by making the crystal longer, although the disagreement observed for the parameter b may not disappear.
0.02
2.4. Position resolution 0.01 0 0
0.5
1
1.5 2 2.5 Energy [GeV]
3
3.5
Fig. 7. Energy resolution sE /E obtained for a 3 3 BSO crystal calorimeter. The solid line represents function (2) with the parameters given by fitting the data. The dashed line indicates an EGS4 simulation result under the same condition in the same detector arrangement. The quoted errors denote fitting uncertainties.
calorimeter. Fig. 7 shows the obtained energy resolutions sE =E (closed circles) and the result of a Monte Carlo simulation based on the EGS4 code (open circles) as a function of the incident electron energy. The simulation was made under the same geometrical condition as that of the experiment, namely electrons were injected on the 4 mm 4 mm area of the central crystal also in the simulation. The energy resolution is parametrized as sE a (2) ¼ pffiffiffiffi b E E
Position information for incident photons/electrons is necessary to provide the momentum vectors of themselves, with which the invariant mass, for example, of a meson decaying into photons and/or electrons (e.g. Z ! 2g, Z ! eþ e g, etc.) can be obtained. The center of gravity X CG for the incident position is simply given with measured energy deposits as P9 E i xi X CG ¼ Pi¼1 (3) 9 i¼1 E i where E i is the energy deposit and xi the x coordinate of the center of the ith crystal. Fig. 8 shows a scatter plot giving the relation between X CG and the impact position X reconstructed with MWPCs for 1.5 GeV/c electrons. The data are fitted with a function X CG ¼ p0 tan p1 ðX p2 Þ
(4)
where p0 ¼ ð3:0 0:1Þ mm and p1 ¼ ð0:114 0:004Þ rad=mm are obtained as a set of common parameters independent of the impact energy. The parameter p2 corresponds to a deviation coming
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6
10 7.5
5 Position Resolution [mm]
XCG [mm]
5 2.5 0 -2.5 -5 -7.5
4
3
2
1
-10 -10 -7.5
-5 -2.5 0 2.5 5 Impact position X [GeV]
7.5
10
Fig. 8. Scatter plot of X CG versus X for 1.5 GeV/c electrons. The solid curve gives the fitted result of function (4) with the energy-independent parameters.
about due to mis-alignment of the calorimeter and is found to be 0:34 mm for the x direction and þ0:21 mm for the y direction in this measurement. The position resolution of the BSO calorimeter is estimated with the RMS deviation of the position given by the inverse function of Eq. (4) with X CG from the MWPC reconstructed position. pffiffiffiffi The result is shown in Fig. 9, indicating a 1= E dependence. The RMS position resolution sx at the center of the calorimeter is thus obtained as " # ð3:2 0:1Þ sx ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð1:3 0:1Þ mm. (5) EðGeVÞ The position resolution is better than 5 mm for incident electrons of the energy greater than 0.5 GeV. 2.5. Electron/pion separation The e=p separation capability is one of the characteristics to be studied for EM calorimeters. A charged pion produced in experiments sometimes gives an energy deposit as large as an electron does due to the nuclear reaction taking place in the calorimeter. In this case the pion is
0 0
0.5
1
1.5 2 Energy [GeV]
2.5
3
3.5
Fig. 9. Position resolution at the center of the calorimeter. The solid line is the fitted result given by Eq. (5). The quoted errors are statistical uncertainties.
misidentified as an electron, if the pion momentum is given. In order to estimate the e=p separation capability, first of all, pions have to be well separated from electrons on the p2 beam line. Three GCCs were employed for this purpose. A hadron beam was defined by the trigger signal of GCC1 \ GCC2 \ GCC3. The detection efficiency of the ith GCC for electrons is given by i ¼
Y 123 Y jk
(6)
where Y 123 represents the number of three-fold coincidence events firing three GCCs simultaneously and Y jk means that of two-fold coincidence events. The obtained efficiencies are 1 ¼ 0:959, 2 ¼ 0:996 and 3 ¼ 0:987, respectively. Thus, the contamination rate of electrons in the hadron beam is found to be ð1 1 Þð1 2 Þ ð1 3 Þ ¼ 2:13 10 6 , which is good enough for the present study. The selection of pions in the hadron beam was made with TOF signals. There were some muons produced at the production target of the p2 beam line as indicated in Fig. 5. The rate of muons to pions in the 0.5 GeV/c hadron beam is 12.6% which is derived from the TOF data. It is almost impossible, however, to evaluate the rate from the
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slightly larger value than that for p at low momenta around 1 GeV/c, while p gives small values less than 10 3 for the pion momentum ranging from 1.0 to 3.0 GeV/c. Similar studies were made previously for CsI(pure) [24] and PWO crystals [25], giving about the same results for pþ and p , respectively, although the 2s range was defined as the region of electrons in those studies.
TOF data for higher momentum beams. But if the main part of the momentum-analyzed muons is assumed to come from pions decaying in the vicinity of the production target, the rate of muons to pions can be estimated. That is 6.3%, 4.2%, 3.1% and 2.1% for the beam momentum of 1.0, 1.5, 2.0 and 3.0 GeV/c, respectively, which are consistent with the previously measured values at KEK [23]. Fig. 10(a) shows the response of the BSO calorimeter for 3.0 GeV/c p and electrons injected onto the area of 20 mm 20 mm on the central crystal. The peak around 200 MeV corresponds to the minimum ionization loss of pions passing through the calorimeter and a broad bump up to 3.0 GeV is due to the nuclear reaction in the BSO crystals. The pion spectrum has an overlap with the electron peak slightly. The misidentification probability Z of pions to electrons (so-called e=p separation factor) may be defined as R hE e iþ3sE hE i 3s Y p ðE ob Þ dE ob ZðEÞ ¼ Re 1 E (7) 0 Y p ðE ob Þ dE ob
3. Summary We have tested a prototype calorimeter consisting of 3 3 BSO crystals using electron and charged pion beams of 0.5, 1.0, 1.5, 2.0 and 3.0 GeV/c. The obtained energy pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi resolution is sE = E ¼ ð0:023 0:003Þ= EðGeVÞ ð0:017 0:002Þ for electrons of the energy from 0.5 to 3.0 GeV. The RMS position resolution is found to ffi be pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi parameterized as sx ¼ ½ð3:2 0:1Þ= EðGeVÞ ð1:3 0:1Þ mm at the center of the crystal in this energy range. The electron/pion separation factor (the misidentification probability of pions to electrons) is estimated to be approximately 10 3 for the pions in the momentum range of 1.0–3.0 GeV/c. The decay time has been measured for a small BSO block cut out of the same ingot providing the crystals under the beam test. The measured values of the decay time are t1 ¼ 30 13 ns with an intensity of 43 4%, and t2 ¼ 106 6 ns with 57 4%.
for the incident pion momentum E, where Y p ðE ob Þ is the number of pions with an observed energy E ob , hE e i the peak deposit-energy of electrons and sE the measured energy resolution for electrons. Fig. 10(b) shows the misidentification probability of pions to electrons as a function of the pion momentum. The separation factor for pþ shows a
Number of Events
10
3
-
e
π-
2
〈Ee 〉± 3σE →
←
10
1
0 (a)
1
2
Energy [GeV]
Pion misidentification probability
0.004 10
3
π+ π-
0.003
0.002
0.001
0
(b)
1
2
3
Momentum [GeV/c]
Fig. 10. (a) Energy spectra for 3 GeV/c p together with 3 GeV/c electrons. (b) Misidentification probability of pions to electrons. The vertical bars show 68.3% CL intervals.
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Clear and colorless single crystals of BSO are now available with such a large size as 70 mm in diameter and 200 mm in length. The BSO crystals are thus a good candidate for the detector element to be used in 4p EM calorimeters in the energy region of several hundred MeV. The remaining key issue about BSO is the cost for mass production of large crystals in large-scale 4p detectors. It, of course, depends on the demand for BSO crystals.
Acknowledgements The authors would like to thank Prof. M. Ishii and Prof. M. Kobayashi for their help in growing up large BSO crystals. Thanks are also due to Prof. M. Taino and Prof. M. Ieiri for their help in preparation for the present experiment at KEK. This work was supported in part by the Grants-in Aid for Scientific Research of the Japanese Ministry of Education and Science (Nos. 15340069 and 17340063). References [1] M. Kobayashi, et al., Nucl. Instr. and Meth. 205 (1983) 133. [2] M. Kobayashi, et al., Nucl. Instr. and Meth. A 372 (1996) 45. [3] M. Kobayashi, et al., Nucl. Instr. and Meth. A 400 (1997) 392.
[4] M.J. Weber, R.R. Monchamp, J. Appl. Phys. 44 (1973) 5495. [5] K. Takagi, T. Fukazawa, Appl. Phys. Lett. 42 (1983) 43. [6] H. Ishibashi, et al., IEEE Trans. Nucl. Sci. NS-36 (1989) 170. [7] C.L. Melcher, et al., IEEE Trans. Nucl. Sci. NS-37 (1990) 161. [8] M. Kobayashi, et al., Nucl. Instr. and Meth. A 306 (1991) 139. [9] W.W. Moses, S.E. Derenzo, IEEE Trans. Nucl. Sci. NS-36 (1989) 173. [10] D.F. Anderson, Nucl. Instr. and Meth. A 287 (1990) 606. [11] S. Anderson, et al., Nucl. Instr. and Meth. A 332 (1993) 373. [12] S. Kubota, et al., Nucl. Instr. and Meth. A 268 (1988) 275. [13] C.L. Woody, et al., IEEE Trans. Nucl. Sci. NS-37 (1990) 492. [14] I-H. Chiang, et al., IEEE Trans. Nucl. Sci. NS-42 (1995) 394. [15] S.E. Derenzo, et al., IEEE Trans. Nucl. Sci. NS-37 (1990) 203. [16] V.G. Baryshevsky, et al., Nucl. Instr. and Meth. A 322 (1992) 231. [17] M. Kobayashi, et al., Nucl. Instr. and Meth. A 333 (1993) 429. [18] O.V. Buyanov, et al., Nucl. Instr. and Meth. A 349 (1994) 62. [19] M. Kobayashi, et al., Nucl. Instr. and Meth. A 373 (1996) 333. [20] M. Ishii, et al., J. Crystal Growth 205 (1999) 191. [21] M. Ishii, et al., Opt. Mater. 19 (2002) 201. [22] M. Moszynski, B. Bengtson, Nucl. Instr. and Meth. 142 (1977) 417. [23] KEK Annual Report, 1976, p. 57. [24] H. Yamazaki, et al., Nucl. Instr. and Meth. A 391 (1997) 427. [25] S. Inaba, et al., Nucl. Instr. and Meth. A 359 (1995) 485.
Nuclear Instruments and Methods in Physics Research A 550 (2005) 258–266 www.elsevier.com/locate/nima
First beam test on a BSO electromagnetic calorimeter H. Shimizua,, F. Miyaharaa, H. Hariub, T. Hayakawaa, T. Ishikawaa, M. Itayab, T. Iwatab, T. Kinoshitaa, M. Moriyab, T. Nakabayashia, T. Sasakib, Y. Tajimab, S. Takitab, M. Yamamotob, H. Yamazakia, H.Y. Yoshidab, Y. Yoshidab a
Laboratory of Nuclear Science, Tohoku University, Sendai 982-0826, Japan b Department of Physics, Yamagata University, Yamagata 990-8560, Japan Received 11 May 2005; accepted 7 June 2005 Available online 6 July 2005
Abstract A beam test has been performed on a prototype electromagnetic calorimeter of bismuth silicate BSO crystals for the first time using electrons of 0.5–3.0 GeV. The calorimeter consists of nine BSO crystals, 22 mm 22 mm 180 mm (15.6 radiation lengths), arranged in a 3 3 array. Each crystal is coupled to a small photomultiplier tube of 20 mm in pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi diameter. The obtained energy resolution is sE =E ¼ ð0:023 0:003Þ= EðGeVÞ ð0:017 0:002Þ for incident electrons of the energy E in GeV. The position information for an electron injected onto the center crystal of the calorimeter array is given withpenergy deposit signals from nine BSO crystals. The RMS position resolution sx is parameterized as ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi sx ¼ ½ð3:2 0:1Þ= EðGeVÞ ð1:3 0:1Þ mm at the center of the crystal. Misidentification probability of pions to electrons is found to be 10 3 for the pion momentum ranging from 1.0 to 3.0 GeV/c. r 2005 Elsevier B.V. All rights reserved. PACS: 29.40.Vj; 25.20.Lj; 29.40.Mc Keywords: Bismuth silicate; BSO
1. Introduction Different types of electromagnetic (EM) calorimeters have been utilized in Nuclear and Particle Physics, depending on the energy of photons to be Corresponding author. Tel.: +81 22 743 3423;
fax: +81 22 743 3401. E-mail address: [email protected] (H. Shimizu).
detected and on the event rate, etc. We are especially interested in a 4p EM calorimeter in the energy region of several hundred MeV for Quark Nuclear Physics experiments to be conducted at SPring-8/LEPS and Laboratory of Nuclear Science, Tohoku University. In this energy region, GSO, BGO and CeF3 are excellent materials for the 4p calorimeter in principle among the available crystals. They have good
0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.06.040
ARTICLE IN PRESS H. Shimizu et al. / Nuclear Instruments and Methods in Physics Research A 550 (2005) 258–266
characteristics such as a good energy resolution, fast response (a little longer decay time for BGO, though), a short radiation length, and no hygroscopicity. However, GSO and BGO are rather expensive for a 4p detector. Presently, it is difficult to grow a single long CeF3 crystal. Bismuth silicate, Bi4 Si3 O12 (BSO) [1], produced by replacing Ge in BGO with Si has been developed as an alternative to BGO. The material cost for BSO is expected to be significantly reduced, compared with that for BGO since the main part of the cost for BGO production comes from high-purity powder of GeO2 which is expensive. BSO resembles BGO in physical and optical properties but has a little different scintillation characteristics. The decay time of BSO is faster than that of BGO, which is welcome for calorimeters, while the light output is 25% [3] of BGO, which is, however, still quite acceptable for detecting several hundred MeV photons. Table 1 shows the characteristics of BSO together with some available crystals for EM calorimeters just for comparison. BSO crystals have not been used up to now notwithstanding their good features as a scintillation material for EM calorimeters because it was difficult to grow a large single crystal before. Employing the vertical Bridgman method, Ishii and his colleagues recently succeeded in growing up single crystals of BSO [2,20,21] with a size of 35 mm in diameter and about 200 mm in length. Subsequently, some additional techniques for mass production of large BSO crystals were put into operation. Now clear single crystals of BSO as large as 70 mm in diameter have become available.
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2. Experiment 2.1. BSO calorimeter The calorimeter used in the present study comprises nine identical BSO crystals arranged in a 3 3 array, as shown in Fig. 1. The size of the crystals is 22 mm 22 mm in cross-section and 180 mm long, which corresponds to 15.6 radiation lengths along the direction of incident photons/ electrons. These single crystals were produced at Futek Furnace Co., Yokohama, where three large BSO ingots of 70 mm in diameter were grown for this purpose. The crystals, all the faces of which were optically polished, were wrapped with 15 mm thick aluminum foil. There are several other wrapping materials such as Teflon, Millipore foil, Tyvek paper and so on. We used aluminum foil
Fig. 1. Schematic view of the prototype BSO calorimeter.
Table 1 Comparison of properties of scintillating crystals
Density ðg=cm3 Þ Radiation length (mm) Decay time (ns) Peak emission (nm) Relative light outputa Hygroscopicity
BSO [1–3]
BGO [4]
GSO(Ce) [5–8]
CeF3 [9–11]
CsI (pure) [12–14]
PWO [15–19]
6.80 11.5 100 480 0.04 No
7.13 11.2 300 480 0.1 No
6.71 13.8 30–60 430 0.2 No
6.16 16.8 10–30 310–340 0.07 No
4.53 18.5 30f , 680s 300f , 480s 0:08f , 0:02s Some
8.28 8.9 10 410–500 0.01 No
f ¼ fast component, s ¼ slow component. a Approximate light yield at room temperature, 1 for NaI(Tl).
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simply because it would be one of the thinnest reflector giving good optical isolation. One end of each crystal was left unwrapped to give an optical contact with a 32 mm long light guide connected to a photomultiplier tube, Hamamatsu R4125GMOD. The optical contact was made by using OKEN6262A optical compound, the refractive index of which was 1.42 at l 450 nm. The decay time was measured by utilizing the conventional single photoelectron method in a separate-scintillator arrangement [22] for a small block of BSO which was cut with a size of 22 mm 22 mm 20 mm from one of the ingots providing the nine crystals used in the beam test. The setup for the measurement of the scintillation decay time is shown in Fig. 2. A 60Co radioactive source, emitting two cascade g-rays (1.17 and 1.33 MeV), was employed in the decay time measurement. A 30 mm thick plastic scintillator was mounted on a 2 in. photomultiplier tube, HAMAMATSU H2431 to detect g-rays from the 60 Co source and to produce the start pulse. The distance between the photocathode of the stop counter, HAMAMATSU H6410, and the BSO block was adjusted so as to give single photoelectrons. The overall time resolution of the system was 0.7 ns for this measurement. The obtained 60Co
Collimater Start counter
BSO
200mm Plastic scintillator
Lead block
Number of Events
260
0
50
100 150 200 250 300 350 Decay time [ns]
Fig. 3. Decay time spectrum of BSO scintillation. The solid curve gives a fit with a sum of two exponential functions and a constant background.
decay time spectrum is shown in Fig. 3. The data were fitted with two exponential functions with a constant background, giving the decay times, t1 ¼ 30 13 ns with an intensity of 43 4% and t2 ¼ 106 6 ns with 57 4%. The very fast component of the decay time of about 2 ns reported by Kobayashi et al. [3] was not observed in the present measurement in which no correction was made for spectral response of the phototubes to different excitation–emission wavelengths. The decay times of scintillation in PWO and NaI(Tl) were also measured as points of reference in the same system. The obtained values for PWO were t1 ¼ 4:8 0:7 ns (95 1%) and t2 ¼ 25 3 ns (5 1%) and that for NaI(Tl) was t ¼ 242 5 ns. These are consistent with known values for PWO and NaI(Tl) scintillators. 2.2. Experimental setup for the beam test
Stop counter Fig. 2. Setup for the measurement of the BSO scintillation decay time.
A beam experiment on the BSO calorimeter was performed at the p2 beam line of the proton synchrotron facility in KEK. The beam momenta used in the experiment were 0.5, 1.0, 1.5, 2.0 and 3.0 GeV/c. The experimental setup is illustrated in
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Plastic greenhouse
MWPC S1
261
S2
S3
S4
BSO crystals CO2 Gas Cherenkov Counter SF6 Gas Cherenkov Counter 11.0 m Fig. 4. Plan view of experimental setup.
5000 π+ Number of Events
Fig. 4. Three plastic scintillation-counters, S1, S2 and S3, were used as a set of trigger counters. Particle identification was made with time of flight (TOF) signals between S1 and S3. Three gas Cherenkov counters (GCC1-3) were installed in the p2 beam line to distinguish electrons/positrons from the other charged particles. The GCC1 was filled with 1.8 atm CO2 gas while 1 atm SF6 gas was used for GCC2 and GCC3. Electrons/positrons were identified by requiring signals of GCC1 \ GCC2 \ GCC3. Four sets of multiwire proportional chambers (MWPC) were employed. Each MWPC has two wire planes, x and y. The wire spacing for all these planes is 2 mm. The trajectory of a charged particle injected onto the calorimeter was given by three sets of MWPCs placed downstream. The temperature dependence of the light output of BSO is as large as 2%= C at room temperature [1]. Therefore, the calorimeter was placed in a thermally controlled plastic greenhouse located at the end of the beam line so as to keep the temperature of BSO crystals unchanged. The temperature in the greenhouse was monitored during the experiment and was kept to be 17 1 C. The momentum spread of the beam in the p2 beam line has been evaluated with TOF spectra given by the time difference between signals from S1 and S4 plastic counters, the thickness of which is 10 and 30 mm, respectively. The distance between S1 and S4 is just 11.0 m. In this measurement the calorimeter was moved down so that the beam was able to pass through the S4
4000
3000 µ+
2000
1000
0
e+
30
32
34
36 38 40 S1-S4 TOF [ns]
42
44
Fig. 5. TOF spectrum for 0.5 GeV/c particles.
counter. The time resolution in the TOF measurement is 0.18 ns which is deduced from the TOF spectra of b ’ 1 pions. Fig. 5 shows one of the TOF spectra for 0.5 GeV/c particles. The momentum 0.5 GeV/c corresponds to a rather low kinetic energy for protons to pass through all the materials such as the air, Cherenkov counters, MWPCs and plastic scintillators on the p2 beam line. Therefore, the data for protons with a momentum of 1 GeV/c ðb ¼ 0:73Þ have been used to estimate the momentum spread of the beam. The TOF spectrum for 1 GeV/c protons shows a beautiful Gaussian shape instead and gives a fitted
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value of sTOF ¼ 0:25 ns, where the broadening effect due to the energy loss of the beam is negligible. The relation between the momentum spread sp =p of the beam and the spread of the TOF spectrum sTOF =T is given by sp =p ¼ g2 sTOF =T, where T denotes TOF and g is the Lorentz factor of the beam. Thus, the momentum spread of the incident beam sp =p is found to be 0.7%, taking the time resolution into account. It is known that the momentum spread of the beam in the p2 beam line is roughly independent of the beam momentum. 2.3. Energy resolution
Number of Events
The obtained energy spectra are shown in Fig. 6 for 0.5, 1.0, 1.5, 2.0 and 3.0 GeV/c electrons incident on the 4 mm 4 mm area of the central crystal. The deposited energy was given by summing up all the signals of the nine crystals. The energy calibration for the crystal detectors was made by using 3.0 GeV/c electrons injected on to the center of each crystal one by one in such a way that the peak corresponding to the deposited energy in one crystal was equivalent to 2157 MeV
which was given by an EGS4 simulation. Every spectrum in Fig. 6 shows a tail on the low energy side, which is mainly due to longitudinal and also transverse EM-shower leakage. In this case the sum of a Gaussian and an additional broader Gaussian function can fit the spectrum empirically. There exist some unknown components making a tail on the high energy side as far as the 3.0 GeV spectrum is concerned. In any case each spectrum is fitted with two Gaussian functions to get the energy resolution. The energy resolution sE =E is deduced by subtracting the effect of the beam-momentum spread from the observed value sob =E ob which may be written as sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi s 2 s 2 sob E b . (1) ¼ þ E ob E Eb Here E b represents the beam energy and sb denotes the energy spread of the beam which is given from the momentum spread sp =p ¼ 0:007 of the p2 beam line. The data are summarized in Table 2, showing a good linearity between the incident energy and the energy deposit in the
150
0.5 GeV 40
1.0 GeV
100
1.5 GeV
80 60 40
20
50 20
0 200
400
0
600
800
[MeV]
0
1000
1250
[MeV]
1500
[MeV]
Number of Events
200
2.0 GeV
3.0 GeV
200
150 150 100
100
50 0 1500
50 1750
2000
[MeV]
0
2500
3000
[MeV]
Fig. 6. Response of the BSO calorimeter for 0.5, 1.0, 1.5, 2.0 and 3.0 GeV/c electrons. The solid curve gives a fit with a sum of two Gaussian functions represented with a dashed and a dotted curve.
ARTICLE IN PRESS H. Shimizu et al. / Nuclear Instruments and Methods in Physics Research A 550 (2005) 258–266 Table 2 Summary of the data obtained in the present beam test Beam momentum E b (GeV/c)
Energy deposit
Energy resolution
E ob (GeV)
sob =E ob
s=E
0.5 1.0 1.5 2.0 3.0
0.470 0.913 1.413 1.859 2.751
0:038 0:010 0:029 0:002 0:027 0:002 0:024 0:001 0:023 0:001
0:038 0:010 0:028 0:002 0:026 0:002 0:023 0:001 0:022 0:001
0.07 Experiment
Energy Resolution σE/E
0.06
Simulation
0.05 0.04 0.03
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where denotes a quadratic sum, a means the statistical fluctuation of the detected energy E given in GeV and b is a constant. By fitting the data of sE =E with function (2), the coefficients are found to be a ¼ 0:023 0:003 and b ¼ 0:017 0:002, while a ¼ 0:019 0:001 and b ¼ 0:014 0:001 are given by the EGS4 simulation. There is a discrepancy between the data and the simulation result. The difference on the parameter a comes from the statistics of scintillation photons which are not taken into account in the simulation, since EGS4 can only handle the transport of electrons and photons in materials. However, the disagreement for the constant term b is not well understood. In general, shower leakage out of the calorimeter mostly accounts for the constant term. The rate of the shower leakage can be estimated to be about 7% from the observed energy deposits given in Table 2. The constant term itself can be reduced by making the crystal longer, although the disagreement observed for the parameter b may not disappear.
0.02
2.4. Position resolution 0.01 0 0
0.5
1
1.5 2 2.5 Energy [GeV]
3
3.5
Fig. 7. Energy resolution sE /E obtained for a 3 3 BSO crystal calorimeter. The solid line represents function (2) with the parameters given by fitting the data. The dashed line indicates an EGS4 simulation result under the same condition in the same detector arrangement. The quoted errors denote fitting uncertainties.
calorimeter. Fig. 7 shows the obtained energy resolutions sE =E (closed circles) and the result of a Monte Carlo simulation based on the EGS4 code (open circles) as a function of the incident electron energy. The simulation was made under the same geometrical condition as that of the experiment, namely electrons were injected on the 4 mm 4 mm area of the central crystal also in the simulation. The energy resolution is parametrized as sE a (2) ¼ pffiffiffiffi b E E
Position information for incident photons/electrons is necessary to provide the momentum vectors of themselves, with which the invariant mass, for example, of a meson decaying into photons and/or electrons (e.g. Z ! 2g, Z ! eþ e g, etc.) can be obtained. The center of gravity X CG for the incident position is simply given with measured energy deposits as P9 E i xi X CG ¼ Pi¼1 (3) 9 i¼1 E i where E i is the energy deposit and xi the x coordinate of the center of the ith crystal. Fig. 8 shows a scatter plot giving the relation between X CG and the impact position X reconstructed with MWPCs for 1.5 GeV/c electrons. The data are fitted with a function X CG ¼ p0 tan p1 ðX p2 Þ
(4)
where p0 ¼ ð3:0 0:1Þ mm and p1 ¼ ð0:114 0:004Þ rad=mm are obtained as a set of common parameters independent of the impact energy. The parameter p2 corresponds to a deviation coming
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6
10 7.5
5 Position Resolution [mm]
XCG [mm]
5 2.5 0 -2.5 -5 -7.5
4
3
2
1
-10 -10 -7.5
-5 -2.5 0 2.5 5 Impact position X [GeV]
7.5
10
Fig. 8. Scatter plot of X CG versus X for 1.5 GeV/c electrons. The solid curve gives the fitted result of function (4) with the energy-independent parameters.
about due to mis-alignment of the calorimeter and is found to be 0:34 mm for the x direction and þ0:21 mm for the y direction in this measurement. The position resolution of the BSO calorimeter is estimated with the RMS deviation of the position given by the inverse function of Eq. (4) with X CG from the MWPC reconstructed position. pffiffiffiffi The result is shown in Fig. 9, indicating a 1= E dependence. The RMS position resolution sx at the center of the calorimeter is thus obtained as " # ð3:2 0:1Þ sx ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð1:3 0:1Þ mm. (5) EðGeVÞ The position resolution is better than 5 mm for incident electrons of the energy greater than 0.5 GeV. 2.5. Electron/pion separation The e=p separation capability is one of the characteristics to be studied for EM calorimeters. A charged pion produced in experiments sometimes gives an energy deposit as large as an electron does due to the nuclear reaction taking place in the calorimeter. In this case the pion is
0 0
0.5
1
1.5 2 Energy [GeV]
2.5
3
3.5
Fig. 9. Position resolution at the center of the calorimeter. The solid line is the fitted result given by Eq. (5). The quoted errors are statistical uncertainties.
misidentified as an electron, if the pion momentum is given. In order to estimate the e=p separation capability, first of all, pions have to be well separated from electrons on the p2 beam line. Three GCCs were employed for this purpose. A hadron beam was defined by the trigger signal of GCC1 \ GCC2 \ GCC3. The detection efficiency of the ith GCC for electrons is given by i ¼
Y 123 Y jk
(6)
where Y 123 represents the number of three-fold coincidence events firing three GCCs simultaneously and Y jk means that of two-fold coincidence events. The obtained efficiencies are 1 ¼ 0:959, 2 ¼ 0:996 and 3 ¼ 0:987, respectively. Thus, the contamination rate of electrons in the hadron beam is found to be ð1 1 Þð1 2 Þ ð1 3 Þ ¼ 2:13 10 6 , which is good enough for the present study. The selection of pions in the hadron beam was made with TOF signals. There were some muons produced at the production target of the p2 beam line as indicated in Fig. 5. The rate of muons to pions in the 0.5 GeV/c hadron beam is 12.6% which is derived from the TOF data. It is almost impossible, however, to evaluate the rate from the
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slightly larger value than that for p at low momenta around 1 GeV/c, while p gives small values less than 10 3 for the pion momentum ranging from 1.0 to 3.0 GeV/c. Similar studies were made previously for CsI(pure) [24] and PWO crystals [25], giving about the same results for pþ and p , respectively, although the 2s range was defined as the region of electrons in those studies.
TOF data for higher momentum beams. But if the main part of the momentum-analyzed muons is assumed to come from pions decaying in the vicinity of the production target, the rate of muons to pions can be estimated. That is 6.3%, 4.2%, 3.1% and 2.1% for the beam momentum of 1.0, 1.5, 2.0 and 3.0 GeV/c, respectively, which are consistent with the previously measured values at KEK [23]. Fig. 10(a) shows the response of the BSO calorimeter for 3.0 GeV/c p and electrons injected onto the area of 20 mm 20 mm on the central crystal. The peak around 200 MeV corresponds to the minimum ionization loss of pions passing through the calorimeter and a broad bump up to 3.0 GeV is due to the nuclear reaction in the BSO crystals. The pion spectrum has an overlap with the electron peak slightly. The misidentification probability Z of pions to electrons (so-called e=p separation factor) may be defined as R hE e iþ3sE hE i 3s Y p ðE ob Þ dE ob ZðEÞ ¼ Re 1 E (7) 0 Y p ðE ob Þ dE ob
3. Summary We have tested a prototype calorimeter consisting of 3 3 BSO crystals using electron and charged pion beams of 0.5, 1.0, 1.5, 2.0 and 3.0 GeV/c. The obtained energy pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi resolution is sE = E ¼ ð0:023 0:003Þ= EðGeVÞ ð0:017 0:002Þ for electrons of the energy from 0.5 to 3.0 GeV. The RMS position resolution is found to ffi be pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi parameterized as sx ¼ ½ð3:2 0:1Þ= EðGeVÞ ð1:3 0:1Þ mm at the center of the crystal in this energy range. The electron/pion separation factor (the misidentification probability of pions to electrons) is estimated to be approximately 10 3 for the pions in the momentum range of 1.0–3.0 GeV/c. The decay time has been measured for a small BSO block cut out of the same ingot providing the crystals under the beam test. The measured values of the decay time are t1 ¼ 30 13 ns with an intensity of 43 4%, and t2 ¼ 106 6 ns with 57 4%.
for the incident pion momentum E, where Y p ðE ob Þ is the number of pions with an observed energy E ob , hE e i the peak deposit-energy of electrons and sE the measured energy resolution for electrons. Fig. 10(b) shows the misidentification probability of pions to electrons as a function of the pion momentum. The separation factor for pþ shows a
Number of Events
10
3
-
e
π-
2
〈Ee 〉± 3σE →
←
10
1
0 (a)
1
2
Energy [GeV]
Pion misidentification probability
0.004 10
3
π+ π-
0.003
0.002
0.001
0
(b)
1
2
3
Momentum [GeV/c]
Fig. 10. (a) Energy spectra for 3 GeV/c p together with 3 GeV/c electrons. (b) Misidentification probability of pions to electrons. The vertical bars show 68.3% CL intervals.
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Clear and colorless single crystals of BSO are now available with such a large size as 70 mm in diameter and 200 mm in length. The BSO crystals are thus a good candidate for the detector element to be used in 4p EM calorimeters in the energy region of several hundred MeV. The remaining key issue about BSO is the cost for mass production of large crystals in large-scale 4p detectors. It, of course, depends on the demand for BSO crystals.
Acknowledgements The authors would like to thank Prof. M. Ishii and Prof. M. Kobayashi for their help in growing up large BSO crystals. Thanks are also due to Prof. M. Taino and Prof. M. Ieiri for their help in preparation for the present experiment at KEK. This work was supported in part by the Grants-in Aid for Scientific Research of the Japanese Ministry of Education and Science (Nos. 15340069 and 17340063). References [1] M. Kobayashi, et al., Nucl. Instr. and Meth. 205 (1983) 133. [2] M. Kobayashi, et al., Nucl. Instr. and Meth. A 372 (1996) 45. [3] M. Kobayashi, et al., Nucl. Instr. and Meth. A 400 (1997) 392.
[4] M.J. Weber, R.R. Monchamp, J. Appl. Phys. 44 (1973) 5495. [5] K. Takagi, T. Fukazawa, Appl. Phys. Lett. 42 (1983) 43. [6] H. Ishibashi, et al., IEEE Trans. Nucl. Sci. NS-36 (1989) 170. [7] C.L. Melcher, et al., IEEE Trans. Nucl. Sci. NS-37 (1990) 161. [8] M. Kobayashi, et al., Nucl. Instr. and Meth. A 306 (1991) 139. [9] W.W. Moses, S.E. Derenzo, IEEE Trans. Nucl. Sci. NS-36 (1989) 173. [10] D.F. Anderson, Nucl. Instr. and Meth. A 287 (1990) 606. [11] S. Anderson, et al., Nucl. Instr. and Meth. A 332 (1993) 373. [12] S. Kubota, et al., Nucl. Instr. and Meth. A 268 (1988) 275. [13] C.L. Woody, et al., IEEE Trans. Nucl. Sci. NS-37 (1990) 492. [14] I-H. Chiang, et al., IEEE Trans. Nucl. Sci. NS-42 (1995) 394. [15] S.E. Derenzo, et al., IEEE Trans. Nucl. Sci. NS-37 (1990) 203. [16] V.G. Baryshevsky, et al., Nucl. Instr. and Meth. A 322 (1992) 231. [17] M. Kobayashi, et al., Nucl. Instr. and Meth. A 333 (1993) 429. [18] O.V. Buyanov, et al., Nucl. Instr. and Meth. A 349 (1994) 62. [19] M. Kobayashi, et al., Nucl. Instr. and Meth. A 373 (1996) 333. [20] M. Ishii, et al., J. Crystal Growth 205 (1999) 191. [21] M. Ishii, et al., Opt. Mater. 19 (2002) 201. [22] M. Moszynski, B. Bengtson, Nucl. Instr. and Meth. 142 (1977) 417. [23] KEK Annual Report, 1976, p. 57. [24] H. Yamazaki, et al., Nucl. Instr. and Meth. A 391 (1997) 427. [25] S. Inaba, et al., Nucl. Instr. and Meth. A 359 (1995) 485.