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A gamma ray detector with lead-scintillator tiles and WLS fiber readout for the experiment at SPring-8 to study the GDH sum rule
Nuclear Instruments and Methods in Physics Research A 481 (2002) 188–199
A gamma ray detector with lead-scintillator tiles and WLS fiber readout for the experiment at SPring-8 to study the GDH sum rule I. Daitoa, M. Gesob, S. Hasegawac, N. Horikawaa, H. Ichiharac,1, T. Iwatac,*, K. Kondoc, D. Menzea,d,2, Y. Miyachia,3, H. Nakayamac,4, S. Tanoshimac,5, A. Wakaic,6 a
b
CIRSE, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan Department of medical radiations sciences, Royal Melbourne Institute of Technology, P.O. Box 71, Bundoora, Vic 3083, Australia c Department of Physics, Nagoya University, PI-group, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan d Physikalisches Institut, Universita.t Bonn, D-53115 Bonn, Germany Received 29 May 2001
Abstract A prototype gamma ray detector for the detection of p0 in the proposed experiment at SPring-8 to study the Gerasimov–Drell–Hearn (GDH) sum rule (GDH experiment) has been built and tested. The detector is fabricated with lead-scintillator tiles and wave length shifter fiber readout. This detector will be placed inside the bore of the superconducting magnet which supplies the polarized target with a 2:5 T magnetic field. High-detection efficiency (above 85%) for the gamma rays is the most important requirement for this detector to be employed in the proposed experiment. It has been tested with minimum ionizing particles, electrons and gamma rays. The results of the tests in comparison with some Monte Carlo simulations are presented. The evaluated efficiency for gamma rays ranging in energy above 50 MeV is higher than 90%. r 2002 Elsevier Science B.V. All rights reserved. PACS: 29.40.Vj Keywords: Lead-scintillator tiles; WLS fibers; Gamma ray detector
*Corresponding author. Tel.: +81-52-789-2895; fax: +81-52-789-2903. E-mail address: [email protected] (T. Iwata). 1 Present address: OLYMPUS OPTICAL CO. LTD., 2-3 Kuboyama-chou, Hachioji, Tokyo 192-8512, Japan. 2 Present address: Physikalisches Institut, Universit.at Bonn, D-53115 Bonn, Germany. 3 Present address: Physics Department, Tokyo Institute of Technology, 2-12-1, Oh-okayama, Meguro, Tokyo 152-8551, Japan. 4 Present address: Department of Physics, Chiba, University, 1-33, Yayoi, Inage-ku, Chiba 263-8522, Japan. 5 Present address: NTT Data, Shiba A-building 7F, 3-2-18, Shiba, Minato-ku, Tokyo 105-0014, Japan. 6 Present address: Akita Industry Promotion Foundation (AIPF), 3-1-1, Sanno, Akita-city, Akita 010-8572, 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 3 6 1 - 4
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1. Introduction The proposed experiment [1] at the SPring-8 facility aims at measuring the helicity-dependent total photoabsorption cross sections of the proton at photon energies between 1.5 and 2:9 GeV: The results will be compared with proposed measurements at other parts of the world to test the validity of the Gerasimov–Drell–Hearn (GDH) sum rule [2]. The polarized photon beam from the Laser–Electron–Photon (LEP) beam line at SPring-8, currently operational [3], will be used as a source for circularly polarized photons of variable energy. A longitudinally polarized proton target is under construction at Nagoya University. The essential feature of the experiment is a complete detection of all hadronic final states using a, 4p detector setup as displayed in Fig. 1. Some detectors will be encompassed inside the bore of the superconducting solenoid magnet of the polarized target. The gamma ray detector to be installed around the target is required to have a satisfactory
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efficiency and a large geometrical coverage for gamma rays of energies higher than 70 MeV which mostly originate from p0 produced in the reaction. This requirement becomes even harder to fulfill due to (1) limited space (about 60 cm) inside the magnet bore where the detector will be placed and (2) the influence of the high magnetic field (2:5 T) of the superconducting magnet of the target. A plausible solution would be to use heavy scintillating crystals such as CsI with photodiodes that are not so sensitive to the high magnetic fields. However, such a solution is very expensive. An alternative to this is a sampling calorimeter type detector consisting of lead and plastic scintillator tiles with WLS fibers for the readout. The WLS fiber couples with a long light guiding fiber which transmits the secondary emitted light to a photomultiplier tube (PMT). This PMT is placed at some distance from the superconducting magnet to avoid the influence of the magnetic field. The design and characteristics of such a prototype gamma ray detector based on the above-mentioned method is described in this article. The
Fig. 1. The setup of the GDH experiment at SPring-8.
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detector’s ability for the detection of minimum ionizing particles, electrons and gamma rays has been tested. The results are displayed.
2. Detector design The design of the gamma ray detector is based on the following requirements to make it suitable for the proposed experimental setup: (1) the thickness of the detector7 along the gamma rays should be less than 20 cm (2) the detector’s efficiency must exceed 85% for gamma rays of energy higher than 70 MeV (3) the dead space between the detectors should be as narrow as possible and (4) the PMT has to be placed at a distance of about 3 m from the magnet. One of the major hurdles faced in the design of this detector is the requirement for a high detection efficiency with only a few radiation lengths compared to that of conventional calorimeters. Accordingly, the number of sampling layers and the thickness of the scintillator and lead tiles are appropriately determined and an elegant light collection method is employed. In the following subsections, the structure of the detector and the readout by WLS fibers will be described. 2.1. Sampling layers and lead tiles The number of the sampling layers and tiles thickness appropriate to obtain high detection efficiency are determined using GEANT simulation [4]. The efficiency was based on the fraction of the events which is the result of an energy deposit higher than the threshold energy of 10 MeV: This threshold energy is set higher than that usually set for conventional calorimeters. The detection efficiency under the condition of the detector’s total thickness to be less than 170 mm was obtained as shown in Fig. 2 as a function of the number of the sampling layers ðns Þ and the ratio of the thickness 7
The space inside the magnet bore, 60 cm in diameter, is occupied by the polarized target cryostat (10 cm), the internal charged particle detector (5 cm 2 ¼ 10 cm) and the gamma ray detector (20 cm 2 ¼ 40 cmÞ:
Fig. 2. Efficiencies (in %) as a function of the number of sampling layers and the ratio R:
of the scintillator and lead tiles ðR ¼ tðscin:Þ= tðleadÞÞ; where the thicknesses are measured in units of mm. The detection efficiency first increases with ns and then saturates. However, in relation to R; it seems that there is a nominal R value which gives a maximum efficiency. This is because a thin lead tile (i.e. a large R value) gives a low gamma interaction probability while on the contrary a thin scintillation tile (i.e. a small R value) results in a small energy deposit, hence reduces the efficiency. Our simulation results show that efficiencies above 85% are achievable with ns value more than 17 and the ratio R ranging between 3 and 5. Accordingly, the ns value of 18 and the R ratio of 5 were selected for the detector’s design. These results lead to the following detector’s configuration: tðscin:Þ ¼ 8:0 mm; tðleadÞ ¼ 1:6 mm and the total detector’s thickness of tðdet:Þ ¼ 5:4X0 (where X0 is the radiation length). 2.2. WLS fiber readout The WLS fiber (Kuraray, type Y-7(100)) of 1 mm diameter with two turns is embedded into a circular groove with a diameter of 160 mm which is machined onto the surface of the scintillator tile
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of SCSN-38 (Kuraray) with an area of 200 mm 200 mm and a thickness of 8 mm: One end of the WLS fiber is mirrored by vacuum evaporation of aluminum to reflect the secondary lights emitted into the opposite direction to the readout. The other end is jointed with an optical cement (BC600 from Bicron) to a light guiding fiber (Clear-PS from Kuraray) of 1 mm diameter and 3 m in length. The end of the light guiding fiber is connected to a PMT (H1161 from Hamamatsu). The scintillator tile is then wrapped with an aluminized Mylar sheet. Before constructing the prototype detector, the light collection efficiency of such a single tile was tested using the 29 MeV proton beam from the SF-Cyclotron at INS Tokyo. The variation path of the incident proton beam on the tile is shown in Fig. 3 and the spatial variation of the light yield is displayed in Fig. 4. An interesting feature found in this test is that the light yields inside the groove circle vary in average by about 20%. It is clear that the light yield outside the groove circle is significantly less than that inside, because the scintillation photons produced outside the groove circle which likely escape from the sides of the tile have not been captured by the WLS fiber. This result leads to the following restrictions on the detector design: the area surrounded by the groove circle has to be made as large as the design
Fig. 3. The incident beam positions.
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Fig. 4. Light yield vs. beam position.
allows and the reflector on the sides should be improved.
3. Detector structure This detector has a trapezoidal cross section and covers a 1/8 section ðDf ¼ 451) in the azimuthal angle as shown in Fig. 5. It consists of 18 sampling layers of different areas, the smallest on the inner side and the largest on the outer side. The smallest layer (No. 1) has its area of 75 200 mm2 and the largest one (No. 18) 210 200 mm2 : Each layer consists of a lead tile of 1:6 mm thickness and a scintillator tile of 8 mm thickness. The lead tiles are made of ‘‘hard lead’’ which is composed of in parts of 96% of lead and 4% antimony. It is known that such a hard lead is malleable hence it is easy to handle during detector fabrication. The scintillator tiles are cut from a BC412 Bicron scintillator plate. This scintillator plate was selected due to its high light output compared to other scintillators such as SCSN-38. Grooves were etched onto the surface of the scintillator tiles by a round edge cutter. Two different shaped grooves are formed in these tiles: racetrack and S-figure type. The first type is adopted for the large area tiles i.e. from No. 6 to 18 as shown in Fig. 12. The groove depth of 2:6 mm allows two turns of the WLS fiber of 1 mm in diameter to be installed in the tile. Based on the results from the single tile test described in the previous section, the diameter
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Fig. 5. The schematic view of the prototype for the gamma ray detector.
of the groove circles is made as large as possible in order to improve the light collection ability. The second type, S-figure type, allows only a single trace through the S-figured groove of depth 1:6 mm: This type is adopted for the first five tiles from No.1 to 5 as displayed in Fig. 11. The width of the grooves in all tiles of both types is 1:1 mm: The grooves were not polished in order to obtain uniform response [5]. The WLS fiber of 1 mm diameter is used in all the tiles. Both ends of the WLS fibers are milled and then polished with fine sandpaper. A small disc made of an aluminized Mylar sheet is glued with the optical cement on one end of the fiber to form a light reflector. The other end of the WLS fiber is jointed, as discussed in the previous section, to a light guiding fiber 3 m long. The other end of the fiber is directly attached to the PMT. The four sides of the scintillator plate are painted with Bicron BC-650 white paint to improve efficiency of light reflection and the top and the bottom are covered with aluminized Mylar sheets. The main properties of the detector and the configuration of the tile/fiber system are summarized in Tables 1 and 2.
Table 1 Summary of the detector properties Scintillator tiles Lead tiles Total thickness WLS fiber Light guiding fiber PMT
BC-412, 8 mm thick Hard lead (antimony 4%), 1:6 mm thick 173 mm (lead : 24:8 mm; plastic scintillator : 148 mmÞ; 5:4X0 Kuraray Y-7 (100), single cladding, 1 mm in diameter Kuraray Clear-PS, single cladding, 1 mm in diameter Hamamatsu H1161, nominal gain 1 106 at 1:5 KV
4. Reflector’s impact on the light yield In this section, the effect of the light reflector formed at the end of the WLS fiber on the light yield is discussed. As it is known from the applications of scintillation and WLS fibers, light reflectors are usually installed at the end of the fibers for the purpose of enhancing the light yield. These light reflectors are formed by either aluminum coating by vacuum evaporation [6,7] or by
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installing an aluminized Mylar sheet [8]. The former method is a tedious one and difficult to fulfill since it requires polishing the fiber surface very well to protect the reflector end. This problem Table 2 Configuration of the tile/fiber system Layer 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Tile dimension ðmm2 Þ 75 200 83 200 91 200 99 200 107 200 115 200 123 200 131 200 139 200 147 200 155 200 163 200 171 200 178 200 186 200 194 200 202 200 210 200
Groove type
WLS fiber length (mm)
S-figure S-figure S-figure S-figure S-figure racetrack racetrack racetrack racetrack racetrack racetrack racetrack racetrack racetrack racetrack racetrack racetrack racetrack
261 231 228 229 230 1025 1041 1059 1075 1093 1109 1127 1143 1162 1177 1196 1211 1230
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will be exacerbated in the case of production of a large number of detectors for the GDH experiment. The second method on the other hand is quite simple and hence it is the method followed for the formation of light reflectors in this prototype detector. The reflector in this case is formed by attaching a small disc made of aluminized Mylar to the WLS fiber’s surface using an optical glue. Such a simple structure is suitable mainly for the production of large numbers of such detectors for the proposed experiment. Moreover, attention should be focused to the fact that it is not essential to have well-polished surfaces since smooth surfaces will be formed using the glue. An experimental test was performed to compare the light yield from the tiles treated with the two different methods mentioned above. The experimental setup is displayed in Fig. 6. The results show that about 30% more light yield was obtained from the fibers with the light reflector made of aluminized Mylar sheet compared to that obtained from the fibers with light reflectors made following the vacuum evaporation technique. Also, it was observed that the light reflector
Fig. 6. Setup for the study of the light output from the WLS fiber. A part of the WLS fiber is illuminated by a light source. The reflector is mounted at the right end of the WLS fiber. The photo diode installed at the left end of the WLS fiber measures the light output.
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Fig. 7. The setup of the test in the beam at the KEK-PS. T1; T2; T3 and T4: TOF counters used for the trigger; C1, C2 and C3: gas Cherenkov counters; Ga: the gamma-ray detector on test; LG: lead glass calorimeter.
produced via vacuum evaporation technique did not improve the light yield of a similar fiber with no reflector at all. This could be caused by the fact that not enough smoothed surface in such a case is obtained. Accordingly, the technique of using aluminized Mylar was selected.
5. Charged particles detection A series of particle detection measurements has been carried out with this detector to determine its detection characteristics. They were performed at the T1 beam line of the proton synchrotron of the KEK-PS. The charged particles of the beam were mainly minimum ionizing particles (pions) and electrons. The measurement procedure and some of the acquired results are presented in the following two subsections alongside some discussions. 5.1. Experimental setup with charged particles
Fig. 8. A typical pulse height distribution for p at 2 GeV=c traversing through the gamma ray detector.
Charged particles ranging from 0.3 to 2 GeV=c were offered in the beam line. The experimental setup is displayed in Fig. 7. Particle identification was accomplished with a time of flight (TOF) system, gas Cherenkov counters and a lead glass calorimeter placed most downstream of the setup. A time resolution of 110 ps for the flying path of 7:4 m was obtained in the TOF system.
27 MeV in all scintillator tiles and about 1:5 MeV in one tile. From this set of data it was observed that the light output normalized by the energy deposit in the scintillator tiles was about 1:03 P:E:=MeV: The number of photoelectrons normalized by the energy deposit may be expressed as
5.2. Minimum ionizing particles detection
where Nph ¼ 104 is the number of scintillation photons per energy deposit of 1 MeV given by a minimum ionizing particle traversing plastic scintillator tiles, Zc to be evaluated here is the conversion efficiency to be evaluated from the scintillation photons into secondary emitted
The light output was studied for p at 2 GeV=c: In Fig. 8 a typical pulse height distribution resulting from the detection of these particles is shown. The particles deposit in average about
NP:E: ¼ Nph Zc O1=2 ð1 þ eR ÞAWLS Ajoint ALG QPMT ð1Þ
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photons in the WLS fiber, O1=2 ¼ 3:1% is the trapping efficiency in the WLS fiber with single cladding, eR ¼ 30% is the reflector’s efficiency obtained in the measurement described in the previous section, AWLS and ALG are the light attenuations in the WLS fiber and the light guiding fiber, respectively, Ajoint is the attenuation at the joint between the fibers, QPMT ¼ 15% is the quantum efficiency of the PMT. Assuming the attenuation length8 in the WLS fiber to be 3:4 m and in the light guiding fiber 10 m; AWLS B0:9 is given from the half length of the WLS fiber to be 0:44 m and ALGF B0:7 from the light guiding fiber 3 m long. Taking Ajoint to be unity, we obtain Zc B2:7%: In the following two subsections, the amount of the light yield obtained in each individual layer and the light yield spatial variations will be presented. 5.2.1. Light yield in an individual layer As described in the section of the detector’s structure, the scintillator tiles are classified according to the shape of the groove of the WLS fiber: one type is called S-figure and the other racetrack type. It is interesting to compare the light yield in these two types. There is a difference in the amount of the light yield in the tiles with the S-figure and those of the racetrack. It is anticipated that the former type would have less light yield due to its shorter WLS path length despite their narrower area which could assist in improving the scintillation light collection. On the other hand, the light yield in the racetrack type tiles is expected to be higher due to their high collection efficiency based on the fact that they contain a longer WLS fiber which results in covering a larger area for the light collection. But this might be offset, somehow, by the light attenuation caused by longer paths traversed by the light collected. The light output from an individual scintillator tile is measured with only one light guiding fiber connected to the PMT. Typical pulse height distributions are displayed in Fig. 9. This measurement indicates that the light output in the tiles of the racetrack type is larger compared to those of 8
Scintillation Materials, Kuraray Co., Ltd.
Fig. 9. The pulse height distributions for a: No. 5 and b: No. 18 layers.
the S-figure type: the averaged light output over the tiles of the racetrack type is 1:8 P:E: and that over the S-figure type is 1.1 P.E. 5.2.2. Spatial variation of light yield The light yield position dependence for the p of 2 GeV=c in each individual tile and in all of the tiles collectively is being investigated. This was accomplished by measuring the light output upon varying the beam incident position on the tiles along the y-axis as indicated in Fig. 5. Although, the central parts of the tile are anticipated to produce more light yield than the peripheral parts of the tile, no significant spatial variation of the light yield was observed experimentally. This spatial light yield variation is found to be less than 10% as displayed in Fig. 10. This was the result for all the layers collectively. For individual layers, the light yield position dependence was only studied for two layers i.e. No. 5 as representative of the S-figure type and No. 18 as the racetrack type representation. The result for No. 5 shows no measurable variation in the amount of light yield obtained as shown in Fig. 11, although the measurement was carried out only at three points. For No. 18, the light yield
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Fig. 10. The light output for all the layers as a function of the incident beam position.
inside the groove circle is little improved than those outside as shown in Fig. 12.
experiment is to achieve high-detection efficiency and much less emphasis is placed on its energy resolution. Since the radiation length of this prototype detector is only 5:4 X0 ; its energy response is expected to show some non-linearity at high energies due to some shower leak. This will affect its energy resolution. During the tests with electrons, the energy response and energy resolution as a function of the incident electron energy are investigated. The energy response was investigated for the electrons ranging from 400 MeV up to 1:4 GeV: A typical pulse height distribution obtained at 600 MeV is displayed in Fig. 13. It is well reproduced by the Monte Carlo simulation using GEANT4 package [9]. A slight variation from linearity in the detector’s energy response, about 5% at 1 GeV; is shown in Fig. 14. The energy resolution s=E of 20–25% for electrons ranging from 400 MeV to 1:4 GeV is achieved in this test as shown in Fig. 15. These resolutions are, as anticipated, apparently worse compared with those given by typical calorimeters pffiffiffielectromagnetic ffi such as s=EB9%= E [10,11].
5.3. Electrons detection The main attractive feature of typical electromagnetic calorimeters is their ability to achieve high energy resolutions. However, the emphasis on the gamma ray detector for the proposed GDH
6. Gamma ray detection efficiency The gamma ray detection efficiency is measured using the tagged photon beam delivered by the
Fig. 11. The light yields at different positions of the incident beam on the No.5 layer.
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Fig. 12. The light yields at different positions of the incident beam on the No. 18 layer.
Fig. 13. A typical pulse height distribution for electrons at 600 MeV: The histogram represents the measurement. The pedestal is located at 110 channel. The closed circles are results of the simulation.
booster synchrotron at Laboratory for Nuclear Science (LNS) of Tohoku University. The beam energy ranges from 600 to 800 MeV and the energy resolution of the tagging system is about 4:7 MeV:
Fig. 14. The energy response of the prototype detector as a function of the incident electron energy. The solid line is the result of a linear fit for the three data points at 400, 500 and 600 MeV:
The experimental setup is displayed in Fig. 16. To veto charged particles in the beam, scintillation counters (T1, T2) and a Gas Cherenkov counter
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Fig. 15. Energy resolution as a function of the incident electron energy. The solid line is the result of a fit by the function ðs=EÞ2 ¼ ða=E 1=2 Þ2 þ b2 with the parameters a ¼ 0:10 and
Fig. 17. The pulse height distribution for gamma rays at 630 MeV (histogram) in comparison with a Monte Carlo simulation (closed circles). The cross hatched area shows the events above the threshold set at channel 140 corresponding to the four photoelectron level. The fraction of the events in the hatched area to the total events gives the detection efficiency of 97:170:2%:
Fig. 16. The experimental setup to measure detection efficiencies for gamma rays at the LNS of Tohoku University. T1, T2: veto counters; C1: gas Cherenkov counter(veto); Ga: the gamma ray detector on test; LG: lead glass calorimeter.
(C1) are used. The prototype gamma ray detector (Ga), is followed by a lead glass calorimeter (LG) which allows the calibration of the photon flux. An event selection was made by requiring that at least one of Ga and LG gives a significant signal, in order to eliminate photons not reaching Ga. The pulse height distribution of the prototype gamma ray detector obtained for photons of 630 MeV shows an asymmetric shape with a tail in the lower energy region due to some leakage of the shower (Fig. 17). It is well reproduced by our simulation using the GEANT4 package [9] based on the experimental data obtained at KEK with electrons taking into account the
Fig. 18. The detection efficiency for gamma rays as a function of energy. The closed square is the measured value. The closed circles are obtained in the simulation. The solid line is an eye guide.
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differences in the experimental conditions. Setting the threshold at the four photoelectron level, the detection efficiency was calculated to be 97:170:2%: We evaluated it also at different energies using the Monte Carlo simulation. The results are shown along with the experimental value in Fig. 18. The detection efficiencies were satisfactorily high for the use in the proposed GDH experiment (i.e. above 90% at the energies higher than 50 MeV).
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would like to thank Dr. H. Yamazaki of LNSTohoku University for his effort in arranging our experiment. We acknowledge the technical staffs of the mechanical workshop of Department of Physics of Nagoya University, in particular, Mr. S. Inoue and Mr. A. Itoh who produced the grooves onto the scintillator tiles. M. Geso acknowledges the travel grants from ANSTOAccess to Major Research Facility & Faculty of Life Sciences-RMIT university 2000.
7. Conclusion The prototype of the gamma ray detector specially designed to be used in the proposed GDH experiment at SPring-8 has been fabricated with lead and scintillator tiles using WLS fiber readout. It has been tested using minimum ionizing particles, electrons and gamma rays. The results of the test were compared with those obtained by a Monte Carlo simulation. The detection efficiency for gamma rays was evaluated based on the simulation and also measured with a tagged photon beam. It is found to be higher than 90% at energies above 50 MeV: This result meets the requirement of the proposed GDH experiment.
Acknowledgements We appreciate the technical staffs of KEK for their help during the test experiment at KEK-PS. We would like to express our gratitude to the accelerator crews of LNS of Tohoku University for their effort to maintain the beam condition. We
References [1] T. Iwata, Symposium on THE GERASIMOV-DRELLHEARN SUM RULE AND THE NUCLEON SPIN STRUCTURE IN THE RESONANCE REGION, Johannes Gutenberg-Universitaet, Mainz, Germany, June 14–17, 2000. [2] S.B. Gerasimov, Yad. Fiz. 2 (1965) 598; S.B. Gerasimov, Sov. J. Nucl. Phys. 2 (1996) 430; S.D. Drell, A.C. Hearn, Phys. Rev. Lett. 16 (1966) 908; M. Hosoda, K. Yamamoto, Prog. Theoret. Phys. 36 (1966) 425. [3] J.K. Ahn, et al., 12th Symposimum on Accelerator Science and Technology, vol. 141–143, 27–29 October 1999, Wako, Japan. [4] R. Brun, et al., GEANT3 Users Guide, CERN= DD=EE=84ð1987Þ: [5] T. Asakawa, et al., Nucl. Instr. and Meth. A 340 (1994) 458–465. [6] P. Annis, et al., Nucl. Instr. and Meth. A 412 (1998) 19–37. [7] S. Kim, Nucl. Instr. and Meth. A 360 (1995) 206–211. [8] D. Adams, et al., Nucl. Phys. B (Proc. Suppl.) 44 (1995) 332–339. [9] K. Amako, et al., Nucl. Instr. and Meth. A 453 (2000) 455–466. [10] W. Hoffman, et al., Nucl. Instr. Meth. 163 (1979) 77. [11] M.A. Schneegans, et al., Nucl. Instr. and Meth. 193 (1982) 445.
A gamma ray detector with lead-scintillator tiles and WLS fiber readout for the experiment at SPring-8 to study the GDH sum rule I. Daitoa, M. Gesob, S. Hasegawac, N. Horikawaa, H. Ichiharac,1, T. Iwatac,*, K. Kondoc, D. Menzea,d,2, Y. Miyachia,3, H. Nakayamac,4, S. Tanoshimac,5, A. Wakaic,6 a
b
CIRSE, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan Department of medical radiations sciences, Royal Melbourne Institute of Technology, P.O. Box 71, Bundoora, Vic 3083, Australia c Department of Physics, Nagoya University, PI-group, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan d Physikalisches Institut, Universita.t Bonn, D-53115 Bonn, Germany Received 29 May 2001
Abstract A prototype gamma ray detector for the detection of p0 in the proposed experiment at SPring-8 to study the Gerasimov–Drell–Hearn (GDH) sum rule (GDH experiment) has been built and tested. The detector is fabricated with lead-scintillator tiles and wave length shifter fiber readout. This detector will be placed inside the bore of the superconducting magnet which supplies the polarized target with a 2:5 T magnetic field. High-detection efficiency (above 85%) for the gamma rays is the most important requirement for this detector to be employed in the proposed experiment. It has been tested with minimum ionizing particles, electrons and gamma rays. The results of the tests in comparison with some Monte Carlo simulations are presented. The evaluated efficiency for gamma rays ranging in energy above 50 MeV is higher than 90%. r 2002 Elsevier Science B.V. All rights reserved. PACS: 29.40.Vj Keywords: Lead-scintillator tiles; WLS fibers; Gamma ray detector
*Corresponding author. Tel.: +81-52-789-2895; fax: +81-52-789-2903. E-mail address: [email protected] (T. Iwata). 1 Present address: OLYMPUS OPTICAL CO. LTD., 2-3 Kuboyama-chou, Hachioji, Tokyo 192-8512, Japan. 2 Present address: Physikalisches Institut, Universit.at Bonn, D-53115 Bonn, Germany. 3 Present address: Physics Department, Tokyo Institute of Technology, 2-12-1, Oh-okayama, Meguro, Tokyo 152-8551, Japan. 4 Present address: Department of Physics, Chiba, University, 1-33, Yayoi, Inage-ku, Chiba 263-8522, Japan. 5 Present address: NTT Data, Shiba A-building 7F, 3-2-18, Shiba, Minato-ku, Tokyo 105-0014, Japan. 6 Present address: Akita Industry Promotion Foundation (AIPF), 3-1-1, Sanno, Akita-city, Akita 010-8572, 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 3 6 1 - 4
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1. Introduction The proposed experiment [1] at the SPring-8 facility aims at measuring the helicity-dependent total photoabsorption cross sections of the proton at photon energies between 1.5 and 2:9 GeV: The results will be compared with proposed measurements at other parts of the world to test the validity of the Gerasimov–Drell–Hearn (GDH) sum rule [2]. The polarized photon beam from the Laser–Electron–Photon (LEP) beam line at SPring-8, currently operational [3], will be used as a source for circularly polarized photons of variable energy. A longitudinally polarized proton target is under construction at Nagoya University. The essential feature of the experiment is a complete detection of all hadronic final states using a, 4p detector setup as displayed in Fig. 1. Some detectors will be encompassed inside the bore of the superconducting solenoid magnet of the polarized target. The gamma ray detector to be installed around the target is required to have a satisfactory
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efficiency and a large geometrical coverage for gamma rays of energies higher than 70 MeV which mostly originate from p0 produced in the reaction. This requirement becomes even harder to fulfill due to (1) limited space (about 60 cm) inside the magnet bore where the detector will be placed and (2) the influence of the high magnetic field (2:5 T) of the superconducting magnet of the target. A plausible solution would be to use heavy scintillating crystals such as CsI with photodiodes that are not so sensitive to the high magnetic fields. However, such a solution is very expensive. An alternative to this is a sampling calorimeter type detector consisting of lead and plastic scintillator tiles with WLS fibers for the readout. The WLS fiber couples with a long light guiding fiber which transmits the secondary emitted light to a photomultiplier tube (PMT). This PMT is placed at some distance from the superconducting magnet to avoid the influence of the magnetic field. The design and characteristics of such a prototype gamma ray detector based on the above-mentioned method is described in this article. The
Fig. 1. The setup of the GDH experiment at SPring-8.
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detector’s ability for the detection of minimum ionizing particles, electrons and gamma rays has been tested. The results are displayed.
2. Detector design The design of the gamma ray detector is based on the following requirements to make it suitable for the proposed experimental setup: (1) the thickness of the detector7 along the gamma rays should be less than 20 cm (2) the detector’s efficiency must exceed 85% for gamma rays of energy higher than 70 MeV (3) the dead space between the detectors should be as narrow as possible and (4) the PMT has to be placed at a distance of about 3 m from the magnet. One of the major hurdles faced in the design of this detector is the requirement for a high detection efficiency with only a few radiation lengths compared to that of conventional calorimeters. Accordingly, the number of sampling layers and the thickness of the scintillator and lead tiles are appropriately determined and an elegant light collection method is employed. In the following subsections, the structure of the detector and the readout by WLS fibers will be described. 2.1. Sampling layers and lead tiles The number of the sampling layers and tiles thickness appropriate to obtain high detection efficiency are determined using GEANT simulation [4]. The efficiency was based on the fraction of the events which is the result of an energy deposit higher than the threshold energy of 10 MeV: This threshold energy is set higher than that usually set for conventional calorimeters. The detection efficiency under the condition of the detector’s total thickness to be less than 170 mm was obtained as shown in Fig. 2 as a function of the number of the sampling layers ðns Þ and the ratio of the thickness 7
The space inside the magnet bore, 60 cm in diameter, is occupied by the polarized target cryostat (10 cm), the internal charged particle detector (5 cm 2 ¼ 10 cm) and the gamma ray detector (20 cm 2 ¼ 40 cmÞ:
Fig. 2. Efficiencies (in %) as a function of the number of sampling layers and the ratio R:
of the scintillator and lead tiles ðR ¼ tðscin:Þ= tðleadÞÞ; where the thicknesses are measured in units of mm. The detection efficiency first increases with ns and then saturates. However, in relation to R; it seems that there is a nominal R value which gives a maximum efficiency. This is because a thin lead tile (i.e. a large R value) gives a low gamma interaction probability while on the contrary a thin scintillation tile (i.e. a small R value) results in a small energy deposit, hence reduces the efficiency. Our simulation results show that efficiencies above 85% are achievable with ns value more than 17 and the ratio R ranging between 3 and 5. Accordingly, the ns value of 18 and the R ratio of 5 were selected for the detector’s design. These results lead to the following detector’s configuration: tðscin:Þ ¼ 8:0 mm; tðleadÞ ¼ 1:6 mm and the total detector’s thickness of tðdet:Þ ¼ 5:4X0 (where X0 is the radiation length). 2.2. WLS fiber readout The WLS fiber (Kuraray, type Y-7(100)) of 1 mm diameter with two turns is embedded into a circular groove with a diameter of 160 mm which is machined onto the surface of the scintillator tile
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of SCSN-38 (Kuraray) with an area of 200 mm 200 mm and a thickness of 8 mm: One end of the WLS fiber is mirrored by vacuum evaporation of aluminum to reflect the secondary lights emitted into the opposite direction to the readout. The other end is jointed with an optical cement (BC600 from Bicron) to a light guiding fiber (Clear-PS from Kuraray) of 1 mm diameter and 3 m in length. The end of the light guiding fiber is connected to a PMT (H1161 from Hamamatsu). The scintillator tile is then wrapped with an aluminized Mylar sheet. Before constructing the prototype detector, the light collection efficiency of such a single tile was tested using the 29 MeV proton beam from the SF-Cyclotron at INS Tokyo. The variation path of the incident proton beam on the tile is shown in Fig. 3 and the spatial variation of the light yield is displayed in Fig. 4. An interesting feature found in this test is that the light yields inside the groove circle vary in average by about 20%. It is clear that the light yield outside the groove circle is significantly less than that inside, because the scintillation photons produced outside the groove circle which likely escape from the sides of the tile have not been captured by the WLS fiber. This result leads to the following restrictions on the detector design: the area surrounded by the groove circle has to be made as large as the design
Fig. 3. The incident beam positions.
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Fig. 4. Light yield vs. beam position.
allows and the reflector on the sides should be improved.
3. Detector structure This detector has a trapezoidal cross section and covers a 1/8 section ðDf ¼ 451) in the azimuthal angle as shown in Fig. 5. It consists of 18 sampling layers of different areas, the smallest on the inner side and the largest on the outer side. The smallest layer (No. 1) has its area of 75 200 mm2 and the largest one (No. 18) 210 200 mm2 : Each layer consists of a lead tile of 1:6 mm thickness and a scintillator tile of 8 mm thickness. The lead tiles are made of ‘‘hard lead’’ which is composed of in parts of 96% of lead and 4% antimony. It is known that such a hard lead is malleable hence it is easy to handle during detector fabrication. The scintillator tiles are cut from a BC412 Bicron scintillator plate. This scintillator plate was selected due to its high light output compared to other scintillators such as SCSN-38. Grooves were etched onto the surface of the scintillator tiles by a round edge cutter. Two different shaped grooves are formed in these tiles: racetrack and S-figure type. The first type is adopted for the large area tiles i.e. from No. 6 to 18 as shown in Fig. 12. The groove depth of 2:6 mm allows two turns of the WLS fiber of 1 mm in diameter to be installed in the tile. Based on the results from the single tile test described in the previous section, the diameter
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Fig. 5. The schematic view of the prototype for the gamma ray detector.
of the groove circles is made as large as possible in order to improve the light collection ability. The second type, S-figure type, allows only a single trace through the S-figured groove of depth 1:6 mm: This type is adopted for the first five tiles from No.1 to 5 as displayed in Fig. 11. The width of the grooves in all tiles of both types is 1:1 mm: The grooves were not polished in order to obtain uniform response [5]. The WLS fiber of 1 mm diameter is used in all the tiles. Both ends of the WLS fibers are milled and then polished with fine sandpaper. A small disc made of an aluminized Mylar sheet is glued with the optical cement on one end of the fiber to form a light reflector. The other end of the WLS fiber is jointed, as discussed in the previous section, to a light guiding fiber 3 m long. The other end of the fiber is directly attached to the PMT. The four sides of the scintillator plate are painted with Bicron BC-650 white paint to improve efficiency of light reflection and the top and the bottom are covered with aluminized Mylar sheets. The main properties of the detector and the configuration of the tile/fiber system are summarized in Tables 1 and 2.
Table 1 Summary of the detector properties Scintillator tiles Lead tiles Total thickness WLS fiber Light guiding fiber PMT
BC-412, 8 mm thick Hard lead (antimony 4%), 1:6 mm thick 173 mm (lead : 24:8 mm; plastic scintillator : 148 mmÞ; 5:4X0 Kuraray Y-7 (100), single cladding, 1 mm in diameter Kuraray Clear-PS, single cladding, 1 mm in diameter Hamamatsu H1161, nominal gain 1 106 at 1:5 KV
4. Reflector’s impact on the light yield In this section, the effect of the light reflector formed at the end of the WLS fiber on the light yield is discussed. As it is known from the applications of scintillation and WLS fibers, light reflectors are usually installed at the end of the fibers for the purpose of enhancing the light yield. These light reflectors are formed by either aluminum coating by vacuum evaporation [6,7] or by
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installing an aluminized Mylar sheet [8]. The former method is a tedious one and difficult to fulfill since it requires polishing the fiber surface very well to protect the reflector end. This problem Table 2 Configuration of the tile/fiber system Layer 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Tile dimension ðmm2 Þ 75 200 83 200 91 200 99 200 107 200 115 200 123 200 131 200 139 200 147 200 155 200 163 200 171 200 178 200 186 200 194 200 202 200 210 200
Groove type
WLS fiber length (mm)
S-figure S-figure S-figure S-figure S-figure racetrack racetrack racetrack racetrack racetrack racetrack racetrack racetrack racetrack racetrack racetrack racetrack racetrack
261 231 228 229 230 1025 1041 1059 1075 1093 1109 1127 1143 1162 1177 1196 1211 1230
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will be exacerbated in the case of production of a large number of detectors for the GDH experiment. The second method on the other hand is quite simple and hence it is the method followed for the formation of light reflectors in this prototype detector. The reflector in this case is formed by attaching a small disc made of aluminized Mylar to the WLS fiber’s surface using an optical glue. Such a simple structure is suitable mainly for the production of large numbers of such detectors for the proposed experiment. Moreover, attention should be focused to the fact that it is not essential to have well-polished surfaces since smooth surfaces will be formed using the glue. An experimental test was performed to compare the light yield from the tiles treated with the two different methods mentioned above. The experimental setup is displayed in Fig. 6. The results show that about 30% more light yield was obtained from the fibers with the light reflector made of aluminized Mylar sheet compared to that obtained from the fibers with light reflectors made following the vacuum evaporation technique. Also, it was observed that the light reflector
Fig. 6. Setup for the study of the light output from the WLS fiber. A part of the WLS fiber is illuminated by a light source. The reflector is mounted at the right end of the WLS fiber. The photo diode installed at the left end of the WLS fiber measures the light output.
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Fig. 7. The setup of the test in the beam at the KEK-PS. T1; T2; T3 and T4: TOF counters used for the trigger; C1, C2 and C3: gas Cherenkov counters; Ga: the gamma-ray detector on test; LG: lead glass calorimeter.
produced via vacuum evaporation technique did not improve the light yield of a similar fiber with no reflector at all. This could be caused by the fact that not enough smoothed surface in such a case is obtained. Accordingly, the technique of using aluminized Mylar was selected.
5. Charged particles detection A series of particle detection measurements has been carried out with this detector to determine its detection characteristics. They were performed at the T1 beam line of the proton synchrotron of the KEK-PS. The charged particles of the beam were mainly minimum ionizing particles (pions) and electrons. The measurement procedure and some of the acquired results are presented in the following two subsections alongside some discussions. 5.1. Experimental setup with charged particles
Fig. 8. A typical pulse height distribution for p at 2 GeV=c traversing through the gamma ray detector.
Charged particles ranging from 0.3 to 2 GeV=c were offered in the beam line. The experimental setup is displayed in Fig. 7. Particle identification was accomplished with a time of flight (TOF) system, gas Cherenkov counters and a lead glass calorimeter placed most downstream of the setup. A time resolution of 110 ps for the flying path of 7:4 m was obtained in the TOF system.
27 MeV in all scintillator tiles and about 1:5 MeV in one tile. From this set of data it was observed that the light output normalized by the energy deposit in the scintillator tiles was about 1:03 P:E:=MeV: The number of photoelectrons normalized by the energy deposit may be expressed as
5.2. Minimum ionizing particles detection
where Nph ¼ 104 is the number of scintillation photons per energy deposit of 1 MeV given by a minimum ionizing particle traversing plastic scintillator tiles, Zc to be evaluated here is the conversion efficiency to be evaluated from the scintillation photons into secondary emitted
The light output was studied for p at 2 GeV=c: In Fig. 8 a typical pulse height distribution resulting from the detection of these particles is shown. The particles deposit in average about
NP:E: ¼ Nph Zc O1=2 ð1 þ eR ÞAWLS Ajoint ALG QPMT ð1Þ
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photons in the WLS fiber, O1=2 ¼ 3:1% is the trapping efficiency in the WLS fiber with single cladding, eR ¼ 30% is the reflector’s efficiency obtained in the measurement described in the previous section, AWLS and ALG are the light attenuations in the WLS fiber and the light guiding fiber, respectively, Ajoint is the attenuation at the joint between the fibers, QPMT ¼ 15% is the quantum efficiency of the PMT. Assuming the attenuation length8 in the WLS fiber to be 3:4 m and in the light guiding fiber 10 m; AWLS B0:9 is given from the half length of the WLS fiber to be 0:44 m and ALGF B0:7 from the light guiding fiber 3 m long. Taking Ajoint to be unity, we obtain Zc B2:7%: In the following two subsections, the amount of the light yield obtained in each individual layer and the light yield spatial variations will be presented. 5.2.1. Light yield in an individual layer As described in the section of the detector’s structure, the scintillator tiles are classified according to the shape of the groove of the WLS fiber: one type is called S-figure and the other racetrack type. It is interesting to compare the light yield in these two types. There is a difference in the amount of the light yield in the tiles with the S-figure and those of the racetrack. It is anticipated that the former type would have less light yield due to its shorter WLS path length despite their narrower area which could assist in improving the scintillation light collection. On the other hand, the light yield in the racetrack type tiles is expected to be higher due to their high collection efficiency based on the fact that they contain a longer WLS fiber which results in covering a larger area for the light collection. But this might be offset, somehow, by the light attenuation caused by longer paths traversed by the light collected. The light output from an individual scintillator tile is measured with only one light guiding fiber connected to the PMT. Typical pulse height distributions are displayed in Fig. 9. This measurement indicates that the light output in the tiles of the racetrack type is larger compared to those of 8
Scintillation Materials, Kuraray Co., Ltd.
Fig. 9. The pulse height distributions for a: No. 5 and b: No. 18 layers.
the S-figure type: the averaged light output over the tiles of the racetrack type is 1:8 P:E: and that over the S-figure type is 1.1 P.E. 5.2.2. Spatial variation of light yield The light yield position dependence for the p of 2 GeV=c in each individual tile and in all of the tiles collectively is being investigated. This was accomplished by measuring the light output upon varying the beam incident position on the tiles along the y-axis as indicated in Fig. 5. Although, the central parts of the tile are anticipated to produce more light yield than the peripheral parts of the tile, no significant spatial variation of the light yield was observed experimentally. This spatial light yield variation is found to be less than 10% as displayed in Fig. 10. This was the result for all the layers collectively. For individual layers, the light yield position dependence was only studied for two layers i.e. No. 5 as representative of the S-figure type and No. 18 as the racetrack type representation. The result for No. 5 shows no measurable variation in the amount of light yield obtained as shown in Fig. 11, although the measurement was carried out only at three points. For No. 18, the light yield
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Fig. 10. The light output for all the layers as a function of the incident beam position.
inside the groove circle is little improved than those outside as shown in Fig. 12.
experiment is to achieve high-detection efficiency and much less emphasis is placed on its energy resolution. Since the radiation length of this prototype detector is only 5:4 X0 ; its energy response is expected to show some non-linearity at high energies due to some shower leak. This will affect its energy resolution. During the tests with electrons, the energy response and energy resolution as a function of the incident electron energy are investigated. The energy response was investigated for the electrons ranging from 400 MeV up to 1:4 GeV: A typical pulse height distribution obtained at 600 MeV is displayed in Fig. 13. It is well reproduced by the Monte Carlo simulation using GEANT4 package [9]. A slight variation from linearity in the detector’s energy response, about 5% at 1 GeV; is shown in Fig. 14. The energy resolution s=E of 20–25% for electrons ranging from 400 MeV to 1:4 GeV is achieved in this test as shown in Fig. 15. These resolutions are, as anticipated, apparently worse compared with those given by typical calorimeters pffiffiffielectromagnetic ffi such as s=EB9%= E [10,11].
5.3. Electrons detection The main attractive feature of typical electromagnetic calorimeters is their ability to achieve high energy resolutions. However, the emphasis on the gamma ray detector for the proposed GDH
6. Gamma ray detection efficiency The gamma ray detection efficiency is measured using the tagged photon beam delivered by the
Fig. 11. The light yields at different positions of the incident beam on the No.5 layer.
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Fig. 12. The light yields at different positions of the incident beam on the No. 18 layer.
Fig. 13. A typical pulse height distribution for electrons at 600 MeV: The histogram represents the measurement. The pedestal is located at 110 channel. The closed circles are results of the simulation.
booster synchrotron at Laboratory for Nuclear Science (LNS) of Tohoku University. The beam energy ranges from 600 to 800 MeV and the energy resolution of the tagging system is about 4:7 MeV:
Fig. 14. The energy response of the prototype detector as a function of the incident electron energy. The solid line is the result of a linear fit for the three data points at 400, 500 and 600 MeV:
The experimental setup is displayed in Fig. 16. To veto charged particles in the beam, scintillation counters (T1, T2) and a Gas Cherenkov counter
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Fig. 15. Energy resolution as a function of the incident electron energy. The solid line is the result of a fit by the function ðs=EÞ2 ¼ ða=E 1=2 Þ2 þ b2 with the parameters a ¼ 0:10 and
Fig. 17. The pulse height distribution for gamma rays at 630 MeV (histogram) in comparison with a Monte Carlo simulation (closed circles). The cross hatched area shows the events above the threshold set at channel 140 corresponding to the four photoelectron level. The fraction of the events in the hatched area to the total events gives the detection efficiency of 97:170:2%:
Fig. 16. The experimental setup to measure detection efficiencies for gamma rays at the LNS of Tohoku University. T1, T2: veto counters; C1: gas Cherenkov counter(veto); Ga: the gamma ray detector on test; LG: lead glass calorimeter.
(C1) are used. The prototype gamma ray detector (Ga), is followed by a lead glass calorimeter (LG) which allows the calibration of the photon flux. An event selection was made by requiring that at least one of Ga and LG gives a significant signal, in order to eliminate photons not reaching Ga. The pulse height distribution of the prototype gamma ray detector obtained for photons of 630 MeV shows an asymmetric shape with a tail in the lower energy region due to some leakage of the shower (Fig. 17). It is well reproduced by our simulation using the GEANT4 package [9] based on the experimental data obtained at KEK with electrons taking into account the
Fig. 18. The detection efficiency for gamma rays as a function of energy. The closed square is the measured value. The closed circles are obtained in the simulation. The solid line is an eye guide.
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differences in the experimental conditions. Setting the threshold at the four photoelectron level, the detection efficiency was calculated to be 97:170:2%: We evaluated it also at different energies using the Monte Carlo simulation. The results are shown along with the experimental value in Fig. 18. The detection efficiencies were satisfactorily high for the use in the proposed GDH experiment (i.e. above 90% at the energies higher than 50 MeV).
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would like to thank Dr. H. Yamazaki of LNSTohoku University for his effort in arranging our experiment. We acknowledge the technical staffs of the mechanical workshop of Department of Physics of Nagoya University, in particular, Mr. S. Inoue and Mr. A. Itoh who produced the grooves onto the scintillator tiles. M. Geso acknowledges the travel grants from ANSTOAccess to Major Research Facility & Faculty of Life Sciences-RMIT university 2000.
7. Conclusion The prototype of the gamma ray detector specially designed to be used in the proposed GDH experiment at SPring-8 has been fabricated with lead and scintillator tiles using WLS fiber readout. It has been tested using minimum ionizing particles, electrons and gamma rays. The results of the test were compared with those obtained by a Monte Carlo simulation. The detection efficiency for gamma rays was evaluated based on the simulation and also measured with a tagged photon beam. It is found to be higher than 90% at energies above 50 MeV: This result meets the requirement of the proposed GDH experiment.
Acknowledgements We appreciate the technical staffs of KEK for their help during the test experiment at KEK-PS. We would like to express our gratitude to the accelerator crews of LNS of Tohoku University for their effort to maintain the beam condition. We
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