- Email: [email protected]
Development status of position-sensitive neutron detectors for J-PARC in JAERI—a comprehensive overview
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 529 (2004) 254–259
Development status of position-sensitive neutron detectors for J-PARC in JAERI—a comprehensive overview Masaki Katagiri Neutron Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan
Abstract We have developed various kinds of position-sensitive neutron detectors in JAERI for the J-PARC project. The neutron imaging detectors using scintillators have been developed aiming for high counting rate, high spatial resolution, high neutron gamma-rays discrimination, and high detection efficiency. The development included various kinds of phosphors, scintillators, and read-out methods. Another is a two-dimensional gaseous neutron detector, which is based on a microstrip technology. The development of the microstrip gas chamber that has individual strip read-out and of the instrument system with a capability of secondary-particle discrimination is underway for a high spatial resolution and high detection efficiency with moderate area coverage. r 2004 Elsevier B.V. All rights reserved. PACS: 29.30.Hs; 29.40.Mc Keywords: Neutron detector; Position sensitive; Scintillator; Gas; Counting rate; Position resolution
1. Introduction The next-generation pulsed neutron sources using high-intensity proton accelerators have made a great deal of progress in Japan (J-PARC project) [1], United States (SNS project) [2], and United Kingdom (ISIS second target station project) [3]. The specifications required for the position-sensitive neutron detectors (PSNDs) at such high-intensity neutron sources are high count rate, high detection efficiency, high spatial resolution, high time resolution, high n/g discrimination, large area, least dead area, and long-term stability. E-mail address: [email protected] (M. Katagiri).
The detectors with such performances should be realized in an acceptable cost. The performances of the detector must be optimized for the instrumentation to be installed, i.e., a fair ‘‘trade-off’’ in performances is also required. The development of position-sensitive scintillation and gaseous detectors has been underway for several years in Japan Atomic Energy Research Institute. We have also developed other types of neutron detectors based on image plates [4,5], photo-stimulated phosphors [6,7], new types of semiconductor detectors [8,9], and superconducting neutron detectors [10,11]. In this paper, we describe recent developments of scintillator and gaseous neutron detectors for the J-PARC project.
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.04.154
ARTICLE IN PRESS M. Katagiri / Nuclear Instruments and Methods in Physics Research A 529 (2004) 254–259
2. Neutron imaging detectors using scintillators Neutron imaging detectors using scintillators have promising performances for high counting rate, high spatial resolution, large angle-coverage, and small dead area. The scintillation detectors were developed and serviced successfully for various instruments at ISIS (see for example [12]). However, the improvement on the detector performances such as spatial resolution, detection efficiency, and counting rate are required when those detectors are used at the high-intensity pulsed sources. To fulfill those requirements, scintillation detectors, in which crossed wavelength shifting (WLS) fibers are used with a photon-counting read-out, have been developed. We have also developed several types of scintillation materials and of read-out methods. The appropriate combination of those would result in an optimal detector for each instrument. 2.1. Scintillators for PSNDs We investigated various kinds of scintillators searching for the well-balanced ones with respect to detection efficiency, counting rate, n/g ratio, and multi-counting. The scintillators investigated were: ZnS and ZnS-series phosphors with neutron converters, short life-time phosphors with neutron converters [13–17], short life-time phosphors containing 10B, glass scintillators containing 10B or 6 Li, 6Li-loaded plastic scintillators [18] and organic phosphors containing 10B [19]. Their neutrondetection characteristics, mainly detection efficiencies, were presented in separate papers. Table 1 summarizes the detection efficiencies for some of the developed scintillators. The ZnS/6LiF scintillators exhibited a superior n/g ratio than the others with acceptable detection efficiency. The devel-
255
oped ZnS/6LiF scintillator (indicated as ‘‘standard’’) exhibited a detection efficiency of 40.7% for thermal neutrons, which was about 1.5 times higher than that of commercial products (Bicron, BC-704) [13]. We estimate that the main reason is transparency of the scintillators. Consequently, we concluded that ZnS or ZnS-series phosphors with neutron converters were preferable in most applications because of their good n/g ratio, detection efficiency, and large yields of luminescent light, although they are limited in count rate due to the slow decay component of B1 ms. We also found that Y2SiO5:Ce3+/6LiF exhibited excellent performance in counting rate (the decay time of which is about 40 ns) with a moderate n/g ratio [13]. 2.2. Read-out methods for PSNDs We have developed various types of read-out methods for the scintillation detectors using WLS fibers according to the required spatial resolution. In all the developed methods, the luminescent light was read-out in photon-counting mode (i.e., without integration of the light quanta at the output of the photomultiplier) to increase neutron detection efficiency. This photon counting is suitable for the cases, where the intensity of the scintillation light reaching the photomultiplier is rather small, such as in the cases when using low luminescent scintillators and/or WLS fibers. The developed methods are; (1) Backside read-out, where the WLS fibers for X- and Y-axis were placed behind the scintillator sheet, for a high spatial resolution of B0.5 mm [20,21]. (2) Backside read-out with a tapered fiber optics placed between the scintillation-sheet and the WLS fibers for ultra-high spatial resolution less than 0.15 mm [21].
Table 1 Neutron detection efficiencies for the phosphor/neutron converter scintillators for thermal neutrons Scintillator
ZnS/6LiF standard (%)
ZnS/6LiF 2 times 6 LiF (%)
ZnS/10B2O3 Heated up to 500 (%)
Y2SiO5:Ce3+ /6LiF (%)
Bicron ZnS/6LiF BC-704 (%)
Neutron detection efficiency
40.7
43.5
29.0
37.1
26.4
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(3) Backside read-out with bundled fibers for a spatial resolution of a few mm [21]. (4) Four-coincidence read-out for pixel scintillators for a spatial resolution of a few mm [22–24]. (5) Crossed fiber read-out, where the WLS fibers were placed before and behind the scintillation-sheet, with centroid finding methods based on coincidence signal processing for a spatial resolution of B0.5 mm [25,26]. We obtained the following results: (1) Detection efficiency more than 40% for thermal neutrons with the developed ZnS/6LiF scintillator [13]. (2) Spatial resolution of B0.5 mm using the crossed/backside fiber read-out with the centroid finding method based on coincidence signal processing [25,26]. (3) Maximum count rate up to 3 Mcps using the Y2SiO5:Ce3+/7Li10 2 B4O7 scintillator with the crossed fiber read-out [26]. (4) n/g ratio of less than 10 7 with ZnS/6LiF scintillators. Consequently, it was confirmed that the optimal neutron detectors could be constructed when we appropriately combine the scintillators with the read-out method according to the specifications required for the associated instrument. 2.3. Electronics for PSNDs
propagated through WLS fibers are amplified at the photomultipliers, and signals more above the discrimination level are converted to digital signals. The incident position of neutron can be calculated in the field programmable gate arrays (FPGA) for high counting rates. The positioning schemes (such as simple centroid, double coincidence, and four-coincidence [23–26]) can easily be implemented into the system by replacing the associated gate programs with that recorded in the EPROMs. Fig. 2 shows the prototype system, which can read input channels of 256 256. The implemented FPGAs operate with the clock frequency of 300 MHz, and they accept the signals of low voltage differential signaling. 2.4. A compact, high-spatial resolution detector A compact high-position resolution detector was developed aiming to reduce the dead area. The idea was to bend the WLS fibers in right angle at the edges of the scintillation-sheet [20]. Fig. 3 shows the illustration of the developed detector using a ZnS:Ag/6LiF scintillator with WLS fibers placed at the backside of the scintillation-sheet. The WLS fibers, which have a side width of 0.5 mm, were bent at right angles with a curvature of about 1 mm. The propagating light was lost about 50% of the incident light to the WLS fiber [20]. Fig. 4 shows the photograph of the developed detector. The detector performances were evaluated using cold neutrons at the beam line of C2-3
We developed a signal-processing system for scintillator detectors using the photon-counting read-out [27]. Fig. 1 shows a schematic diagram of the signal-processing system. The light signals
Neutron imaging device using two kinds of WLS fibers based on photon counting method
64ch PMT
64ch PMT
FPGA signal processing circuit
PMT amplifier
Discriminator
NIM-FAST To TTL
FPGA
PMT amplifier
Discriminator
NIM-FAST To TTL
FPGA
Data acquistion system for TOF experiments
Y-axis
EEROM for Xaxis processing program EEROM for Yaxis processing program X-axis
ADC modulated interface
Fig. 1. Schematic diagram of the signal-processing system.
Fig. 2. A signal-processing system with 256 256 channels.
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257
Neutron ZnS:Ag:/6LiF Fiber part bent in 90 degree
Wavelength shifting fibers for X-axis readout
Wavelength shifting fibers for X-axis readout
Multi-anode Photomultiplietr
Fig. 3. A compact detector using a ZnS: Ag/6LiF scintillator with the WLS fibers bent in a right angle at the edges. Fig. 5. Examples of neutron images measured by the developed detector in TOF mode. The width of the WLS fibers is 0.5 mm both in the X- and Y-axes.
3. He-3 gas neutron imaging detector
Fig. 4. Photograph of the compact high-position resolution neutron imaging detector.
of the JRR-3 reactor in JAERI. The spatial resolution was 0.6 and 0.7 mm for the X- and Yaxis, respectively. The detector was also tested using pulsed neutrons at KEK. Fig. 5 shows some of the neutron images for corresponding time slices using the collimated neutron beam of 1 1 mm. The detector measured a well-collimated beam when the neutron energy decreased.
We have been developing microstrip gas chamber (MSGC) detectors using helium-3 gas as neutron converter. The specifications expected for the MSGC detector are high detection efficiency (>60% for thermal neutrons), high spatial resolution (o1 mm), high n/g discrimination ratio (>107), high counting rate (>106 cps/ module), and a moderate area (20 20 cm2, maximum). A large amount of heavy gas such as CF4 has to be filled into achieve a high spatial resolution in a conventional positioning scheme, such as charge-division or delay-line chain. However, achieving a gas gain of more than ten in such a gas condition would require an electric field strength of B104 V/mm between the microstrip electrodes, thereby making the detector unstable due to the dielectric breakdown at the surface of the substrate. To solve these problems and achieve the specifications mentioned above, we developed the MSGC system, in which all the signal channels are read-out individually, and the incident positions of
ARTICLE IN PRESS 258
M. Katagiri / Nuclear Instruments and Methods in Physics Research A 529 (2004) 254–259
the neutrons are determined by the instrument system with a capability of secondary particle discrimination (InSPaD). The InSPaD identifies the particles proton and triton created in the nuclear reaction 3He + n - p + T on the basis of the track length, and determines the incident position of the neutron accurately, i.e., the spatial resolution of the system is not limited in the track length of the secondary particles. A simulation result showed a high spatial resolution less than 1 mm regardless of the gas condition using the InSPaD [28,29]. Full description of the InSPaD can be found in Refs. [29,30]. With this system, one can use a ‘‘light’’ gas condition without loss of spatial resolution; thereby the stability of the detector should be further increased as well. Fig. 6 shows the proto-type of the microstrip substrate, which measures 50 50 mm mounted on the PCB socket board for individual read-out [31]. The MSGC was manufactured by TOSHIBA Co., which can deal with charge-up at high counting rate by appropriately adjusting the resistivity of the polyimide substrate using an organic titanium coating [32]. We have made two of the high pressure gas chambers; one is for the test (Fig. 7) and the other is for the proto-type. The signal channels of 160 and 561 can be fed through out of the chambers so that most/all the
Fig. 6. Microstrip substrate mounted on the PCB socket board for individual read-out.
Fig. 7. Test gas chamber for individually read-out, which withstands up to 8 atm.
signals from each anode strip can be read-out individually. The gas chambers withstand up to 8 atm, and have windows made of aluminum with a thickness of 3 mm. For each signal channels, the signal pulses were amplified, shaped and discriminated individually. We do not measure the pulse height, but only measure the pattern and the numbers of the fired channels (i.e. hit detection) event by event to ensure high count rate. The feasibility of the hit-detection method was well confirmed both in X-ray [31] and in neutron detection [33,34]. In the InSPaD, the voltage levels of the discriminators were set identical for each channel and appropriately so that the numbers of suprathreshold channels should become different for a proton and a triton. The identification of the proton and the triton can be done using the numbers of suprathreshold channels, and the incident position of neutron can be determined by a simple logical calculation. The InSPaD realizes the identification of the particles with a simple, fast and cost-effective method, thus, ensuring high count rate, and moreover increases the signal-to-noise ratio of the detector system,
ARTICLE IN PRESS M. Katagiri / Nuclear Instruments and Methods in Physics Research A 529 (2004) 254–259
which is attributable to the relatively high discrimination levels to differentiate a proton and a triton. Our MSGC system with the InSPaD is still under development, but the operational principle was confirmed using a test detector system [34]. We are going to proceed to the next stage development of the proto-type detector system to obtain the neutron images in the near future.
4. Conclusions We have developed two-dimensional neutron detectors; scintillation detectors with the WLS fiber read-out and the MSGC detector with individual read-out. The basic performances of the developed scintillation detectors were confirmed in experiments. We are going to transfer the developed technology to industrial companies to produce proto-types for the J-PARC instruments. As for the MSGC detector, we are going to develop the prototype and demonstrate the highperformance detector system based on the secondary particles identification.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
see http://j-parc.jp/index.html. see http://www.sns.gov/. see http://www.isis.rl.ac.uk/TargetStation2/. M. Katagiri, et al., Nucl. Instr. and Meth. A 461 (2001) 207. M. Katagiri, et al., Nucl. Instr. and Meth. A 477 (2002) 179. K. Sakasai, et al., Appl. Phys. A 74 (2003) S1589. K. Sakasai, et al., Nucl. Instr. and Meth. A, in these proceedings. T. Nakamura, et al., Nucl. Instr. and Meth. A, these proceedings. T. Nakamura, et al., Rev. Sci. Instrum. 75 (2004) 340.
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[10] T. Nakamura, et al., Nucl. Instr. and Meth. A 520 (2004) 67. [11] T. Nakamura, et al., Nucl. Instr. and Meth. A, in these proceedings. [12] The ISIS Facility Annual Report 2002–2003, Rutherford Appleton Laboratory Report, Report Number RAL-TR2003-050. [13] M. Katagiri, et al., Nucl. Instr. and Meth. A, in these proceedings. [14] M. Katagiri, et al., Nucl. Instr. and Meth. A 513 (2004) 374. [15] N. Kubota, et al., Nucl. Instr. and Meth. A, in these proceedings. [16] M. Matsubayashi, et al., Nucl. Instr. and Meth. A, in these proceedings. [17] T. Kojima, et al., Nucl. Instr. and Meth. A, in these proceedings. [18] M. Katagiri, et al, Nucl. Instr. and Meth. A, in these proceedings. [19] H. Kamaya, et al, Nucl. Instr. and Meth. A, in these proceedings. [20] M. Katagiri, et al., Nucl. Instr. and Meth. A, in these proceedings. [21] M. Katagiri, et al., ESS 03-136-M1, Vol. III, 2003, p. 435. [22] K. Toh, et al., JAERI-conf 2001-002 (2001) 627. [23] K. Toh, et al., Appl. Phys. A 74 (Suppl.) (2002) S1601. [24] K. Toh, et al., Nucl. Instr. and Meth. A 485 (2002) 571. [25] M. Katagiri, et al., Appl. Phys. A 74 (Suppl.) (2002) S1604. [26] M. Katagiri, et al., Nucl. Instr. and Meth. A 513 (2004) 374. [27] M. Ebine, et al., Nucl. Instr. and Meth. A, in these proceedings. [28] H. Yamagishi, et al., Nucl. Instr. and Meth. A, in these proceedings. [29] H. Yamagishi, et al., Rev. Sci. Instrum., in press. [30] H. Yamagishi, et al., Japanese patent pending, P2000253967/P2002-62360A, 2000. [31] T. Tanimori, et al., Nucl. Instr. and Meth. A 436 (1999) 188. [32] A. Ochi, et al., Nucl. Instr. and Meth. A 471 (2001) 264. [33] T. Nakamura, et al., Proceedings of the 16th International Collaboration on Advanced Neutron Source, Neuss, 2003, p. 441. [34] T. Nakamura, et al., Nucl. Instr. and Meth. A, in these proceedings.
Nuclear Instruments and Methods in Physics Research A 529 (2004) 254–259
Development status of position-sensitive neutron detectors for J-PARC in JAERI—a comprehensive overview Masaki Katagiri Neutron Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan
Abstract We have developed various kinds of position-sensitive neutron detectors in JAERI for the J-PARC project. The neutron imaging detectors using scintillators have been developed aiming for high counting rate, high spatial resolution, high neutron gamma-rays discrimination, and high detection efficiency. The development included various kinds of phosphors, scintillators, and read-out methods. Another is a two-dimensional gaseous neutron detector, which is based on a microstrip technology. The development of the microstrip gas chamber that has individual strip read-out and of the instrument system with a capability of secondary-particle discrimination is underway for a high spatial resolution and high detection efficiency with moderate area coverage. r 2004 Elsevier B.V. All rights reserved. PACS: 29.30.Hs; 29.40.Mc Keywords: Neutron detector; Position sensitive; Scintillator; Gas; Counting rate; Position resolution
1. Introduction The next-generation pulsed neutron sources using high-intensity proton accelerators have made a great deal of progress in Japan (J-PARC project) [1], United States (SNS project) [2], and United Kingdom (ISIS second target station project) [3]. The specifications required for the position-sensitive neutron detectors (PSNDs) at such high-intensity neutron sources are high count rate, high detection efficiency, high spatial resolution, high time resolution, high n/g discrimination, large area, least dead area, and long-term stability. E-mail address: [email protected] (M. Katagiri).
The detectors with such performances should be realized in an acceptable cost. The performances of the detector must be optimized for the instrumentation to be installed, i.e., a fair ‘‘trade-off’’ in performances is also required. The development of position-sensitive scintillation and gaseous detectors has been underway for several years in Japan Atomic Energy Research Institute. We have also developed other types of neutron detectors based on image plates [4,5], photo-stimulated phosphors [6,7], new types of semiconductor detectors [8,9], and superconducting neutron detectors [10,11]. In this paper, we describe recent developments of scintillator and gaseous neutron detectors for the J-PARC project.
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.04.154
ARTICLE IN PRESS M. Katagiri / Nuclear Instruments and Methods in Physics Research A 529 (2004) 254–259
2. Neutron imaging detectors using scintillators Neutron imaging detectors using scintillators have promising performances for high counting rate, high spatial resolution, large angle-coverage, and small dead area. The scintillation detectors were developed and serviced successfully for various instruments at ISIS (see for example [12]). However, the improvement on the detector performances such as spatial resolution, detection efficiency, and counting rate are required when those detectors are used at the high-intensity pulsed sources. To fulfill those requirements, scintillation detectors, in which crossed wavelength shifting (WLS) fibers are used with a photon-counting read-out, have been developed. We have also developed several types of scintillation materials and of read-out methods. The appropriate combination of those would result in an optimal detector for each instrument. 2.1. Scintillators for PSNDs We investigated various kinds of scintillators searching for the well-balanced ones with respect to detection efficiency, counting rate, n/g ratio, and multi-counting. The scintillators investigated were: ZnS and ZnS-series phosphors with neutron converters, short life-time phosphors with neutron converters [13–17], short life-time phosphors containing 10B, glass scintillators containing 10B or 6 Li, 6Li-loaded plastic scintillators [18] and organic phosphors containing 10B [19]. Their neutrondetection characteristics, mainly detection efficiencies, were presented in separate papers. Table 1 summarizes the detection efficiencies for some of the developed scintillators. The ZnS/6LiF scintillators exhibited a superior n/g ratio than the others with acceptable detection efficiency. The devel-
255
oped ZnS/6LiF scintillator (indicated as ‘‘standard’’) exhibited a detection efficiency of 40.7% for thermal neutrons, which was about 1.5 times higher than that of commercial products (Bicron, BC-704) [13]. We estimate that the main reason is transparency of the scintillators. Consequently, we concluded that ZnS or ZnS-series phosphors with neutron converters were preferable in most applications because of their good n/g ratio, detection efficiency, and large yields of luminescent light, although they are limited in count rate due to the slow decay component of B1 ms. We also found that Y2SiO5:Ce3+/6LiF exhibited excellent performance in counting rate (the decay time of which is about 40 ns) with a moderate n/g ratio [13]. 2.2. Read-out methods for PSNDs We have developed various types of read-out methods for the scintillation detectors using WLS fibers according to the required spatial resolution. In all the developed methods, the luminescent light was read-out in photon-counting mode (i.e., without integration of the light quanta at the output of the photomultiplier) to increase neutron detection efficiency. This photon counting is suitable for the cases, where the intensity of the scintillation light reaching the photomultiplier is rather small, such as in the cases when using low luminescent scintillators and/or WLS fibers. The developed methods are; (1) Backside read-out, where the WLS fibers for X- and Y-axis were placed behind the scintillator sheet, for a high spatial resolution of B0.5 mm [20,21]. (2) Backside read-out with a tapered fiber optics placed between the scintillation-sheet and the WLS fibers for ultra-high spatial resolution less than 0.15 mm [21].
Table 1 Neutron detection efficiencies for the phosphor/neutron converter scintillators for thermal neutrons Scintillator
ZnS/6LiF standard (%)
ZnS/6LiF 2 times 6 LiF (%)
ZnS/10B2O3 Heated up to 500 (%)
Y2SiO5:Ce3+ /6LiF (%)
Bicron ZnS/6LiF BC-704 (%)
Neutron detection efficiency
40.7
43.5
29.0
37.1
26.4
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256
(3) Backside read-out with bundled fibers for a spatial resolution of a few mm [21]. (4) Four-coincidence read-out for pixel scintillators for a spatial resolution of a few mm [22–24]. (5) Crossed fiber read-out, where the WLS fibers were placed before and behind the scintillation-sheet, with centroid finding methods based on coincidence signal processing for a spatial resolution of B0.5 mm [25,26]. We obtained the following results: (1) Detection efficiency more than 40% for thermal neutrons with the developed ZnS/6LiF scintillator [13]. (2) Spatial resolution of B0.5 mm using the crossed/backside fiber read-out with the centroid finding method based on coincidence signal processing [25,26]. (3) Maximum count rate up to 3 Mcps using the Y2SiO5:Ce3+/7Li10 2 B4O7 scintillator with the crossed fiber read-out [26]. (4) n/g ratio of less than 10 7 with ZnS/6LiF scintillators. Consequently, it was confirmed that the optimal neutron detectors could be constructed when we appropriately combine the scintillators with the read-out method according to the specifications required for the associated instrument. 2.3. Electronics for PSNDs
propagated through WLS fibers are amplified at the photomultipliers, and signals more above the discrimination level are converted to digital signals. The incident position of neutron can be calculated in the field programmable gate arrays (FPGA) for high counting rates. The positioning schemes (such as simple centroid, double coincidence, and four-coincidence [23–26]) can easily be implemented into the system by replacing the associated gate programs with that recorded in the EPROMs. Fig. 2 shows the prototype system, which can read input channels of 256 256. The implemented FPGAs operate with the clock frequency of 300 MHz, and they accept the signals of low voltage differential signaling. 2.4. A compact, high-spatial resolution detector A compact high-position resolution detector was developed aiming to reduce the dead area. The idea was to bend the WLS fibers in right angle at the edges of the scintillation-sheet [20]. Fig. 3 shows the illustration of the developed detector using a ZnS:Ag/6LiF scintillator with WLS fibers placed at the backside of the scintillation-sheet. The WLS fibers, which have a side width of 0.5 mm, were bent at right angles with a curvature of about 1 mm. The propagating light was lost about 50% of the incident light to the WLS fiber [20]. Fig. 4 shows the photograph of the developed detector. The detector performances were evaluated using cold neutrons at the beam line of C2-3
We developed a signal-processing system for scintillator detectors using the photon-counting read-out [27]. Fig. 1 shows a schematic diagram of the signal-processing system. The light signals
Neutron imaging device using two kinds of WLS fibers based on photon counting method
64ch PMT
64ch PMT
FPGA signal processing circuit
PMT amplifier
Discriminator
NIM-FAST To TTL
FPGA
PMT amplifier
Discriminator
NIM-FAST To TTL
FPGA
Data acquistion system for TOF experiments
Y-axis
EEROM for Xaxis processing program EEROM for Yaxis processing program X-axis
ADC modulated interface
Fig. 1. Schematic diagram of the signal-processing system.
Fig. 2. A signal-processing system with 256 256 channels.
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257
Neutron ZnS:Ag:/6LiF Fiber part bent in 90 degree
Wavelength shifting fibers for X-axis readout
Wavelength shifting fibers for X-axis readout
Multi-anode Photomultiplietr
Fig. 3. A compact detector using a ZnS: Ag/6LiF scintillator with the WLS fibers bent in a right angle at the edges. Fig. 5. Examples of neutron images measured by the developed detector in TOF mode. The width of the WLS fibers is 0.5 mm both in the X- and Y-axes.
3. He-3 gas neutron imaging detector
Fig. 4. Photograph of the compact high-position resolution neutron imaging detector.
of the JRR-3 reactor in JAERI. The spatial resolution was 0.6 and 0.7 mm for the X- and Yaxis, respectively. The detector was also tested using pulsed neutrons at KEK. Fig. 5 shows some of the neutron images for corresponding time slices using the collimated neutron beam of 1 1 mm. The detector measured a well-collimated beam when the neutron energy decreased.
We have been developing microstrip gas chamber (MSGC) detectors using helium-3 gas as neutron converter. The specifications expected for the MSGC detector are high detection efficiency (>60% for thermal neutrons), high spatial resolution (o1 mm), high n/g discrimination ratio (>107), high counting rate (>106 cps/ module), and a moderate area (20 20 cm2, maximum). A large amount of heavy gas such as CF4 has to be filled into achieve a high spatial resolution in a conventional positioning scheme, such as charge-division or delay-line chain. However, achieving a gas gain of more than ten in such a gas condition would require an electric field strength of B104 V/mm between the microstrip electrodes, thereby making the detector unstable due to the dielectric breakdown at the surface of the substrate. To solve these problems and achieve the specifications mentioned above, we developed the MSGC system, in which all the signal channels are read-out individually, and the incident positions of
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the neutrons are determined by the instrument system with a capability of secondary particle discrimination (InSPaD). The InSPaD identifies the particles proton and triton created in the nuclear reaction 3He + n - p + T on the basis of the track length, and determines the incident position of the neutron accurately, i.e., the spatial resolution of the system is not limited in the track length of the secondary particles. A simulation result showed a high spatial resolution less than 1 mm regardless of the gas condition using the InSPaD [28,29]. Full description of the InSPaD can be found in Refs. [29,30]. With this system, one can use a ‘‘light’’ gas condition without loss of spatial resolution; thereby the stability of the detector should be further increased as well. Fig. 6 shows the proto-type of the microstrip substrate, which measures 50 50 mm mounted on the PCB socket board for individual read-out [31]. The MSGC was manufactured by TOSHIBA Co., which can deal with charge-up at high counting rate by appropriately adjusting the resistivity of the polyimide substrate using an organic titanium coating [32]. We have made two of the high pressure gas chambers; one is for the test (Fig. 7) and the other is for the proto-type. The signal channels of 160 and 561 can be fed through out of the chambers so that most/all the
Fig. 6. Microstrip substrate mounted on the PCB socket board for individual read-out.
Fig. 7. Test gas chamber for individually read-out, which withstands up to 8 atm.
signals from each anode strip can be read-out individually. The gas chambers withstand up to 8 atm, and have windows made of aluminum with a thickness of 3 mm. For each signal channels, the signal pulses were amplified, shaped and discriminated individually. We do not measure the pulse height, but only measure the pattern and the numbers of the fired channels (i.e. hit detection) event by event to ensure high count rate. The feasibility of the hit-detection method was well confirmed both in X-ray [31] and in neutron detection [33,34]. In the InSPaD, the voltage levels of the discriminators were set identical for each channel and appropriately so that the numbers of suprathreshold channels should become different for a proton and a triton. The identification of the proton and the triton can be done using the numbers of suprathreshold channels, and the incident position of neutron can be determined by a simple logical calculation. The InSPaD realizes the identification of the particles with a simple, fast and cost-effective method, thus, ensuring high count rate, and moreover increases the signal-to-noise ratio of the detector system,
ARTICLE IN PRESS M. Katagiri / Nuclear Instruments and Methods in Physics Research A 529 (2004) 254–259
which is attributable to the relatively high discrimination levels to differentiate a proton and a triton. Our MSGC system with the InSPaD is still under development, but the operational principle was confirmed using a test detector system [34]. We are going to proceed to the next stage development of the proto-type detector system to obtain the neutron images in the near future.
4. Conclusions We have developed two-dimensional neutron detectors; scintillation detectors with the WLS fiber read-out and the MSGC detector with individual read-out. The basic performances of the developed scintillation detectors were confirmed in experiments. We are going to transfer the developed technology to industrial companies to produce proto-types for the J-PARC instruments. As for the MSGC detector, we are going to develop the prototype and demonstrate the highperformance detector system based on the secondary particles identification.
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