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A Gd-based gaseous electron multiplier detector for neutron scattering applications
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 579 (2007) 71–74 www.elsevier.com/locate/nima
A Gd-based gaseous electron multiplier detector for neutron scattering applications D.D. DiJulioa, A.I. Hawaria,, R. Berlinerb a
Department of Nuclear Engineering, North Carolina State University, Raleigh, NC 27695, USA b Instrumentation Associates, 2 Davis Drive, Research Triangle Park, NC 27709, USA Available online 4 April 2007
Abstract The optimum configuration of a Gaseous Electron Multiplier (GEM) neutron detector using a CsI–Gd–Kapton–Gd–CsI neutron converter is investigated using Monte-Carlo simulations. Neutrons absorbed in the converter produce secondary electrons that are emitted from the CsI layers. The detector can be assembled from multiple modules where each module consists of the neutron converter, several cascaded GEMs and anode pickup plates on both sides. Position sensitive anodes and localization electronics are then used to detect, timestamp and record the resulting signal. For a single GEM module, the performance can be assessed by estimating the secondary electron (SE) leakage from the converter sandwich. The simulations show that the optimum for a single module, double-sided detector would have a neutron converter composed of 0.1 mm CsI on top of 3 mm Gd plated on each side of a 7.5 mm thick Kapton foil. This detector would have a SE yield of approximately 0.6 SE/neutron and neutron absorption of 60%. Significant enhancements of the SE yield can be obtained for detectors composed of multiple modules with thinner Gd converters. The multi-module design allows for enhanced SE leakage from each converter while maintaining the high neutron absorption efficiency of thicker converters and dividing the detector count rate among multiple sets of decoding electronics. r 2007 Elsevier B.V. All rights reserved. PACS: 28.20.Cz; 29.40.Cs; 29.40.n Keywords: GEM detector; Neutron scattering; Neutron detector; Gd
1. Introduction Neutron scattering methods provide basic information on the structure and dynamics of materials in many different fields of science. The next generation neutron sources will generate neutron fluxes of the order of 30 greater than that of current sources imposing new requirements on detector systems [1]. Detectors with higher counting rate capabilities and better position resolution will be needed. The Gaseous Electron Multiplier (GEM) neutron detector is one of a number of new concepts that are currently being investigated for the next generation neutron sources [2]. The GEM is a kapton foil that is copper coated on both sides and pierced electro-chemically Corresponding author. Tel.: +1 919 515 4598; fax: +1 919 513 1276.
E-mail address: [email protected] (A.I. Hawari). 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.04.015
with a high density of micrometer sized holes. By applying a potential of several hundred volts between the metal layers, electrons in the gas above the foil are drifted into the holes where gas multiplication and electron gain occur. Gains in excess of 103 have been achieved with a single GEM and even higher gains can be reached by using multiple GEM structures in series [3]. 2. Detector concept In this work, we describe calculations to optimize the design of a multi-module neutron detector. Each module consists of the neutron converter, several cascaded GEMs as electron amplifiers and position sensitive anode pickup plates on both sides of the neutron converter foil. The CsI–Gd–Kapton–Gd–CsI sandwich converts incoming neutrons to a detectable electronic signal [4]. Kapton forms the mechanical support for the much thinner Gd and
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D.D. DiJulio et al. / Nuclear Instruments and Methods in Physics Research A 579 (2007) 71–74
CsI layers. Low-energy electrons emitted from CsI drift through the cascade of GEM foils and are amplified. Position sensitive anodes and localization electronics are then used to detect, timestamp and record the resulting signal. 3. Performance optimization
Fig. 1. Optimization of a single Gd–CsI converter. The Gd thickness refers to the total thickness of both layers while the CsI thickness refers to the thickness of each layer. The SE yield is relative to incident neutron.
Secondary Electron Yield/primary electron
The main issue in detecting thermal neutrons is the conversion of the neutron to an electrical signal. Gd is an attractive converter material because of its high neutron absorption cross-section for thermal neutrons (49,700 b for natural Gd at a neutron energy of 0.0253 eV). When a neutron is captured by a Gd nucleus, conversion electrons are emitted with a probability of 87.5% and an energy in the range of 29–180 keV [5]. For a single GEM module, signal optimization is obtained by maximizing secondary electron (SE) leakage from the converter sandwich. PENELOPE, a Monte-Carlo code for electron/photon/positron transport was used to simulate the energy loss of primary electrons following neutron absorption in the Gd layers [6]. The material model employed is a multilayered cylindrical structure in which each material of the converter foil is one of the layers in the cylinder. The cylinder is perpendicular to the direction of travel of the neutrons and is symmetric under rotations about its axis. In order to account for the absorption of the neutrons, the program samples the position of primary electrons exponentially within the Gd layers. The emitted electrons have an energy sampled from the Gd conversion electron spectrum and their direction is isotropic. The resulting depth–dose distribution in the CsI layers, defined as the average energy deposited per unit depth per primary electron, was also calculated by PENELOPE. The SE yield was obtained by integrating the depth–dose distribution with an exponential weighting factor to model the SE escape probability. This model of SE production requires two empirical parameters: the energy to produce a SE and its escape depth (18.5 eV/SE and 200 A˚ escape depth) [7]. These have been obtained by comparison to the results of Akkerman and co-workers [8].
3.0 0.1 micron Gd 0.015 absorption 1 micron Gd 0.14 absorption 3 micron Gd 0.36 absorption 6 micron Gd 0.60 absorption
2.5 2.0 1.5 1.0 0.5 0.0 0.0
0.5
1.0 CsI Thickness (µm)
1.5
2.0
4. Simulations and results
Fig. 2. The SE yield (relative to primary electron) for a single Gd–CsI converter. The CsI thickness corresponds to the thickness of each layer.
Results of the performance optimization calculations described above are presented in Fig. 1. In all of the calculations, the Gd thickness was identical on both sides of the Kapton support foil. The results show that for a single neutron converter sandwich, the maximum SE yield of approximately 0.6 SE (per incident neutron) is obtained for CsI layers near 0.1 mm thickness. The optimum total Gd thickness rises to a maximum around 6 mm. As the Kapton foil thickness is decreased, the SE production increases but for practical reasons a 7.5 mm Kapton foil will be used. Simulations have also shown that for the Gd thicknesses of interest, i.e., less than 6 mm, the exact position of the Kapton within the total thickness of Gd does not
significantly affect the number of secondary electrons produced. The competition between Gd thickness, SE yield and detector efficiency is illustrated in Fig. 2, which shows the SE yield (per primary electron) at different Gd thicknesses as a function of CsI thickness. Approximately 87.5% of the absorbed neutrons result in an energetic conversion (primary) electron. Thinner Gd layers lead to greatly enhanced SE yield at the expense of lower neutron absorption and hence, detector efficiency. Further enhancement of the total SE yield can be obtained by
ARTICLE IN PRESS D.D. DiJulio et al. / Nuclear Instruments and Methods in Physics Research A 579 (2007) 71–74
Secondary Electron Yield/neutron
3.0
Table 1 Data obtained for multiple equal Gd thickness modules
0.1µm Gd 1µm Gd 2µm Gd
2.5
4µm Gd
Gd thickness/ module (mm)
Number of modules
Neutron absorption efficiency (%)
Total SE/f
0.1 0.5 1 2 3 6
60 12 6 3 2 1
60 60 60 60 60 60
1.46 1.21 1.15 1.06 0.96 0.67
6µm Gd
2.0
73
10µm Gd
1.5 1.0 0.5 0.0 1
10 Number of Modules
50
Fig. 3. The SE yield (relative to incident neutron) as a function of the number of equal thickness Gd modules.
configuring the detector to have multiple lower neutron efficiency modules or by employing isotopically enriched Gd for the neutron converter [4].
Table 2 Data for multiple different Gd thickness modules First layer Gd thickness (mm)a
Total Gd thickness (mm)
Number of modules
Neutron absorption efficiency (%)
Total SE/f
0.1 0.2 0.4 0.6 0.8 1
26.7 20 15.3 10.7 8.7 6
98 40 16 9 6 4
98 95 90 80 73 60
1.21 1.63 1.75 1.59 1.42 1.16
a
Successive Gd modules have increasing thickness.
4.1. Multiple modules, equal Gd thickness
4.2. Multiple modules, different Gd thicknesses
Optimization of the detector based on multiple modules of the same Gd thickness requires that the SE yield (per incident neutron) on the detector be calculated. It is possible to calculate the SE yield for a detector containing multiple modules by combining the results from Fig. 1 and using
An alternative detector design constraint is to require that each module in a multi-module detector have the same neutron detection efficiency. This ensures that each module will be subject to the same dead-time corrections and considerations in high-count rate applications. This requirement is equivalent to requiring that each module yield the same number of SE/incident neutron or equivalently, have the same neutron absorption. Increasing the total thickness of the Gd from one module to the next in the direction of the incident beam can achieve this end. In this case, the number of modules allowed and the total neutron absorption across the detector depends on the total Gd thickness of the first module. Table 2 shows the results of these calculations for various detectors using a different thickness for the Gd converter in the first module. A comparison of the results in Tables 1 and 2 shows that the maximum SE/f that is reported in Table 1 (i.e., 1.46) can be nearly achieved (1.42) with a significantly less number of modules (6 modules vs. 60 modules) if the thickness of Gd is allowed to vary.
SE ¼ f
X N P SE e a ðn1Þx f x n¼1
(1)
where N P is the total number of modules, n is the respective module, a is the macroscopic neutron absorption crosssection of Gd, x is the Gd thickness for each module, and (SE/f)x is the SE yield for a single module as shown in Fig. 1. The results of these calculations are presented in Fig. 3. They show that saturation is reached for thicker Gd layers at a lower number of modules. For example, if each module contains 2 mm Gd then building more than 10 modules exhibits no enhancement in the secondary electron yield. Increasing the number of modules and decreasing the Gd thickness of each module results in higher SE yield at the cost of increased electrical and mechanical complexity. Table 1 illustrates the gain in SE yield that is obtained for multiple module detectors at 60% neutron absorption. Ultimately, the number of detector modules employed will depend on the cost of the decoding electronics and the level of mechanical and electronic complexity that can be tolerated in the application.
5. Conclusions Monte–Carlo simulations were performed to optimize the converter structure and the number of modules of a Gd-based GEM detector. It was found that the SE yield (per incident neutron) is enhanced while maintaining high efficiency by using modules with equal thickness Gd layers. However, further reduction in the number of modules can
ARTICLE IN PRESS 74
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be made by using Gd converters that increase in thickness successively. Acknowledgement This work is supported by DOE SBIR Phase-II Grant DE-FG02-03ER83685. References [1] M. Johnson, International Workshop on Position Sensitive Neutron Detectors: Status and Perspectives, Hahn-Meitner-Institue, Berlin, Germany, 2001, pp. 28–30.
[2] F. Sauli, Nucl. Instr. and Meth. 386 (1997) 531. [3] A. Bressan, A. Hoch, M. Pagano, P. Ropelewski, L. Sauli, F. Biagi, S. Buzulutskov, A. Gruwe, M. De Lentdecker, G. Moermann, D.A. Sharma, Nucl. Instr. and Meth. 424 (1999) 321. [4] B. Gebauer, Ch. Schulz, Th. Wilpert., Nucl. Instr. and Meth. 392 (1997) 68. [5] R.C. Greenwood, C.W. Reich, H.A. Baader, H.R. Koch, D. Breitig, O.W.B. Schult, B. Fogelberg, A. Ba¨cklin, W. Mampe, T. Von Eg1dy, K. Schreckenbach, Nucl. Phys. A 304 (1978) 327. [6] F. Salvat, J.M. Fernandex-Varea, E. Acosta, J. Sempau, in: Workshop Proceedings, Issyles-Moulineaux, France, 7–10 July 2003. [7] R. Berliner, Bull. Am. Phys. Soc., K1.171, Montreal, Canada, March 2004. [8] A. Akkerman, A. Gibrekhterman, A. Breskin, R. Chechik, J. Appl. Phys. 72 (11) (1992) 5429.
Nuclear Instruments and Methods in Physics Research A 579 (2007) 71–74 www.elsevier.com/locate/nima
A Gd-based gaseous electron multiplier detector for neutron scattering applications D.D. DiJulioa, A.I. Hawaria,, R. Berlinerb a
Department of Nuclear Engineering, North Carolina State University, Raleigh, NC 27695, USA b Instrumentation Associates, 2 Davis Drive, Research Triangle Park, NC 27709, USA Available online 4 April 2007
Abstract The optimum configuration of a Gaseous Electron Multiplier (GEM) neutron detector using a CsI–Gd–Kapton–Gd–CsI neutron converter is investigated using Monte-Carlo simulations. Neutrons absorbed in the converter produce secondary electrons that are emitted from the CsI layers. The detector can be assembled from multiple modules where each module consists of the neutron converter, several cascaded GEMs and anode pickup plates on both sides. Position sensitive anodes and localization electronics are then used to detect, timestamp and record the resulting signal. For a single GEM module, the performance can be assessed by estimating the secondary electron (SE) leakage from the converter sandwich. The simulations show that the optimum for a single module, double-sided detector would have a neutron converter composed of 0.1 mm CsI on top of 3 mm Gd plated on each side of a 7.5 mm thick Kapton foil. This detector would have a SE yield of approximately 0.6 SE/neutron and neutron absorption of 60%. Significant enhancements of the SE yield can be obtained for detectors composed of multiple modules with thinner Gd converters. The multi-module design allows for enhanced SE leakage from each converter while maintaining the high neutron absorption efficiency of thicker converters and dividing the detector count rate among multiple sets of decoding electronics. r 2007 Elsevier B.V. All rights reserved. PACS: 28.20.Cz; 29.40.Cs; 29.40.n Keywords: GEM detector; Neutron scattering; Neutron detector; Gd
1. Introduction Neutron scattering methods provide basic information on the structure and dynamics of materials in many different fields of science. The next generation neutron sources will generate neutron fluxes of the order of 30 greater than that of current sources imposing new requirements on detector systems [1]. Detectors with higher counting rate capabilities and better position resolution will be needed. The Gaseous Electron Multiplier (GEM) neutron detector is one of a number of new concepts that are currently being investigated for the next generation neutron sources [2]. The GEM is a kapton foil that is copper coated on both sides and pierced electro-chemically Corresponding author. Tel.: +1 919 515 4598; fax: +1 919 513 1276.
E-mail address: [email protected] (A.I. Hawari). 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.04.015
with a high density of micrometer sized holes. By applying a potential of several hundred volts between the metal layers, electrons in the gas above the foil are drifted into the holes where gas multiplication and electron gain occur. Gains in excess of 103 have been achieved with a single GEM and even higher gains can be reached by using multiple GEM structures in series [3]. 2. Detector concept In this work, we describe calculations to optimize the design of a multi-module neutron detector. Each module consists of the neutron converter, several cascaded GEMs as electron amplifiers and position sensitive anode pickup plates on both sides of the neutron converter foil. The CsI–Gd–Kapton–Gd–CsI sandwich converts incoming neutrons to a detectable electronic signal [4]. Kapton forms the mechanical support for the much thinner Gd and
ARTICLE IN PRESS 72
D.D. DiJulio et al. / Nuclear Instruments and Methods in Physics Research A 579 (2007) 71–74
CsI layers. Low-energy electrons emitted from CsI drift through the cascade of GEM foils and are amplified. Position sensitive anodes and localization electronics are then used to detect, timestamp and record the resulting signal. 3. Performance optimization
Fig. 1. Optimization of a single Gd–CsI converter. The Gd thickness refers to the total thickness of both layers while the CsI thickness refers to the thickness of each layer. The SE yield is relative to incident neutron.
Secondary Electron Yield/primary electron
The main issue in detecting thermal neutrons is the conversion of the neutron to an electrical signal. Gd is an attractive converter material because of its high neutron absorption cross-section for thermal neutrons (49,700 b for natural Gd at a neutron energy of 0.0253 eV). When a neutron is captured by a Gd nucleus, conversion electrons are emitted with a probability of 87.5% and an energy in the range of 29–180 keV [5]. For a single GEM module, signal optimization is obtained by maximizing secondary electron (SE) leakage from the converter sandwich. PENELOPE, a Monte-Carlo code for electron/photon/positron transport was used to simulate the energy loss of primary electrons following neutron absorption in the Gd layers [6]. The material model employed is a multilayered cylindrical structure in which each material of the converter foil is one of the layers in the cylinder. The cylinder is perpendicular to the direction of travel of the neutrons and is symmetric under rotations about its axis. In order to account for the absorption of the neutrons, the program samples the position of primary electrons exponentially within the Gd layers. The emitted electrons have an energy sampled from the Gd conversion electron spectrum and their direction is isotropic. The resulting depth–dose distribution in the CsI layers, defined as the average energy deposited per unit depth per primary electron, was also calculated by PENELOPE. The SE yield was obtained by integrating the depth–dose distribution with an exponential weighting factor to model the SE escape probability. This model of SE production requires two empirical parameters: the energy to produce a SE and its escape depth (18.5 eV/SE and 200 A˚ escape depth) [7]. These have been obtained by comparison to the results of Akkerman and co-workers [8].
3.0 0.1 micron Gd 0.015 absorption 1 micron Gd 0.14 absorption 3 micron Gd 0.36 absorption 6 micron Gd 0.60 absorption
2.5 2.0 1.5 1.0 0.5 0.0 0.0
0.5
1.0 CsI Thickness (µm)
1.5
2.0
4. Simulations and results
Fig. 2. The SE yield (relative to primary electron) for a single Gd–CsI converter. The CsI thickness corresponds to the thickness of each layer.
Results of the performance optimization calculations described above are presented in Fig. 1. In all of the calculations, the Gd thickness was identical on both sides of the Kapton support foil. The results show that for a single neutron converter sandwich, the maximum SE yield of approximately 0.6 SE (per incident neutron) is obtained for CsI layers near 0.1 mm thickness. The optimum total Gd thickness rises to a maximum around 6 mm. As the Kapton foil thickness is decreased, the SE production increases but for practical reasons a 7.5 mm Kapton foil will be used. Simulations have also shown that for the Gd thicknesses of interest, i.e., less than 6 mm, the exact position of the Kapton within the total thickness of Gd does not
significantly affect the number of secondary electrons produced. The competition between Gd thickness, SE yield and detector efficiency is illustrated in Fig. 2, which shows the SE yield (per primary electron) at different Gd thicknesses as a function of CsI thickness. Approximately 87.5% of the absorbed neutrons result in an energetic conversion (primary) electron. Thinner Gd layers lead to greatly enhanced SE yield at the expense of lower neutron absorption and hence, detector efficiency. Further enhancement of the total SE yield can be obtained by
ARTICLE IN PRESS D.D. DiJulio et al. / Nuclear Instruments and Methods in Physics Research A 579 (2007) 71–74
Secondary Electron Yield/neutron
3.0
Table 1 Data obtained for multiple equal Gd thickness modules
0.1µm Gd 1µm Gd 2µm Gd
2.5
4µm Gd
Gd thickness/ module (mm)
Number of modules
Neutron absorption efficiency (%)
Total SE/f
0.1 0.5 1 2 3 6
60 12 6 3 2 1
60 60 60 60 60 60
1.46 1.21 1.15 1.06 0.96 0.67
6µm Gd
2.0
73
10µm Gd
1.5 1.0 0.5 0.0 1
10 Number of Modules
50
Fig. 3. The SE yield (relative to incident neutron) as a function of the number of equal thickness Gd modules.
configuring the detector to have multiple lower neutron efficiency modules or by employing isotopically enriched Gd for the neutron converter [4].
Table 2 Data for multiple different Gd thickness modules First layer Gd thickness (mm)a
Total Gd thickness (mm)
Number of modules
Neutron absorption efficiency (%)
Total SE/f
0.1 0.2 0.4 0.6 0.8 1
26.7 20 15.3 10.7 8.7 6
98 40 16 9 6 4
98 95 90 80 73 60
1.21 1.63 1.75 1.59 1.42 1.16
a
Successive Gd modules have increasing thickness.
4.1. Multiple modules, equal Gd thickness
4.2. Multiple modules, different Gd thicknesses
Optimization of the detector based on multiple modules of the same Gd thickness requires that the SE yield (per incident neutron) on the detector be calculated. It is possible to calculate the SE yield for a detector containing multiple modules by combining the results from Fig. 1 and using
An alternative detector design constraint is to require that each module in a multi-module detector have the same neutron detection efficiency. This ensures that each module will be subject to the same dead-time corrections and considerations in high-count rate applications. This requirement is equivalent to requiring that each module yield the same number of SE/incident neutron or equivalently, have the same neutron absorption. Increasing the total thickness of the Gd from one module to the next in the direction of the incident beam can achieve this end. In this case, the number of modules allowed and the total neutron absorption across the detector depends on the total Gd thickness of the first module. Table 2 shows the results of these calculations for various detectors using a different thickness for the Gd converter in the first module. A comparison of the results in Tables 1 and 2 shows that the maximum SE/f that is reported in Table 1 (i.e., 1.46) can be nearly achieved (1.42) with a significantly less number of modules (6 modules vs. 60 modules) if the thickness of Gd is allowed to vary.
SE ¼ f
X N P SE e a ðn1Þx f x n¼1
(1)
where N P is the total number of modules, n is the respective module, a is the macroscopic neutron absorption crosssection of Gd, x is the Gd thickness for each module, and (SE/f)x is the SE yield for a single module as shown in Fig. 1. The results of these calculations are presented in Fig. 3. They show that saturation is reached for thicker Gd layers at a lower number of modules. For example, if each module contains 2 mm Gd then building more than 10 modules exhibits no enhancement in the secondary electron yield. Increasing the number of modules and decreasing the Gd thickness of each module results in higher SE yield at the cost of increased electrical and mechanical complexity. Table 1 illustrates the gain in SE yield that is obtained for multiple module detectors at 60% neutron absorption. Ultimately, the number of detector modules employed will depend on the cost of the decoding electronics and the level of mechanical and electronic complexity that can be tolerated in the application.
5. Conclusions Monte–Carlo simulations were performed to optimize the converter structure and the number of modules of a Gd-based GEM detector. It was found that the SE yield (per incident neutron) is enhanced while maintaining high efficiency by using modules with equal thickness Gd layers. However, further reduction in the number of modules can
ARTICLE IN PRESS 74
D.D. DiJulio et al. / Nuclear Instruments and Methods in Physics Research A 579 (2007) 71–74
be made by using Gd converters that increase in thickness successively. Acknowledgement This work is supported by DOE SBIR Phase-II Grant DE-FG02-03ER83685. References [1] M. Johnson, International Workshop on Position Sensitive Neutron Detectors: Status and Perspectives, Hahn-Meitner-Institue, Berlin, Germany, 2001, pp. 28–30.
[2] F. Sauli, Nucl. Instr. and Meth. 386 (1997) 531. [3] A. Bressan, A. Hoch, M. Pagano, P. Ropelewski, L. Sauli, F. Biagi, S. Buzulutskov, A. Gruwe, M. De Lentdecker, G. Moermann, D.A. Sharma, Nucl. Instr. and Meth. 424 (1999) 321. [4] B. Gebauer, Ch. Schulz, Th. Wilpert., Nucl. Instr. and Meth. 392 (1997) 68. [5] R.C. Greenwood, C.W. Reich, H.A. Baader, H.R. Koch, D. Breitig, O.W.B. Schult, B. Fogelberg, A. Ba¨cklin, W. Mampe, T. Von Eg1dy, K. Schreckenbach, Nucl. Phys. A 304 (1978) 327. [6] F. Salvat, J.M. Fernandex-Varea, E. Acosta, J. Sempau, in: Workshop Proceedings, Issyles-Moulineaux, France, 7–10 July 2003. [7] R. Berliner, Bull. Am. Phys. Soc., K1.171, Montreal, Canada, March 2004. [8] A. Akkerman, A. Gibrekhterman, A. Breskin, R. Chechik, J. Appl. Phys. 72 (11) (1992) 5429.