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Study of uniform drift electric field used for boron-lined honeycomb neutron detector
Nuclear Inst. and Methods in Physics Research, A (
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Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Study of uniform drift electric field used for boron-lined honeycomb neutron detector Zhujun Fang a,b , Yigang Yang a,b ,∗, Yulan Li a,b , Zhi Zhang a,b , Xuewu Wang a,b a b
Department of Engineering Physics, Tsinghua University, Beijing, PR China Key Laboratory of Particle & Radiation Imaging, Tsinghua University, Ministry of Education, Beijing, PR China
ARTICLE Keywords: Field-cage Drift electric field Field uniformity
INFO
ABSTRACT A thermal neutron detector design with a boron-lined honeycomb as the neutron converter was proposed, which can be probably a 3 He alternative detector used in neutron scattering. This detector demands a high uniformity drift electric field due to the electron migration process, which simplifies the detector structure and improves the detector robustness. To research the influencing parameters to the drift electric field, the Garfield 9 and Maxwell 11 simulations were carried out. The simulation results demonstrated that both the field-cage pitch and the width of field strips determined the uniformity of drift electric field. The field-cage with optimized structure for the boron-lined honeycomb neutron detector was also designed, with 2 mm pitch, 1.5 mm width of inner field strip and 0.5 mm width of outer mirror field strip. The result of neutron experiment shows an electron migration efficiency increase of 12.4% and a neutron detection efficiency increase of 26%, demonstrating the effectiveness of field-cage optimization and drift electric field uniformity promotion.
Contents 1. 2. 3. 4. 5. 6. 7.
Introduction ....................................................................................................................................................................................................... Simulation of the influence of electric field distortion on electron migration efficiency ............................................................................................... Simulation of influence of field-cage parameters on drift electric field....................................................................................................................... Field-cage optimization........................................................................................................................................................................................ Experimental evaluation of optimized drift electric field .......................................................................................................................................... Discussion .......................................................................................................................................................................................................... Conclusions ........................................................................................................................................................................................................ Acknowledgment ................................................................................................................................................................................................ References..........................................................................................................................................................................................................
1. Introduction In the past several years, the worldwide shortage of 3 He gas has become a non-negligible problem for neutron detection in scientific research and industry, triggering the research of a variety of 3 He alternative neutron detectors [1–3]. A thermal neutron detector design using a boron-lined honeycomb structure as the neutron converter has been proposed and developed [4]. The experimental results demonstrate that it is a promising detector with high neutron detection efficiency and good spatial resolution, indicating it can probably be used in neutron scattering. In the proposed design, the neutron detection process is divided into three steps, as shown in Fig. 1. First, the incident thermal neutrons are absorbed by the boron layer on the inner surface of the honeycomb holes, and the generated 𝛼 particles or 7 Li will produce
1 2 2 4 4 4 5 5 5
electron–ion pairs in the working gas. Second, these ionized electrons will be driven out of the honeycomb holes by an electric field applied through a field-cage, and then fed into the holes of a gaseous electron multiplier (GEM) foil [5]. Finally, the electrons are multiplied by the GEM and collected by the anode to form the electric signal. The introduction of electron migration process removes the necessity of manufacturing hundreds to thousands of anode wires that are common in traditional boron-lined detectors, simplifying the detector’s structure and improving its robustness. However, it also employs a drift electric field of high uniformity to effectively drive the electrons migrating along the honeycomb holes with a 30:1 aspect ratio (100 mm length and 3.2 mm diameter). The non-parallelism between the drift electric field and the honeycomb hole axis could lead to the loss of electrons caused by adsorption on the honeycomb walls along with
∗ Corresponding author at: Department of Engineering Physics, Tsinghua University, Beijing, PR China. E-mail address: [email protected] (Y. Yang).
https://doi.org/10.1016/j.nima.2018.10.109 Received 31 July 2018; Received in revised form 14 October 2018; Accepted 15 October 2018 Available online xxxx 0168-9002/© 2018 Elsevier B.V. All rights reserved.
Please cite this article in press as: Z. Fang, et al., Study of uniform drift electric field used for boron-lined honeycomb neutron detector, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.10.109.
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Fig. 1. Schematic of honeycomb converter based detector and neutron detection process. Fig. 3. Influence of electron migration efficiency with different 𝐸r ∕𝐸∕∕ ratios (𝐸∕∕ is set as a constant, 200 V/cm).
should be suppressed to reduce the fraction of the electric field distortion space, with the aim of optimizing the field-cage. In this research, the influence of the non-uniformity of the drift electric field on the electron migration efficiency was researched with Garfield 9 simulation [14]. The three main parameters of the field-cage, i.e., the field-cage pitch and the widths of the inner and outer field strips, were simulated and analyzed using Maxwell 11 simulation [15] to evaluate the influence on electric distortion. Based on the simulation results, a new field-cage with optimized structure was designed. A set of neutron experiments were also conducted to evaluate the effect of field-cage optimization. 2. Simulation of the influence of electric field distortion on electron migration efficiency To evaluate the uniformity of the drift electric field, two parameters can be studied: the strength value of the electric field parallel to the drift direction of electrons and that of the electric field vertical to the drift direction of electrons, which are denoted as 𝐸∕∕ and 𝐸r , respectively, as shown in Fig. 2. It can be seen in Fig. 2 that the equipotential surfaces no longer remain parallel near the field-cage, leading to the distortion of the drift electric field. This kind of distortion exerts a harmful influence on the electron migration by misleading the electrons to hit the honeycomb holes wall and be adsorbed. To evaluate the influence of drift electric field distortion on electron migration efficiency, Garfield 9 simulation was carried out, with the results shown in Fig. 3. 𝐸∕∕ set to 200 V/cm as a constant, which equals the electric field strength used in the experiment, while 𝐸r rises from 0 to 20 V/cm. It can be seen that there will be a significant decrease of electron migration efficiency when the 𝐸r ∕𝐸∕∕ ratio is 0.01 or larger, indicating that the 𝐸r in the drift electric field should not be larger than 2 V/cm in order to realize an acceptable electron migration efficiency.
Fig. 2. Electric distortion near the field-cage with mirror strips simulated using Maxwell 11.
the migration process [6], resulting in a decreased number of electrons collected by the GEM and a decreased neutron detection efficiency. A guarantee of uniformity of the drift electric field, not only in this detector, but also in a time projection chamber (TPC) [7], drift chamber, or other 3 He alternative detectors with an independent neutron converter [8,9], is also essential to achieve good detection performance. Thus, the realization of uniform drift electric field is a universal problem and is worth researching. A field-cage is commonly used to form a uniform drift electric field [10–12] when there always is an electric distortion present, as shown in Fig. 2. The reason is that the contribution of each field strip to the electric field cannot be deemed as the far-field approximation in the space near the field-cage, resulting in a non-uniform electric field. Some research has indicated that significant electric distortion would appear in the space within 2–3 times the pitch from the field-cage, and that a field-cage with mirror strips could realize a smaller electric distortion [13]. Although it is possible to make a large field-cage and use the intermediate part of the region as the drift space to realize a small electric distortion [11], in the 3 He alternative detectors, this idea does not work because the detector volume is so valuable that as many boron-lined neutron converters as possible should be placed to improve the global detection efficiency. Thus, the problem of electric distortion
3. Simulation of influence of field-cage parameters on drift electric field Maxwell 11 simulation was used to simulate the effect of each fieldcage parameter on the electric distortion, with a 105-mm-long and 115mm-high square field-cage that can accommodate a typical honeycomb converter based neutron detector. The thickness of the inner and outer copper layers is 35 μm and the substrate thickness is 150 μm, which is 2
Please cite this article in press as: Z. Fang, et al., Study of uniform drift electric field used for boron-lined honeycomb neutron detector, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.10.109.
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Fig. 4. Relationship between field-cage pitch and 𝐸r and 𝐸∕∕ values.
Fig. 5. Relationship between field strip width and 𝐸r and 𝐸∕∕ values, keeping the pitch at 5 mm and the sum of inner and outer field strip widths equal to the pitch.
Fig. 6. Relationship between field strip width and 𝐸r and 𝐸∕∕ values, keeping the pitch at 5 mm.
a typical parameter in the flexible printed circuit board manufacturing process. The pitch of the field-cage should first be researched because it affects the range of distorted drift electric field. Maxwell 11 simulation results demonstrate that a smaller pitch of the field-cage could help
reduce the volume of space in which the drift electric field is distorted, as shown in Fig. 4. The simulation results show that 𝐸r will be lower than 2 V/cm from 6 mm along the field-cage wall with a 5-mm fieldcage pitch, while the range could even decrease to 3.8 mm with a 3mm field-cage pitch and to be 3.2 mm with a 2-mm field-cage pitch. A 3
Please cite this article in press as: Z. Fang, et al., Study of uniform drift electric field used for boron-lined honeycomb neutron detector, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.10.109.
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Fig. 7. Influence of grounded detector shell on drift electric field, with different inner and outer field strip widths.
Fig. 8. Photograph of optimized field-cage and array chip resistor chain.
smaller field-cage pitch is thus necessary to realize a more uniform drift electric field. Fig. 9. Energy spectra of non-optimized and optimized field-cages, measured in a thermal neutron field.
With the same field-cage pitch, the selection of the inner and outer field strip widths may have an impact on the drift electric field. Fig. 5 shows the Maxwell 11 simulation results, indicating that the 𝐸r and 𝐸∕∕ values exhibit extremely small differences when keeping the sum of the inner and outer field strip widths equal to the pitch. Consequently, the sum of the inner and outer field strip widths, but not the ratio between them, should be of concern when aiming for reduced distortion of the drift electric field.
5. Experimental evaluation of optimized drift electric field To evaluate the drift electric field optimization, the optimized fieldcage and the non-optimized field-cage (5-mm pitch) were tested with the boron-lined honeycomb neutron detector in a thermal neutron field. Fig. 9 shows their measured energy spectra (of the neutron count sum of each 10 channels) and calculated as Fig. 10 with the calculated neutron count-rate ratio (of the neutron count sum of each 100 channels) between the neutron detectors with optimized and non-optimized fieldcages, demonstrating that the optimized field-cage can realize both higher electron migration and neutron detection efficiencies. With the same threshold of 55 keV, the neutron detection efficiency with an optimized field-cage is 26% higher than that of the non-optimized fieldcage. The electron migration efficiency was also measured at Hokkaido University with non-optimized field-cage and at China Mianyang Research Reactor (CMRR) with an optimized field-cage, as shown in Fig. 11. The results indicate that the electron migration efficiency exhibits an average increase of 12.4%, demonstrating a significant promotion of efficiency.
Fig. 6 shows the simulation results with various values of the sum of the inner and outer field strip widths, confirming that the smaller field strip width could result in less electric distortion. However, the grounded detector shell surrounds the field-cage, and if the small field strips cannot shield the side face completely, the drift electric field will show an apparent distortion, which is shown in Fig. 7. This result demonstrates that the sum of the inner and outer field strip widths should equal the field-cage pitch.
4. Field-cage optimization According to the simulation results, the field-cage is optimized with a pitch of 2 mm, an inner field strip width of 1.5 mm, and an outer mirror field strip width of 0.5 mm. In this case, the non-negligible electric distortion is limited within 3.2 mm from the field-cage. The array chip resistor is introduced to reduce the welding difficulty and field-cage complexity, as shown in Fig. 8.
6. Discussion The electric field produced by a field-cage is an approximation of the uniform electric field. The introduction of field strips of a 4
Please cite this article in press as: Z. Fang, et al., Study of uniform drift electric field used for boron-lined honeycomb neutron detector, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.10.109.
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significant electric distortion, leading to a limited electron migration efficiency with a long migration length. Therefore, the electron migration curve only reflects the electron migration efficiency near the field-cage, but not in the entire drift area. To address this problem, setting the incident neutron coming into the detector from the incident surface is essential, and using a neutron converter with a smaller height is also useful as well. In this case, more incident neutrons can be absorbed in the area with a smaller electric distortion, realizing a higher electron migration efficiency and a higher neutron detection efficiency. 7. Conclusions A newly designed boron-lined honeycomb neutron converter based detector has been formerly proposed as the 3 He alternative detector. In the proposed detector, a uniform electric field applied with the fieldcage is necessary to drive electrons out of the holes of the honeycomb converter to the GEM foil. The influence of electric field distortion on the migration of electrons was evaluated using Garfield 9 simulation, the results of which indicate that the electric distortion can be accepted if the value of the radial electric field strength 𝐸r , which is vertical to the drift direction, is smaller than 2 V/cm, when the drift electric strength 𝐸∕∕ , which is along the direction of hole axis, has a typical value of 200 V/cm. Maxwell 11 simulations demonstrated that smaller field-cage pitch and smaller field strips are beneficial to the suppression of electric distortion, while the summation of inner and outer field strip widths should be the same as the field-cage pitch. The field-cage was optimized with a 2-mm pitch and 1.5-and 0.5-mm widths of the inner field strip and outer mirror field strip, respectively. The neutron experiments show an electron migration efficiency increase of 12.4% and a neutron detection efficiency increase of 26%, demonstrating the effectiveness of field-cage optimization and drift electric field uniformity promotion.
Fig. 10. Count-rate ratio between optimized and non-optimized field-cages.
Acknowledgment This research is supported by National Natural Science Foundation of China (Grant No. 11735008). References [1] M. Henske, et al., The 10 B based jalousie neutron detector – an alternative for 3 He filled position sensitive counter tubes, Nucl. Instrum. Methods Phys. Res. A 686 (2012) 151–155. [2] J. Bitch, et al.,10 B4 C Multi-grid as an alternative to 3 He for large area neutron detectors, IEEE Trans. Nucl. Sci. 60 (2013) 871–878. [3] J.L. Lacy, et al., Boron-coated straws as a replacement for 3 He-based neutron detectors, Nucl. Instrum. Methods Phys. Res. A 652 (2011) 359–363. [4] Z. Fang, et al., Research on a neutron detector with a boron-lined honeycomb neutron converter, IEEE Trans. Nucl. Sci. 64 (4) (2017) 1048–1055. [5] F. Sauli, GEM: a new concept for electron amplification in gas detectors, Nucl. Instrum. Methods Phys. Res. A 386 (1997) 531–534. [6] S.F. Biagi, Monte carlo simulation of electron drift and diffusion in counting gases under the influence of electric and magnetic fields, Nucl. Instrum. Methods Phys. Res. A 421 (1999) 234–240. [7] M. Huang, et al., A Fast Neutron Spectrometer Based on GEM-TPC, in: 2012 IEEE Nuclear Science Symposium and Medical Imaging Conference Record, 2012. [8] G. Albani, et al., Evolution in boron-based GEM detectors for diffraction measurements: from planar to 3D converters, Meas. Sci. Technol. 27 (2016) 115902. [9] A. Pietropaolo, et al., A new 3 He-free thermal neutrons detector concept based on the GEM technology, Nucl. Instrum. Methods Phys. Res. A 729 (2013) 117–126. [10] H. Kubo, et al., Development of a time projection chamber with a micro-pixel electrodes, Nucl. Instrum. Methods Phys. Res. A 513 (2003) 94–98. [11] M. Killenberg, et al., Development of a GEM-based high resolution TPC for the ILC, Nucl. Instrum. Methods Phys. Res. A 573 (2007) 183–186. [12] H. Kim, et al., Performance of a high-pressure xenon ionization chamber for environmental radiation monitoring, Radiat. Meas. 43 (2008) 659–663. [13] T. Behnke, et al., A lightweight field-cage for a large TPC prototype for the ILC, J. Instrum. 5 (10) (2010) 10011. [14] R. Veenhof, Garfield, recent developments, Nucl. Instrum. Methods Phys. Res. A 419 (1998) 726–730. [15] Maxwell 3D Ansoft, Four Station Square, Suite200, Pittsburgh, PA 15219, USA.
Fig. 11. Electron migration efficiency curves with the non-optimized and optimized fieldcages.
certain width sets the same voltage potential in the surrounding area, leading to a bending of the electric equipotential surface in the space. Compared with the non-optimized field-cage, the optimized field-cage pitch can reduce the electric distortion significantly, at the cost of increased field-cage structural complexity. The 8P4R array chip resistor was introduced to the field-cage manufacturing process, reducing the number of resistors by 75% and the number of solder joints by 50%. In this case, the optimized field-cage has a similar structural complexity as the non-optimized field-cage, while the non-negligible electric distortion range is reduced from 6 to 3.2 mm from the inner surface of field-cage, corresponding to a distorted electric field volume decrease of 47%. Fig. 11 shows that the electron migration efficiency exhibits a significant increase, with an electron migration length of less than 50 mm, while the efficiency shows a very small difference when the migration length is larger than 60 mm. Maxwell 11 simulation shows that the intermediate part of the drift area exhibits a trivial electric distortion, while the space near the field-cage exhibits a more significant distortion. In the electron migration curve measurement, the neutron beam incident into the detector from the detector side face, which is close to the field-cage, leading to the absorption of most of the incident neutrons near the field-cage. Thus, most of the electrons suffer from
5 Please cite this article in press as: Z. Fang, et al., Study of uniform drift electric field used for boron-lined honeycomb neutron detector, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.10.109.
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Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Study of uniform drift electric field used for boron-lined honeycomb neutron detector Zhujun Fang a,b , Yigang Yang a,b ,∗, Yulan Li a,b , Zhi Zhang a,b , Xuewu Wang a,b a b
Department of Engineering Physics, Tsinghua University, Beijing, PR China Key Laboratory of Particle & Radiation Imaging, Tsinghua University, Ministry of Education, Beijing, PR China
ARTICLE Keywords: Field-cage Drift electric field Field uniformity
INFO
ABSTRACT A thermal neutron detector design with a boron-lined honeycomb as the neutron converter was proposed, which can be probably a 3 He alternative detector used in neutron scattering. This detector demands a high uniformity drift electric field due to the electron migration process, which simplifies the detector structure and improves the detector robustness. To research the influencing parameters to the drift electric field, the Garfield 9 and Maxwell 11 simulations were carried out. The simulation results demonstrated that both the field-cage pitch and the width of field strips determined the uniformity of drift electric field. The field-cage with optimized structure for the boron-lined honeycomb neutron detector was also designed, with 2 mm pitch, 1.5 mm width of inner field strip and 0.5 mm width of outer mirror field strip. The result of neutron experiment shows an electron migration efficiency increase of 12.4% and a neutron detection efficiency increase of 26%, demonstrating the effectiveness of field-cage optimization and drift electric field uniformity promotion.
Contents 1. 2. 3. 4. 5. 6. 7.
Introduction ....................................................................................................................................................................................................... Simulation of the influence of electric field distortion on electron migration efficiency ............................................................................................... Simulation of influence of field-cage parameters on drift electric field....................................................................................................................... Field-cage optimization........................................................................................................................................................................................ Experimental evaluation of optimized drift electric field .......................................................................................................................................... Discussion .......................................................................................................................................................................................................... Conclusions ........................................................................................................................................................................................................ Acknowledgment ................................................................................................................................................................................................ References..........................................................................................................................................................................................................
1. Introduction In the past several years, the worldwide shortage of 3 He gas has become a non-negligible problem for neutron detection in scientific research and industry, triggering the research of a variety of 3 He alternative neutron detectors [1–3]. A thermal neutron detector design using a boron-lined honeycomb structure as the neutron converter has been proposed and developed [4]. The experimental results demonstrate that it is a promising detector with high neutron detection efficiency and good spatial resolution, indicating it can probably be used in neutron scattering. In the proposed design, the neutron detection process is divided into three steps, as shown in Fig. 1. First, the incident thermal neutrons are absorbed by the boron layer on the inner surface of the honeycomb holes, and the generated 𝛼 particles or 7 Li will produce
1 2 2 4 4 4 5 5 5
electron–ion pairs in the working gas. Second, these ionized electrons will be driven out of the honeycomb holes by an electric field applied through a field-cage, and then fed into the holes of a gaseous electron multiplier (GEM) foil [5]. Finally, the electrons are multiplied by the GEM and collected by the anode to form the electric signal. The introduction of electron migration process removes the necessity of manufacturing hundreds to thousands of anode wires that are common in traditional boron-lined detectors, simplifying the detector’s structure and improving its robustness. However, it also employs a drift electric field of high uniformity to effectively drive the electrons migrating along the honeycomb holes with a 30:1 aspect ratio (100 mm length and 3.2 mm diameter). The non-parallelism between the drift electric field and the honeycomb hole axis could lead to the loss of electrons caused by adsorption on the honeycomb walls along with
∗ Corresponding author at: Department of Engineering Physics, Tsinghua University, Beijing, PR China. E-mail address: [email protected] (Y. Yang).
https://doi.org/10.1016/j.nima.2018.10.109 Received 31 July 2018; Received in revised form 14 October 2018; Accepted 15 October 2018 Available online xxxx 0168-9002/© 2018 Elsevier B.V. All rights reserved.
Please cite this article in press as: Z. Fang, et al., Study of uniform drift electric field used for boron-lined honeycomb neutron detector, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.10.109.
Z. Fang et al.
Nuclear Inst. and Methods in Physics Research, A (
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Fig. 1. Schematic of honeycomb converter based detector and neutron detection process. Fig. 3. Influence of electron migration efficiency with different 𝐸r ∕𝐸∕∕ ratios (𝐸∕∕ is set as a constant, 200 V/cm).
should be suppressed to reduce the fraction of the electric field distortion space, with the aim of optimizing the field-cage. In this research, the influence of the non-uniformity of the drift electric field on the electron migration efficiency was researched with Garfield 9 simulation [14]. The three main parameters of the field-cage, i.e., the field-cage pitch and the widths of the inner and outer field strips, were simulated and analyzed using Maxwell 11 simulation [15] to evaluate the influence on electric distortion. Based on the simulation results, a new field-cage with optimized structure was designed. A set of neutron experiments were also conducted to evaluate the effect of field-cage optimization. 2. Simulation of the influence of electric field distortion on electron migration efficiency To evaluate the uniformity of the drift electric field, two parameters can be studied: the strength value of the electric field parallel to the drift direction of electrons and that of the electric field vertical to the drift direction of electrons, which are denoted as 𝐸∕∕ and 𝐸r , respectively, as shown in Fig. 2. It can be seen in Fig. 2 that the equipotential surfaces no longer remain parallel near the field-cage, leading to the distortion of the drift electric field. This kind of distortion exerts a harmful influence on the electron migration by misleading the electrons to hit the honeycomb holes wall and be adsorbed. To evaluate the influence of drift electric field distortion on electron migration efficiency, Garfield 9 simulation was carried out, with the results shown in Fig. 3. 𝐸∕∕ set to 200 V/cm as a constant, which equals the electric field strength used in the experiment, while 𝐸r rises from 0 to 20 V/cm. It can be seen that there will be a significant decrease of electron migration efficiency when the 𝐸r ∕𝐸∕∕ ratio is 0.01 or larger, indicating that the 𝐸r in the drift electric field should not be larger than 2 V/cm in order to realize an acceptable electron migration efficiency.
Fig. 2. Electric distortion near the field-cage with mirror strips simulated using Maxwell 11.
the migration process [6], resulting in a decreased number of electrons collected by the GEM and a decreased neutron detection efficiency. A guarantee of uniformity of the drift electric field, not only in this detector, but also in a time projection chamber (TPC) [7], drift chamber, or other 3 He alternative detectors with an independent neutron converter [8,9], is also essential to achieve good detection performance. Thus, the realization of uniform drift electric field is a universal problem and is worth researching. A field-cage is commonly used to form a uniform drift electric field [10–12] when there always is an electric distortion present, as shown in Fig. 2. The reason is that the contribution of each field strip to the electric field cannot be deemed as the far-field approximation in the space near the field-cage, resulting in a non-uniform electric field. Some research has indicated that significant electric distortion would appear in the space within 2–3 times the pitch from the field-cage, and that a field-cage with mirror strips could realize a smaller electric distortion [13]. Although it is possible to make a large field-cage and use the intermediate part of the region as the drift space to realize a small electric distortion [11], in the 3 He alternative detectors, this idea does not work because the detector volume is so valuable that as many boron-lined neutron converters as possible should be placed to improve the global detection efficiency. Thus, the problem of electric distortion
3. Simulation of influence of field-cage parameters on drift electric field Maxwell 11 simulation was used to simulate the effect of each fieldcage parameter on the electric distortion, with a 105-mm-long and 115mm-high square field-cage that can accommodate a typical honeycomb converter based neutron detector. The thickness of the inner and outer copper layers is 35 μm and the substrate thickness is 150 μm, which is 2
Please cite this article in press as: Z. Fang, et al., Study of uniform drift electric field used for boron-lined honeycomb neutron detector, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.10.109.
Z. Fang et al.
Nuclear Inst. and Methods in Physics Research, A (
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Fig. 4. Relationship between field-cage pitch and 𝐸r and 𝐸∕∕ values.
Fig. 5. Relationship between field strip width and 𝐸r and 𝐸∕∕ values, keeping the pitch at 5 mm and the sum of inner and outer field strip widths equal to the pitch.
Fig. 6. Relationship between field strip width and 𝐸r and 𝐸∕∕ values, keeping the pitch at 5 mm.
a typical parameter in the flexible printed circuit board manufacturing process. The pitch of the field-cage should first be researched because it affects the range of distorted drift electric field. Maxwell 11 simulation results demonstrate that a smaller pitch of the field-cage could help
reduce the volume of space in which the drift electric field is distorted, as shown in Fig. 4. The simulation results show that 𝐸r will be lower than 2 V/cm from 6 mm along the field-cage wall with a 5-mm fieldcage pitch, while the range could even decrease to 3.8 mm with a 3mm field-cage pitch and to be 3.2 mm with a 2-mm field-cage pitch. A 3
Please cite this article in press as: Z. Fang, et al., Study of uniform drift electric field used for boron-lined honeycomb neutron detector, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.10.109.
Z. Fang et al.
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Fig. 7. Influence of grounded detector shell on drift electric field, with different inner and outer field strip widths.
Fig. 8. Photograph of optimized field-cage and array chip resistor chain.
smaller field-cage pitch is thus necessary to realize a more uniform drift electric field. Fig. 9. Energy spectra of non-optimized and optimized field-cages, measured in a thermal neutron field.
With the same field-cage pitch, the selection of the inner and outer field strip widths may have an impact on the drift electric field. Fig. 5 shows the Maxwell 11 simulation results, indicating that the 𝐸r and 𝐸∕∕ values exhibit extremely small differences when keeping the sum of the inner and outer field strip widths equal to the pitch. Consequently, the sum of the inner and outer field strip widths, but not the ratio between them, should be of concern when aiming for reduced distortion of the drift electric field.
5. Experimental evaluation of optimized drift electric field To evaluate the drift electric field optimization, the optimized fieldcage and the non-optimized field-cage (5-mm pitch) were tested with the boron-lined honeycomb neutron detector in a thermal neutron field. Fig. 9 shows their measured energy spectra (of the neutron count sum of each 10 channels) and calculated as Fig. 10 with the calculated neutron count-rate ratio (of the neutron count sum of each 100 channels) between the neutron detectors with optimized and non-optimized fieldcages, demonstrating that the optimized field-cage can realize both higher electron migration and neutron detection efficiencies. With the same threshold of 55 keV, the neutron detection efficiency with an optimized field-cage is 26% higher than that of the non-optimized fieldcage. The electron migration efficiency was also measured at Hokkaido University with non-optimized field-cage and at China Mianyang Research Reactor (CMRR) with an optimized field-cage, as shown in Fig. 11. The results indicate that the electron migration efficiency exhibits an average increase of 12.4%, demonstrating a significant promotion of efficiency.
Fig. 6 shows the simulation results with various values of the sum of the inner and outer field strip widths, confirming that the smaller field strip width could result in less electric distortion. However, the grounded detector shell surrounds the field-cage, and if the small field strips cannot shield the side face completely, the drift electric field will show an apparent distortion, which is shown in Fig. 7. This result demonstrates that the sum of the inner and outer field strip widths should equal the field-cage pitch.
4. Field-cage optimization According to the simulation results, the field-cage is optimized with a pitch of 2 mm, an inner field strip width of 1.5 mm, and an outer mirror field strip width of 0.5 mm. In this case, the non-negligible electric distortion is limited within 3.2 mm from the field-cage. The array chip resistor is introduced to reduce the welding difficulty and field-cage complexity, as shown in Fig. 8.
6. Discussion The electric field produced by a field-cage is an approximation of the uniform electric field. The introduction of field strips of a 4
Please cite this article in press as: Z. Fang, et al., Study of uniform drift electric field used for boron-lined honeycomb neutron detector, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.10.109.
Z. Fang et al.
Nuclear Inst. and Methods in Physics Research, A (
)
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significant electric distortion, leading to a limited electron migration efficiency with a long migration length. Therefore, the electron migration curve only reflects the electron migration efficiency near the field-cage, but not in the entire drift area. To address this problem, setting the incident neutron coming into the detector from the incident surface is essential, and using a neutron converter with a smaller height is also useful as well. In this case, more incident neutrons can be absorbed in the area with a smaller electric distortion, realizing a higher electron migration efficiency and a higher neutron detection efficiency. 7. Conclusions A newly designed boron-lined honeycomb neutron converter based detector has been formerly proposed as the 3 He alternative detector. In the proposed detector, a uniform electric field applied with the fieldcage is necessary to drive electrons out of the holes of the honeycomb converter to the GEM foil. The influence of electric field distortion on the migration of electrons was evaluated using Garfield 9 simulation, the results of which indicate that the electric distortion can be accepted if the value of the radial electric field strength 𝐸r , which is vertical to the drift direction, is smaller than 2 V/cm, when the drift electric strength 𝐸∕∕ , which is along the direction of hole axis, has a typical value of 200 V/cm. Maxwell 11 simulations demonstrated that smaller field-cage pitch and smaller field strips are beneficial to the suppression of electric distortion, while the summation of inner and outer field strip widths should be the same as the field-cage pitch. The field-cage was optimized with a 2-mm pitch and 1.5-and 0.5-mm widths of the inner field strip and outer mirror field strip, respectively. The neutron experiments show an electron migration efficiency increase of 12.4% and a neutron detection efficiency increase of 26%, demonstrating the effectiveness of field-cage optimization and drift electric field uniformity promotion.
Fig. 10. Count-rate ratio between optimized and non-optimized field-cages.
Acknowledgment This research is supported by National Natural Science Foundation of China (Grant No. 11735008). References [1] M. Henske, et al., The 10 B based jalousie neutron detector – an alternative for 3 He filled position sensitive counter tubes, Nucl. Instrum. Methods Phys. Res. A 686 (2012) 151–155. [2] J. Bitch, et al.,10 B4 C Multi-grid as an alternative to 3 He for large area neutron detectors, IEEE Trans. Nucl. Sci. 60 (2013) 871–878. [3] J.L. Lacy, et al., Boron-coated straws as a replacement for 3 He-based neutron detectors, Nucl. Instrum. Methods Phys. Res. A 652 (2011) 359–363. [4] Z. Fang, et al., Research on a neutron detector with a boron-lined honeycomb neutron converter, IEEE Trans. Nucl. Sci. 64 (4) (2017) 1048–1055. [5] F. Sauli, GEM: a new concept for electron amplification in gas detectors, Nucl. Instrum. Methods Phys. Res. A 386 (1997) 531–534. [6] S.F. Biagi, Monte carlo simulation of electron drift and diffusion in counting gases under the influence of electric and magnetic fields, Nucl. Instrum. Methods Phys. Res. A 421 (1999) 234–240. [7] M. Huang, et al., A Fast Neutron Spectrometer Based on GEM-TPC, in: 2012 IEEE Nuclear Science Symposium and Medical Imaging Conference Record, 2012. [8] G. Albani, et al., Evolution in boron-based GEM detectors for diffraction measurements: from planar to 3D converters, Meas. Sci. Technol. 27 (2016) 115902. [9] A. Pietropaolo, et al., A new 3 He-free thermal neutrons detector concept based on the GEM technology, Nucl. Instrum. Methods Phys. Res. A 729 (2013) 117–126. [10] H. Kubo, et al., Development of a time projection chamber with a micro-pixel electrodes, Nucl. Instrum. Methods Phys. Res. A 513 (2003) 94–98. [11] M. Killenberg, et al., Development of a GEM-based high resolution TPC for the ILC, Nucl. Instrum. Methods Phys. Res. A 573 (2007) 183–186. [12] H. Kim, et al., Performance of a high-pressure xenon ionization chamber for environmental radiation monitoring, Radiat. Meas. 43 (2008) 659–663. [13] T. Behnke, et al., A lightweight field-cage for a large TPC prototype for the ILC, J. Instrum. 5 (10) (2010) 10011. [14] R. Veenhof, Garfield, recent developments, Nucl. Instrum. Methods Phys. Res. A 419 (1998) 726–730. [15] Maxwell 3D Ansoft, Four Station Square, Suite200, Pittsburgh, PA 15219, USA.
Fig. 11. Electron migration efficiency curves with the non-optimized and optimized fieldcages.
certain width sets the same voltage potential in the surrounding area, leading to a bending of the electric equipotential surface in the space. Compared with the non-optimized field-cage, the optimized field-cage pitch can reduce the electric distortion significantly, at the cost of increased field-cage structural complexity. The 8P4R array chip resistor was introduced to the field-cage manufacturing process, reducing the number of resistors by 75% and the number of solder joints by 50%. In this case, the optimized field-cage has a similar structural complexity as the non-optimized field-cage, while the non-negligible electric distortion range is reduced from 6 to 3.2 mm from the inner surface of field-cage, corresponding to a distorted electric field volume decrease of 47%. Fig. 11 shows that the electron migration efficiency exhibits a significant increase, with an electron migration length of less than 50 mm, while the efficiency shows a very small difference when the migration length is larger than 60 mm. Maxwell 11 simulation shows that the intermediate part of the drift area exhibits a trivial electric distortion, while the space near the field-cage exhibits a more significant distortion. In the electron migration curve measurement, the neutron beam incident into the detector from the detector side face, which is close to the field-cage, leading to the absorption of most of the incident neutrons near the field-cage. Thus, most of the electrons suffer from
5 Please cite this article in press as: Z. Fang, et al., Study of uniform drift electric field used for boron-lined honeycomb neutron detector, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.10.109.