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Improving thermal-neutron detection efficiency of silicon neutron detectors using the combined layers of 10B4C on 6LiF
Nuclear Inst. and Methods in Physics Research, A 932 (2019) 50–55
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Improving thermal-neutron detection efficiency of silicon neutron detectors using the combined layers of 10 B4 C on 6 LiF Yong Jiang, Jian Wu, Yanpeng Yin, Yi Lu, Kunlin Wu, Xiaoqiang Fan, Jiarong Lei ∗ Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, PR China CAEP Key Laboratory of Neutron Physics, Mianyang 621900, PR China
ARTICLE
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Keywords: Neutron detector Silicon detector Detection efficiency Gamma-ray discrimination ability 10 B4 C/6 LiF converter
ABSTRACT We tested a layered structure of 10 B4 C on 6 LiF (10 B4 C/6 LiF) neutron converter for use with silicon neutron detectors. The combined layers effectively improve the efficiency of thermal-neutron detection over that of traditional single-layer converters. Silicon neutron detectors were constructed from a 2 cm × 2 cm silicon planar detector and coupled with removable neutron converters made from layers of 10 B4 C, 6 LiF, or 10 B4 C/6 LiF deposited on ceramics. The responses of these devices were tested with an 241 Am–Li neutron source and the accompanying background gamma rays. The capability of the devices to discriminate neutron events from background gamma rays was tested by varying the bias voltages of the detectors when irradiated with an 241 Am–Li neutron source and with 137 Cs or 60 Co gamma sources. The silicon neutron detector using the 10 B C/6 LiF converter of 1.2-μm-thick 10 B C and 21.1-μm-thick 6 LiF achieved the expected thermal-neutron 4 4 detection efficiency of 5.6%. The compact size, simple construction, and low bias voltage of these devices make them promising for neutron detector applications like neutron dosimeters and beam monitors.
1. Introduction Since neutrons have no charge, they cannot be detected directly via the electron–hole pairs produced in direct interactions with a semiconductor material. Therefore, silicon detectors are typically covered with a neutron converter, such as a film of 10 B, 10 B4 C, 6 Li, or 6 LiF, to detect thermal neutrons. Several research groups [1–6] did early works on the development of silicon semiconductor neutron detectors. Recently, a great deal of progress has been made in increasing these detectors’ thermal-neutron detection efficiency and their ability to reject background gamma rays. For example, researchers at Kansas State University [7–10] have developed microstructured semiconductor neutron detectors (MSNDs) that achieve high-efficiency neutron detection by increasing the probability for reaction products to reach the silicon semiconductor. Other researchers [11–14] have developed stacked planar silicon neutron detectors for the same purpose. Furthermore, research has been done for reducing the sensitive volume of the silicon detector using silicon on insulator (SOI) [15,16], epitaxial silicon [17], or silicon carbide [18] to decrease the detector’s sensitivity to background gamma rays. However, the capacitance and charge collection efficiency (CCE) of these methods are worse than traditional planar silicon detectors. Since thermal-neutron detection efficiency is limited by the self-absorption effect of the converter layer itself, the maximum reported detection
efficiency is 4.0% for 10 B-coated planar silicon neutron detectors and 4.6% for 6 LiF-coated devices [1]. Though the pure 6 Li achieved the highest efficiency of 11.5% for the thickness of 100 μm, it was difficult to handle due to its corrosive and active characteristics. D. S. McGregor [1] described a planer GaAs semiconductor neutron detector using combined layers of 1.1 μm 10 B/ 30 μm 6 LiF, and obtained a thermalneutron detection efficiency of 6.3%. However, the influence of gamma background had not been reported because the 250 μm-thickness bulk semi-insulation (SI) GaAs was extremely sensitive to the accompanying gamma. Compared with GaAs detector, the CCE of silicon detector is larger and obviously less sensitive to gamma ray, so it is a better choice for the fabrication of high-efficiency semiconductor neutron detectors. However, to the best of our knowledge, there is hardly any research on silicon neutron detectors using combined layers of 10 B4 C/ 6 LiF and the influence of gamma background on neutron events. The two neutron interactions typically used to detect thermal neutrons are the 10 B (n, 𝛼) 7 Li reaction and the 6 Li (n, 𝛼) T reaction, which result in two secondary charged particles traveling in opposite directions [1]. When neutrons are absorbed by 10 B, 94% of the 10 B (n, 𝛼) 7 Li reactions leave second-order products in the first excited state, and the energies of the alpha and 7 Li particles, respectively, are 1.47 and 0.84 MeV after they relax to the ground state by releasing gamma rays. For the remaining 6% of the reaction products resulting
∗ Correspondence to: Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, 64 Mianshan Road, Mianyang, Sichuan Province, China. E-mail addresses: [email protected], [email protected] (J. Lei).
https://doi.org/10.1016/j.nima.2019.04.051 Received 5 December 2018; Received in revised form 21 March 2019; Accepted 10 April 2019 Available online 16 April 2019 0168-9002/© 2019 Published by Elsevier B.V.
Y. Jiang, J. Wu, Y. Yin et al.
Nuclear Inst. and Methods in Physics Research, A 932 (2019) 50–55
Fig. 2. (a) The Silicon detector (20 × 20 mm2 ) packaged in a printed circuit board alongside a 10 B4 C/6 LiF converter on a thin ceramic substrate, (b) 10 B4 C/6 LiF converter installed over the surface of the silicon detector, separated by 0.5 mm.
construction, compact size, low cost, and operation at low bias voltages. These advantages make them useful for application in portable neutron dosimeters and neutron-beam monitors. Also, 3 He-gas-based detector could be replaced with our stacked silicon neutron detectors. Fig. 1. Schematic cross-section of a silicon detector coated with a combined layers 10 B4 C/6 LiF converter. The 10 B4 C layer is closest to the silicon detector surface, and a layer of 6 LiF is coated onto the 10 B4 C layer.
2. Detector fabrication and experimental techniques The semiconductor detector that we used for device characterization and measurements is a PIN silicon detector with an active area of (20 × 20) mm2 . The aluminum entrance window is 200 nm thick. The silicon detector is constructed from floating-zone N-type silicon with a crystal orientation of ⟨111⟩, thickness of 500 μm, and resistivity ⩾8000 Ω cm. To form the converter, a 1.2-μm-thick 10 B4 C converter layer with 92% isotopic enriched 10 B was deposited on a thin ceramic substrate by an electron beam evaporator. The thickness of the 10 B4 C converter deposition was controlled accurately using a quartz oscillator (INFICON SQC-310). Using the same procedure, 6.4- and 21.1-μm-thick 6 LiF converters with 95% isotopic enriched 6 Li were deposited on the ceramic substrates of the same size. The layered structure converters of 1.2-μm-thick 10 B4 C + 6.4-μm-thick 6 LiF and 1.2-μm-thick 10 B4 C + 21.1-μm-thick 6 LiF were fabricated on the ceramic substrates by laying the 10 B4 C layer upon the 6 LiF layer. As the photographs in Fig. 2 show, the silicon chips were packaged on printed circuit board (PCB) substrates and connected to negative and positive terminals with silver-epoxy adhesive and gold wire bonding, respectively. A PCB frame around the silicon surface was included so that the neutron converters could be placed on top of the ceramic substrates. A 0.5-mm gap was left between the neutron converter and the detector surface. The PCB substrate and the neutron converter on its ceramic substrate were encased in an aluminum box to shield from visible light and electromagnetic radiation and were operated in vacuum conditions to eliminate any effect because of the 0.5-mm gap. The maximum energy deposited in the 0.5-mm air gap layer is 154 keV for 840 keV 7 Li particles from the 10 B (n, 𝛼) 7 Li reactions, calculated using SRIM2011. When the vacuum is pumped to 100 Pa, the energy loss is only 0.2 keV, and the effect of the air gap is completely negligible. The optimal structure is that the combined layers are deposited directly on the surfaces of the silicon detectors. The purpose of using the removable converter layer is to recognize the effect of gamma background. To evaluate the electrical performance of the silicon detector, its I–V characteristics were recorded with a source measuring unit (2450 SourceMeter, Keithley) at room temperature (24 ◦ C. For calibration of the electronic chain with the silicon detector, the alpha spectra were recorded using a 226 Ra source with an activity of 3300 Bq. This source included five alpha-emitting isotopes 226 Ra (4.774 MeV), 210 Po (5.304 MeV), 222 Rn (5.489 MeV), 218 Po (6.001 MeV), and 214 Po (7.687 MeV). The 226 Ra source was placed in front of the bare silicon surface with a separation of approximately 1 mm, and a 1.5-mmdiameter spot collimator was used to increase the energy resolution of
in the ground state, the energies of the alpha and 7 Li particles are 1.78 and 1.02 MeV, respectively. When neutrons are absorbed by 6 Li, the energies of the alpha and triton particles are 2.05 and 2.73 MeV, respectively. The average penetration ranges in the 6 LiF material of the triton and alpha particles are 32.1 and 6.1 μm, respectively, and the average ranges in the 10 B4 C material for the alpha and 7 Li particles are from 1.6 to 4.3 μm. The microscopic thermal neutron (0.0259 eV) absorption cross-sections of the 10 B (n, 𝛼) 7 Li reaction and the 6 Li (n, 𝛼) T reaction are 3840 and 940 b, respectively. For thermal neutrons, the 10 B (n, 𝛼) 7 Li reaction is generally more likely than the 6 Li (n, 𝛼) T reaction because of its larger absorption cross-section. However, the 6 Li (n, 𝛼) T reaction has a longer average range than the 10 B (n, 𝛼) 7 Li reaction because of the higher energy of the reaction products. The products of the 6 Li (n, 𝛼) T reaction clearly have longer ranges within the 6 LiF material than the products of the 10 B (n, 𝛼) 7 Li reaction within the 10 B4 C material. Therefore, neutron detection efficiency can be increased using a 10 B4 C/6 LiF converter. Calculated using Geant4, the maximum efficiency is 3.4% for the 10 B4 C converter with a layer thickness of 2 μm when the setting of minimum detectable threshold is 200 keV. The efficiency is further reduced if the layer thickness is lower or higher than the thickness. The maximum efficiency is 4.4% for the 6 LiF converter with layer thickness of 25 μm. The maximum efficiency is 6.4% for the combined layers of 2 μm10 B4 C + 25 μm 6 LiF. In the detector that we have developed, the 10 B4 C layer is closer to the surface of the silicon detector, so charged particles from the 10 B (n, 𝛼) 7 Li reactions enter the silicon detector directly. However, the longerrange charged particles from the 6 Li (n, 𝛼) T reaction can still reach the silicon detector after passing through the 10 B4 C layer. Compared with a single-layer converter coating of 6 LiF or 10 B4 C, our design for a 10 B C/6 LiF converter should achieve more-efficient neutron detection, 4 as Fig. 1 illustrates. However, if the 6 LiF layer is deposited upon the 10 B C, it will actually reduce efficiency instead of increasing efficiency, 4 because charged particles from the 10 B (n, 𝛼) 7 Li reactions cannot pass through a 6 LiF layer larger than 5 μm and enter the silicon detector. The present paper reports on the spectra and detection efficiency of a Positive-Intrinsic-Negative (PIN) silicon detector when coupled with three different converters: 10 B4 C, 6 LiF, and a combination of the two. We also tested these detectors’ response to thermal neutrons and gamma rays with the silicon detector operating at a range of reversebias voltages. Such neutron detectors offer the advantages of simple 51
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Nuclear Inst. and Methods in Physics Research, A 932 (2019) 50–55
Fig. 4. Spectra of the detector response to a
Fig. 3. I–V characteristic of the silicon detector.
the alpha test. The CCE and signal-to-noise ratio (SNR) of the silicon detector were tested with an 241 Am alpha source with an activity of 1600 Bq. We define the SNR as the ratio between the baseline output and the average amplitude that corresponds to the 241 Am alpha particles. The output pulses were recorded under vacuum using a standard charge-sensitive preamplifier installed with an Ortec 671 amplifier. The amplifier’s unipolar output was passed on to an ORTEC 926 multichannel analyzer. An Ortec 428 voltage supply was used to apply the bias for the measurements. The 241 Am–Li neutron source that we employed emits 5.2 × 104 neutrons/s, with an average neutron energy of 480 keV. To enrich the low-energy neutrons, a 5-cm-thick polyethylene plate was used to soften the original neutrons. The thermal-neutron fluence rate of detector measurement position was 14.7 n cm2 s−1 and was measured using a 6 Li glass-scintillator neutron detector. The gamma response of the silicon neutron detector was tested with a 4.2 × 104 Bq 137 Cs gamma-ray source and a 5.0 × 103 Bq 60 Co gamma-ray source placed directly over the detector. Neutron-response measurements were performed with three neutron converters installed to clarify how well the 10 B4 C/ 6 LiF converter improves detection efficiency. The capability of the detector to reject background gamma rays was tested while varying the bias voltage and irradiating the detector with various sources. To collect enough data for statistical analysis and ensure the stability of the system, each round of measurements was divided into two runs of 2500 s, and the data were combined later.
226 Ra
alpha source.
Fig. 5. Charge collection efficiency as a function of the applied reverse voltage, responding to an 241 Am alpha source.
3. Results and discussion Fig. 3 shows the leakage current of the silicon detector at a room temperature of 24 ◦ C. From the I–V characteristics, we find that the detector has a remarkably low leakage current of a few nA. The performance of the silicon detector for alpha-particle detection is plotted in Fig. 4, which shows an excellent energy resolution of 32 keV for alpha particles at 7.687 MeV (∼0.41%). The energy response of the measurement system was calibrated using a 226 Ra alpha source, the output energy of which corresponds to the channel peaks. Fig. 5 plots the CCE of the silicon detector as a function of the reverse bias. Full charge collection is reached at about 20 V, where the depletion region has a maximum thickness of nearly 500 μm. Fig. 6 plots the SNR of the silicon detector as a function of the reverse bias. The SNR is high at 31, with a bias of 10 V. These results
Fig. 6. Signal-to-noise ratio as a function of the applied reverse voltage as measured with an 241 Am alpha source.
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Fig. 9. Spectrum of the silicon detector with a 6.4-μm 6 LiF converter film.
Fig. 7. Spectrum of the silicon detector without a converter film.
Fig. 8. Spectrum of the silicon detector with a 1.2-μm-thick
10 B C 4
Fig. 10. Spectrum of the silicon detector with a 21.1-μm 6 LiF converter film.
converter film.
respectively. The triton peaks from the 6 Li(n, 𝛼)T reaction, a continuous alpha spectrum, and the same background gamma rays as the previous measurements are apparent. Compared with these two spectra, the thinner 6 LiF converter had a clearer triton peak, though the thicker 6 LiF converter had greater efficiency, which agrees with the results reported by Pappalardo et al. [6]. When compared with the 10 B4 C converter results shown in Fig. 8, the second-order products from the 6 Li(n, 𝛼)T reaction were obviously more numerous than those from the 10 B(n, 𝛼)7 Li reaction because of the greater reaction energy of the 6 Li(n, 𝛼)T reaction. The net neutron count rates (after subtracting the background gamma rays) of the 6.4 and 21.1 μm 6 LiF converters were 1.27 and 2.52 cps, respectively. The results from the measurements with two 10 B4 C/ 6 LiF converters installed, of 1.2 μm 10 B4 C + 6.4 μm 6 LiF and 1.2 μm 10 B4 C + 21.1 μm 6 LiF, are shown in Figs. 11 and 12, respectively. The two spectra of 10 B4 C/ 6 LiF converters resemble the result if we were to add the spectra recorded from a single 10 B4 C converter and from a single 6 LiF converter. The spectrum in the region from 2 to 2.7 MeV clearly indicates triton particles from the 6 Li(n, 𝛼)T reaction, and the spectrum in the region from 0.9 to 1.6 MeV represents the main products from the 10 B(n, 𝛼)7 Li reaction and part of the alpha particles from the 6 Li(n, 𝛼)T reaction. Since the products from the 6 Li(n, 𝛼)T reaction have a longer penetration range than the products from the 10 B(n, 𝛼)7 Li reaction, we judge that the 1.2-μm-thick 10 B4 C converter slightly reduced the energy
demonstrate the high-energy resolution and high SNR of the silicon detector. The first run of measurements was intended to evaluate the response of the detector to background gamma rays with only a blank ceramic substrate installed and when exposed to moderated neutrons emitted from the 241 Am–Li source. Fig. 7 shows the spectrum of the background gamma-ray response with the silicon detector operating under a reverse-bias voltage of 10 V. These gamma rays showed a steep continuous attenuation in the low-energy region of the spectrum (below 0.9 MeV), whereas the response to gamma rays drops suddenly down from 0.9 MeV to the maximum energy around 1.7 MeV. The overall count rate of the background gamma-ray spectrum was 3.98 cps. In Fig. 8, the energy spectrum was recorded with the same 241 Am–Li neutron source and experimental conditions, but with the 1.2-μm-thick 10 B C converter installed. In Fig. 8, the spectrum is clearly increased in 4 the region above 0.9 MeV because of the contribution of the products from the 10 B(n, 𝛼)7 Li reaction, and the low-energy part of the spectrum still shows the contribution of background gamma rays that appears in Fig. 7. The overall count rate of the 1.2-μm-thick 10 B4 C converter spectrum was 4.91 cps. The difference between these two count rates, of 0.93 cps, isolates the contribution of the 1.2-μm-thick 10 B4 C converter. Following the same procedure used in the measurements described above, the energy spectra were recorded with the two 6 LiF converters with thicknesses of 6.4 and 21.1 μm, shown in Figs. 9 and 10, 53
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Nuclear Inst. and Methods in Physics Research, A 932 (2019) 50–55
Fig. 11. Spectrum of the silicon detector with a 1.2 μm film.
Fig. 12. Spectrum of the silicon detector with a 1.2 μm film.
10
Fig. 13. Spectra from the 1.2 μm 10 B4 C + 6.4 μm 6 LiF detector response to an 241 Am– Li neutron source (black squares), 137 Cs gamma source (green triangles) and 60 Co gamma source (blue inverted hollow triangles), and background gamma rays (without converter) with an 241 Am–Li neutron source (red circles), when operated at a reverse voltage of 2 V.
B4 C + 6.4 μm 6 LiF converter
10 B C 4
+ 21.1 μm 6 LiF converter Fig. 14. Spectra from the 1.2 μm 10 B4 C + 6.4 μm 6 LiF detector response to 241 Am– Li neutron (black squares), 137 Cs gamma rays (green triangles) and 60 Co gamma (inverted blue hollow triangles), and background gamma rays (without converter) with an 241 Am–Li neutron (red circles), when operated at a reverse bias of 4 V.
of the products from the 6 LiF converter before they reach the silicon detector. The net neutron count rates of the 1.2 μm 10 B4 C + 6.4 μm 6 LiF and 1.2 μm 10 B4 C + 21.1 μm 6 LiF converters were 2.13 and 3.29 cps, respectively, and thermal-neutron detection efficiency of 5.6% was obtained for the 1.2 μm 10 B4 C + 21.1 μm 6 LiF converters via the division of the count rates of the 6 Li glass-scintillator. These experimental results lead us to conclude that the neutron detection efficiency with the 10 B4 C/6 LiF converter installed is clearly better than the efficiency with a single 10 B4 C or 6 LiF converter installed. Fig. 13 allows a direct comparison of four spectra recorded from the silicon detector coated with the 1.2 μm 10 B4 C + 6.4 μm 6 LiF converter, operated at a reverse-bias voltage of 2 V. The spectra were recorded under irradiation with the 137 Cs gamma source, 60 Co gamma source, 241 Am–Li neutron source, and without any converter and under radiation from the 241 Am–Li neutron source. The same set of measurement conditions was applied, but with bias voltages of 4, 6, and 8 V, to record the data shown in Figs. 14 to 16. The spectra shown in Figs. 13 to 16 indicate that the sensitivity to gamma rays from 137 Cs or 60 Co increases along with the bias voltage, which was expected because the depletion region of the silicon thickens at higher bias voltages. The spectra of neutron reactions from the 10 B C/6 LiF converter are slightly shifted to higher energies as the bias 4
voltages increase, since the CCE also increases at high bias voltages. The spectra without a converter installed, which we previously assumed to indicate background gamma rays, shift dramatically to a highenergy region as the bias voltage increases. This shift indicates that the response in this case includes a small part of fast neutrons that interact directly with the abundant silicon material; this behavior is also confirmed in the literature [3,6]. In all cases, the net neutron count rates at each bias voltage were constant. Consequently, the experimental results demonstrate that the silicon neutron detector that we tested is very effective at discriminating gamma rays from neutron counts when operated at a bias of 2 V. 4. Conclusion The combined layers 10 B4 C/6 LiF neutron converter installed on top of a silicon detector proved effective at increasing the silicon detector’s thermal-neutron detection efficiency. The expected efficiency of 5.6% was realized by installing a pair of ceramic substrates coated with 1.2-μm-thick 10 B4 C and 21.1-μm-thick 6 LiF layers on top of the 54
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Acknowledgments This work was supported by the Key Laboratory of Neutron Physics of CAEP, China [grant number 2013AC01], the National Natural Science Foundation of China [Grant No. 11475151, 11605174, 11775198] and the Foundation for Development of Science and Technology of China Academy of Engineering Physics [Grant No. 2015B0103009]. References [1] D.S. McGregor, M.D. Hammig, Y.H. Yang, H.K. Gersch, R.T. Klann, Design considerations for thin film coated semiconductor thermal neutron detectors—I: basics regarding alpha particle emitting neutron reactive films, Nucl. Instrum. Methods Phys. Res. A 500 (2003) 272–308. [2] J. Schelten, M. Balzhauser, F. Hongesberg, R. Engels, R. Reinartz, A new neutron detector development based on silicon semiconductor and6 LiF converter, Physica B 234–236 (1997) 1084–1086. [3] Bernard F. Phlips, Francis J. Kub, Elena I. Novikova, Eric A. Wulf, Carrie Fitzgerald, Neutron detection using large area silicon detectors, Nucl. Instrum. Methods A 579 (2007) 173–176. [4] M. Barbagallo, L. Cosentino, V. Forcina, C. Marchetta, A. Pappalardo, P. Peerani, C. Scirè, S. Scirè, M. Schillaci, S. Vaccaro, G. Vecchio, Thermal neutron detection using a silicon pad detector and6 LiF removable converters, Rev. Sci. Instrum. 84 (2013) 033503. [5] K.H. Kang, H.B. Jeon, G.N. Kim, H. Park, Response of a photodiode coupled with boron for neutron detection, J. Korean Phys. Soc. 65 (2014) 1374–1378. [6] A. Pappalardo, M. Barbagallo, L. Cosentino, C. Marchetta, et al., Characterization of the silicon +6 LiF thermal neutron detection technique, Nucl. Instrum. Methods Phys. Res. A 810 (2016) 6–13. [7] J. Kenneth Shultis, Douglas S. McGregor, Efficiencies of Coated and Perforated Semiconductor Neutron Detectors, IEEE, 2004, pp. 4569–4574. [8] S.L. Bellinger, W.J. McNcil, D.S. McGregor, Variant Designs and Characteris tics of Improved Microstructured Solid-State Neutron detectors, in: 2009 IEEE Nuclear Science Symposium Conference Record, 2009, Vol. N14-4, pp. 986-989. [9] R.G. Fronk, S.L. Bellinger, L.C. Henson, D.E. Huddleston, T.R. Ochs, T.J. Sobering, D.S. McGregor, High-efficency microstructured semiconductor neutron detectors for direct3 He raplacement, Nucl. Instrum. Methods Phys. Res. A 779 (2015) 25–32. [10] D.S. McGregor, S.L. Bellinger, R.G. Fronk, L. Henson, D. Huddleston, T. Ochs, J.K. Shultis, T.J. Sobering, R.D. Taylor, Development of compact high efficiency microstructured semiconductor neutron detectors, Radiat. Phasics Chem. 116 (2015) 32–37. [11] M. Daniel, Efficient scalable solid-state neutron detector, Rev. Sci. Instrum. 86 (2015) 065103. [12] X. Gao, LiI Fenghua, Lu Min, Y. Jiang, Li Cheng, Characteristics of Si-PIN nuclear radiation detectors stacked in series and parallel, Sci. China Technol. Sci. 58 (2015) 1–6. [13] A. Ahmed, S. Burdin, G. Casse, H. van Zalinge, S. Powel, J. Rees, A. Smith, I. Tsurin, GAMBE: multipurpose sandwich detector for neutrons and photons, Proc. SPIE 9969 (2016) 99690E. [14] A. Pappalardo, C. Vasi, P. Finocchiaro, Direct comparison between solid state silicon+6 LiF and3 He gas tube neutron detectors, Results Phys. 6 (2016) 12–13. [15] Mythili Subramanian, Bernard Phlips, Fritz Kub, Characteristics of a silicon on insulator neutron detector, in: 2009 IEEE Nuclear Science Symposium Conference Record, 2009, Vol. N25-29, pp. 1306-1309. [16] G. Pellegrini, F. Garcia, J. Balbuena, E. Cabruja, M. Lozano, R. Orava, M. Ullan, Fabrication and simulation of novel ultra-thin 3D silicon detectors, Nucl. Instrum. Methods Phys. Res. A 604 (2009) 115–118. [17] Arvind Singh, Anita Topkar, Thin epitaxial silicon PIN detectors for thermal neutron detection with improved gamma (𝛾) discrimination, AIP Conf. Proc. 1731 (2016) 060022. [18] C. Manfredotti, A.L. Giudice, F. Fasolo, E. Vittone, C. Paolini, F. Fizzotti, A. Zanini, G. Wagner, C. Lanzieri, SiC Detectors for neutron monitoring, Nucl. Instrum. Methods Phys. Res. A 552 (2005) 131–137.
Fig. 15. Spectra from the 1.2 μm 10 B4 C + 6.4 μm 6 LiF detector response to 241 Am–Li neutron (black squares), 137 Cs gamma rays (green triangles) and 60 Co gamma rays (blue inverted hollow triangles), and background gamma-ray response (with no converter) with 241 Am–Li neutron (red circles), when operated at a reverse-bias voltage of 6 V.
Fig. 16. Spectra from the 1.2 μm 10 B4 C + 6.4 μm 6 LiF converter with 241 Am–Li neutron (black squares), 137 Cs gamma (green triangles) and 60 Co gamma (blue inverted hollow triangles), and background gamma rays (without converter) with 241 Am–Li neutron (red circles), when operated at a reverse voltage of 8 V.
silicon detector. The silicon neutron detector achieved was surprisingly effective at discriminating between background gamma rays and neutron counts when operated at a bias voltage of 2 V. In a future work, we will address the peeling of thicker coatings of 10 B4 C/6 LiF converters deposited on the ceramic substrates, and we are planning to build stacked silicon neutron detectors to further improve detection efficiency.
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Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Improving thermal-neutron detection efficiency of silicon neutron detectors using the combined layers of 10 B4 C on 6 LiF Yong Jiang, Jian Wu, Yanpeng Yin, Yi Lu, Kunlin Wu, Xiaoqiang Fan, Jiarong Lei ∗ Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, PR China CAEP Key Laboratory of Neutron Physics, Mianyang 621900, PR China
ARTICLE
INFO
Keywords: Neutron detector Silicon detector Detection efficiency Gamma-ray discrimination ability 10 B4 C/6 LiF converter
ABSTRACT We tested a layered structure of 10 B4 C on 6 LiF (10 B4 C/6 LiF) neutron converter for use with silicon neutron detectors. The combined layers effectively improve the efficiency of thermal-neutron detection over that of traditional single-layer converters. Silicon neutron detectors were constructed from a 2 cm × 2 cm silicon planar detector and coupled with removable neutron converters made from layers of 10 B4 C, 6 LiF, or 10 B4 C/6 LiF deposited on ceramics. The responses of these devices were tested with an 241 Am–Li neutron source and the accompanying background gamma rays. The capability of the devices to discriminate neutron events from background gamma rays was tested by varying the bias voltages of the detectors when irradiated with an 241 Am–Li neutron source and with 137 Cs or 60 Co gamma sources. The silicon neutron detector using the 10 B C/6 LiF converter of 1.2-μm-thick 10 B C and 21.1-μm-thick 6 LiF achieved the expected thermal-neutron 4 4 detection efficiency of 5.6%. The compact size, simple construction, and low bias voltage of these devices make them promising for neutron detector applications like neutron dosimeters and beam monitors.
1. Introduction Since neutrons have no charge, they cannot be detected directly via the electron–hole pairs produced in direct interactions with a semiconductor material. Therefore, silicon detectors are typically covered with a neutron converter, such as a film of 10 B, 10 B4 C, 6 Li, or 6 LiF, to detect thermal neutrons. Several research groups [1–6] did early works on the development of silicon semiconductor neutron detectors. Recently, a great deal of progress has been made in increasing these detectors’ thermal-neutron detection efficiency and their ability to reject background gamma rays. For example, researchers at Kansas State University [7–10] have developed microstructured semiconductor neutron detectors (MSNDs) that achieve high-efficiency neutron detection by increasing the probability for reaction products to reach the silicon semiconductor. Other researchers [11–14] have developed stacked planar silicon neutron detectors for the same purpose. Furthermore, research has been done for reducing the sensitive volume of the silicon detector using silicon on insulator (SOI) [15,16], epitaxial silicon [17], or silicon carbide [18] to decrease the detector’s sensitivity to background gamma rays. However, the capacitance and charge collection efficiency (CCE) of these methods are worse than traditional planar silicon detectors. Since thermal-neutron detection efficiency is limited by the self-absorption effect of the converter layer itself, the maximum reported detection
efficiency is 4.0% for 10 B-coated planar silicon neutron detectors and 4.6% for 6 LiF-coated devices [1]. Though the pure 6 Li achieved the highest efficiency of 11.5% for the thickness of 100 μm, it was difficult to handle due to its corrosive and active characteristics. D. S. McGregor [1] described a planer GaAs semiconductor neutron detector using combined layers of 1.1 μm 10 B/ 30 μm 6 LiF, and obtained a thermalneutron detection efficiency of 6.3%. However, the influence of gamma background had not been reported because the 250 μm-thickness bulk semi-insulation (SI) GaAs was extremely sensitive to the accompanying gamma. Compared with GaAs detector, the CCE of silicon detector is larger and obviously less sensitive to gamma ray, so it is a better choice for the fabrication of high-efficiency semiconductor neutron detectors. However, to the best of our knowledge, there is hardly any research on silicon neutron detectors using combined layers of 10 B4 C/ 6 LiF and the influence of gamma background on neutron events. The two neutron interactions typically used to detect thermal neutrons are the 10 B (n, 𝛼) 7 Li reaction and the 6 Li (n, 𝛼) T reaction, which result in two secondary charged particles traveling in opposite directions [1]. When neutrons are absorbed by 10 B, 94% of the 10 B (n, 𝛼) 7 Li reactions leave second-order products in the first excited state, and the energies of the alpha and 7 Li particles, respectively, are 1.47 and 0.84 MeV after they relax to the ground state by releasing gamma rays. For the remaining 6% of the reaction products resulting
∗ Correspondence to: Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, 64 Mianshan Road, Mianyang, Sichuan Province, China. E-mail addresses: [email protected], [email protected] (J. Lei).
https://doi.org/10.1016/j.nima.2019.04.051 Received 5 December 2018; Received in revised form 21 March 2019; Accepted 10 April 2019 Available online 16 April 2019 0168-9002/© 2019 Published by Elsevier B.V.
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Fig. 2. (a) The Silicon detector (20 × 20 mm2 ) packaged in a printed circuit board alongside a 10 B4 C/6 LiF converter on a thin ceramic substrate, (b) 10 B4 C/6 LiF converter installed over the surface of the silicon detector, separated by 0.5 mm.
construction, compact size, low cost, and operation at low bias voltages. These advantages make them useful for application in portable neutron dosimeters and neutron-beam monitors. Also, 3 He-gas-based detector could be replaced with our stacked silicon neutron detectors. Fig. 1. Schematic cross-section of a silicon detector coated with a combined layers 10 B4 C/6 LiF converter. The 10 B4 C layer is closest to the silicon detector surface, and a layer of 6 LiF is coated onto the 10 B4 C layer.
2. Detector fabrication and experimental techniques The semiconductor detector that we used for device characterization and measurements is a PIN silicon detector with an active area of (20 × 20) mm2 . The aluminum entrance window is 200 nm thick. The silicon detector is constructed from floating-zone N-type silicon with a crystal orientation of ⟨111⟩, thickness of 500 μm, and resistivity ⩾8000 Ω cm. To form the converter, a 1.2-μm-thick 10 B4 C converter layer with 92% isotopic enriched 10 B was deposited on a thin ceramic substrate by an electron beam evaporator. The thickness of the 10 B4 C converter deposition was controlled accurately using a quartz oscillator (INFICON SQC-310). Using the same procedure, 6.4- and 21.1-μm-thick 6 LiF converters with 95% isotopic enriched 6 Li were deposited on the ceramic substrates of the same size. The layered structure converters of 1.2-μm-thick 10 B4 C + 6.4-μm-thick 6 LiF and 1.2-μm-thick 10 B4 C + 21.1-μm-thick 6 LiF were fabricated on the ceramic substrates by laying the 10 B4 C layer upon the 6 LiF layer. As the photographs in Fig. 2 show, the silicon chips were packaged on printed circuit board (PCB) substrates and connected to negative and positive terminals with silver-epoxy adhesive and gold wire bonding, respectively. A PCB frame around the silicon surface was included so that the neutron converters could be placed on top of the ceramic substrates. A 0.5-mm gap was left between the neutron converter and the detector surface. The PCB substrate and the neutron converter on its ceramic substrate were encased in an aluminum box to shield from visible light and electromagnetic radiation and were operated in vacuum conditions to eliminate any effect because of the 0.5-mm gap. The maximum energy deposited in the 0.5-mm air gap layer is 154 keV for 840 keV 7 Li particles from the 10 B (n, 𝛼) 7 Li reactions, calculated using SRIM2011. When the vacuum is pumped to 100 Pa, the energy loss is only 0.2 keV, and the effect of the air gap is completely negligible. The optimal structure is that the combined layers are deposited directly on the surfaces of the silicon detectors. The purpose of using the removable converter layer is to recognize the effect of gamma background. To evaluate the electrical performance of the silicon detector, its I–V characteristics were recorded with a source measuring unit (2450 SourceMeter, Keithley) at room temperature (24 ◦ C. For calibration of the electronic chain with the silicon detector, the alpha spectra were recorded using a 226 Ra source with an activity of 3300 Bq. This source included five alpha-emitting isotopes 226 Ra (4.774 MeV), 210 Po (5.304 MeV), 222 Rn (5.489 MeV), 218 Po (6.001 MeV), and 214 Po (7.687 MeV). The 226 Ra source was placed in front of the bare silicon surface with a separation of approximately 1 mm, and a 1.5-mmdiameter spot collimator was used to increase the energy resolution of
in the ground state, the energies of the alpha and 7 Li particles are 1.78 and 1.02 MeV, respectively. When neutrons are absorbed by 6 Li, the energies of the alpha and triton particles are 2.05 and 2.73 MeV, respectively. The average penetration ranges in the 6 LiF material of the triton and alpha particles are 32.1 and 6.1 μm, respectively, and the average ranges in the 10 B4 C material for the alpha and 7 Li particles are from 1.6 to 4.3 μm. The microscopic thermal neutron (0.0259 eV) absorption cross-sections of the 10 B (n, 𝛼) 7 Li reaction and the 6 Li (n, 𝛼) T reaction are 3840 and 940 b, respectively. For thermal neutrons, the 10 B (n, 𝛼) 7 Li reaction is generally more likely than the 6 Li (n, 𝛼) T reaction because of its larger absorption cross-section. However, the 6 Li (n, 𝛼) T reaction has a longer average range than the 10 B (n, 𝛼) 7 Li reaction because of the higher energy of the reaction products. The products of the 6 Li (n, 𝛼) T reaction clearly have longer ranges within the 6 LiF material than the products of the 10 B (n, 𝛼) 7 Li reaction within the 10 B4 C material. Therefore, neutron detection efficiency can be increased using a 10 B4 C/6 LiF converter. Calculated using Geant4, the maximum efficiency is 3.4% for the 10 B4 C converter with a layer thickness of 2 μm when the setting of minimum detectable threshold is 200 keV. The efficiency is further reduced if the layer thickness is lower or higher than the thickness. The maximum efficiency is 4.4% for the 6 LiF converter with layer thickness of 25 μm. The maximum efficiency is 6.4% for the combined layers of 2 μm10 B4 C + 25 μm 6 LiF. In the detector that we have developed, the 10 B4 C layer is closer to the surface of the silicon detector, so charged particles from the 10 B (n, 𝛼) 7 Li reactions enter the silicon detector directly. However, the longerrange charged particles from the 6 Li (n, 𝛼) T reaction can still reach the silicon detector after passing through the 10 B4 C layer. Compared with a single-layer converter coating of 6 LiF or 10 B4 C, our design for a 10 B C/6 LiF converter should achieve more-efficient neutron detection, 4 as Fig. 1 illustrates. However, if the 6 LiF layer is deposited upon the 10 B C, it will actually reduce efficiency instead of increasing efficiency, 4 because charged particles from the 10 B (n, 𝛼) 7 Li reactions cannot pass through a 6 LiF layer larger than 5 μm and enter the silicon detector. The present paper reports on the spectra and detection efficiency of a Positive-Intrinsic-Negative (PIN) silicon detector when coupled with three different converters: 10 B4 C, 6 LiF, and a combination of the two. We also tested these detectors’ response to thermal neutrons and gamma rays with the silicon detector operating at a range of reversebias voltages. Such neutron detectors offer the advantages of simple 51
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Fig. 4. Spectra of the detector response to a
Fig. 3. I–V characteristic of the silicon detector.
the alpha test. The CCE and signal-to-noise ratio (SNR) of the silicon detector were tested with an 241 Am alpha source with an activity of 1600 Bq. We define the SNR as the ratio between the baseline output and the average amplitude that corresponds to the 241 Am alpha particles. The output pulses were recorded under vacuum using a standard charge-sensitive preamplifier installed with an Ortec 671 amplifier. The amplifier’s unipolar output was passed on to an ORTEC 926 multichannel analyzer. An Ortec 428 voltage supply was used to apply the bias for the measurements. The 241 Am–Li neutron source that we employed emits 5.2 × 104 neutrons/s, with an average neutron energy of 480 keV. To enrich the low-energy neutrons, a 5-cm-thick polyethylene plate was used to soften the original neutrons. The thermal-neutron fluence rate of detector measurement position was 14.7 n cm2 s−1 and was measured using a 6 Li glass-scintillator neutron detector. The gamma response of the silicon neutron detector was tested with a 4.2 × 104 Bq 137 Cs gamma-ray source and a 5.0 × 103 Bq 60 Co gamma-ray source placed directly over the detector. Neutron-response measurements were performed with three neutron converters installed to clarify how well the 10 B4 C/ 6 LiF converter improves detection efficiency. The capability of the detector to reject background gamma rays was tested while varying the bias voltage and irradiating the detector with various sources. To collect enough data for statistical analysis and ensure the stability of the system, each round of measurements was divided into two runs of 2500 s, and the data were combined later.
226 Ra
alpha source.
Fig. 5. Charge collection efficiency as a function of the applied reverse voltage, responding to an 241 Am alpha source.
3. Results and discussion Fig. 3 shows the leakage current of the silicon detector at a room temperature of 24 ◦ C. From the I–V characteristics, we find that the detector has a remarkably low leakage current of a few nA. The performance of the silicon detector for alpha-particle detection is plotted in Fig. 4, which shows an excellent energy resolution of 32 keV for alpha particles at 7.687 MeV (∼0.41%). The energy response of the measurement system was calibrated using a 226 Ra alpha source, the output energy of which corresponds to the channel peaks. Fig. 5 plots the CCE of the silicon detector as a function of the reverse bias. Full charge collection is reached at about 20 V, where the depletion region has a maximum thickness of nearly 500 μm. Fig. 6 plots the SNR of the silicon detector as a function of the reverse bias. The SNR is high at 31, with a bias of 10 V. These results
Fig. 6. Signal-to-noise ratio as a function of the applied reverse voltage as measured with an 241 Am alpha source.
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Fig. 9. Spectrum of the silicon detector with a 6.4-μm 6 LiF converter film.
Fig. 7. Spectrum of the silicon detector without a converter film.
Fig. 8. Spectrum of the silicon detector with a 1.2-μm-thick
10 B C 4
Fig. 10. Spectrum of the silicon detector with a 21.1-μm 6 LiF converter film.
converter film.
respectively. The triton peaks from the 6 Li(n, 𝛼)T reaction, a continuous alpha spectrum, and the same background gamma rays as the previous measurements are apparent. Compared with these two spectra, the thinner 6 LiF converter had a clearer triton peak, though the thicker 6 LiF converter had greater efficiency, which agrees with the results reported by Pappalardo et al. [6]. When compared with the 10 B4 C converter results shown in Fig. 8, the second-order products from the 6 Li(n, 𝛼)T reaction were obviously more numerous than those from the 10 B(n, 𝛼)7 Li reaction because of the greater reaction energy of the 6 Li(n, 𝛼)T reaction. The net neutron count rates (after subtracting the background gamma rays) of the 6.4 and 21.1 μm 6 LiF converters were 1.27 and 2.52 cps, respectively. The results from the measurements with two 10 B4 C/ 6 LiF converters installed, of 1.2 μm 10 B4 C + 6.4 μm 6 LiF and 1.2 μm 10 B4 C + 21.1 μm 6 LiF, are shown in Figs. 11 and 12, respectively. The two spectra of 10 B4 C/ 6 LiF converters resemble the result if we were to add the spectra recorded from a single 10 B4 C converter and from a single 6 LiF converter. The spectrum in the region from 2 to 2.7 MeV clearly indicates triton particles from the 6 Li(n, 𝛼)T reaction, and the spectrum in the region from 0.9 to 1.6 MeV represents the main products from the 10 B(n, 𝛼)7 Li reaction and part of the alpha particles from the 6 Li(n, 𝛼)T reaction. Since the products from the 6 Li(n, 𝛼)T reaction have a longer penetration range than the products from the 10 B(n, 𝛼)7 Li reaction, we judge that the 1.2-μm-thick 10 B4 C converter slightly reduced the energy
demonstrate the high-energy resolution and high SNR of the silicon detector. The first run of measurements was intended to evaluate the response of the detector to background gamma rays with only a blank ceramic substrate installed and when exposed to moderated neutrons emitted from the 241 Am–Li source. Fig. 7 shows the spectrum of the background gamma-ray response with the silicon detector operating under a reverse-bias voltage of 10 V. These gamma rays showed a steep continuous attenuation in the low-energy region of the spectrum (below 0.9 MeV), whereas the response to gamma rays drops suddenly down from 0.9 MeV to the maximum energy around 1.7 MeV. The overall count rate of the background gamma-ray spectrum was 3.98 cps. In Fig. 8, the energy spectrum was recorded with the same 241 Am–Li neutron source and experimental conditions, but with the 1.2-μm-thick 10 B C converter installed. In Fig. 8, the spectrum is clearly increased in 4 the region above 0.9 MeV because of the contribution of the products from the 10 B(n, 𝛼)7 Li reaction, and the low-energy part of the spectrum still shows the contribution of background gamma rays that appears in Fig. 7. The overall count rate of the 1.2-μm-thick 10 B4 C converter spectrum was 4.91 cps. The difference between these two count rates, of 0.93 cps, isolates the contribution of the 1.2-μm-thick 10 B4 C converter. Following the same procedure used in the measurements described above, the energy spectra were recorded with the two 6 LiF converters with thicknesses of 6.4 and 21.1 μm, shown in Figs. 9 and 10, 53
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Fig. 11. Spectrum of the silicon detector with a 1.2 μm film.
Fig. 12. Spectrum of the silicon detector with a 1.2 μm film.
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Fig. 13. Spectra from the 1.2 μm 10 B4 C + 6.4 μm 6 LiF detector response to an 241 Am– Li neutron source (black squares), 137 Cs gamma source (green triangles) and 60 Co gamma source (blue inverted hollow triangles), and background gamma rays (without converter) with an 241 Am–Li neutron source (red circles), when operated at a reverse voltage of 2 V.
B4 C + 6.4 μm 6 LiF converter
10 B C 4
+ 21.1 μm 6 LiF converter Fig. 14. Spectra from the 1.2 μm 10 B4 C + 6.4 μm 6 LiF detector response to 241 Am– Li neutron (black squares), 137 Cs gamma rays (green triangles) and 60 Co gamma (inverted blue hollow triangles), and background gamma rays (without converter) with an 241 Am–Li neutron (red circles), when operated at a reverse bias of 4 V.
of the products from the 6 LiF converter before they reach the silicon detector. The net neutron count rates of the 1.2 μm 10 B4 C + 6.4 μm 6 LiF and 1.2 μm 10 B4 C + 21.1 μm 6 LiF converters were 2.13 and 3.29 cps, respectively, and thermal-neutron detection efficiency of 5.6% was obtained for the 1.2 μm 10 B4 C + 21.1 μm 6 LiF converters via the division of the count rates of the 6 Li glass-scintillator. These experimental results lead us to conclude that the neutron detection efficiency with the 10 B4 C/6 LiF converter installed is clearly better than the efficiency with a single 10 B4 C or 6 LiF converter installed. Fig. 13 allows a direct comparison of four spectra recorded from the silicon detector coated with the 1.2 μm 10 B4 C + 6.4 μm 6 LiF converter, operated at a reverse-bias voltage of 2 V. The spectra were recorded under irradiation with the 137 Cs gamma source, 60 Co gamma source, 241 Am–Li neutron source, and without any converter and under radiation from the 241 Am–Li neutron source. The same set of measurement conditions was applied, but with bias voltages of 4, 6, and 8 V, to record the data shown in Figs. 14 to 16. The spectra shown in Figs. 13 to 16 indicate that the sensitivity to gamma rays from 137 Cs or 60 Co increases along with the bias voltage, which was expected because the depletion region of the silicon thickens at higher bias voltages. The spectra of neutron reactions from the 10 B C/6 LiF converter are slightly shifted to higher energies as the bias 4
voltages increase, since the CCE also increases at high bias voltages. The spectra without a converter installed, which we previously assumed to indicate background gamma rays, shift dramatically to a highenergy region as the bias voltage increases. This shift indicates that the response in this case includes a small part of fast neutrons that interact directly with the abundant silicon material; this behavior is also confirmed in the literature [3,6]. In all cases, the net neutron count rates at each bias voltage were constant. Consequently, the experimental results demonstrate that the silicon neutron detector that we tested is very effective at discriminating gamma rays from neutron counts when operated at a bias of 2 V. 4. Conclusion The combined layers 10 B4 C/6 LiF neutron converter installed on top of a silicon detector proved effective at increasing the silicon detector’s thermal-neutron detection efficiency. The expected efficiency of 5.6% was realized by installing a pair of ceramic substrates coated with 1.2-μm-thick 10 B4 C and 21.1-μm-thick 6 LiF layers on top of the 54
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Acknowledgments This work was supported by the Key Laboratory of Neutron Physics of CAEP, China [grant number 2013AC01], the National Natural Science Foundation of China [Grant No. 11475151, 11605174, 11775198] and the Foundation for Development of Science and Technology of China Academy of Engineering Physics [Grant No. 2015B0103009]. References [1] D.S. McGregor, M.D. Hammig, Y.H. Yang, H.K. Gersch, R.T. Klann, Design considerations for thin film coated semiconductor thermal neutron detectors—I: basics regarding alpha particle emitting neutron reactive films, Nucl. Instrum. Methods Phys. Res. A 500 (2003) 272–308. [2] J. Schelten, M. Balzhauser, F. Hongesberg, R. Engels, R. Reinartz, A new neutron detector development based on silicon semiconductor and6 LiF converter, Physica B 234–236 (1997) 1084–1086. [3] Bernard F. Phlips, Francis J. Kub, Elena I. Novikova, Eric A. Wulf, Carrie Fitzgerald, Neutron detection using large area silicon detectors, Nucl. Instrum. Methods A 579 (2007) 173–176. [4] M. Barbagallo, L. Cosentino, V. Forcina, C. Marchetta, A. Pappalardo, P. Peerani, C. Scirè, S. Scirè, M. Schillaci, S. Vaccaro, G. Vecchio, Thermal neutron detection using a silicon pad detector and6 LiF removable converters, Rev. Sci. Instrum. 84 (2013) 033503. [5] K.H. Kang, H.B. Jeon, G.N. Kim, H. Park, Response of a photodiode coupled with boron for neutron detection, J. Korean Phys. Soc. 65 (2014) 1374–1378. [6] A. Pappalardo, M. Barbagallo, L. Cosentino, C. Marchetta, et al., Characterization of the silicon +6 LiF thermal neutron detection technique, Nucl. Instrum. Methods Phys. Res. A 810 (2016) 6–13. [7] J. Kenneth Shultis, Douglas S. McGregor, Efficiencies of Coated and Perforated Semiconductor Neutron Detectors, IEEE, 2004, pp. 4569–4574. [8] S.L. Bellinger, W.J. McNcil, D.S. McGregor, Variant Designs and Characteris tics of Improved Microstructured Solid-State Neutron detectors, in: 2009 IEEE Nuclear Science Symposium Conference Record, 2009, Vol. N14-4, pp. 986-989. [9] R.G. Fronk, S.L. Bellinger, L.C. Henson, D.E. Huddleston, T.R. Ochs, T.J. Sobering, D.S. McGregor, High-efficency microstructured semiconductor neutron detectors for direct3 He raplacement, Nucl. Instrum. Methods Phys. Res. A 779 (2015) 25–32. [10] D.S. McGregor, S.L. Bellinger, R.G. Fronk, L. Henson, D. Huddleston, T. Ochs, J.K. Shultis, T.J. Sobering, R.D. Taylor, Development of compact high efficiency microstructured semiconductor neutron detectors, Radiat. Phasics Chem. 116 (2015) 32–37. [11] M. Daniel, Efficient scalable solid-state neutron detector, Rev. Sci. Instrum. 86 (2015) 065103. [12] X. Gao, LiI Fenghua, Lu Min, Y. Jiang, Li Cheng, Characteristics of Si-PIN nuclear radiation detectors stacked in series and parallel, Sci. China Technol. Sci. 58 (2015) 1–6. [13] A. Ahmed, S. Burdin, G. Casse, H. van Zalinge, S. Powel, J. Rees, A. Smith, I. Tsurin, GAMBE: multipurpose sandwich detector for neutrons and photons, Proc. SPIE 9969 (2016) 99690E. [14] A. Pappalardo, C. Vasi, P. Finocchiaro, Direct comparison between solid state silicon+6 LiF and3 He gas tube neutron detectors, Results Phys. 6 (2016) 12–13. [15] Mythili Subramanian, Bernard Phlips, Fritz Kub, Characteristics of a silicon on insulator neutron detector, in: 2009 IEEE Nuclear Science Symposium Conference Record, 2009, Vol. N25-29, pp. 1306-1309. [16] G. Pellegrini, F. Garcia, J. Balbuena, E. Cabruja, M. Lozano, R. Orava, M. Ullan, Fabrication and simulation of novel ultra-thin 3D silicon detectors, Nucl. Instrum. Methods Phys. Res. A 604 (2009) 115–118. [17] Arvind Singh, Anita Topkar, Thin epitaxial silicon PIN detectors for thermal neutron detection with improved gamma (𝛾) discrimination, AIP Conf. Proc. 1731 (2016) 060022. [18] C. Manfredotti, A.L. Giudice, F. Fasolo, E. Vittone, C. Paolini, F. Fizzotti, A. Zanini, G. Wagner, C. Lanzieri, SiC Detectors for neutron monitoring, Nucl. Instrum. Methods Phys. Res. A 552 (2005) 131–137.
Fig. 15. Spectra from the 1.2 μm 10 B4 C + 6.4 μm 6 LiF detector response to 241 Am–Li neutron (black squares), 137 Cs gamma rays (green triangles) and 60 Co gamma rays (blue inverted hollow triangles), and background gamma-ray response (with no converter) with 241 Am–Li neutron (red circles), when operated at a reverse-bias voltage of 6 V.
Fig. 16. Spectra from the 1.2 μm 10 B4 C + 6.4 μm 6 LiF converter with 241 Am–Li neutron (black squares), 137 Cs gamma (green triangles) and 60 Co gamma (blue inverted hollow triangles), and background gamma rays (without converter) with 241 Am–Li neutron (red circles), when operated at a reverse voltage of 8 V.
silicon detector. The silicon neutron detector achieved was surprisingly effective at discriminating between background gamma rays and neutron counts when operated at a bias voltage of 2 V. In a future work, we will address the peeling of thicker coatings of 10 B4 C/6 LiF converters deposited on the ceramic substrates, and we are planning to build stacked silicon neutron detectors to further improve detection efficiency.
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