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Thin-film neutron detector based on CdTe and 6Li layers
Nuclear Inst. and Methods in Physics Research, A (
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Thin-film neutron detector based on CdTe and 6 Li layers Alvin Compaan a,b ,∗, Ford B. Cauffiel a , Anthony J. Matthews a , Song Cheng d , Ambalanath Shan c , Robert D. Fisher a , Victor V. Plotnikov b , Ralph Becker a a
Lithium Innovations Co., LLC, 3171 N. Republic Blvd., Toledo, OH 43615, United States Lucintech Inc, 1510 N. Westwood Ave, Toledo, OH 43606, United States 2425 Cheltenham Rd., Toledo, OH 43606, United States d Department of Physics and Astronomy, The University of Toledo, Toledo, OH 43606, United States b c
ARTICLE Keywords: Neutron Detector CdTe Li-6 Thin-film Radiation
INFO
ABSTRACT We report an innovative, low-cost, solid-state neutron detector based on the technology of thin-film CdTe solar cells activated with layers of isotopically enriched metallic Li-6. The technology leverages recent advances in thin-film solar-module manufacturing and lithium sources for electrochromic window fabrication. Our detector inherently has very high rejection of gamma events and strong directional sensitivity; it is readily scalable in size from personal radiation monitors to radiation portal monitors. The thermal neutron response of a double detector stack is competitive with and/or exceeds responses reported for other portable detectors such as a small He-3 tube, a CLYC-based scintillator, or a microstructured semiconductor detector.
Contents 1. 2. 3.
4. 5. 6.
Introduction ....................................................................................................................................................................................................... Design and fabrication details............................................................................................................................................................................... Detector operation .............................................................................................................................................................................................. 3.1. Pulse height spectra for thermal neutron beam............................................................................................................................................ 3.2. Radiation hardness tests using Po-210 alphas.............................................................................................................................................. 3.3. Gamma rejection ..................................................................................................................................................................................... 3.4. Angular dependence ................................................................................................................................................................................ PRD configuration and electronics ........................................................................................................................................................................ MCNP modeling of detector performance .............................................................................................................................................................. Conclusions ........................................................................................................................................................................................................ References..........................................................................................................................................................................................................
1. Introduction Although semiconductor diodes offer many advantages for the detection of high energy photons (X-rays and gamma rays) and energetic charged particles, the detection of neutrons requires the use of sensitizing elements. These sensitizing elements should have nuclei with large neutron reaction cross sections that preferably yield charged particle daughter products. These include 3 He, 6 Li, 10 B, and 157 Gd. 3 He has a large thermal neutron cross section (5330 barns) but as a gas is difficult to couple efficiently to the semiconductor and furthermore is constrained by availability and cost. 157 Gd has a very large thermal neutron cross section (240,000 b) but after neutron capture has a complex decay scheme with multiple gamma rays and three internal conversion electrons with energies of 29, 71, and 78 keV [1] that do
1 2 3 3 3 4 4 4 5 5 5
not facilitate detection of neutrons while discriminating against other sources of gamma rays. Thus, it has been recognized that 10 B and 6 Li are the remaining attractive choices for sensitizing elements to couple with semiconductor detectors. 10 B has the larger thermal neutron capture cross section (3840 barns) but with charged daughter products of 4 He and 7 Li that have relatively low combined energies of 2.8 MeV (4%) or 2.31 MeV (96%). The resulting short transport distances [2] of the reaction particles are not good for planar structures but suggest more complex micro-structuring to achieve high efficiency as has been done, e.g., by McGregor, et al. [3,4]. Thermal neutron capture by 6 Li has a smaller cross section of 940 barns and results in 4 He (alpha) plus 3 H (triton) with much higher combined energy of 4.8 MeV. The larger reaction energy for the 6 Li neutron capture coupled with the smaller nuclear charges
∗ Corresponding author at: Lucintech Inc, 1510 N. Westwood Ave, Toledo, OH 43606, United States. E-mail address: [email protected] (A. Compaan).
https://doi.org/10.1016/j.nima.2018.08.086 Received 31 July 2018; Accepted 23 August 2018 Available online xxxx 0168-9002/© 2018 Elsevier B.V. All rights reserved.
Please cite this article in press as: A. Compaan, et al., Thin-film neutron detector based on CdTe and 6 Li layers, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.08.086.
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Fig. 1. Detection geometry showing 6 Li activation foil surrounded by two CdTe thin-film detectors. (1.5 μm polymer separation layers between Li and CdTe detectors not shown.) Fig. 2. Two-pixel CdTe detector plate with ∼30 cm2 active area.
of the daughter products of 6 Li (triton, alpha) vs. 10 B (alpha, lithium) result in much longer range of the daughter products that need to be detected by the semiconductor diode. The above considerations, together with our interest in exploiting low-cost, large-area, thin-film deposition technology for the semiconductor detector components have led us to choose 6 Li for the sensitizing layer. In work reported here, we have leveraged recent advances in the technology of thin-film CdTe solar cells which is inexpensive and scalable to larger areas [5]. Furthermore, we have leveraged our capability to fabricate and handle thin sheets of lithium with a rolling mill operating in a dry environment. Most exploratory use of 6 Li for detectors has focused on the use of LiF which does not require handling in a low humidity atmosphere. The number density of Li nuclei in LiF is 1.33 times higher than Li metal, but most importantly the high electron density of LiF leads to much higher stopping power and shorter charged particle ranges for the alpha and triton as they escape the sensitizing layer on the way to detection in the semiconductor diode. Thus, the characteristics of 6 Li metal are particularly attractive for the planar structure of the thin-film CdTe structure. We have procured 95% isotopically enriched 6 Li from Oak Ridge National Lab for this project. The disadvantage of smaller thermal neutron cross section for 6 Li than that of 3 He or 10 B, is partly compensated by the fact that the planar geometry and thin-film structure of the Lithium Innovations’ design facilitates multi-layer stacking for enhanced sensitivity. We report here on our patented [6,7], low-cost, solid-state neutron detector based on the technology of thin-film CdTe solar cells activated with layers of isotopically enriched metallic 6 Li. The technology leverages recent advances in thin-film solar-module manufacturing and lithium sources for electrochromic window fabrication. Our detector avoids the use of expensive and rare 3 He gas as well as scintillator materials that require discrimination against gamma events. It is readily scalable in size from personal radiation monitors to radiation portal monitors.
the CdS/CdTe diode structure followed by a thin gold contact layer. Although a 2 μm CdTe layer is typically used for solar cells and gives complete absorption of visible light, we have found that a ∼20 μm CdTe layer provides excellent charge collection even though the range of a 2.75 MeV triton in CdTe is somewhat larger at 35 μm. We have found that thicker layers of CdTe lead to somewhat poorer charge collection particularly from tritons or alphas that enter the CdTe at large angles and deposit their energies very near the back contact. It should be noted that the CdTe diode structures are polycrystalline when deposited on FTO-coated glass which allows these structures to be scaled to large areas. We have prepared these devices by magnetron sputtering of CdS and CdTe, but for the 15–25 μm thick CdTe layers best for detectors we have found that close-spaced sublimation is faster and more convenient. Substrate temperature during CdS sputtering is typically 250 ◦ C; CdTe deposition is typically done at 550 ◦ C; and the metal back contact at room temperature. All detector structures (glass/TCO/CdS/CdTe) received an ‘‘activation’’ treatment which involves annealing at ∼390 ◦ C for 120 min in air with vapors of CdCl2 . This activation is typically used for producing the highest performance solar cells as well [8–10]. In operation, although this detector structure shows good charge collection without external bias, some improvement is observed with the use of reverse bias of 3 to 18 V. Typically thin-film coatings may exhibit occasional pinholes or isolated regions of ‘‘weak diodes’’ which can contribute to shunt currents that increase the baseline noise in the detector. We have used shunt passivation techniques to achieve shunt currents of less than 10 nA/cm2 at 10 V reverse bias. Thin-film detector structures were prepared as plates having a total area of ∼50 cm2 followed by defining two detector pixels each having an area of 15 cm2 . We have found it advantageous to fabricate on each plate two series-interconnected pixels using standard three-scribe interconnects typical for thin-film CdTe solar cells. In this way we achieve a detector area of 30 cm2 with a capacitance equivalent to that of a 7.5 cm2 pixel, since the series capacitance scales as 1/Cef f = 1/C1 + 1/C2 . Fig. 2 shows the layout of the two detector pixels including two contact pads for the monolithically series-connected pixels. Although we have prepared thinner foils of 6 Li for small area detectors, we have found it convenient for the larger detectors to use 6 Li foils prepared by a rolling mill in a dry nitrogen or argon atmosphere followed by encapsulating the foils with the glass/detector structures
2. Design and fabrication details The basic planar module stack of the Lithium Innovations neutron detector array LiNDA® detector is shown in Fig. 1. The 95% enriched 6 Li layer is surrounded front and back with CdS/CdTe diodes that are essentially modified solar cells. The basic diode structure used for the detector elements consists of a 1 mm soda-lime glass substrate coated with SnO2 :F (FTO) which serves as one electrode, followed by 2
Please cite this article in press as: A. Compaan, et al., Thin-film neutron detector based on CdTe and 6 Li layers, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.08.086.
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Fig. 3. Pulse height data for 5 cm2 detectors with incident thermal neutrons. Left panel: 14 μm CdTe with 9 V bias; right panel: 21 μm CdTe with 18 V bias. Data are shown for both 20 μm and 40 μm 6 Li foils. Note: Data obtained at OSU Research Reactor using a vacuum enclosure and commercial electronics. Thermal neutron flux estimated at ∼4×106 n/cm2 /s. MCA conversion gain ∼100 channel/volt. High energy tails due to pulse pile-up.
on both sides. Hermeticity is assured with a suitable epoxy seal around the glass edges and the four fine wires leading from the two contact pads on each detector ‘‘plate’’. Extended durability studies indicated that if the 6 Li foil is in direct contact with the gold back contacts of the two opposing detectors, some deterioration of performance will occur. Thus a very thin (1.5 μm) polymer separation film was used between the lithium foil and the back contact of the detectors. 3. Detector operation The energy of the charged daughter products of the neutron capture reaction [1] n + 6 Li → 4 He (2.05 MeV) + 3 H (2.75 MeV) is deposited in the CdTe absorber layers after passing through the 1.5 μm separator and entering the CdTe through the ∼15 nm back contacts. The 2.75 MeV triton range in Li is 116 μm and in CdTe is 35 μm; the 2.05 MeV alpha range in Li is 20.3 μm and in CdTe is 6.4 μm. Thus sheets of 6 Li in the range of 40 to 100 μm will allow most of the tritons to enter the CdTe detector and some of the alphas. The thermal neutron range in pure 6 Li is ∼200 μm, so stacks of 2 to 4 modules can be used to achieve high sensitivity. We have been able to scale the area of the detector elements to greater than 30 cm2 using a series connection configuration to achieve good signal-to-noise ratios for the custom pulse-counting electronics. Fig. 2 shows one of the detector plates ready for assembly with a 6 Li neutron activation foil and a second detector plate into a completed module as sketched in Fig. 1. Note that each module is fabricated on glass which provides a good hermetic barrier to protect the lithium metal layer and the CdTe detector structure. The assembly needs only a good edge seal for packaging. This detector structure is based on low-cost polycrystalline thin-film CdTe photovoltaics technologies and can readily be scaled to larger areas with increased responsivity to neutrons. Our modeling of detector performance as a function of 6 Li thickness shows that the optimum thickness of metallic lithium is 80 to 100 μm yielding thermal neutron efficiencies of ∼20% for a single module (single 6 Li foil surrounded by back-to-back detectors). For vertical stacks using several modules, the optimum lithium thickness is less and efficiencies can exceed 50%. See below.
Fig. 4. Radiation-induced changes observed for high activity Po-210 alpha source.
detector. Data in Fig. 3 are shown for both 20 μm and 40 μm thick foils of 6 Li which are both substantially less than the 116 μm range of a 2.75 MeV
triton in lithium. Thus the spectra shown in Fig. 3 show a well-defined but broad peak with a weak tail extending to lower energies. This tail represents tritons that exit the 6 Li non-normal to the surface and so have decreased energies. Note that thicker Li foils yield higher count rates as expected. No attempt was made to calibrate beam flux and detector position from run to run, but clearly visible is the improved signal to noise the results from using 21 μm vs. 14 μm of CdTe as well as increasing the bias voltage from 9 V to 18 V. 3.2. Radiation hardness tests using Po-210 alphas In order to test the radiation hardness of our detector system, Lithium Innovations obtained a 210 Po source (NRD Nuclespot Ionizer) [12]. This source emits a high flux of alpha particles with energy of 5.3 MeV. Note that the charged particles generated in the n +6 Li reaction consist of an alpha with energy 2.05 MeV and a triton with energy 2.75 MeV so the high activity Po-210 source provides an excellent basis for an accelerated life test of our detector. The test was set up to produce a count rate of about 375,000 counts per second. The results are shown in Fig. 4 with data acquired over 720 s for total counts of about 2.6 × 108 . Note that the observed count rate is very stable with a slight decrease over time. We have found that this decrease almost fully recovers if the alpha particle flux is removed. This indicates that the slight decrease is likely not due to radiationinduced damage in the CdTe but may arise from an external effect such as charging on the surface or ionization of air. Thus we confirm that our thin-film detector should have extremely long life in typical personal
3.1. Pulse height spectra for thermal neutron beam Early versions of the CdTe detector were made as 5 cm2 dots using a pogo-pin contact to the gold with a Cremat 111 preamplifier mounted inside a custom aluminum vacuum chamber, followed by an external CR 200 shaping amplifier to feed signals to an Ortec pulse height analyzer (Easy MCA-2K) [11]. A variety of 6 Li foils were mounted on a rotating wheel inside the vacuum chamber approximately 2 cm in front of the 3
Please cite this article in press as: A. Compaan, et al., Thin-film neutron detector based on CdTe and 6 Li layers, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.08.086.
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Fig. 5. Angular dependence of detector response to 252 Cf source at 28 cm. HDPE sheets in front of and behind the detector had thickness of 1.2 cm in front and 1.9 cm behind. Left panel: observed data; right panel: calculated data from MCNP modeling.
radiation monitoring situations and is likely to find applications also in very intense radiation environments. 3.3. Gamma rejection In many monitoring and search operations it is highly desirable to avoid false positives that may be triggered by intense gamma sources that are not special nuclear materials (fissionable sources). Thus, high gamma-ray rejection is often extremely important for neutron detectors. The planar, thin-film geometry of this detector produces extremely low gamma response without depending on pulse-shape discrimination. The high gamma rejection ratio can be understood from the difference between the low energy loss per unit length of ∼MeV electrons compared with those of ∼MeV tritons or alpha particles. The range of a 2.75 MeV triton in CdTe is 35 μm compared with 1230 μm for a 1 MeV electron [2]. Consequently, energetic electrons produced by the photoelectric effect or Compton scattering will deposit very little energy in a 20 μm thick film of CdTe except for the rare trajectory that is oriented parallel to the film plane. These geometrical factors lead to a detector that has excellent discrimination against gammas. Measurements at PNNL and in our lab have shown gamma rejection ratios of ∼3 × 10−8 for 60 Co. In the presence of a neutron source, the gamma-ray absolute rejection ratio for neutrons (GARRn) was measured as 1.00 +∕− 0.0153.
Fig. 6. The picture above depicts test units each consisting of four detector modules in two stacks of two modules. Pre-amplifiers are embedded in the Faraday cage enclosure along with the detectors. Electronics associated with the microcontroller and shaping amplifiers is visible in the upper portion of the photo. Total assembly has 8 detector plates/8 independent channels and 60 cm2 of detector cross section.
and measured results in the left panel. The results are in reasonable agreement. Comparison of simulated results for 1.2 cm thick HDPE moderators vs. 0.5 cm thick moderators clearly shows that the finite thickness of the HDPE moderators needed to produce thermal neutrons limits the angular sensitivity. In a thermal neutron beam or moderated source however, without the need for moderators surrounding the detector, the intrinsic angular sensitivity would be very high. This points toward the possible use of two or three detectors mounted orthogonally to produce a monitor with very high directional sensitivity.
3.4. Angular dependence The planar nature of our thin-film detector leads to a strong angular dependence which could be a very useful feature for some applications. The response was tested with an unmoderated 252 Cf source mounted at 28 cm distance. Measurements were taken with a two-module detector surrounded with two sheets of HDPE moderator. In the front (between the source and detector), we placed a 1.2 cm thick sheet and behind a 1.9 cm sheet of HDPE each with area of 9.5 cm × 13.7 cm, somewhat larger than the two-pixel detector of 5.5 cm × 5.5 cm. Fig. 5 shows the response to an unmoderated 252 Cf source as the detector is rotated 180 degrees. The simulation result in Fig. 5 is based on the following conditions: two detector modules stacked, each with CdTe/Li6/CdTe structure (20 μm/60 μm/20 μm), were placed between two HDPE moderators: one on the zero-degree direction facing the source 9.5 cm × 13.7 cm × 1.2 cm (thick) and the other on the 180 degree direction with 9.5 cm × 13.7 cm × 1.9 cm (thick). A 252 Cf source with a 2.0 ×104 n/s activity was placed 28 cm away at different angles. Our previous simulations have shown that appropriate HDPE thickness to moderate fast neutrons without significantly attenuating thermal neutron yields is around 5 cm to 7 cm thick for HDPE. Fig. 5 shows simulation results in the right panel
4. PRD configuration and electronics A configuration has been assembled of two double-module stacks of 60 cm2 detectors as shown in Fig. 6. This assembly has dimensions close to those of a standard personal radiation detector (PRD). The image shows two preamplifier boards, each with four preamplifier stages, serving the four detectors of each double-module stack directly underneath. (One detector ‘‘module’’ consists of one foil of 6 Li surrounded and sealed top and bottom with the CdTe detectors fabricated on 1 mm glass.) Tests have been performed on both a double-module stack at PNNL with various fission sources and in-house with a 252 Cf source on both a double-module stack and two double-module stacks side by side. The detector with a single, double-module stack consists of two modules vertically stacked with an active area of 30 cm2 . Each module has the structure (glass/TCO/CdS/CdTe/gold/poly/6 Li/poly/gold/CdTe/CdS/TCO/ 4
Please cite this article in press as: A. Compaan, et al., Thin-film neutron detector based on CdTe and 6 Li layers, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.08.086.
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Table 1 Calculated detector efficiency for modules with stacks of 10 μm CdTe/40 μm 6 Li/10 μm CdTe.
Stack 1 Stack 2 Stack 3 Stack 4
Detecting tritons only
Detecting both tritons and alphas
Individual
Cumulative
Individual
Cumulative
0.149 0.117 0.093 0.073
0.149 0.266 0.359 0.432
0.190 0.144 0.110 0.083
0.190 0.334 0.444 0.527
Individual
Cumulative
0.204 0.118 0.068 0.040 0.023
0.204 0.322 0.390 0.430 0.453
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Table 3 Calculated triton detection efficiency with module stacks of 20 μm CdTe/100 μm 6 Li/20 μm CdTe.
Stack 1 Stack 2 Stack 3 Stack 4 Stack 5
Table 2 Calculated triton detection efficiency with module stacks of 20 μm CdTe/80 μm 6 Li/20 μm CdTe.
Stack 1 Stack 2 Stack 3 Stack 4 Stack 5
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Individual
Cumulative
0.211 0.110 0.058 0.030 0.016
0.211 0.321 0.378 0.408 0.424
with 80 μm and 100 μm lithium foils. In addition most of our studies have used CdTe films of 20 μm thickness. Tables 2 and 3 show the modeling for modules composed of 20 μm CdTe layers and 6 Li layers of 80 μm (Table 2) and 100 μm (Table 3). Note that this modeling assumes 100% thermal neutrons incident and shows that for a single module (glass/CdTe/6 Li/CdTe/glass), the efficiency for triton detection rises from 14.9% at 40 μm of Li to 20.4% at 80 μm to 21.1% at 100 μm. By contrast, there is little difference among the two-module stacks for these three thicknesses, all with efficiencies near 32%. For four-module stacks the modeled efficiency for tritons only decreases from 43.2% to 43% to 40.8% for stacks using 40, 80, and 100 μm of 6 Li. Thus, modeling also shows efficiencies above 50% are achievable with stacks using 40 μm sheets of lithium if both alphas and tritons are detected, but lithium sheets in the 80 to 100 μm range give much higher response for one and two-module stacks. Increasing the number of modules with thinner 6 Li and CdTe can yield higher thermal neutron efficiencies but comes at higher overhead of electronics since each double-pixel detector is independently read.
glass), with each of the two thin-film CdTe detectors defined into two, 15 cm2 pixels and series interconnected by laser scribing. ‘‘Poly’’ refers to the 1.5 μm thick polymer separation layers. We have designated a double module stack as LiNDA 2.3. An early version of this configuration was tested at PNNL when one of the four CdTe detector read-outs was not functioning and subsequently tested in our laboratory with a 31 ng 252 Cf source. The results reported here are from our own measurements but are consistent with the PNNL tests. Using a standard PMMA phantom (30 cm × 30 cm × 15 cm) behind the LiNDA 2.3 and the unmoderated source at 25 cm. We find the neutron response to be 0.5 cps/neutron/cm2 s) or equivalently 2.3 × 10−3 cps/ng. This compares favorably with the response from an RDT Domino [13] detector of 5.8 × 10−4 cps/ng. It should be noted that the Domino active area is 4 cm2 compared with the LiNDA 2.3 active area of 30 cm2 . We have not measured directly the response of LiNDA 2.3 to thermal neutrons but estimate it to be approximately 3.1 cps/nv. This thermal neutron response is about 60% of that of a 0.75 inch × 1.5 inch tube filled with 3 He. As discussed below, the responsivity can readily be enhanced by forming stacks of these thin-film detector modules and the fabrication technology is ideal for low-cost manufacturing of larger detector arrays with pixels defined and interconnected by laser scribing similar to methods in common use in the PV industry.
6. Conclusions We have described a new type of semiconductor-based, neutron detector design that leverages recent developments in large-area, thinfilm PV and lithium sources for electrochromic windows. Initial test results have been described for our (LiNDA® ) detector stack with 252 Cf and other spontaneous fission sources. Some of the data were obtained as part of a recent test sequence at the Pacific Northwest National Lab and other data were obtained with our own 252 Cf source. We find that the thermal neutron response of a double detector stack is competitive with and/or exceeds responses reported for other portable detectors such as a small 3 He tube, a CLYC-based scintillator, or a microstructured semiconductor detector. The planar geometry of our detector leads naturally to a very high gamma rejection ratio (∼108 ) and also to strong directional sensitivity. This low-cost, thin-film neutron detector design has excellent potential for a wide range of applications for monitoring and discovery of special nuclear materials. This work was supported in part by the U.S. Department of Homeland Security, Domestic Nuclear Detection Office (USA), under competitively awarded contract numbers HSHQDC-16-C-00042 and HSHQDCN-16C-00010. This support does not constitute an expressed or implied endorsement on the part of the Government.
5. MCNP modeling of detector performance We have modeled the detector performance for a variety of thicknesses of CdTe and 6 Li. The Monte Carlo neutral particle transport code (MCNP6) [14] modeling accounts not only for the propagation of neutrons through the 6 Li and the escape of tritons and alphas from the 6 Li and the energy deposition in the CdTe, but also the loss of thermal neutrons due to the reaction with the 12% abundant 113 Cd isotope. Thicker CdTe layers increase the total energy collected from the ionization produced by the energetic tritons and alphas but also decrease the detector capacitance which is beneficial for lowering the noise input to the preamplifiers. Table 1 summarizes the detection probability separately for alphas and tritons in each of the two 10 μm thick CdTe detectors facing a 6 Li sensitizing foil 40 μm thick. This is a configuration that is nearly ideal for stacking multiple detectors to achieve the highest possible detection efficiency. The Table shows that a stack of four such detector elements should reach 52.7% thermal neutron detection efficiency. Although we have fabricated ∼10 cm2 buttons of 6 Li with thickness as low as 10 μm, fabricating uniform, 50 cm2 and larger lithium sheets with thickness less than 100 μm is challenging to fabricate with our inertatmosphere rolling mill. Therefore we have done most of our studies
References [1] http://www.oecd-nea.org/janisweb/book/neutrons/Si29/MT4/renderer/243. [2] https://www.nist.gov/pml/stopping-power-range-tables-electrons-protons-andhelium-ions. [3] See for example, D.S. McGregor, R.T. Klann, H.K. Gersch, Y.H. Yang, Nucl. Instrum. Methods A 466 (2001) 126. [4] R.G. Fronk, S.L. Bellinger, L.C. Henson, T.R. Ochs, C.T. Smith, J.K. Shultis, D.S. McGregor, Nucl. Instrum. Methods A 804 (2015) 201. [5] See for example, M.A. Green, K. Emery, Y. Hishikawa, W. Warta, E.D. Dunlop, Solar cell efficiency tables (version 47), Progr. Photovolt: Res. Appl. 24 (2016) 3. [6] F.B. Cauffiel, A.D. Compaan, V.V. Plotnikov, P.G. Chamberlin, J.M. Stayancho, A. Shan, US Patent No. 9, 590, 128, Particle detector and method of detecting particles, (3/7/2017). [7] F.B. Cauffiel, A.D Compaan, V.V Plotnikov, A. Shan, A.J Matthews, R.D Fisher, S. Cheng, US Patent No. 9,923,115, Particle detector and method of making the same, (3/20/2018).
5 Please cite this article in press as: A. Compaan, et al., Thin-film neutron detector based on CdTe and 6 Li layers, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.08.086.
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[8] A.D. Compaan, Materials challenges for terrestrial thin-film photovoltaics, J. Mater. (2007) 32–37. [9] S. Pookpanratana, X. Liu, N.R. Paudel, L. Weinhardt, M. Bär, Y. Zhang, A. Ranasinghe, F. Khan, M. Blum, W. Yang, A.D. Compaan, C. Heske, Effects of postdeposition treatments on surfaces of CdTe/CdS solar cells, Appl. Phys. Lett. 97 (2010) 172109.
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[10] N.R. Paudel, K.A. Wieland, A.D. Compaan, Ultrathin CdS/CdTe solar cells by sputtering, Sol. Energy Mater. Sol. Cells 105 (2012) 109–112. [11] Ametek/Ortec, 801 South Illinois Ave, Oak Ridge, TN 37830. [12] Amstat Industries, 25685 Hillview Ct, Mundelein, IL 60060. [13] Radiation Detector Technologies, Inc., 4615 S Dwight Dr, Manhattan, KS 66502. [14] https://mcnp.lanl.gov/.
6 Please cite this article in press as: A. Compaan, et al., Thin-film neutron detector based on CdTe and 6 Li layers, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.08.086.
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Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Thin-film neutron detector based on CdTe and 6 Li layers Alvin Compaan a,b ,∗, Ford B. Cauffiel a , Anthony J. Matthews a , Song Cheng d , Ambalanath Shan c , Robert D. Fisher a , Victor V. Plotnikov b , Ralph Becker a a
Lithium Innovations Co., LLC, 3171 N. Republic Blvd., Toledo, OH 43615, United States Lucintech Inc, 1510 N. Westwood Ave, Toledo, OH 43606, United States 2425 Cheltenham Rd., Toledo, OH 43606, United States d Department of Physics and Astronomy, The University of Toledo, Toledo, OH 43606, United States b c
ARTICLE Keywords: Neutron Detector CdTe Li-6 Thin-film Radiation
INFO
ABSTRACT We report an innovative, low-cost, solid-state neutron detector based on the technology of thin-film CdTe solar cells activated with layers of isotopically enriched metallic Li-6. The technology leverages recent advances in thin-film solar-module manufacturing and lithium sources for electrochromic window fabrication. Our detector inherently has very high rejection of gamma events and strong directional sensitivity; it is readily scalable in size from personal radiation monitors to radiation portal monitors. The thermal neutron response of a double detector stack is competitive with and/or exceeds responses reported for other portable detectors such as a small He-3 tube, a CLYC-based scintillator, or a microstructured semiconductor detector.
Contents 1. 2. 3.
4. 5. 6.
Introduction ....................................................................................................................................................................................................... Design and fabrication details............................................................................................................................................................................... Detector operation .............................................................................................................................................................................................. 3.1. Pulse height spectra for thermal neutron beam............................................................................................................................................ 3.2. Radiation hardness tests using Po-210 alphas.............................................................................................................................................. 3.3. Gamma rejection ..................................................................................................................................................................................... 3.4. Angular dependence ................................................................................................................................................................................ PRD configuration and electronics ........................................................................................................................................................................ MCNP modeling of detector performance .............................................................................................................................................................. Conclusions ........................................................................................................................................................................................................ References..........................................................................................................................................................................................................
1. Introduction Although semiconductor diodes offer many advantages for the detection of high energy photons (X-rays and gamma rays) and energetic charged particles, the detection of neutrons requires the use of sensitizing elements. These sensitizing elements should have nuclei with large neutron reaction cross sections that preferably yield charged particle daughter products. These include 3 He, 6 Li, 10 B, and 157 Gd. 3 He has a large thermal neutron cross section (5330 barns) but as a gas is difficult to couple efficiently to the semiconductor and furthermore is constrained by availability and cost. 157 Gd has a very large thermal neutron cross section (240,000 b) but after neutron capture has a complex decay scheme with multiple gamma rays and three internal conversion electrons with energies of 29, 71, and 78 keV [1] that do
1 2 3 3 3 4 4 4 5 5 5
not facilitate detection of neutrons while discriminating against other sources of gamma rays. Thus, it has been recognized that 10 B and 6 Li are the remaining attractive choices for sensitizing elements to couple with semiconductor detectors. 10 B has the larger thermal neutron capture cross section (3840 barns) but with charged daughter products of 4 He and 7 Li that have relatively low combined energies of 2.8 MeV (4%) or 2.31 MeV (96%). The resulting short transport distances [2] of the reaction particles are not good for planar structures but suggest more complex micro-structuring to achieve high efficiency as has been done, e.g., by McGregor, et al. [3,4]. Thermal neutron capture by 6 Li has a smaller cross section of 940 barns and results in 4 He (alpha) plus 3 H (triton) with much higher combined energy of 4.8 MeV. The larger reaction energy for the 6 Li neutron capture coupled with the smaller nuclear charges
∗ Corresponding author at: Lucintech Inc, 1510 N. Westwood Ave, Toledo, OH 43606, United States. E-mail address: [email protected] (A. Compaan).
https://doi.org/10.1016/j.nima.2018.08.086 Received 31 July 2018; Accepted 23 August 2018 Available online xxxx 0168-9002/© 2018 Elsevier B.V. All rights reserved.
Please cite this article in press as: A. Compaan, et al., Thin-film neutron detector based on CdTe and 6 Li layers, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.08.086.
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Fig. 1. Detection geometry showing 6 Li activation foil surrounded by two CdTe thin-film detectors. (1.5 μm polymer separation layers between Li and CdTe detectors not shown.) Fig. 2. Two-pixel CdTe detector plate with ∼30 cm2 active area.
of the daughter products of 6 Li (triton, alpha) vs. 10 B (alpha, lithium) result in much longer range of the daughter products that need to be detected by the semiconductor diode. The above considerations, together with our interest in exploiting low-cost, large-area, thin-film deposition technology for the semiconductor detector components have led us to choose 6 Li for the sensitizing layer. In work reported here, we have leveraged recent advances in the technology of thin-film CdTe solar cells which is inexpensive and scalable to larger areas [5]. Furthermore, we have leveraged our capability to fabricate and handle thin sheets of lithium with a rolling mill operating in a dry environment. Most exploratory use of 6 Li for detectors has focused on the use of LiF which does not require handling in a low humidity atmosphere. The number density of Li nuclei in LiF is 1.33 times higher than Li metal, but most importantly the high electron density of LiF leads to much higher stopping power and shorter charged particle ranges for the alpha and triton as they escape the sensitizing layer on the way to detection in the semiconductor diode. Thus, the characteristics of 6 Li metal are particularly attractive for the planar structure of the thin-film CdTe structure. We have procured 95% isotopically enriched 6 Li from Oak Ridge National Lab for this project. The disadvantage of smaller thermal neutron cross section for 6 Li than that of 3 He or 10 B, is partly compensated by the fact that the planar geometry and thin-film structure of the Lithium Innovations’ design facilitates multi-layer stacking for enhanced sensitivity. We report here on our patented [6,7], low-cost, solid-state neutron detector based on the technology of thin-film CdTe solar cells activated with layers of isotopically enriched metallic 6 Li. The technology leverages recent advances in thin-film solar-module manufacturing and lithium sources for electrochromic window fabrication. Our detector avoids the use of expensive and rare 3 He gas as well as scintillator materials that require discrimination against gamma events. It is readily scalable in size from personal radiation monitors to radiation portal monitors.
the CdS/CdTe diode structure followed by a thin gold contact layer. Although a 2 μm CdTe layer is typically used for solar cells and gives complete absorption of visible light, we have found that a ∼20 μm CdTe layer provides excellent charge collection even though the range of a 2.75 MeV triton in CdTe is somewhat larger at 35 μm. We have found that thicker layers of CdTe lead to somewhat poorer charge collection particularly from tritons or alphas that enter the CdTe at large angles and deposit their energies very near the back contact. It should be noted that the CdTe diode structures are polycrystalline when deposited on FTO-coated glass which allows these structures to be scaled to large areas. We have prepared these devices by magnetron sputtering of CdS and CdTe, but for the 15–25 μm thick CdTe layers best for detectors we have found that close-spaced sublimation is faster and more convenient. Substrate temperature during CdS sputtering is typically 250 ◦ C; CdTe deposition is typically done at 550 ◦ C; and the metal back contact at room temperature. All detector structures (glass/TCO/CdS/CdTe) received an ‘‘activation’’ treatment which involves annealing at ∼390 ◦ C for 120 min in air with vapors of CdCl2 . This activation is typically used for producing the highest performance solar cells as well [8–10]. In operation, although this detector structure shows good charge collection without external bias, some improvement is observed with the use of reverse bias of 3 to 18 V. Typically thin-film coatings may exhibit occasional pinholes or isolated regions of ‘‘weak diodes’’ which can contribute to shunt currents that increase the baseline noise in the detector. We have used shunt passivation techniques to achieve shunt currents of less than 10 nA/cm2 at 10 V reverse bias. Thin-film detector structures were prepared as plates having a total area of ∼50 cm2 followed by defining two detector pixels each having an area of 15 cm2 . We have found it advantageous to fabricate on each plate two series-interconnected pixels using standard three-scribe interconnects typical for thin-film CdTe solar cells. In this way we achieve a detector area of 30 cm2 with a capacitance equivalent to that of a 7.5 cm2 pixel, since the series capacitance scales as 1/Cef f = 1/C1 + 1/C2 . Fig. 2 shows the layout of the two detector pixels including two contact pads for the monolithically series-connected pixels. Although we have prepared thinner foils of 6 Li for small area detectors, we have found it convenient for the larger detectors to use 6 Li foils prepared by a rolling mill in a dry nitrogen or argon atmosphere followed by encapsulating the foils with the glass/detector structures
2. Design and fabrication details The basic planar module stack of the Lithium Innovations neutron detector array LiNDA® detector is shown in Fig. 1. The 95% enriched 6 Li layer is surrounded front and back with CdS/CdTe diodes that are essentially modified solar cells. The basic diode structure used for the detector elements consists of a 1 mm soda-lime glass substrate coated with SnO2 :F (FTO) which serves as one electrode, followed by 2
Please cite this article in press as: A. Compaan, et al., Thin-film neutron detector based on CdTe and 6 Li layers, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.08.086.
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Fig. 3. Pulse height data for 5 cm2 detectors with incident thermal neutrons. Left panel: 14 μm CdTe with 9 V bias; right panel: 21 μm CdTe with 18 V bias. Data are shown for both 20 μm and 40 μm 6 Li foils. Note: Data obtained at OSU Research Reactor using a vacuum enclosure and commercial electronics. Thermal neutron flux estimated at ∼4×106 n/cm2 /s. MCA conversion gain ∼100 channel/volt. High energy tails due to pulse pile-up.
on both sides. Hermeticity is assured with a suitable epoxy seal around the glass edges and the four fine wires leading from the two contact pads on each detector ‘‘plate’’. Extended durability studies indicated that if the 6 Li foil is in direct contact with the gold back contacts of the two opposing detectors, some deterioration of performance will occur. Thus a very thin (1.5 μm) polymer separation film was used between the lithium foil and the back contact of the detectors. 3. Detector operation The energy of the charged daughter products of the neutron capture reaction [1] n + 6 Li → 4 He (2.05 MeV) + 3 H (2.75 MeV) is deposited in the CdTe absorber layers after passing through the 1.5 μm separator and entering the CdTe through the ∼15 nm back contacts. The 2.75 MeV triton range in Li is 116 μm and in CdTe is 35 μm; the 2.05 MeV alpha range in Li is 20.3 μm and in CdTe is 6.4 μm. Thus sheets of 6 Li in the range of 40 to 100 μm will allow most of the tritons to enter the CdTe detector and some of the alphas. The thermal neutron range in pure 6 Li is ∼200 μm, so stacks of 2 to 4 modules can be used to achieve high sensitivity. We have been able to scale the area of the detector elements to greater than 30 cm2 using a series connection configuration to achieve good signal-to-noise ratios for the custom pulse-counting electronics. Fig. 2 shows one of the detector plates ready for assembly with a 6 Li neutron activation foil and a second detector plate into a completed module as sketched in Fig. 1. Note that each module is fabricated on glass which provides a good hermetic barrier to protect the lithium metal layer and the CdTe detector structure. The assembly needs only a good edge seal for packaging. This detector structure is based on low-cost polycrystalline thin-film CdTe photovoltaics technologies and can readily be scaled to larger areas with increased responsivity to neutrons. Our modeling of detector performance as a function of 6 Li thickness shows that the optimum thickness of metallic lithium is 80 to 100 μm yielding thermal neutron efficiencies of ∼20% for a single module (single 6 Li foil surrounded by back-to-back detectors). For vertical stacks using several modules, the optimum lithium thickness is less and efficiencies can exceed 50%. See below.
Fig. 4. Radiation-induced changes observed for high activity Po-210 alpha source.
detector. Data in Fig. 3 are shown for both 20 μm and 40 μm thick foils of 6 Li which are both substantially less than the 116 μm range of a 2.75 MeV
triton in lithium. Thus the spectra shown in Fig. 3 show a well-defined but broad peak with a weak tail extending to lower energies. This tail represents tritons that exit the 6 Li non-normal to the surface and so have decreased energies. Note that thicker Li foils yield higher count rates as expected. No attempt was made to calibrate beam flux and detector position from run to run, but clearly visible is the improved signal to noise the results from using 21 μm vs. 14 μm of CdTe as well as increasing the bias voltage from 9 V to 18 V. 3.2. Radiation hardness tests using Po-210 alphas In order to test the radiation hardness of our detector system, Lithium Innovations obtained a 210 Po source (NRD Nuclespot Ionizer) [12]. This source emits a high flux of alpha particles with energy of 5.3 MeV. Note that the charged particles generated in the n +6 Li reaction consist of an alpha with energy 2.05 MeV and a triton with energy 2.75 MeV so the high activity Po-210 source provides an excellent basis for an accelerated life test of our detector. The test was set up to produce a count rate of about 375,000 counts per second. The results are shown in Fig. 4 with data acquired over 720 s for total counts of about 2.6 × 108 . Note that the observed count rate is very stable with a slight decrease over time. We have found that this decrease almost fully recovers if the alpha particle flux is removed. This indicates that the slight decrease is likely not due to radiationinduced damage in the CdTe but may arise from an external effect such as charging on the surface or ionization of air. Thus we confirm that our thin-film detector should have extremely long life in typical personal
3.1. Pulse height spectra for thermal neutron beam Early versions of the CdTe detector were made as 5 cm2 dots using a pogo-pin contact to the gold with a Cremat 111 preamplifier mounted inside a custom aluminum vacuum chamber, followed by an external CR 200 shaping amplifier to feed signals to an Ortec pulse height analyzer (Easy MCA-2K) [11]. A variety of 6 Li foils were mounted on a rotating wheel inside the vacuum chamber approximately 2 cm in front of the 3
Please cite this article in press as: A. Compaan, et al., Thin-film neutron detector based on CdTe and 6 Li layers, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.08.086.
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Fig. 5. Angular dependence of detector response to 252 Cf source at 28 cm. HDPE sheets in front of and behind the detector had thickness of 1.2 cm in front and 1.9 cm behind. Left panel: observed data; right panel: calculated data from MCNP modeling.
radiation monitoring situations and is likely to find applications also in very intense radiation environments. 3.3. Gamma rejection In many monitoring and search operations it is highly desirable to avoid false positives that may be triggered by intense gamma sources that are not special nuclear materials (fissionable sources). Thus, high gamma-ray rejection is often extremely important for neutron detectors. The planar, thin-film geometry of this detector produces extremely low gamma response without depending on pulse-shape discrimination. The high gamma rejection ratio can be understood from the difference between the low energy loss per unit length of ∼MeV electrons compared with those of ∼MeV tritons or alpha particles. The range of a 2.75 MeV triton in CdTe is 35 μm compared with 1230 μm for a 1 MeV electron [2]. Consequently, energetic electrons produced by the photoelectric effect or Compton scattering will deposit very little energy in a 20 μm thick film of CdTe except for the rare trajectory that is oriented parallel to the film plane. These geometrical factors lead to a detector that has excellent discrimination against gammas. Measurements at PNNL and in our lab have shown gamma rejection ratios of ∼3 × 10−8 for 60 Co. In the presence of a neutron source, the gamma-ray absolute rejection ratio for neutrons (GARRn) was measured as 1.00 +∕− 0.0153.
Fig. 6. The picture above depicts test units each consisting of four detector modules in two stacks of two modules. Pre-amplifiers are embedded in the Faraday cage enclosure along with the detectors. Electronics associated with the microcontroller and shaping amplifiers is visible in the upper portion of the photo. Total assembly has 8 detector plates/8 independent channels and 60 cm2 of detector cross section.
and measured results in the left panel. The results are in reasonable agreement. Comparison of simulated results for 1.2 cm thick HDPE moderators vs. 0.5 cm thick moderators clearly shows that the finite thickness of the HDPE moderators needed to produce thermal neutrons limits the angular sensitivity. In a thermal neutron beam or moderated source however, without the need for moderators surrounding the detector, the intrinsic angular sensitivity would be very high. This points toward the possible use of two or three detectors mounted orthogonally to produce a monitor with very high directional sensitivity.
3.4. Angular dependence The planar nature of our thin-film detector leads to a strong angular dependence which could be a very useful feature for some applications. The response was tested with an unmoderated 252 Cf source mounted at 28 cm distance. Measurements were taken with a two-module detector surrounded with two sheets of HDPE moderator. In the front (between the source and detector), we placed a 1.2 cm thick sheet and behind a 1.9 cm sheet of HDPE each with area of 9.5 cm × 13.7 cm, somewhat larger than the two-pixel detector of 5.5 cm × 5.5 cm. Fig. 5 shows the response to an unmoderated 252 Cf source as the detector is rotated 180 degrees. The simulation result in Fig. 5 is based on the following conditions: two detector modules stacked, each with CdTe/Li6/CdTe structure (20 μm/60 μm/20 μm), were placed between two HDPE moderators: one on the zero-degree direction facing the source 9.5 cm × 13.7 cm × 1.2 cm (thick) and the other on the 180 degree direction with 9.5 cm × 13.7 cm × 1.9 cm (thick). A 252 Cf source with a 2.0 ×104 n/s activity was placed 28 cm away at different angles. Our previous simulations have shown that appropriate HDPE thickness to moderate fast neutrons without significantly attenuating thermal neutron yields is around 5 cm to 7 cm thick for HDPE. Fig. 5 shows simulation results in the right panel
4. PRD configuration and electronics A configuration has been assembled of two double-module stacks of 60 cm2 detectors as shown in Fig. 6. This assembly has dimensions close to those of a standard personal radiation detector (PRD). The image shows two preamplifier boards, each with four preamplifier stages, serving the four detectors of each double-module stack directly underneath. (One detector ‘‘module’’ consists of one foil of 6 Li surrounded and sealed top and bottom with the CdTe detectors fabricated on 1 mm glass.) Tests have been performed on both a double-module stack at PNNL with various fission sources and in-house with a 252 Cf source on both a double-module stack and two double-module stacks side by side. The detector with a single, double-module stack consists of two modules vertically stacked with an active area of 30 cm2 . Each module has the structure (glass/TCO/CdS/CdTe/gold/poly/6 Li/poly/gold/CdTe/CdS/TCO/ 4
Please cite this article in press as: A. Compaan, et al., Thin-film neutron detector based on CdTe and 6 Li layers, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.08.086.
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Table 1 Calculated detector efficiency for modules with stacks of 10 μm CdTe/40 μm 6 Li/10 μm CdTe.
Stack 1 Stack 2 Stack 3 Stack 4
Detecting tritons only
Detecting both tritons and alphas
Individual
Cumulative
Individual
Cumulative
0.149 0.117 0.093 0.073
0.149 0.266 0.359 0.432
0.190 0.144 0.110 0.083
0.190 0.334 0.444 0.527
Individual
Cumulative
0.204 0.118 0.068 0.040 0.023
0.204 0.322 0.390 0.430 0.453
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Table 3 Calculated triton detection efficiency with module stacks of 20 μm CdTe/100 μm 6 Li/20 μm CdTe.
Stack 1 Stack 2 Stack 3 Stack 4 Stack 5
Table 2 Calculated triton detection efficiency with module stacks of 20 μm CdTe/80 μm 6 Li/20 μm CdTe.
Stack 1 Stack 2 Stack 3 Stack 4 Stack 5
)
Individual
Cumulative
0.211 0.110 0.058 0.030 0.016
0.211 0.321 0.378 0.408 0.424
with 80 μm and 100 μm lithium foils. In addition most of our studies have used CdTe films of 20 μm thickness. Tables 2 and 3 show the modeling for modules composed of 20 μm CdTe layers and 6 Li layers of 80 μm (Table 2) and 100 μm (Table 3). Note that this modeling assumes 100% thermal neutrons incident and shows that for a single module (glass/CdTe/6 Li/CdTe/glass), the efficiency for triton detection rises from 14.9% at 40 μm of Li to 20.4% at 80 μm to 21.1% at 100 μm. By contrast, there is little difference among the two-module stacks for these three thicknesses, all with efficiencies near 32%. For four-module stacks the modeled efficiency for tritons only decreases from 43.2% to 43% to 40.8% for stacks using 40, 80, and 100 μm of 6 Li. Thus, modeling also shows efficiencies above 50% are achievable with stacks using 40 μm sheets of lithium if both alphas and tritons are detected, but lithium sheets in the 80 to 100 μm range give much higher response for one and two-module stacks. Increasing the number of modules with thinner 6 Li and CdTe can yield higher thermal neutron efficiencies but comes at higher overhead of electronics since each double-pixel detector is independently read.
glass), with each of the two thin-film CdTe detectors defined into two, 15 cm2 pixels and series interconnected by laser scribing. ‘‘Poly’’ refers to the 1.5 μm thick polymer separation layers. We have designated a double module stack as LiNDA 2.3. An early version of this configuration was tested at PNNL when one of the four CdTe detector read-outs was not functioning and subsequently tested in our laboratory with a 31 ng 252 Cf source. The results reported here are from our own measurements but are consistent with the PNNL tests. Using a standard PMMA phantom (30 cm × 30 cm × 15 cm) behind the LiNDA 2.3 and the unmoderated source at 25 cm. We find the neutron response to be 0.5 cps/neutron/cm2 s) or equivalently 2.3 × 10−3 cps/ng. This compares favorably with the response from an RDT Domino [13] detector of 5.8 × 10−4 cps/ng. It should be noted that the Domino active area is 4 cm2 compared with the LiNDA 2.3 active area of 30 cm2 . We have not measured directly the response of LiNDA 2.3 to thermal neutrons but estimate it to be approximately 3.1 cps/nv. This thermal neutron response is about 60% of that of a 0.75 inch × 1.5 inch tube filled with 3 He. As discussed below, the responsivity can readily be enhanced by forming stacks of these thin-film detector modules and the fabrication technology is ideal for low-cost manufacturing of larger detector arrays with pixels defined and interconnected by laser scribing similar to methods in common use in the PV industry.
6. Conclusions We have described a new type of semiconductor-based, neutron detector design that leverages recent developments in large-area, thinfilm PV and lithium sources for electrochromic windows. Initial test results have been described for our (LiNDA® ) detector stack with 252 Cf and other spontaneous fission sources. Some of the data were obtained as part of a recent test sequence at the Pacific Northwest National Lab and other data were obtained with our own 252 Cf source. We find that the thermal neutron response of a double detector stack is competitive with and/or exceeds responses reported for other portable detectors such as a small 3 He tube, a CLYC-based scintillator, or a microstructured semiconductor detector. The planar geometry of our detector leads naturally to a very high gamma rejection ratio (∼108 ) and also to strong directional sensitivity. This low-cost, thin-film neutron detector design has excellent potential for a wide range of applications for monitoring and discovery of special nuclear materials. This work was supported in part by the U.S. Department of Homeland Security, Domestic Nuclear Detection Office (USA), under competitively awarded contract numbers HSHQDC-16-C-00042 and HSHQDCN-16C-00010. This support does not constitute an expressed or implied endorsement on the part of the Government.
5. MCNP modeling of detector performance We have modeled the detector performance for a variety of thicknesses of CdTe and 6 Li. The Monte Carlo neutral particle transport code (MCNP6) [14] modeling accounts not only for the propagation of neutrons through the 6 Li and the escape of tritons and alphas from the 6 Li and the energy deposition in the CdTe, but also the loss of thermal neutrons due to the reaction with the 12% abundant 113 Cd isotope. Thicker CdTe layers increase the total energy collected from the ionization produced by the energetic tritons and alphas but also decrease the detector capacitance which is beneficial for lowering the noise input to the preamplifiers. Table 1 summarizes the detection probability separately for alphas and tritons in each of the two 10 μm thick CdTe detectors facing a 6 Li sensitizing foil 40 μm thick. This is a configuration that is nearly ideal for stacking multiple detectors to achieve the highest possible detection efficiency. The Table shows that a stack of four such detector elements should reach 52.7% thermal neutron detection efficiency. Although we have fabricated ∼10 cm2 buttons of 6 Li with thickness as low as 10 μm, fabricating uniform, 50 cm2 and larger lithium sheets with thickness less than 100 μm is challenging to fabricate with our inertatmosphere rolling mill. Therefore we have done most of our studies
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5 Please cite this article in press as: A. Compaan, et al., Thin-film neutron detector based on CdTe and 6 Li layers, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.08.086.
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6 Please cite this article in press as: A. Compaan, et al., Thin-film neutron detector based on CdTe and 6 Li layers, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.08.086.