Gamma Rays Rejection in a Gadolinium Based Semiconductor Neutron Detector

Gamma Rays Rejection in a Gadolinium based Semiconductor Neutron Detector

Dissertation

Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University

Praneeth Kandlakunta, M.S.

Graduate Program in Nuclear Engineering

The Ohio State University

2014

Dissertation Committee:

Prof. Dr. Lei Cao, Advisor

Prof. Dr. Don Miller

Prof. Dr. Thomas Blue

Prof. Dr. Shaurya Prakash

Praneeth Kandlakunta

2014

Abstract

Gadolinium (Gd) is a competent neutron conversion material due to its extremely large neutron capture cross-section, which makes it an attractive choice for thermal neutron detection as well as certain medical applications, such as Gd neutron capture therapy (GdNCT). However, the principal secondary particles that generate electron-hole

(e-h) pairs in a semiconductor detector following Gd neutron capture are low energy internal conversion electrons (ICEs). Detailed information about the low energy electron spectrum emitted after Gd neutron capture is fundamental for evaluating the conversion efficiency of Gd for neutron detection as well as accurately determining dose delivery to the target and healthy tissues in GdNCT.

However, the suitability of Gd for neutron conversion over other competing materials such as boron (B) and lithium (Li) is still debated owing to issues associated with the low energy of ICEs and high gamma interaction probability of Gd. The detection of low energy ICEs, although emitted in abundance, is prone to be interfered by external and/or internal gamma rays, such as the activated 43 keV K-X rays, given the high atomic number (Z) of Gd. A method for separation of gamma rays is thus highly essential when

Gd is used in the format of thin film semiconductor detector for neutron detection.

The objectives of this research are, to study the feasibility of using Gd for neutron detection, and to develop a gamma ray rejection scheme for a Gd based semiconductor ii neutron detector and investigate the efficacy of the rejection scheme for separation of gamma rays. In this dissertation, a gamma ray rejection scheme designed using two identical semiconductor detectors (twin-detector), Gd neutron conversion layer and polyethylene electron separator has been investigated. Monte Carlo (MC) simulations of neutron and gamma ray interaction with the twin-detector structure were extensively performed. The simulation results validated the hypothesis of the rejection method and demonstrated effective separation of neutron induced ICEs from gamma rays. A comprehensive set of experiments were performed to evaluate the neutron and gamma sensitivity of Gd and test the practicability of the proposed neutron-gamma (n-γ) separation method. The experimental results agreed well with simulations and supported the feasibility of the gamma rejection scheme. Results further established the suitability of Gd for neutron detection, indicating a neutron sensitivity much superior to its gamma sensitivity, and demonstrated the effectiveness of n-γ separation using the proposed method.

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Dedication

This document is dedicated to my parents and my beloved sister.

Acknowledgments

I would like to express my deep gratitude to Dr. Lei Cao, my research advisor, for his invaluable guidance and suggestions, and for his constant encouragement and support throughout the course of my research at The Ohio State University. I am also thankful to

Dr. Cao for the many productive discussions we had on research topics.

I sincerely thank Dr. Don Miller, Dr. Thomas Blue and Dr. Shaurya Prakash for their constructive inputs to my research and for serving on my dissertation committee.

My special appreciation goes to the staff at The Ohio State University Nuclear

Reactor Laboratory and Dr. Jie Qiu for their assistance in performing experiments at the reactor. I am grateful also to Dr. Greg Downing for his assistance in performing experiments at the cold neutron depth profiling facility at the NIST Center for Neutron

Research.

I would also like to thank the faculty of Nuclear Engineering Program, Dr. Xiaodong

Sun, Dr. Tunc Aldemir and Prof. Brian Hajek, and also Rob Tayloe for their valuable teachings, which motivated my learning.

I wish to extend my sincere appreciation to my classmates Danyal Turkoglu,

Padhraic Mulligan, Jinghui Wang and others in the Nuclear Engineering Program for their timely help and assistance.

I also wish to thank all my friends who stood by and supported me in my educational and research endeavors. Finally, I am very deeply grateful to my parents for their love, and their continuous support and encouragement throughout my academic and research work, without which I could not have made it this far.

Vita

December 2008………………....B.E.(Hons.), Birla Institute of Technology and Science–

Pilani, India.

June 2012……………………….M.S., The Ohio State University

September 2010 to present……..Graduate Research Associate, Nuclear Engineering

Program, Department of Mechanical and Aerospace

Engineering, The Ohio State University

Publications

Journal articles

 P. Kandlakunta, L.R. Cao, “Neutron Conversion Efficiency and Gamma Interference with Using Gadolinium”, Journal of Radioanalytical and Nuclear Chemistry (under review).

 Kandlakunta, P., Cao, L. R., Mulligan, P. “Measurement of internal conversion electrons from Gd neutron capture.” Nuclear Instruments and Methods in Physics Research Section A, 705, 36 (2013).

 Praneeth Kandlakunta, Lei Cao, "Gamma-Ray Rejection, or Detection, with Gadolinium as a Converter," Radiation Protection Dosimetry, 151 (3), 2012, 586- 590.

Conference proceedings (peer reviewed):

 Lei R. Cao, Praneeth Kandlakunta, "Measure Internal Conversion Electron Spectrum of Gadolinium Neutron Capture Using Neutron Beam." In: Transactions of American Nuclear Society, (Aug 2013) 108, p.267 - 269.

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 Praneeth Kandlakunta, Padhraic Mulligan, Danyal Turkoglu, Lei Cao, “A Neutron Flux Monitor for a Reactor Neutron Beam Facility”, IEEE Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC) Record, 2012, Anaheim, CA, USA.

 Praneeth Kandlakunta, Danyal Turkoglu, Padhraic Mulligan, Lei Cao, "A Neutron Beam Monitor for a Neutron Depth Profiling Facility." American Nuclear Society Annual Meeting 2012, Chicago, IL, USA.

 J. Ralston, P. Kandlakunta, L. Cao, "Electron Emission Following 157Gd Neutron Capture." American Nuclear Society Annual Meeting 2012, Chicago, IL, USA.

 Praneeth Kandlakunta, Lei Cao, "A Neutron Detector with Gamma Discrimination." In: Transactions of the American Nuclear Society. Vol. 105. Washington, D.C., USA. (2011):335-336.

 Padhraic L. Mulligan, Danyal J. Turkoglu, Praneeth Kandlakunta, Lei Cao, "Improving Neutron Depth Profiling at the Ohio State University Using Multiple Detectors." In: Transactions of the American Nuclear Society. Vol. 104. Hollywood, FL, USA (2011): 227-229.

 Jinghui Wang, Praneeth Kandlakunta, Thomas F. Kent, John Carlin, Daniel R. Hoy, Roberto C. Myers, Lei Cao, "A Gadolinium Doped Superlattice GaN Schottky Diode for Neutron Detection." In: Transactions of the American Nuclear Society. Vol. 104. Hollywood, FL, USA (2011): 207-209.

 D. Turkoglu, P. Kandlakunta, P. Mulligan, L. Cao, J. Zhang, B.T. Sang, R.G. Downing, “Development of a Neutron Depth Profiling Facility for Characterizing Advanced Reactor Materials,” Summer Meeting of the American Nuclear Society, ANS transactions vol. 103, Hollywood, FL, U.S.A., June 26-30, 2011.

 D. Turkoglu, J. Burke, P. Kandlakunta, L. Cao, “Development of an External Neutron Beam Facility at The Ohio State University,” 13th International Conference on Modern Trends in Activation Analysis, College Station, TX, U.S.A., March 13-18, 2011.

 D. Turkoglu, J. Strah, P. Kandlakunta, L. Cao, "Development of an External Neutron Beam Facility at the Ohio State University." In: Transactions of the American Nuclear Society. Vol.102. Las Vegas, NV, USA (2010).

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Fields of Study

Major Field: Nuclear Engineering

Table of Contents

Abstract ...... ii Dedication ...... iv Acknowledgments...... v Vita ...... vii List of Tables ...... xiii List of Figures ...... xiv Chapter 1. Introduction ...... 1 1.1. Background ...... 2 1.2. Neutron detection ...... 4 1.3. Solid-state neutron detectors ...... 4 1.4. Neutron converter materials ...... 7 1.5. Summary ...... 10 Chapter 2. Interaction of neutrons with Gd ...... 11 2.1. Prompt gamma rays ...... 12 2.2. Internal conversion electrons (ICEs) ...... 16 2.3. Energy spectrum of Gd(n,γ)Gd* reaction products in a thin film semiconductor 21 2.4. Range of IC electrons (ICEs) ...... 24 2.5. Optimal thickness of Gd for neutron converter coating ...... 31 2.6. Summary ...... 36 Chapter 3. Interaction of gamma rays with Gd ...... 38 3.1. Evaluation of photoelectric interaction of Gd ...... 39 3.1.1. Experimental setup...... 41 3.1.2. Results and discussion ...... 44 3.2. Summary ...... 46 Chapter 4. Gamma ray rejection technique ...... 48 x

4.1. Literature review of gamma ray discrimination techniques ...... 49 4.1.1. Pulse shape discrimination (PSD) ...... 49 4.1.1.1. Rise-time discrimination...... 51 4.1.1.2. Charge comparison ...... 52 4.1.1.3. Comparison of the two techniques ...... 53 4.1.1.4. PSD in semiconductors ...... 54 4.1.1.5. Advantages and limitations ...... 55 4.1.2. Pulse height discrimination ...... 56 4.1.3. Coincidence detection method ...... 58 4.1.4. Spectrum subtraction technique ...... 59 4.2. Proposed gamma ray discrimination scheme ...... 61 4.3. Summary ...... 63 Chapter 5. Simulation of the gamma rejection method ...... 64 5.1. Neutron interaction ...... 64 5.2. Gamma ray interaction ...... 66 5.3. Summary ...... 71 Chapter 6. Experimental study ...... 72 6.1. Evaluation of neutron sensitivity of gadolinium ...... 72 6.1.1. Measurement of the ICE energy spectrum ...... 73 6.1.1.1. Experimental setup ...... 74 6.1.1.2. Results and discussion ...... 75 6.1.2. Experimental validation of optimal thickness of Gd ...... 80 6.1.3. Comparison of Gd, B and Li neutron reaction energy deposition rates...... 83 6.2. Evaluation of gamma ray sensitivity of gadolinium ...... 87 6.3. Evaluation of the gamma rejection scheme ...... 91 6.3.1. Experiments in gamma radiation field ...... 91 6.3.2. Experiment in mixed beta-gamma (β-γ) radiation field ...... 94 6.3.3. Experiment in mixed n-γ radiation field ...... 96 6.4. Further study ...... 101 6.4.1. Experiments in a low intensity neutron field ...... 102

6.4.1.1. Experimental setup ...... 102 6.4.1.2. PuBe source characterization ...... 105 6.4.2. Depth profiling using ICEs ...... 109 6.5. Summary ...... 113 Chapter 7. Conclusions ...... 116 References ...... 119 Appendix A: Detector calibration for identifying the ICE peak energies ...... 129 Appendix B: Attenuation of gamma rays in polyethylene ...... 132 Appendix C: Si PIN photodiode for Gd ICE measurement ...... 134 C.1. Experimental setup ...... 136 C.2. 241Am measurement ...... 137 C.3. 57Co measurement...... 139 C.4. 14C measurement ...... 140 C.5. Summary ...... 141 Appendix D: Evaluation of ICE escape efficiency ...... 142 D.1. C++ program to generate MCNP5 input file ...... 142 D.2. MCNP5 input file ...... 145 D.3. Matlab script for evaluating the optimal thickness ...... 149 Appendix E: MCNP5 input for gamma rejection simulation ...... 151

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List of Tables

Table 1. A comparison of nuclear reaction energies, isotopic and elemental neutron capture cross sections of the most common neutron converter materials [37, 45-47]...... 8 Table 2. List of characteristic x-rays emitted during the atomic relaxation of Gd ...... 17 Table 3. List of nuclear excitation energy levels, prompt gamma ray energies and intensities, and IC coefficients of 158Gd* and 156Gd*...... 19 Table 4. List of prompt gamma ray energies and corresponding ICE intensities of 158Gd* and 156Gd*...... 20 Table 5. Comparison of calculated ICE intensities of Gd with those from the literature. 20 Table 6. Energy-ranges of ICEs emitted in Gd neutron capture in Si, Gd, and polyethylene according to various range definitions (all units in µm)...... 28 Table 7. Specifications of the Si detector and the 57Co gamma ray source used in the Gd gamma ray activation experiment...... 41 Table 8. Specifications of the Si charged particle detectors used in the ICE measurements...... 75 Table 9. List of specifications, characteristic parameters and energy deposition rates of Gd, B and Li compounds used in the experiments...... 86 Table 10. List of different measurements performed to evaluate the gamma ray sensitivity of a Gd based semiconductor detector...... 88 Table 11. PuBe nuclide concentrations in grams used in Origen-Arp calculation...... 107 Table 12. Specifications of the Si PIN photodiode ...... 134 Table 13. Pulse processing settings applied in the measurement of 241Am, 57Co and 14C energy spectra...... 137

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List of Figures

Fig. 1. Schematic illustration of indirect-conversion (left) and direct-conversion (right) configurations of semiconductor neutron detectors...... 5 Fig. 2. Geometry used in SWORD simulation indicating the thermal neutron beam, Gd foil and HPGe detector assembly...... 12 Fig. 3. Energy spectrum of Gd* prompt gamma rays obtained in a HPGe detector using SWORD simulation...... 13 Fig. 4. Experimental standard energy spectrum of neutron induced prompt gamma rays from natural Gd (reproduced from Revay, 2004 [54])...... 13 Fig. 5. Comparison of the SWORD simulation result (right) with the experimental prompt gamma ray spectrum of Gd (left) in the energy range (a) ≤ 300 keV (b) 600-1000 keV (c) 1000-1400 keV ...... 14 Fig. 6. Schematic illustration of the three lowest and primary de-excitation paths of 158Gd* and 156Gd* leading to prompt gamma ray and ICE emissions...... 18 Fig. 7. Geometry used in the MCNP5 model to observe ICE energy spectrum in a Si detector...... 21 Fig. 8. Energy spectrum from a Si detector in response to neutron activation of Gd obtained using F8 pulse height tally in MCNP5...... 22 Fig. 9. Energy spectrum of electrons in a Si detector obtained using a volumetric isotropic source of ICEs with uniform distribution across a Gd foil in MCNP5...... 23 Fig. 10. Energy spectrum from a Si detector indicating the characteristic ICE peaks obtained using SWORD simulation, and compared with the MCNP5 result...... 24 Fig. 11. Left: Electron transmission (ηT) plotted against electron beam energy (E) for constant film thickness [62]. Right: Electron transmission (ηT) plotted against film thickness (t) for constant electron beam energy [60]...... 25 Fig. 12. Graphic showing the penetration of 71 keV ICEs in a 50 µm thick Si slab in CASINO simulation. Trajectories in blue correspond to the transmitted whereas those in red correspond to the backscattered electrons...... 29 Fig. 13. Graphic showing the penetration of 71 keV ICEs in a 50 µm thick Si slab in PENELOPE calculation...... 30 Fig. 14. Distribution of the MPD of 71 keV ICEs in Si obtained using CASINO simulation, and average path length of these electrons in Si computed using PENELOPE...... 30 Fig. 15. Schematic illustration of the geometry used in MCNP5 simulation to compute the total number of electrons escaping from each layer of a thick Gd slab...... 33

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Fig. 16. A schematic representation of the number of Gd layers penetrated by electrons starting from the neutron capture sites in order to reach the detector, in both forward emission and backward emission scenarios...... 34 Fig. 17. ICE escape efficiency calculated using the method outlined in this section for neutron detection in front- and back-illumination cases. As shown in the inset, the orientation of the detector with respect to the neutron path shows the corresponding efficiency curve...... 35 Fig. 18. Photon interaction cross sections for Gd generated using PENELOPE [64]. .... 38 Fig. 19. Energy spectra of the Gd covered and bare Si detectors in response to 122 keV gamma rays obtained using MCNP5 simulation...... 40 Fig. 20. Vacuum instrumentation facility in the Nuclear Analysis and Radiation Sensor (NARS) laboratory at The Ohio State University...... 42 Fig. 21. Al base plate inside the vacuum chamber with multiple detector and source mounts ...... 42 Fig. 22. Block diagram of the instrumentation system in NARS lab showing the vacuum chamber, the dry vacuum pump and the components of the digital DAQ system...... 43 Fig. 23. Experimental geometry inside the vacuum chamber showing the 57Co gamma ray source, Gd foil, polyethylene cap and two small area Si charged particle detectors. . 44 Fig. 24. Energy spectra of Gd covered and polyethylene covered Si detectors obtained in response to 57Co gamma rays...... 45 Fig. 25. Gamma ray rejection scheme with two semiconductor detectors, a Gd layer and a polyethylene layer...... 62 Fig. 26. Geometry used in SWORD simulation in CONFIG1 to model the twin-detector structure and thermal neutron (0.0253 eV) source beam...... 65 Fig. 27. Energy spectra of electrons from the two Si detectors obtained using SWORD simulation in CONFIG1...... 65 Fig. 28. Energy spectra of electrons from the two Si detectors obtained using SWORD simulation in CONFIG2...... 66 Fig. 29. Geometry used in MCNP5 simulation in CONFIG1 to model the twin-detector structure and gamma ray source beam...... 67 Fig. 30. Energy spectra of the two Si detectors in response to 57Co gamma rays obtained using MCNP5 simulation in CONFIG1...... 68 Fig. 31. Energy spectra of the two Si detectors in response to 57Co gamma rays obtained using MCNP5 simulation in CONFIG2...... 68 Fig. 32. Energy spectra of the two Si detectors in response to 235U gamma rays obtained using MCNP5 simulation in CONFIG1...... 70 Fig. 33. Energy spectra of the two Si detectors in response to 235U gamma rays obtained using MCNP5 simulation in CONFIG2...... 70 Fig. 34. Experimental setup inside a large stainless-steel high-vacuum chamber used to perform the Gd ICE measurements...... 75 Fig. 35. Schematic illustration of the experimental setup and the DAQ electronics used in the ICE measurements...... 76 Fig. 36. ICE energy spectrum measured using Si detectors during neutron activation of a thin Gd foil...... 78 xv

Fig. 37. ICE energy spectrum measured using Si detectors during neutron activation of a thin Gd foil at a higher neutron flux and gamma dose rate...... 79 Fig. 38. ICE energy spectra obtained during neutron activation of 5, 25, and 75 µm thick Gd foils in front- and back-illumination...... 81 Fig. 39. Energy spectrum of electrons in the Si detector obtained from 0.0253 eV thermal neutron absorption in Gd foils of different thicknesses in (a) back-illumination (b) front- illumination...... 82 Fig. 40. A comparison of the energy spectra from Gd, B and Li neutron capture reactions in a Si detector...... 85 Fig. 41. Energy spectrum of a Si detector obtained under different experimental conditions during the evaluation of gamma ray sensitivity of Gd. Results are from measurements (a) i, ii, and iv, (b) i, ii, and vi, and (c) i, iii, and v...... 88 Fig. 42. Experimental geometry reproducing the twin-detector scheme for rejection of external gamma rays; a 57Co source was used in the experiment...... 92 Fig. 43. Energy spectra of the two Si charged particle detectors obtained from the experiment studying the gamma ray rejection scheme using 57Co source...... 92 Fig. 44. Energy spectra of the two Si charged particle detectors obtained from the experiment studying the gamma ray rejection scheme using 133Ba source...... 93 Fig. 45. Experimental geometry reproducing the twin-detector scheme for rejection of external gamma rays; 14C and 57Co sources were used in the experiment...... 95 Fig. 46. Energy spectra of the two Si charged particle detectors and the differential spectrum obtained from the experiment to evaluate the gamma ray rejection scheme using 14C beta and 57Co gamma ray sources...... 96 Fig. 47. Experimental setup inside the high-vacuum chamber at OSURR reproducing the twin-detector gamma rejection scheme; a 57Co source was used to enhance the gamma ray background in the measurement...... 98 Fig. 48. Energy spectra of Si detectors obtained during neutron irradiation of the twin detector setup; no external gamma source was used in the measurement...... 99 Fig. 49. Energy spectra of Si detectors obtained during neutron irradiation of the twin detector setup; 57Co source was used in the measurement to enhance the gamma ray background...... 100 Fig. 50. The plutonium-beryllium (PuBe) neutron howitzer ...... 102 Fig. 51. Top - SolidWorks 3D model of the PuBe source facility in NARS lab; Bottom - PuBe source station in its final form after construction...... 103 Fig. 52. Graphite and sapphire rods and Al enclosure assembly built for the purpose of neutron collimation: Top - HDPE rings built for enclosing the graphite and sapphire rods. Bottom - final collimator assembly (components are separated for visibility)...... 104 Fig. 53. Neutron activation foils kit purchased from Shieldwerx (left), and the Au, In, Sc, Cu, Ti and Fe metal foils used in the activation study (right)...... 106 Fig. 54. Initial guess spectrum obtained from Origen-Arp and the PuBe source neutron spectrum unfolded using SAND-2...... 108 Fig. 55. Comparison of neutron spectra obtained from SAND-2 unfolding and MCNP5 simulation...... 108

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Fig. 56. Energy spectra obtained from a Si detector during neutron activation of Gd2O3/PDMS thin film and a bare substrate, and in a background measurement...... 110 Fig. 57. 71 keV electron trajectories inside a slab of Gd2O3/PDMS. Trajectories in blue represent the transmitted electrons whereas those in red represent the back-scattered electrons. The zoomed portion of the figure (inset) shows the narrow electron beam (10 nm radius) penetrating the slab after striking it orthogonally...... 112 Fig. 58. Depth profiling analysis for the Gd2O3/PDMS film. Figure shows the histogram of count rate against the depth of origin of 71 keV ICEs in the thin film...... 112 Fig. 59. Energy spectra of button-sized 241Am and 57Co sources used to calibrate the Si detectors. Measurements were made separately, but the spectra are overlaid...... 130 Fig. 60. Energy spectra of 59.5 keV and 122 keV gamma rays from a Si detector obtained using MCNP5 simulation...... 131 Fig. 61. Left - Energy spectra of 29-246 keV gamma rays from a Si detector obtained using MCNP5 simulation. Right - Energy deposition spectrum of the gamma rays in the polyethylene layer...... 133 Fig. 62. Dark current vs. reverse voltage characteristics of PD 1 ...... 135 Fig. 63. Dark current vs. reverse voltage characteristics of PD 2 ...... 136 Fig. 64. Detector-source geometry used in the measurement of 241Am, 57Co and 14C energy spectra by the photodiode detector...... 137 Fig. 65. Energy spectrum of 241Am α-particles obtained using the photodiode...... 138 Fig. 66. Energy spectrum of 241Am gamma rays obtained using the photodiode...... 138 Fig. 67. Comparison of the 241Am α-spectra measured using the photodiode and a large area Si charged particle detector...... 139 Fig. 68. Energy spectrum of 57Co gamma rays measured using the photodiode...... 140 Fig. 69. A comparison of 14C β-spectra measured using the photodiode and a large area Si detector...... 141

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Chapter 1. Introduction

Since the terrorist attacks on 9/11, there has been an increased concern about the proliferation and illicit usage of special nuclear material1 (SNM) [1], which, when used destructively in nuclear weapons has the potential to inflict unprecedented amounts of damage to national and global interests. Thus, the detection and control of SNM is highly essential for global nuclear threat reduction. A characteristic of the SNM that favors their detection is the emission of neutrons, although in small quantity, with unique signatures.

For example, plutonium (Pu), a SNM used for nuclear weapons, is a significant source of spontaneous fission neutrons [2]. In view of these aspects, neutron detection becomes an integral part of prevention of proliferation of SNMs and plays a central role in the interdiction of radiological and nuclear threats for homeland security.

Apart from homeland security [3] and nuclear non-proliferation [4], neutron detection is applied in various other areas including nuclear industry [5], radiation dosimetry [6, 7], experimental physics studies such as fusion [8], small-angle neutron scattering [9], spallation neutron source [10], neutron radiography [11], and ITER (formerly

International Thermonuclear Experimental Reactor) [12] etc.

1Special nuclear material (SNM) refers to fissile materials such as plutonium (Pu), uranium (U) enriched in the isotope 233U or isotope 235U and any other material that the Nuclear Regulatory Commission determines to be SNM, but does not include source material. 1

1.1. Background

Neutron detection involves the transduction of a charge-free neutron to charged particles; in this case, a thin-film semiconductor provides an ideal detection platform for secondary charge generation and collection [13]. However, compressed helium-3 (3He) has been widely used as a neutron detection medium due to high neutron sensitivity, high gamma ray insensitivity and non-toxicity possessed by 3He. Radiation portal monitoring systems which are extensively deployed in homeland security and non-proliferation applications use 3He filled proportional counters for neutron detection. The widespread usage of 3He in homeland security, apart from other research (e.g., cryogenics [14],

Spallation Neutron Source [10] and medical applications such as He-3 contrast magnetic resonance imaging (MRI) [15]) resulted in a long standing shortage of 3He supply. In this context, there is an urgent requirement of an effective 3He alternative. While a false negative is intolerable in homeland security detection applications, a false positive signal from detectors can also have a significant negative societal impact. This necessitates neutron detection techniques to mitigate false alarms in the presence of a large gamma ray background [2]. Hence, any potential neutron detection technology needs to address the two fundamental requirements, namely, high neutron detection efficiency and good gamma ray rejection.

Although the conventional 3He gas proportional counters offer very high thermal

(energy < 0.5 electron-volts (eV)) neutron detection efficiency, the requirements of long term stability and convenience in the replacement of highly pure 3He gas limit their efficiency to ~15-30% [16]. These neutron detectors are constrained in use by the 2 requirements of high pressure and high voltage operation, sensitivity to microphonics and large device foot print. Semiconductor detectors in contrast, are characterized by low voltage operation, inherent insensitivity to microphonics, smaller device footprint and excellent stability. Thus, a semiconductor detection medium coupled with an efficient neutron sensitive material (i.e., neutron converter) provides a preferred alternative to the current 3He gas based technology in applications that require field-deployment and portability of compact neutron detectors.

Boron-10 (10B), lithium-6 (6Li) and gadolinium (Gd) are the most commonly used neutron converter materials for thermal neutron detection either in solid form (e.g., doping) or as coating in Schottky, PN junction or a resistive-type of semiconductor material [17-33]. Cadmium (Cd) has also been used for thermal neutron detection in a cadmium zinc telluride semiconductor [34], but the high energy neutron capture gamma rays of Cd require large detectors for efficient detection. While Gd possesses the largest thermal neutron cross section of all naturally occurring elements, much larger compared to that of B and Li, the internal conversion electrons (ICEs) released in Gd(n,γ)Gd* reaction possess much lower energy than the highly energetic charged particles produced in 10B(n,α)7Li and 6Li(n,α)3H reactions. In addition, the gamma ray sensitivity is a drawback when Gd (Z=64) is used for neutron detection. In this dissertation, the aforementioned issues concerning the use of Gd in neutron detection are investigated in detail and methods developed for effective discrimination of gamma rays in a Gd based semiconductor neutron detector.

1.2. Neutron detection

Since neutrons are electrically neutral, detection of neutrons requires a conversion medium (i.e., neutron converter) with high neutron interaction probability to convert the charge-free neutron into electrically charged particles. Neutron interaction with matter occurs primarily in two different ways [35]. Firstly, the incoming neutron is scattered by a nucleus. If the recoil nucleus acquires sufficient energy from the neutron, it ionizes the matter surrounding the interaction site. This type of interaction facilitates fast neutron

(typically > 100 keV) detection and is efficient for neutrons interacting with lighter nuclei such as hydrogen (H) and He. In the second type of interaction, a neutron with sufficiently low energy induces a nuclear reaction in the material, releasing charged particles (including electrons) or gamma rays. Some of these reactions require a threshold on the neutron energy, though most reactions occur at thermal energies. The secondary charged particles or gamma rays further interact directly with atomic electrons in a detector and generate, for example, ionized charge carriers in semiconductor and gas detectors or visible light in case of scintillation detectors. The charge carriers in a semiconductor detector are electron-hole (e-h) pairs which possess much higher mobility than the positive-ions of the electron-ion pairs in a gaseous detector.

1.3. Solid-state neutron detectors

As discussed earlier, a semiconductor material provides ideal platform for charged particle detection, which is attributed to the many desirable characteristics such as smaller detector dimensions due to densities much higher than gaseous media and greater

4 stopping power to charged particles, faster response times, low voltage operation etc. The success of semiconductor detectors over other detector types is also due to several unique and interesting properties including availability of signals in direct electrical form, possibility of integrating detector and readout electronics on a common substrate, extremely precise position measurement with high readout speed, and simultaneous precise measurement of energy and position [36]. In addition, the ionization energy, for example in silicon (Si), is an order of magnitude smaller than that in gases (~30 eV), which allows for creation of large number of e-h pairs. Consequently, semiconductor detectors offer superior energy resolution to charged particle radiation, which is restricted by the total number of charge carriers available for generation of an electrical pulse [37].

Semiconductor neutron detectors are most commonly used in either PN junction or

Schottky based configurations, which are further classified into indirect-conversion (also known as thin-film coated/conversion layer) and direct-conversion (also known as solid- form) devices (Fig. 1). An overview of these two device types is presented in this section

[13].

Fig. 1. Schematic illustration of indirect-conversion (left) and direct-conversion (right) configurations of semiconductor neutron detectors. 5

In indirect-conversion semiconductor devices [17, 24-30], thin film of a neutron converter material is placed in contact with the semiconductor (Fig. 1, left) so that the neutron capture reaction products (i.e., charged particles) may traverse through the converter and create e-h pairs in the adjacent depletion region. In this configuration, a fraction of the particles' energy is unavoidably lost in the converter, without contributing to the electrical pulse generation. The energy loss of the charged particle depends on the neutron capture site in the converter, ranging from a minimum at the converter- semiconductor interface to a maximum at the farthest point of the end of range in the converter from the interface. In direct-conversion semiconductors [30-33], the neutron converter material and the depletion region are the same, so that most or all of the charge particle reaction products' energy is available for charge generation. Consequently, direct- conversion devices are likely to offer the highest detection efficiencies. Nevertheless, materials suitable for direct-conversion semiconductors i.e., in which the bulk constituent is an isotope with large neutron capture cross section that also yields energetic charged particles upon neutron capture are less common, with few studies reported on all-boron carbide devices [31, 38, 39]. In addition, such materials suffer from both immature processing technology and intrinsic high defect level. For this reason, achieving very high charge collection and detection efficiencies with direct conversion semiconductor neutron detectors has been a great challenge. In contrast, indirect-conversion devices are based on semiconductors with mature processing technology, and well-understood material, electrical and electronic properties and thus, may be realized with very high charge collection efficiencies for neutron capture reaction products.

1.4. Neutron converter materials

The probability of interaction of a material with neutron is referred to as its neutron cross section and is measured in units of barns (1 barn (b) = 10-24 cm2). Since the neutron cross section of nuclides is generally high at lower neutron energies, neutron detectors can be made increasingly efficient by moderating the incoming neutrons. Apart from 3He, the most commonly used neutron converter materials are B, Li, 235U and Gd (Table 1). As

3He is out of supply, it remains questionable as to which converter material and detector combination offers the effective alternative to 3He.

Indirect-conversion semiconductor neutron detectors based on B and Li compounds suffer from low detection efficiencies due to the conflicting long neutron mean free path and the short range of charged particles in B and Li [40]. Such inherent limitation in using B and Li in the simplest indirect-conversion configuration creates a strong requirement for more advanced designs such as those in three-dimensional configuration e.g., via-hole, perforated, pillar structured [23, 41, 42] etc., to maximize the detection efficiency. In contrast, the past research indicated higher detection efficiency for Si detectors coupled with Gd converter films (i.e., indirect conversion) due to the detection of ICEs [19, 21, 43, 44]. The 0.0253 eV2 thermal neutron cross section of Gd is about

48,707 barns, largest of all naturally occurring elements. Such a large thermal neutron cross section of Gd is predominantly due to 157Gd with 253,757 barns and 155Gd with

60,740 barns [45], which are present in 15.65% and 14.84% natural abundance,

2The 0.0253 eV kinetic energy corresponds to the most probable velocity of neutrons in thermal equilibrium at room temperature (20 °C). 7 respectively. This makes Gd a suitable material for neutron conversion and a promising neutron detection alternative to 3He.

Cross section @ 0.0253 eV (b) Principal Element/ Nuclear Q-value Elemental useful Isotope reaction (keV) (with natural energies (keV) Isotopic isotopic abundance) He/3He 3He (n,p)3H 191; 573 764 5315.9 0.00728 B/10B 10B(n,α)7Li 840; 1472 2790 3842.5 764.7 Li/6Li 6Li(n,α)3H 2055; 2727 4782 954.7 71.6 U/235U 235U(n,f) ~1.7 × 105 2.07 × 105 585.1 4.21 Gd/157Gd 157Gd(n,γ) 158Gd* 29; 71; 78 7937 253757 48707 Gd/155Gd 155Gd(n,γ)156Gd* 39; 81; 88 8536 60740 Table 1. A comparison of nuclear reaction energies, isotopic and elemental neutron capture cross sections of the most common neutron converter materials [37, 45-47].

In recent times, Gd has also attracted greater interest for applications in Gd neutron capture therapy (NCT), partly due to the success of Gd contrast enhancement agents in

MRI and to the limitations of boron NCT [48]. Although the range of most commonly emitted electrons from the Gd(n,γ)Gd* reaction exceeds the dimension of a single cell, very low energy electrons are also emitted including the Auger electrons [49], which makes them highly ionizing over the volume of a single cell, thereby reducing the gamma ray dose to the surrounding tissue [50]. Thus, these electrons may induce double strand breakage of DNA when Gd is in close proximity [51]. A detailed understanding of the electron spectra is also essential, besides the neutron detection, for accurate determination of dose delivery to the tumor and surrounding healthy tissue.

As discussed in section 1.1, there are certain limitations intrinsic to Gd neutron conversion which counteract the advantages of its large neutron cross section. The prompt gamma rays of Gd(n,γ)Gd* reaction are mostly of high energy, while the ICEs, which are released as a spin-off of Gd* nuclear de-excitation are mainly characterized by low energies. Since a thin film semiconductor detector is almost transparent to high energy gamma rays, the low energy ICEs represent the principal neutron information carriers for e-h pair generation in the semiconductor. In addition, the high gamma interaction probability of Gd further complicates the detection of ICEs in a mixed neutron and gamma (n-γ) radiation environment. These issues are discussed in detail in the forthcoming chapters.

The Chapter 2 presents a background of Gd neutron absorption reaction, the reaction products' energy spectra, the associated Monte Carlo (MC) simulations, and a discussion of some principal parameters such as the ICE range and optimal thickness of Gd for Gd based indirect-conversion detectors. Chapter 3 presents an overview of Gd interaction with gamma rays and a discussion of the gamma interaction probability of Gd using MC simulations and experiments, and concludes by setting a framework for the current research. In Chapter 4, motivation for this research is presented and literature on gamma ray discrimination techniques is extensively reviewed. The Chapter 4 concludes with a discussion of the proposed method of gamma ray rejection in Gd based semiconductor neutron detectors. In Chapter 5, a detailed evaluation and substantiation of the proposed rejection method is discussed using MC simulations. In Chapter 6, extensive experimental evaluation of the neutron and gamma sensitivity of Gd and the proposed

9 rejection method are discussed. Chapter 7 presents the concluding statements of this dissertation.

1.5. Summary

The shortage in 3He supply caused by its widespread usage and demand in a multitude of applications has spurred the research for effective 3He neutron detection alternatives. Gd possesses the largest thermal neutron cross section of all naturally occurring elements and has been commonly applied as a neutron converter in semiconductor detectors. It is supposed that a semiconductor material coupled with a suitable neutron converter such as Gd will be a promising alternative to the current 3He based proportional counter neutron detectors. However, the ICEs released from Gd neutron capture are prone to interference from low energy gamma rays and present a challenge in terms of charge extraction in a thin film semiconductor.

The detection of SNM typically involves neutron deficient and gamma abundant radiation environment, necessitating strong gamma rejection capability for the neutron detectors in operation. A suitable method of detection of Gd ICEs that also allows for effective gamma ray separation will facilitate the application of Gd in neutron detection for homeland security. In addition, the understanding of physics of ICEs will help in the study of GdNCT.

Chapter 2. Interaction of neutrons with Gd

Gd undergoes radiative capture (n,γ) reaction with neutrons, which takes place in the following manner described for reactions in 157Gd and 155Gd [46, 47].

Unlike 10B and 6Li neutron capture reactions, which release heavy charged particles with distinct energies, the Gd(n,γ)Gd* reaction results in an assortment of emissions consisting of prompt gamma rays, low energy internal conversion (IC) and Auger electrons, and characteristic x-rays. The prompt gamma rays are characterized by a broad energy spectrum extending up to 7.9 MeV with a total of 1664 spectral lines. For example, following neutron absorption in 157Gd nucleus, several isomeric transitions occur, which result in the emission of an average of 3.288 photons from an energy spectrum containing 390 spectral lines. These photons have a wide range of energies with a mean value of 2.394 MeV [52].

The following section describes a MC simulation of neutron induced Gd prompt gamma rays and compares the result with an experimentally determined spectrum.

2.1. Prompt gamma rays

The simulation is performed using SWORD 4.0 software package [53] to observe the prompt gamma ray spectrum from Gd. A 25 µm thick Gd foil is placed in the center of a

0.0253 eV3 thermal neutron beam in front of a large volume coaxial high purity germanium (HPGe) detector, as shown in Fig. 2. The HPGe detector is created as an instance of the standard detector model present in the detector library of the software.

The Gd(n,γ)Gd* reaction energy spectrum is recorded in the detector for a total of 2 × 106 particle histories (i.e., source neutrons) and is presented in Fig. 3.

Coaxial HPGe detector assembly Coaxial HPGe core

Gd foil

Dewar shell 0.0253 eV thermal neutron disk source

Fig. 2. Geometry used in SWORD simulation indicating the thermal neutron beam, Gd foil and HPGe detector assembly.

The Fig. 3 indicates the complex spectrum of Gd prompt gamma rays from simulation, which extends up to 7.9 MeV in energy. It can be seen that most of the highly

intense prompt gamma rays are characterized by energies greater than 100 keV, at which

3The 0.0253 eV kinetic energy corresponds to the most probable velocity of neutrons in thermal equilibrium at room temperature (20 °C). 12 the sensitivity of a thin film semiconductor is very low.

2D Graph 1

104

103

102

Counts per Counts bin

101

100 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8000 Energy (keV) (bin size = 1 keV) Fig. 3. Energy spectrum of Gd* prompt gamma rays obtained in a HPGe detector using SWORD simulation.

Fig. 4. Experimental standard energy spectrum of neutron induced prompt gamma rays from natural Gd (reproduced from Revay, 2004 [54]). 13

The simulated prompt gamma ray spectrum is compared with the experimental standard [54] shown in Fig. 4. The simulation, in general, shows good agreement with the standard spectrum, however some deviations are observed due to the differences in detector-source geometry, target Gd sample composition, neutron flux etc. To further illustrate the level of agreement between the simulation and the experimental spectrum, three energy ranges in the spectra are considered and compared separately as shown in

Fig. 5.

Energy (keV) (a) continued

Fig. 5. Comparison of the SWORD simulation result (right) with the experimental prompt gamma ray spectrum of Gd (left) in the energy range (a) ≤ 300 keV (b) 600-1000 keV (c) 1000-1400 keV

Fig. 5 continued

Energy (keV) (b)

Energy (keV) (c)

The above illustrations clearly indicate the good level of agreement between experimental and simulated spectra with significant overlap of the gamma ray spectral

15 lines; thus, further modeling and experimental study using SWORD as the simulation tool is validated.

2.2. Internal conversion electrons (ICEs)

Some of the nuclear de-excitations involve the emission of atomic electrons through

IC, which competes with gamma ray emission and is significant in high Z nuclides such as Gd. During IC, the nuclear excitation energy is transferred to an orbital electron, following which the electron is ejected from its atomic shell with kinetic energy equal to the difference between the excitation energy and the electron shell binding energy.

Unlike the prompt gamma rays, the ICEs of Gd* do not feature a broad energy spectrum and are emitted primarily from the lowest three energy level transitions of Gd* nucleus.

The IC on K,L or M shell results in electron emissions in the energy range 29-246 keV

[55]. Following IC, a vacancy is created in the otherwise full electron shell that leads to the excited state of the atom. The atomic excitation energy is released through the emission of characteristic x-rays (in radiative transition) or Auger electrons (in non- radiative transition). Characteristic x-rays are emitted when an outer-shell electron fills the inner-shell vacancy, with energy equal to the difference between the binding energy of the two shells. Auger electrons are emitted when this transition energy, instead of emerging as x-ray, is used to eject another outer shell electron. The list of characteristic x-rays emitted from an excited Gd atom [56] is shown in Table 2.

Energy Relative X-ray (keV) Intensity (%) Kα1 43.00 100 Kα2 42.31 56 Kβ1 48.70 20 Kβ2 49.96 7 Kβ3 48.56 10 Lα1 6.057 100 Lβ1 6.713 62 Lβ2,15 7.103 21 Lγ1 7.786 11 Table 2. List of characteristic x-rays emitted during the atomic relaxation of Gd

While Auger electrons' emission competes with that of characteristic x-rays, the non- radiative transition probabilities remain lower compared to that of the radiative transitions in Gd atom. In fact, the most probable Auger electrons have energies less than

10 keV [49] and so, are not of use for neutron signal generation in a semiconductor. Due to the small volume and low Z number of common semiconductor materials (e.g., Si), a thin film semiconductor is almost transparent to the high energy prompt gamma rays.

Thus, ICEs represent the principal neutron information carriers for signal generation in such detector. Hence, it is essential to develop precise understanding of the emission spectrum of these electrons.

The following schematic summarizes the three primary Gd nuclear de-excitations indicating the energy levels, and the corresponding prompt gamma ray and ICE energies.

157 1 158 * Gd + n { Gd } Energy levels Prompt γ-rays 539.0 keV ICEs 277.5 keV 261.5 keV K – 22 7 keV 182.0 keV K – 13 1 keV 79.5 keV L – 17 3 keV

79.5 keV M – 18 0 keV 0 keV K – 29 keV {158Gd} L – 71 keV

M – 78 keV

155 1 156 * Gd + n { Gd } Prompt Energy levels γ-rays 584.7 keV ICEs 296.5 keV 288.2 keV K – 24 6 keV 199.2 keV K – 14 9 keV 89.0 keV L – 19 1 keV

89.0 keV M – 19 8 keV 0 keV K – 39 keV {156Gd} L – 81 keV M – 88 keV

Fig. 6. Schematic illustration of the three lowest and primary de-excitation paths of 158Gd* and 156Gd* leading to prompt gamma ray and ICE emissions.

The three most probable low lying energy levels in 158Gd* and 156Gd*, the prompt gamma ray energies and intensities, and the IC coefficients of corresponding atomic shells are retrieved from the nuclear data sheets [46, 47] and using BrICC code package

[57], and are summarized in Table 3. The notation and definitions of symbols used in the table are presented below.

E - Energy level of the excited nuclear state

Eγ - Energy of the gamma ray

Iγ - Gamma ray emission intensity (per neutron absorption)

αtot - Total IC coefficient; the IC coefficient, α is defined as the ratio of the electron emission rate to the gamma emission rate [57].

αK, αL and αM are the IC coefficients of K, L and M shells, respectively.

αN+ - Sum of IC coefficients of all shells from N to the outermost one.

Using the values listed in Table 3, the ICE intensities are derived using the following relationships, with the notations described against the expressions.

ICE(tot) = Iγ αtot; ICE(tot) is the total ICE intensity (per neutron absorption)

ICE(K) = Iγ αK

ICE(L) = Iγ αL

ICE(M) = Iγ αM; ICE(K), ICE(L) and ICE(M) are the intensities of ICEs ejected from K, L and M

shells (per neutron absorption), respectively.

ICE(N+) = Iγ αN+; ICE(N+) is the intensity of ICEs ejected from N to the outermost shell (per

neutron absorption).

I'CE - ICE intensity in natural Gd (per neutron absorption)

E Eγ Iγ αtot αK αL αM αN+ 79.5 79.5 0.09748 5.93 2.02 3.02 0.714 0.18 158Gd* 261.5 182 0.1833 0.308 0.2 0.084 0.018 - 539 277.5 0.01128 - 0.1 - - -

156Gd* 89 89 0.209 3.88 1.559 1.79 0.422 0.1066 288.2 199.2 0.316 0.225 0.1565 0.0531 0.01224 0.00314 584.7 296.5 0.0253 0.0625 0.0477 0.01151 0.00261 0.00068 Table 3. List of nuclear excitation energy levels, prompt gamma ray energies and intensities, and IC coefficients of 158Gd* and 156Gd*.

158Gd* 156Gd* Eγ 79.5 182 277.5 89 199.2 296.5 ICE(tot) 0.5781 0.0565 - 0.8109 0.0711 0.0016

ICE(K) 0.1969 0.0367 0.0011 0.3258 0.0495 0.0012 ICE(L) 0.2944 0.0154 - 0.3741 0.0168 0.0003 ICE(M) 0.0696 0.0033 - 0.0882 0.0039 0.0001 ICE(N+) 0.0175 - - 0.0223 0.0010 0.0000

I'CE(tot) 0.4714 0.0460 - 0.1497 0.0131 0.0003 I'CE(K) 0.1606 0.0299 0.0009 0.0601 0.0091 0.0002 I'CE(L) 0.2401 0.0126 - 0.0691 0.0031 0.0001 I'CE(M) 0.0568 0.0027 - 0.0163 0.0007 0.0000 I'CE(N+) 0.0143 - - 0.0041 0.0002 0.0000 Table 4. List of prompt gamma ray energies and corresponding ICE intensities of 158Gd* and 156Gd*.

The following table lists a comparison of the ICE intensities of natural Gd calculated above, with those reported in literature [55].

ICE energy Intensity per ICE energy Intensity per (keV) thermal neutron (keV) thermal neutron from 158Gd* This Ref. from 156Gd* This Ref. work work 29 0.1606 0.0982 39 0.0601 0.0419 71 0.2401 0.2680 81 0.0691 0.0497 78 0.0568 0.0617 88 0.0163 0.0116 131 0.0299 0.0341 149 0.0091 0.0084 173 0.0126 0.0146 191 0.0031 0.0030 180 0.0027 0.0031 198 0.0007 0.0006 228 0.0009 0.0040 246 0.0002 0.0002 Total yield 0.5036 0.4837 Total yield 0.1586 0.1154 Table 5. Comparison of calculated ICE intensities of Gd with those from the literature.

2.3. Energy spectrum of Gd(n,γ)Gd* reaction products in a thin film semiconductor

The energy spectrum of Gd(n,γ)Gd* reaction products was computed using MC simulations for further understanding of neutron signal generation in a thin film semiconductor. MCNP5 [58] code package was used to perform the simulations4. The geometry model represented a 0.0253 eV monoenergetic thermal neutron source beam striking a Gd foil (10 µm) orthogonally (Fig. 7). Energy spectrum of the reaction products is computed in a planar Si detector (200 µm) using F8 pulse height tally in

MCNP5.

Si detector

Gd foil

0.0253 eV thermal neutron disk source Fig. 7. Geometry used in the MCNP5 model to observe ICE energy spectrum in a Si detector.

4 The choice of using MCNP5 against SWORD software for simulations was rather intentional. 21

4x10-6

3x10-6

2x10-6

10-6

Counts per source particle per bin

0 0 100 200 300 400 500 600 700 800 900 1000 Energy (keV) (bin size = 1 keV)

Fig. 8. Energy spectrum from a Si detector in response to neutron activation of Gd obtained using F8 pulse height tally in MCNP5.

The detector spectrum in Fig. 8 can be attributed only to the Gd prompt gamma rays and x-rays, as evidently no ICE features are seen. The simulation result indicated the deficiency of data libraries in MCNP5 required to model ICE emission and transport.

This was confirmed using a separate simulation in which ICEs were used as the source particles in place of thermal neutrons. The model represented an isotropic source of ICEs with uniform sampling across the volume of Gd foil, with a-priori assumption of neutron absorption in the foil. The ICE source energy distribution is taken from Harms et al [55] and listed previously in Table 5. The simulation result from the modified MCNP5 model is shown in Fig. 9. As expected, the detector spectrum in this case showed the characteristic ICE peaks, in contrast to the previous simulation based on thermal neutron source model.

Comparison of pulse height tallies obtained using explicit thermal neutron interaction and conversion electron source

0.0007

0.0006

0.0005

0.0004

0.0003

0.0002

0.0001

Counts per source particle per bin 0.0000 0 50 100 150 200 250 300 Energy (keV) (bin size = 1 keV)

Fig. 9. Energy spectrum of electrons in a Si detector obtained using a volumetric isotropic source of ICEs with uniform distribution across a Gd foil in MCNP5.

Since ICE emission from Gd neutron capture couldn't be modeled directly by

MCNP5, the simulations were repeated in SWORD and the energy spectrum was recorded in a Si detector. The detector spectrum is shown in Fig. 10 indicating the ICE energy peaks, along with a comparison against that from the modified MCNP5 simulation. Results indicated good agreement between the SWORD and the modified

MCNP5 simulations, and thus, validated the applicability of SWORD for modeling ICE emission from Gd and energy deposition in a semiconductor. Hence, subsequent simulations on Gd neutron absorption were performed in SWORD software instead of

MCNP5.

2D Graph 2

0.0008 SWORD 71 keV MCNP5

0.0006

29 keV 0.0004 39 keV 78 keV

131 keV

Intensity (arb.Intensity units) 0.0002 173 keV

0.0000 0 50 100 150 200 250 300 Energy (keV)

Fig. 10. Energy spectrum from a Si detector indicating the characteristic ICE peaks obtained using SWORD simulation, and compared with the MCNP5 result.

2.4. Range of IC electrons (ICEs)

The amount of ICE energy deposited in a semiconductor with Gd conversion layer depends on the thickness of Gd layer, owing to self-absorption of ICEs in Gd prior to reaching the detector. It implies that a thicker conversion layer results in greater absorption of the ICEs. However, a thinner one, while minimizing the energy loss of

ICEs, also restricts the neutron absorption efficiency. Thus, the Gd conversion layer thickness needs to be optimized for maximum neutron detection efficiency, and the ICEs range in Gd provides a valuable insight into this evaluation. A brief discussion on electron interaction and range in solid materials is presented below.

Electrons with energies in the keV range traveling in a solid undergo inelastic scattering collisions with atomic electrons in the material and are elastically scattered

24 through large angles primarily by collisions with the nuclei. The electron trajectory in a solid therefore, does not behave as a straight line like that of heavy charged particles, but is zigzagging and tortuous. Several definitions of electron range in solid materials have been reported in the literature [59-62], some of which are summarized below.

 A fraction of incident electrons that penetrate a film of given thickness is

measured as a function of electron energy (E). From the absorption curve ( vs.

E), a critical energy (Ec) is defined by extrapolating the linear portion of the curve

and a threshold energy (Eth) is defined at which the penetration is first measured

(Fig. 11, left) [62]. The film thickness is defined as the ‘practical range’ of

electrons in the material corresponding to critical energy Ec and as the ‘maximum

range’ at the threshold energy Eth [60].

1.0 1.0

ηT ηT constant constant 0.5 0.5 thickness energy

0 0 Eth Ec R E e t

Fig. 11. Left: Electron transmission (ηT) plotted against electron beam energy (E) for constant film thickness [62]. Right: Electron transmission (ηT) plotted against film thickness (t) for constant electron beam energy [60].

 The fraction of incident electrons is measured for films of various

thicknesses (t) for constant incident electron energy (E). In the curve obtained by

plotting vs. t (Fig. 11, right), the extrapolated straight-line segment of the curve

intersects the abscissa at the so-called ‘extrapolated range’ for the energy E [60].

 Electron range was defined as the mass thickness (i.e., mass per unit area) that

reduces the most probable energy of the transmitted electrons to zero, whereas a

'mean electron range' was defined as the mass thickness that reduces the mean

energy of the transmitted electrons to zero [59].

 The continuous slowing down approximation (CSDA) estimates the electron

range as the average of total distance traveled by the electron along its path in the

material.

 The maximum penetration depth (MPD) of an electron is the straight-line distance

from the beginning of the electron’s track to its end. Nevertheless, the average

MPD of electrons in the material is smaller than the CSDA range due to multiple

scattering deflections of electrons [63].

The ICE ranges in Gd, Si and polyethylene (will be used as beta particle shield), which are closely related to the discussions presented in this dissertation, are evaluated in this section. The ranges of Gd* ICEs are calculated, in various definitions, both theoretically and by MC simulations. The CSDA range was computed by the integral of the inverse stopping power [60] as described below.

The rate of change of electron kinetic energy in a material along a path s is given by

24 where is the Avogadro's number (0.6022 × 10 ),

is the electron charge, 26

and are the atomic number and atomic weight of the material respectively,

is the material density

is a constant (=1.1658)

is a mean excitation energy for electron energy loss in the solid, given by

By setting in eq. (1), the equation can be rewritten as

The mean electron path length or the Bohr-Bethe range is then given by

where, is the energy of electrons in the incident beam,

However, for numerical evaluation in MATLAB, the integral in eq. (4) was discretized into summation form as indicated below.

where, is the total number of energy intervals,

is the size of each energy interval (step-size), such that and, the

intervals are separated by nodes , ,

corresponds to interval and is evaluated at the energy point,

For the average path length and average MPD calculations, two MC codes-

Penetration and Energy Loss of Positrons and Electrons (PENELOPE) [64] and Monte

Carlo Simulation of Electrons in Solids (CASINO) [65] were used, respectively. All the calculated values are tabulated below.

Energy Gadolinium Silicon Polyethylene (keV) Av.path CSDA Av. Av.path CSDA Av. Av.path CSDA Av. length MPD length MPD length MPD 29 4.47 4.95 1.2 9.33 9.80 4.13 16.3 18.6 10.7 39 7.36 8.22 1.86 15.6 16.7 7.16 27.6 31.8 18.5 71 20.1 23.4 5.03 43.8 49.2 21.0 79.1 95.3 54.8 78 23.5 27.6 5.90 51.4 58.4 24.7 93.1 113 64.8 81 25.0 29.5 6.29 54.8 62.6 26.4 99.4 121.5 69.3 88 28.7 34.2 7.26 63.0 72.8 30.6 114.6 141.6 80.2 Table 6. Energy-ranges of ICEs emitted in Gd neutron capture in Si, Gd, and polyethylene according to various range definitions (all units in µm).

The theoretical CSDA values of the electron range in these materials were computed using an effective atomic number and effective atomic weight for the material. The analytical expressions for both were extracted from Tabata et al [61]. As shown in Table

6, the average path lengths of electrons in the material are always larger than the MPD owing to multiple scattering deflections [63]. The theoretical values (CSDA) and the average path length derived from PENELOPE agree because both deal with the total path length.

As an example, the distribution of MPD and the average path length of 71 keV electrons (from 158Gd*) in a 50 µm thick slab of Si is considered (Fig. 12, Fig. 13, Fig.

14). Si was chosen because the range of ICEs in this detector material is the most

28 important parameter. In contrast to a detector in which 10B or 6Li is applied, where the neutron-induced heavy charged particles traverse a straight, short path, the effective path length of electrons in a detector (e.g. Si) is twice as long as its average MPD. This indicates a longer effective ionizing path of electron than heavy ions in detection medium that is in favor of generating more e–h pairs in the detector, thus compensating for the lesser ionizing power of electrons.

Fig. 12. Graphic showing the penetration of 71 keV ICEs in a 50 µm thick Si slab in CASINO simulation. Trajectories in blue correspond to the transmitted whereas those in red correspond to the backscattered electrons.

Fig. 13. Graphic showing the penetration of 71 keV ICEs in a 50 µm thick Si slab in PENELOPE calculation. 2D Graph 1

Maximum penetration depth simulated by CASINO 0.003 Average path length computed by PENELOPE

0.002

0.001

Intensity (arb. units)

0.000 0 5 10 15 20 25 30 35 40 45 50 Distance (µm)

Fig. 14. Distribution of the MPD of 71 keV ICEs in Si obtained using CASINO simulation, and average path length of these electrons in Si computed using PENELOPE.

2.5. Optimal thickness of Gd for neutron converter coating

As discussed earlier, indirect-conversion semiconductor neutron detectors with thicker conversion layer provide higher neutron absorption efficiency, but at the expense of increased self-absorption of charged particles in the conversion layer. The overall detection efficiency is thus a function of neutron absorption efficiency and the charged particle escape probability. The conflict between the neutron’s long mean free path and the charged particles’ short range in either B or Li compounds, which limits the detection efficiency, is also the case when Gd thin-film coating configuration is applied. Similarly, a thicker Gd layer offers higher neutron capture efficiency, whereas a thinner one offers a better chance for the ICEs to escape.

McGregor et al. extensively studied 10B and 6Li based semiconductor detector designs for optimal film thickness and maximum neutron detection efficiency [40, 41]. In this section, a single thin-film neutron converter coated semiconductor detector is discussed. The terminologies used in this discussion are defined based on the geometry shown in the inset of Fig. 17. The Gd-coated side is considered as the front-side of the detector, while the other side is regarded as the back-side. Thus, the geometry when neutrons are incident on the front- and back-sides is called front- and back-illumination, respectively. Intuitively, not only does a tradeoff exist between Gd’s neutron absorption and escape probability of ICEs, the direction of incident neutrons is also a variable in the overall device detection efficiency. The numerical evaluation of the optimized Gd coating layer thickness is presented in this section.

The optimal thickness is determined by calculating a parameter, which takes into account the combined effect of the increase in neutron absorption efficiency and the decrease in ICE escape probability, as the thickness of Gd is increased. The intrinsic neutron absorption efficiency of Gd is calculated as a function of thickness ( using the equation,

(7)

where is the macroscopic neutron absorption cross section of Gd.

A Gd slab of thickness T is divided into n sub-layers, each of thickness dt, such that

. The integral absorption efficiency at each layer is calculated by considering the total thickness of Gd up to that layer, i.e.,

, (8) for i = 0 to n, where , at first layer, at the second layer and so on

th up to at the n layer, which is the total thickness of Gd.

The incremental neutron absorption efficiency of each layer is then calculated using the equation

, for i = 1 to n. (9)

The number of ICEs resulting from neutron absorptions in each layer is calculated using the equation

, (10)

Where N is the number of incoming neutrons and 0.6 is the net ICE yield from a single neutron absorption.

The number of electrons that escape from each layer into the succeeding layer is calculated using a MCNP5 simulation. In the simulation, a 30 µm thick Gd slab of 1 cm2 cross sectional area was subdivided into a number of layers and bombarded with ICEs emerging from a planar isotropic source. The energy distribution of the ICE source is taken from Harms et al [55]. The geometry used in the simulation is shown in Fig. 15. A computer program in C++ was written to generate the MCNP input file with step size dt as the user-input to the program. The number of electrons escaping from each layer into the succeeding layer per source particle is computed using the F1 electron current tally.

Planar isotropic source of ICEs

Gd slab of 30 µm divided into number of layers

Fig. 15. Schematic illustration of the geometry used in MCNP5 simulation to compute the total number of electrons escaping from each layer of a thick Gd slab.

For a Gd coated semiconductor detector, the ICEs must penetrate or escape through the thickness of Gd starting from the reaction sites, in order to deposit energy into the

33 semiconductor. Thus, the maximum neutron detection efficiency occurs for a Gd layer with the highest electron escape efficiency.

The 'net' ICE escape efficiency for a given thickness T of Gd is then calculated using the equations

(11)

i.e., , (12) for the detection of forward emitted electrons, since electrons emitted in layer i need to traverse n-i+1 layers to reach the detector (Fig. 16).

Neutron path Gd slab of thickness T (= ndt)

Layer 1 2 3 …………i…………...n-1 n

Detector for backward Detector for forward emitted electrons …… ..… emitted electrons

1 2 Number of layers penetrated n-i+1 by forward emitted electrons, n-2 starting from the reaction n-1 sites in each layer n

And,

(7)

, (8) for the detection of backward emitted electrons, since electrons emitted in layer i need to traverse only i layers to reach the detector (Fig. 16).

Fig. 17. ICE escape efficiency calculated using the method outlined in this section for neutron detection in front- and back-illumination cases. As shown in the inset, the orientation of the detector with respect to the neutron path shows the corresponding efficiency curve.

The escape efficiency for the front-illumination case is evaluated for different thicknesses (T) from 1 to 30 µm using dt=0.5 µm. The maximum value of the escape efficiency occurs at T=5 µm (Fig. 17), because further increase of Gd film will only

35 block ICEs from reaching the detector. A further reduction in step-size (dt) increases the accuracy of the evaluation and that of ICE escape efficiency. For the back-illumination, the escape efficiency reaches the maximum value at T ≈ 25 µm and remains constant with further increase in thickness. However, about 90% of this maximum value is attained at 5

μm and the increase in efficiency from 5 to 25 μm is only marginal. Hence, for a device operating in a field containing neutrons in all directions, 5 µm is the recommended optimized Gd coating thickness. Results from above analysis agree with other investigations [43, 66-69].

2.6. Summary

The reaction products of Gd(n,γ)Gd* reaction encompass a convoluted spectrum of prompt gamma rays and the competing ICEs, characteristic x-rays and Auger electrons.

The prompt gamma rays, although released in abundance, are mostly of high energy, which makes a thin film semiconductor almost insensitive to them. However, ICEs are also emitted with high intensity primarily from the lowest three energy states of Gd* and represent the principal neutron signal in a thin semiconductor. This hypothesis has been confirmed by a MC simulation of Gd(n,γ)Gd* reaction energy spectrum in a thin Si detector, which indicated predominant energy deposition of ICEs, characterized by three main peaks at 29 keV, 71 keV and 78 keV in the spectrum.

This chapter also discussed the range and MPD of the prominent ICEs in Gd, Si and polyethylene. The average MPD of ICEs (29-78 keV) in Gd is found to be ~6 µm. The optimal thickness of Gd for neutron converter coating on a semiconductor detector is evaluated as 5 µm, considering both front and back neutron illumination of the detector. 36

MC simulations performed in SWORD for various thicknesses of Gd in both front- and back-illumination cases (discussed in section 6.1.2) supported this evaluation.

Chapter 3. Interaction of gamma rays with Gd

Owing to its high Z number (64), Gd possesses a good interaction probability with gamma rays (Fig. 18). Although the low Z number of certain semiconductor detectors make them insensitive to high energy gamma rays, those detectors incorporated with Gd for neutron conversion may be associated with inherent gamma ray background. 2D Graph 1

108 Rayleigh scattering 107 Compton scattering 6 10 Photoelectric absorption 105 Pair production 104 Total cross section 103 102 101 100 10-1 10-2 Cross-section (barns) 10-3 10-4 10-5 10-6 10-1 100 101 102 103 104 105 106 Energy (keV)

Fig. 18. Photon interaction cross sections for Gd generated using PENELOPE [64].

In photoelectric (PE) absorption, an incident photon ionizes one of the atomic shells of Gd, ejecting an orbital electron. The process requires the incident photon energy to be

38 greater than the shell binding energy, so the excess energy appears as the kinetic energy of the ejected electron. The atomic relaxation of Gd following PE absorption primarily involves the emission of characteristic x-rays, prominently from the K-shell. At photon energy as low as 60 keV, Gd has a PE cross section of 2,437.5 b [70] and x-ray production cross section of 1828 b [71-73] for the K-shell. The PE cross section for a given atomic shell tends to decrease as the energy of the photon increases. This trend results in characteristic saw-tooth absorption edges in the photoelectric cross-section

(Fig. 18) as the binding energy of each atomic shell is attained. For a high Z material such as Gd, PE absorption is still significant at photon energies above 100 keV [63], as also indicated in Fig. 18.

3.1. Evaluation of photoelectric interaction of Gd

The PE interaction probability of Gd is demonstrated with a simple MCNP5 simulation using 122 keV gamma ray disk source (i.e., beam of photons), a 10 µm thick

Gd foil and a thin Si detector (30 µm). Simulations are performed both with and without

(i.e., bare) the Gd foil covering the Si detector and the energy spectra of the detector are obtained (Fig. 19).

As expected, the energy spectrum of the bare Si detector indicates a much lower response due to the small volume and low Z number of the detector. However, energy spectrum of the detector covered by Gd contains several important features. Firstly, an intense energy peak is observed at 72 keV due to photoelectron emission from K-shell.

The 72 keV peak broadening is attributed to the energy loss of photoelectrons in Gd prior to reaching the detector. The other energy peaks at 6 keV and 43 keV are due to x-ray 39 emissions from L and K shells of Gd, respectively. The hump in the 20-40 keV energy region of the spectrum could be attributed in part to the Compton scattered electrons from the atomic shells of Gd. Thus, the simulation demonstrates a significant enhancement in gamma ray sensitivity of a thin film semiconductor owing to gamma activation of Gd.

While the study further provides insight into a practical means of boosting gamma ray detection efficiency with Gd as a converter, the main objective of this study is regarding gamma ray rejection when detecting neutrons.

2D Graph 1

5x10-5 Si detector covered by Gd Si detector 4x10-5

3x10-5

2x10-5

10-5

Counts per source particle per bin 0 0 20 40 60 80 100 120 140 Energy (keV) (bin size = 1 keV)

Fig. 19. Energy spectra of the Gd covered and bare Si detectors in response to 122 keV gamma rays obtained using MCNP5 simulation.

The gamma activation of Gd is also studied experimentally using a button sized radioactive gamma ray source, two commercial Si detectors and a thin Gd foil. For the purpose of this experiment, a 57Co gamma ray source, two identical Si charged particle

40 detectors and a 25 µm thick Gd foil were used. The detector and source specifications are listed in Table 7.

Detector Source CANBERRA button sized ORTEC ULTRA series Model Model radioactive 57Co calibration (CU-12-100U) source Type Ion implanted Type Isotropic Contact 500 A° boron implantation Depletion 10 µCi (at the time of the ≥ 100 µm Activity depth experiment) Active 14 keV, 122.1 keV and 25 mm2 γ-rays area 136.5 keV 12 keV for 5.486 MeV Resolution alpha (α)-particles Table 7. Specifications of the Si detector and the 57Co gamma ray source used in the Gd gamma ray activation experiment.

3.1.1. Experimental setup

The experiment was performed at a vacuum instrumentation facility built for this work at Nuclear Analysis and Radiation Sensor (NARS) Lab of the Ohio State University

(OSU). The experimental setup consisted of an in-house built aluminum (Al) vacuum system, and a 4-channel, 14-bit analog-to-digital converter (ADC) 100 MS/s digitizer

(N6724, CAEN SpA.) based data acquisition (DAQ) system (Fig. 20). The vacuum chamber was built primarily for radiation measurement and detector evaluation studies, but it can also be used as shielding from electromagnetic interference and ambient light.

The vacuum chamber, when evacuated using an advanced dry vacuum pump, is capable of achieving moderate vacuum level (10 to 1 millitorr (mtorr)), which is essential for 41 experiments involving minimal energy loss of charged particle radiation. Such a setup is also prerequisite for evaluation of detectors highly sensitive to visible light (e.g., photodiodes, photomultiplier tubes etc.).

Fig. 20. Vacuum instrumentation facility in the Nuclear Analysis and Radiation Sensor (NARS) laboratory at The Ohio State University.

Fig. 21. Al base plate inside the vacuum chamber with multiple detector and source mounts 42

The vacuum chamber houses a breadboard type Al mounting plate on which multiple detector and source mounts, and holders can be positioned firmly and in flexible geometries (Fig. 21). Detectors can be connected separately to 4 channels of a multichannel charge sensitive preamplifier outside the vacuum chamber.

Vacuum USB link for remote chamber High voltage operation power supply (HVPS)

Host PC Pre- Digitizer amplifier Data transfer link (USB) Dry vacuum pump

Fig. 22. Block diagram of the instrumentation system in NARS lab showing the vacuum chamber, the dry vacuum pump and the components of the digital DAQ system.

A block diagram illustration of the instrumentation system is presented in Fig. 22.

The figure shows the vacuum chamber connected to the dry vacuum pump, four independent detector channels of the digital DAQ system and the components of the

DAQ system which include charge sensitive preamplifier, high voltage power supply

(HVPS) and the digitizer interfaced with a host PC. The HVPS and the digitizer parameters can be adjusted remotely using control software installed on the PC.

3.1.2. Results and discussion

During the experiment, one of the Si detectors was covered by Gd foil, while the other detector was covered with a polyethylene cap (350 µm) to block photo- and

Compton-electrons from reaching the detector. The experimental geometry is illustrated in Fig. 23, which shows the 57Co button-sized source, Gd foil, polyethylene cap and the two detectors positioned on Al mounting structures.

Fig. 23. Experimental geometry inside the vacuum chamber showing the 57Co gamma ray source, Gd foil, polyethylene cap and two small area Si charged particle detectors.

The measurement was performed in the light-tight Al chamber as the Si detectors are highly sensitive to visible light and may cause potential damage to the preamplifier circuit under large reverse bias current. After evacuating the chamber to ~20 mtorr, the detectors' bias voltage was raised to -49.5 V. The detector pulse height data was acquired and stored on a PC using a digital pulse processing control software (DPHA). The energy

44 spectra of the detectors is obtained and shown in Fig. 24. Similar to the simulation result, the energy spectrum of Gd covered detector indicated a higher response compared to the polyethylene covered5 detector. However, the experimental spectra differ from that of the simulation, which could be due to - differences in geometrical setup and detector and Gd foil thicknesses between the simulation and the experiment, limited detector energy resolution and gamma rays backscattering inside the Al chamber. Also noteworthy is the energy loss of photoelectrons in Gd prior to reaching the detector due to a thicker Gd foil, which, in the case of experiment may have a significant effect on the energy spectrum shape.

2D Graph 2

0.06 Si detector covered by Gd Si detector covered by polyethylene 0.05

0.04

0.03

0.02

Counts per bin per second 0.01

0.00 20 40 60 80 100 120 140 Energy (keV) (bin size = 1.051 keV)

Fig. 24. Energy spectra of Gd covered and polyethylene covered Si detectors obtained in response to 57Co gamma rays.

5MC simulations presented in 'Appendix B: Attenuation of gamma rays in polyethylene' showed negligible attenuation of gamma rays (> 15 keV) by polyethylene (350 µm). 45

The high gamma interaction cross section of Gd also allows the conversion of high energy gamma rays into low energy K-X rays. Although a thin film semiconductor detector is almost transparent to high energy gamma rays, it is relatively sensitive to low energy K-X rays. It is supposed that the low energy K-X rays emitted after gamma activation of Gd interfere with the ICE signal in a semiconductor detector. A gamma-ray discrimination method is thus indispensable for a solid-state neutron detector based on

Gd.

While the gamma sensitivity of Gd is a potential drawback in neutron detection scenarios, it presents a suitable method of gamma ray detection with Gd as a converter to transduce high- or medium- energy gamma rays to low energy K-X rays. Subsequently, the detection of gamma rays is enabled using a thin film semiconductor detector.

Although the energy information of incident gamma rays is unattainable with such a detection mode, it suffices many application scenarios, in which only the intensity of gamma rays is of interest.

3.2. Summary

The gamma ray sensitivity of neutron detectors is a critical issue in homeland security applications, which stipulate high gamma ray rejection efficiency. Despite the small volume and low Z number of a semiconductor material (e.g., Si), the high gamma interaction probability of Gd potentially leads to inherent gamma ray background in Gd based semiconductor neutron detectors. Furthermore, the gamma activation products of

Gd could interfere with the low energy ICE signal in a thin film semiconductor. Hence,

46 an effective method for gamma ray separation is essential for a semiconductor neutron detector based on Gd.

While the gamma sensitivity of Gd presents an issue in the context of neutron detection, it points to a practical mode of gamma ray detection using a small volume semiconductor, as exemplified by the simulation and experiments in this chapter.

Chapter 4. Gamma ray rejection technique

Gadolinium based semiconductor neutron detectors bear the limitations of low energy of ICEs and high gamma interaction cross section of Gd. A 25 µm thick Gd film glued to a 500 µm thick cadmium telluride (CdTe) substrate has been developed as a thermal neutron detector for neutron imaging devices [74]. The gamma ray background in the energy spectrum was separable from Gd and Cd neutron capture gamma peaks due to high detector resolution (< 4 keV). Nonetheless, large volume of CdTe detector and Gd foil may still pose an issue when the detector is used in gamma abundant radiation environment.

A Si heterojunction diode with gadolinium oxide (Gd2O3) doped in hafnium oxide

(HfO2) films was reported as a potential neutron pulse counter with negligible gamma ray interference [75]. The thickness of the neutron sensitive HfO2 film was less than 100 nm, due to which the gamma sensitivity of the detector was much lower than that of neutrons.

However, such thin layers are evidently not in favor of high neutron detection efficiency.

Even so, the need for a high neutron detection efficiency and good gamma ray rejection of a Gd based semiconductor detector has not been well addressed. A high mass fraction of Gd in either coating or doping form not only improves the neutron sensitivity but also enhances the gamma ray sensitivity of the detector. In view of these aspects, a gamma ray discrimination capability is deemed essential for a Gd based semiconductor

48 neutron detector.

The main focus of the current research is to develop a gamma ray rejection technique for a Gd based semiconductor neutron detector and a method for real time discrimination of gamma rays in such a detector based on the proposed technique. In the following sections, a literature review of existing gamma ray discrimination techniques in solid state neutron detectors is presented, which provided a basis to the gamma ray rejection method proposed in section 4.2.

4.1. Literature review of gamma ray discrimination techniques

A few gamma ray discrimination schemes in solid-state neutron detectors have been investigated and applied to the real detector. Among them, pulse shape discrimination

(PSD), which has been widely applied in plastic and liquid scintillation neutron detectors and considered effective for neutron-gamma (n-γ) separation in solid state neutron detectors [76], is reviewed in detail in this chapter.

The gamma ray discrimination techniques covered here are -

 Pulse shape discrimination

 Pulse height discrimination

 Coincidence detection method

 Spectrum subtraction technique

4.1.1. Pulse shape discrimination (PSD)

Ionizing radiation can be detected by means of scintillation light produced in certain materials as a result of radiation interaction with the material. For most of the organic 49 scintillating materials (scintillators), the scintillation light is composed of a longer lived component in addition to prompt fluorescence. The decay time of the longer lived component is characterized by several hundred nanoseconds whereas it is only a few nanoseconds for the prompt decay time [37]. An important characteristic of the delayed component is that the fraction of light contained in it often depends on the type of the exciting radiation. The decay times of the scintillation light produced by heavily ionizing radiation such as α-particles or protons is longer than that due to electrons [77]. In other words, the intensity of the slower component of light pulse generated in an organic scintillator when excited by recoil protons is different from that by electrons [78]. Such feature can be used to differentiate different particles that deposit the same energy in the detector [37]. This technique is called pulse shape discrimination (PSD). Roush M.L. first used this technique for the separation of gamma ray background from neutron signal [79].

Gamma ray discrimination was achieved in a proton recoil spectrometer using two variations of PSD, a ‘dividing method’ and a ‘rise time method’, exploiting the difference in specific ionization caused by a recoil proton and a gamma ray induced fast electron

[80]. Two most common techniques that exploit the aforementioned property of organic scintillators for n-γ separation using analog electronics are - the rise time discrimination technique [79, 81] and the charge comparison technique [82, 83]. However, in recent decades, the advancement in digital signal processing and the development of fast ADCs enabled digital implementation of several types of PSD techniques such as pulse gradient analysis based [84], artificial neural network based [85] and frequency domain based

(e.g., wavelet transform [86, 87]) methods. Both rise-time and charge comparison

50 methods have been extensively studied over years, compared for better gamma ray discrimination performance [88-91] and even customized in some cases into a hybrid

PSD technique for performance enhancement [92].

4.1.1.1. Rise-time discrimination

The rise-time discrimination may be performed in different ways. A voltage pulse is differentiated, and the time between the start of the pulse and the point where it crosses the baseline (zero) is measured. The measured time interval remains the same for all pulses with a fixed rise-time, regardless of the pulse amplitude. Thus, pulses with different rise-times can be separated. This method was discussed in detail by Fulle et al

[93]. The concept of constant fraction of pulse height triggering used in the operation of a constant fraction discriminator circuit was explained by Gedcke et al [94]. Two such constant fraction timing discriminators can be employed to identify the point at which a pulse crosses levels that are different fractions of its maximum amplitude and thus be separated [37]. A method that has been predominantly studied is the zero crossing technique [79, 81, 95]. In this method, the anode pulse is passed through a pulse shaping circuit (such as a CR-RC-CR or doubly delay line shaper) to generate a bipolar pulse. The time interval between the start of the pulse and the zero-crossover point is independent of the pulse amplitude, but is a function only of the pulse shape and rise-time and can be used to discriminate between pulses with different shapes [37]. Rise-time discrimination method is based on the underlying principle that the gamma induced light scintillations contain a much larger component of the output from the fast decay than from the slower

51 decay component, when compared to the same relative intensities of light output produced from neutron induced scintillations [96].

Digital implementation of the zero crossing technique has been made possible with the evolution of digital signal processing and the development of fast ADCs. Recently, an algorithm for digital implementation of the conventional zero-crossover method has been described and shown to offer better performance than the charge comparison method in the low-energy range [97].

4.1.1.2. Charge comparison

The charge comparison method has been implemented in detail by Sabbah et al [98].

In this method, the voltage pulse is integrated over two different time windows, one is usually the entire duration of the pulse and the other is only a small time region. The ratio of these charges will be approximately the same for pulses with identical shape [37]. A direct comparison of the relative amounts of the fast and slow components of the scintillation light was first performed by Morris et al in 1976 [96]. This preliminary investigation based on a digital implementation produced satisfactory level of discrimination between neutrons from a PuBe source and gamma rays from a 60Co source and also proved to be a promising method for performing PSD. More recently, a digital charge comparison method was applied for discrimination between fast neutron and gamma ray interactions in a liquid organic scintillation neutron detector (BC501 A) [99]; the current pulse was integrated over two different time intervals, the total pulse duration and the tail portion of the pulse (corresponding to the delayed component) using an integrating charge to digital converter (QDC). 52

Neutrons and gamma rays were successfully separated using analog PSD modules but with a tradeoff on the maximum allowable count rate and data reprocessing ability. A digital PSD system using a commercial transient recorder card was investigated for n-γ separation in three different organic scintillators, and the charge comparison method was applied simultaneously for gamma discrimination and pulse height analysis [100]. The current pulses were integrated on two different time intervals corresponding to the fast and slow components, and ratio of the two charges was used to classify an event as neutron induced or gamma induced. Good data reprocessing and high count rate capabilities were demonstrated using this digital PSD system, and its advantages over analog PSD modules in high intensity radiation measurements were discussed.

4.1.1.3. Comparison of the two techniques

Both zero crossing and charge comparison were applied to a B loaded plastic scintillator for discrimination between fast neutrons, thermal neutrons and gamma rays

[91]. From a detailed comparison of the two methods, it was shown that the charge comparison method works correctly for discrimination between only fast neutrons and gamma rays, whereas the zero crossing method enables discrimination between all three interactions (i.e., fast neutron with hydrogen, thermal neutron with boron and gamma ray with the plastic scintillator).

Digital pulse shape analysis algorithms were developed based on the charge comparison method and the zero-crossover method for discrimination between fast neutrons and gamma rays in a liquid scintillator (BC-501) [101]. The algorithms were compared against each other and separately to an analog PSD unit based on zero- 53 crossover method. The digital algorithms were found to provide better discrimination than the analog unit. The effect of sampling frequency and number of bits resolution of the ADC on the discrimination performance of the system was studied in detail.

Besides organic scintillators, PSD techniques have also been used for effective n-γ separation in inorganic scintillators [102, 103]. Lee et al. characterized, tested and validated an application specific integration circuit (ASIC) based readout system for its

PSD capability in use with Cs2LiYCl6:Ce inorganic scintillator. Their study involved using a waveform digitizer at first to obtain optimized charge integration windows for

PSD, and then applying them to the ASIC based readout system. Yamazaki et al. demonstrated the PSD capability of Ce:LiCaAlF6 scintillator to detect neutrons under an intense high energy gamma ray field using the rise time information of the scintillation light pulses.

4.1.1.4. PSD in semiconductors

The voltage pulse induced by ionizing radiation in a semiconductor detector contains the necessary information to identify the particle type [104]. The rise time of the voltage pulse induced in a semiconductor depends primarily on the charge transit time. This charge transit time in turn is dependent on the distance traversed by the e-h pairs inside the semiconductor depletion region before ultimately reaching the electrodes. The pulse rise time also depends on the range of the primary interacting particle i.e., the charge and mass of the particle. Heavy charged particles such as alphas or protons give rise to a faster voltage pulse owing to their shorter range in a semiconductor and consequently, localized charge generation and faster charge collection, than a lighter charged particle 54 such as electron. The charge created by a fast electron is dispersed over a larger volume compared to that by a heavy particle. Hence, the charge collection time in the case of a heavy charged particle is lesser than in the case of a fast electron.

It follows that, the electronic signals induced by particles with different stopping power such as α-particles, protons and electrons, are characterized by different shapes.

Thus, the voltage pulses resulting from electrons can be distinguished from those due to protons or alpha particles by exploiting the differences in corresponding charge transit times in a semiconductor detector [105].

4.1.1.5. Advantages and limitations

The advantages of PSD, followed by its limitations when applied in a Gd based semiconductor neutron detector are discussed in this section.

 Recent advancements in digital signal processing have greatly simplified the

execution of PSD in the digital domain compared to the analog domain [106].

 Field programmable gate array (FPGA) based digital DAQ systems are

commercially available with embedded PSD software.

 PSD using digital signal processing enables real time separation and event-by-

event classification of neutrons and gamma rays, as demonstrated in some cases

[106, 107].

The applicability of PSD for n-γ separation in a Gd based semiconductor neutron detector, however, is not practical due to the following reasons.

1. Unlike the neutron capture reaction in 10B, 6Li and 3He, where charged particles

with distinct energies are released, neutron absorption in Gd results in the 55

emission of complex spectrum of prompt gamma rays, ICEs (primarily in the

range 29–246 keV), characteristic x-rays and Auger electrons with a variety of

emission intensities. As discussed earlier, the ICEs are released in only about 60%

of the neutron absorption reactions in Gd. Voltage pulses induced by these low

energy electrons and x-rays may not be discernible from those of background

gamma rays. Also, it is unlikely to observe voltage pulses with distinct rise and

decay times considering the wide range of low energy ICEs with varied emission

intensities.

2. In a semiconductor detector, the principal neutron signal in the form of ICEs is

identical in type to the gamma ray induced signal in the form of secondary

electrons (SEs). Hence, PSD between neutrons and gamma rays in effect, implies

the same between ICEs and gamma ray induced SEs. This scenario differs from

the PSD applications reported in literature, where discrimination between

particles such as neutron induced proton, alpha or triton and gamma ray induced

SE was studied in general. Since the specific ionization induced by ICEs and SEs

in a semiconductor detector are identical, the associated pulse shapes are not

expected to be different.

In view of the aspects discussed above, the execution of PSD for n-γ separation when Gd is applied is severely restricted.

4.1.2. Pulse height discrimination

Pulse height discrimination (PHD) between neutrons and gamma rays in a detector is based on the amplitude of the voltage pulses induced by corresponding ionization events. 56

Since the magnitude of electric charge created in a detector is proportional to the energy deposited by the incident radiation, the amplitude of the voltage pulse (or pulse height) in turn is proportional to the deposited energy. In a neutron detection event, the energy deposited in the detector originates from the reaction energy of the nuclear reaction occurring in the neutron converter. Gamma ray discrimination with pulse height can be achieved if the energy deposition of the nuclear reaction products is sufficiently greater than that of gamma rays. Separation of gamma rays using this technique is relatively simple, in that it requires only a low level discriminator setting on the detector pulse height, which greatly simplifies the detector electronics.

PHD can be used for effective n-γ separation in thin film semiconductor detectors based on 10B(n,α)7Li and 6Li(n,α)3H conversion reactions. This is due to the highly energetic charged particles emitted in these reactions against the low energy deposition of background gamma rays. A Si diode slow neutron detector based on 6LiF converter was demonstrated with low gamma ray sensitivity [18]. A small detector active volume, in combination with a sufficiently high PHD setting enabled the discrimination of gamma ray events from slow neutron events. Indirect-conversion semiconductor detectors based on 10B and 6Li neutron conversion layers indicated effective gamma ray rejection using

PHD, even with low thermal neutron detection efficiency [108].

Although the total Q-value of 157Gd(n,γ)158Gd* and 155Gd(n,γ)156Gd* are as high as

7.937 MeV and 8.536 MeV, respectively, the principal neutron signal in a thin film semiconductor originates mostly from the low energy ICEs. While it has been proven to be effectively detectable in semiconductor devices, the ICE signal falls in the range of

57 background signal induced by gamma rays. Consequently, a low level threshold on detector pulse height may not only eliminate the background signal but also result in a loss of the low energy ICE signal.

4.1.3. Coincidence detection method

Detection in coincidence mode enables separation of a signal from undesirable background or random noise by operating two or more detectors in coincidence. Events are either registered or rejected based on whether the detectors in a coincidence system produce pulses in concurrence (i.e., within an accepted time window).

A compact portable neutron detector was designed using LiI(Eu) scintillation crystal and photodiode, and coincident double photodiode readout was implemented to discriminate between neutrons and gamma rays [109]. The reaction energy of 4.78 MeV for 6Li neutron conversion (6Li(n,α)3H) was sufficient to achieve energy discrimination of gamma ray events from neutron events in the scintillator crystal. However, direct interaction of gamma rays inside the photodiode could create pulses with amplitudes large enough to interfere with the neutron signal.

An electronic neutron sensor comprising two Si diodes facing each other and separated by a thin LiF layer coated on each diode was reported by Ndoye et al [110].

The α- and 3H- particles from 6Li nuclear reaction are emitted in coincidence in diametrically opposite directions. A coincidence method for the detection of these particles, in conjunction with an electronic threshold setting eliminated the gamma ray induced events from the detector spectrum.

The γ-γ coincidence approach in neutron detection greatly eliminates the gamma ray background and achieves a good signal-to-noise ratio [111]. However, in the present study, the reaction products of Gd(n,γ)Gd* encompass a complex spectrum of prompt gamma rays with competing ICEs, Auger electrons, and characteristic x-rays. Although emitted in coincidence, most of the prompt gamma rays are characterized by high energy and low emission intensities [112], which would result in a low coincidence detection efficiency with small volume semiconductor detectors.

4.1.4. Spectrum subtraction technique

A gamma ray discrimination method using spectrum subtraction for a Si PIN diode neutron detector was reported by Aoyama et al [6]. Two Si PIN photodiodes were separated by a 25 μm thick Gd foil, a 2 mm Lucite (plastic) board and a tin (Sn) foil. The

Gd foil was placed outside the sensitive surface of the first diode and the Sn (Z=50) foil having roughly the same mass thickness as that of Gd foil was attached to the sensitive surface of the second diode. This canceled out the generation and absorption of Compton electrons in the Gd foil when the two diode signals were subtracted, while the Lucite board prevented the ICEs of Gd neutron capture from entering the second photodiode.

Thus, gamma ray separation was achieved.

Fernandez et al investigated the separation of neutron signal from the gamma ray component in mixed n-γ fields using differential pulse analysis techniques with a double silicon diode [113]. A detector structure having two Si diodes positioned on either side of a single silicon substrate with a 40 µm thick polyethylene converter in the front was used for real time neutron dosimetry. Subtraction of the integrated spectrum obtained in the 59 rear Si diode from that obtained in the front one produced a spectrum mainly due to protons originating in the polyethylene converter.

An electronic neutron dosimeter using two passivated implanted Si detectors and a B implanted polyethylene converter was reported by Barelaud et al [114]. Differential response of the two detectors eliminated the gamma ray component in the final dosimeter response. Gamma ray interference to the dosimeter response in a neutron field was investigated by Paul et al [115]. Their study demonstrated that several dosimeter parameters such as depletion depth of the diodes, threshold voltage and constitution of the sensor (i.e., structure and materials used for its composition) must be optimized in order to achieve discrimination of the greatest number of gamma ray pulses.

A similar principle for separation of neutrons and gamma rays was adopted for a prototype miniature thermal neutron detector designed using a 6LiF converter and scintillating fibers [116]. In order to remove the gamma ray contribution in the amplitude

(energy) spectrum, a difference of the amplitude spectra obtained with and without the

6Li neutron converter was considered. An alternative design was also proposed, which involved two such identical detectors positioned side by side, one with and one without the converter.

Although gamma ray discrimination using spectrum subtraction was proven effective, an inherent drawback in the technique is that it is an offline procedure. In other words, neutron and gamma ray events are not separable in real time. In addition, event- by-event classification of neutrons and gamma rays is not achieved using spectrum subtraction.

4.2. Proposed gamma ray discrimination scheme

Although the small volume and low Z number of a thin-film semiconductor detector

(e.g., Si) make it relatively insensitive to external gamma rays, those low-energy external gamma rays that fall within the ICE energy range could lead to false positive neutron detection. One such application where this could present an issue is in SNM detection, which typically occurs in environments that are “neutron signal starved”, but “gamma signal abundant” [2]. To mitigate the likelihood of a false detection due to gamma rays, a twin-detector scheme is proposed using Gd as a neutron converter, and two detectors to identify and rejecting external gamma rays, as shown in Fig. 25. Rejection is achieved by introducing a layer of Gd foil and another layer of an electron separator material

(polyethylene) into the composite detector scheme. Detector 1 is placed in direct contact with a Gd foil, allowing the detection of ICEs, as well as gamma rays produced by neutron absorption in Gd. A second detector is placed on the rear of the Gd layer, but uses a polyethylene layer of appropriate thickness to stop all Gd generated ICEs and

Auger electrons from reaching detector 2. In this configuration, detector 2 is sensitive only to gamma rays. Hence, detector 1 generates a combined signal induced by both neutrons and gamma rays, whereas detector 2 produces a signal induced only by gamma rays. Subtracting the two detector signals would yield a net signal induced solely by neutrons.

Polyethylene Detector 1 separator

Prompt Neutrons gamma rays IC electrons Gd* K X-rays

Background gamma rays

Gd foil Detector 2

Fig. 25. Gamma ray rejection scheme with two semiconductor detectors, a Gd layer and a polyethylene layer.

Some limitations of the proposed method were also identified during a preliminary investigation and are presented below.

 The proposed gamma ray rejection scheme is heavily dependent on the detection

of ICE signal for registering neutron events. Although a large number of prompt

gamma rays in addition to characteristic x-rays are released in Gd neutron

reaction, the associated component of the neutron signal is effectively cancelled

out when subtracting the two detector signals.

 The direct interaction of external gamma rays with detectors 1&2 is supposed to

generate identical response in the detectors. In addition, K-X rays emitted from

gamma ray activation of Gd induce identical response in the detectors.

Subtraction of the two detector signals effectively cancels out the identical

gamma ray component in the resultant signal. However, electrons emitted due to

PE absorption in Gd penetrate detector 1 but are blocked by the polyethylene

separator from reaching detector 2. Thus, a residual gamma ray component due to

photoelectrons still exists in the resultant signal after subtraction.

 Unlike the case in PSD, event by event separation of neutrons and gamma rays is

not achieved with the proposed method.

4.3. Summary

This chapter discussed the purpose and objectives of current research. The research objectives are identified as, developing a gamma ray rejection technique and a method for achieving real time separation of neutrons and gamma rays in a Gd based semiconductor neutron detector. A literature review of the existing gamma ray discrimination techniques in solid state detectors is presented. The limitations of each discrimination technique in the context of n-γ separation in a Gd based semiconductor detector are discussed.

A technique for rejection of gamma rays when using Gd in a semiconductor neutron detector was discussed. The proposed method is based on the isolation of ICE signal by subtracting gamma ray signal of one detector from mixed ICE and gamma ray signal of a second detector. Although event-by-event classification of neutrons and gamma rays is not achieved using the proposed method, real time separation of the events is still achievable by operating the detectors in current mode.

The following chapters discuss the feasibility of the proposed gamma ray discrimination technique in detail. The underlying principles of the discrimination method are demonstrated using MC simulations and the simulation results are validated experimentally.

Chapter 5. Simulation of the gamma rejection method

Extensive simulations were performed to demonstrate the feasibility of the proposed gamma ray rejection scheme and develop fundamental understanding of the method.

5.1. Neutron interaction

Thermal neutron interactions with the twin-detector were simulated in SWORD by a model of 0.0253 eV monoenergetic neutron beam orthogonally striking two Si detectors of 100 µm thickness, separated by 5 µm thick Gd and 350 µm thick polyethylene layers.

The twin detector (DET) structure can understood using the short form notation

'DET1GdPolyethyleneDET2'. Simulations were performed in two different cases based on the direction of the particle beam (BEAM) with respect to the twin-detector. The case in which the particle beam first strikes detector 1 (i.e., DET1) surface is termed as

CONFIG1 and understood using the notation BEAM→DET1GdPolyethyleneDET2. In the other case termed as CONFIG2, the direction of the particle beam is antiparallel to that in CONFIG1 and thus, strikes the detector 2 (i.e., DET2) surface first. The

CONFIG2 can be understood using the notation DET1GdPolyethyleneDET2←BEAM.

The energy spectra of the detectors were recorded in both cases. The detector-source geometry in CONFIG1 is illustrated in Fig. 26. The detector energy spectra from

CONFIG1 and CONFIG2 are shown in Fig. 27 and Fig. 28, respectively.

Polyethylene Si detector 1 separator (100 µm) (350 µm)

0.0253 eV thermal Gd foil Si detector 2 neutron disk source (100 µm) (5 µm)

Fig. 26. Geometry used in SWORD simulation in CONFIG1 to model the twin-detector structure and thermal neutron (0.0253 eV) source beam. 2D Graph 3

4000 Si detector 1 Si detector 2

3000

2000

Counts per bin 1000

0 0 50 100 150 200 250 300 Energy (keV) (bin size = 1 keV)

Fig. 27. Energy spectra of electrons from the two Si detectors obtained using SWORD simulation in CONFIG1.

While detector 1 energy spectrum shows the characteristic ICE peaks, the energy spectrum from detector 2 indicates that the ICEs are fully stopped by the polyethylene layer before reaching the detector (Fig. 27, Fig. 28). This confirms that subtraction of the two detector spectra generates a detector response due primarily to ICEs.

Not surprisingly, neutron sensitivity of the twin-detector structure is higher in

CONFIG1 (Fig. 27) compared to that in CONFIG2 (Fig. 28), which is also exemplified by the net counts in ICE peaks of the spectra. However, the corresponding energy spectra of the detectors are identical in both the cases without regard to the twin-detector orientation to neutrons. This result indicates that the proposed gamma ray discrimination technique is insensitive to the direction of incident neutrons.

2D Graph 2

2500 Si detector 1 Si detector 2 2000

1500

1000

Counts per bin

500

0 0 50 100 150 200 250 300 Energy (keV) (bin size = 1 keV)

Fig. 28. Energy spectra of electrons from the two Si detectors obtained using SWORD simulation in CONFIG2.

5.2. Gamma ray interaction

A question remains regarding whether spectrum subtraction works for external gamma rays. In other words, whether the two detectors have the same response to gamma rays originated internally from Gd neutron capture and from the external gamma rays.

The proposed gamma-ray rejection scheme is based on the assumption of identical gamma ray response in both detectors; hence, subtracting the two detector signals effectively cancels out gamma ray interference in the final detector response.

Nevertheless, accurate simulations are required to substantiate this hypothesis.

Simulations were performed as in the neutron interaction case, with a gamma ray source substituting the thermal neutron source. A disk gamma ray source was created in MCNP5 for modeling gamma ray interaction with the twin-detector in both configurations,

CONFIG1 (Fig. 29) and CONFIG2.

Polyethylene Si detector 1 separator (100 µm) (350 µm)

Gamma ray Si detector 2 disk source Gd foil (100 µm) (5 µm)

Fig. 29. Geometry used in MCNP5 simulation in CONFIG1 to model the twin-detector structure and gamma ray source beam.

In the simulation, 57Co was chosen as the gamma ray source in order to demonstrate the sensitivity of a thin semiconductor detector to low energy gamma rays. Energy spectra of the detectors were recorded in both CONFIG1 and CONFIG2, and are presented in Fig. 30 and Fig. 31, respectively.

2D Graph 3

6x10-5 Si detector 1 Si detector 2

5x10-5

4x10-5

3x10-5

2x10-5

10-5

Counts per source particle per bin 0 25 50 75 100 125 150 175 200 Energy (keV) (bin size = 1 keV)

Fig. 30. Energy spectra of the two Si detectors in response to 57Co gamma rays obtained using MCNP5 simulation in CONFIG1. 2D Graph 4

Si detector 1 6x10-5 Si detector 2

5x10-5

4x10-5

3x10-5

2x10-5

10-5

Counter per source particle per bin 0 25 50 75 100 125 150 175 200 Energy (keV) (bin size = 1 keV)

Fig. 31. Energy spectra of the two Si detectors in response to 57Co gamma rays obtained using MCNP5 simulation in CONFIG2.

The spectra in Fig. 30 indicate the two 57Co gamma peaks (122.1 keV and 136.5 keV6) and K-X ray peaks (42.3 keV and 43.0 keV) from gamma activation of Gd. The spectral features of detector 1 from ~50–75 keV and ~80–120 keV could be attributed to the energy deposition of photoelectrons ejected from Gd following PE absorption of

122.1 keV photons. The emitted photoelectrons are completely stopped by the polyethylene layer, thus, the corresponding features in the detector 2 spectrum are absent.

Nonetheless, a differential spectrum due to the subtraction of detector 2 spectrum from that of detector 1 indicates a significantly reduced overall gamma ray response.

Most of the discussion pertaining to CONFIG1 remains applicable to the detector spectra in CONFIG2, except for the energy deposition of photoelectrons in detector 1, which is much higher than in the former case. This indeed is explained by the more intense photoelectron peaks in detector 1 spectrum of Fig. 31. Consequently, the differential energy spectrum is expected to contain a significant residue of the gamma ray component. This result indicates that CONFIG2 is not as effective a configuration as the

CONFIG1.

In order to further understand the directional dependence of gamma sensitivity of the twin-detector, simulations were repeated with 235U gamma ray source in both configurations. 235U is characterized by a wide spectrum of gamma rays (32 - 795 keV7) resulting from α-decay to thorium-231 (231Th), and the understanding of the twin-detector response to 235U gamma rays is in direct relevance to SNM detection application. A point isotropic source is used in the simulations in order to represent a more generic scenario.

6 The 14.0 keV gamma ray peak is not shown in the spectra. 7 http://atom.kaeri.re.kr/cgi-bin/decay?U-235%20A 69

The detector spectra from the simulations in CONFIG1 and CONFIG2 are presented below. 2D Graph 1

3.5x10-7 Si detector 1 Si detector 2 3.0x10-7

2.5x10-7

2.0x10-7

1.5x10-7

10-7

5.0x10-8

Counts per source particle per bin 0 0 50 100 150 200 250 300 Energy (keV) (bin size = 1 keV)

Fig. 32. Energy spectra of the two Si detectors in response to 235U gamma rays obtained using MCNP5 simulation in CONFIG1.

2D Graph 1

4x10-7 Si detector 1 Si detector 2

3x10-7

2x10-7

10-7

Counts per source particle per bin 0 0 50 100 150 200 250 300 Energy (keV) (bin size = 1 keV)

Fig. 33. Energy spectra of the two Si detectors in response to 235U gamma rays obtained using MCNP5 simulation in CONFIG2. 70

As observed earlier, the detector spectra are almost identical in CONFIG1 (Fig. 32), while the CONFIG2 spectra (Fig. 33) indicate higher response in detector 1 owing to increased energy deposition of photoelectrons. The simulations thus confirm that the gamma ray separation method is most effective in CONFIG1 and is limited by the enhanced gamma sensitivity of detector 1 in CONFIG2.

5.3. Summary

MC simulations were performed for comprehensive evaluation of the gamma ray discrimination method. Gd neutron capture reaction energy spectra of detector 1 indicate the dependence of neutron sensitivity on the direction of neutrons, in that the sensitivity is higher in CONFIG1 than in CONFIG2. The result is also in agreement with the numerical evaluation of section 2.5. Detector 2 spectra remain unaffected due to the polyethylene electron separator.

In contrast, the gamma ray sensitivity of detector 1 is enhanced in CONFIG2, and is much higher than that of detector 2, when compared to the CONFIG1. However, gamma ray separation is still feasible in either configuration and it is concluded that, the discrimination method is most effective in CONFIG1, which also allows for higher neutron sensitivity of the detector.

Chapter 6. Experimental study

The simulations discussed in the previous chapter clearly demonstrated the function of the proposed gamma ray rejection scheme. The focus in this chapter is on experimental validation of the hypotheses and simulation results presented in previous chapters, thereby gaining a better understanding of the neutron and gamma ray sensitivity of Gd, and the effectiveness of the gamma ray separation technique. Experiments were performed for comprehensive evaluation of neutron and gamma sensitivity of Gd, and for studying gamma ray separation using the twin-detector based rejection scheme. Different experimental procedures, observations and results from each experiment are discussed in the present chapter.

6.1. Evaluation of neutron sensitivity of gadolinium

Neutron sensitivity of Gd is quantified in terms of the energy deposition spectra of

ICEs in a small volume semiconductor detector. After obtaining the ICE spectrum with high accuracy, the measurements were repeated for Gd foils of different thicknesses to validate the numerical evaluation of the optimal thickness of Gd coating (section 2.5). In addition, the energy deposition spectra and energy deposition rates of charged particles of

Gd, B and Li neutron capture reactions in a semiconductor detector are compared, and the efficacy of Gd neutron conversion against that of the competing B and Li is evaluated.

6.1.1. Measurement of the ICE energy spectrum

Results from MC simulations and discussions presented in previous chapter confirmed the role of ICEs as principal neutron signal carriers in a thin film semiconductor. Although the simulation results are fairly convincing, experiments were performed for validation and practical understanding of the neutron sensitivity of Gd in a semiconductor detector. In the following section, a brief summary of past research on Gd

ICE measurement is presented in order to gain a standpoint on the current experimental work.

The ICE spectra from Gd neutron capture were first reported by Fiegl et al [66, 117], in which Si surface barrier detectors coupled with natural Gd and 157Gd foil converters were used as thermal neutron detectors. The spectra of both forward and backward emitted ICEs and the sum spectra were measured when Gd foils of different thicknesses were placed on front and rear surfaces of the detector, and sandwiched between two detectors, respectively. Schulte et al measured the ICE spectrum from the front detector of a large area Si-Gd-Si sandwich detector based on Si photodiodes [118]. Electronic noise due to large detector capacitance necessitated a lower level threshold of 40 keV in the electron energy spectrum. The energy spectra of ICEs were also demonstrated by

Petrillo et al [119] using crystalline Si diodes coupled to Gd and Gd2O3 foil converters with medium-speed readout electronics. Nonetheless, all the previously reported spectra in general, indicated low energy resolution and high noise threshold. Subsequently,

Bruckner et al measured the energy spectra of Gd neutron reaction products at different neutron wavelengths in a position sensitive Si planar detector with lower noise and higher 73 energy resolution [43]. However, gamma ray interference to the neutron signal in Gd based solid-state neutron detectors has not been well addressed.

6.1.1.1. Experimental setup

The experimental studies were performed at the OSU Research Reactor (OSURR), which is a 500 kW thermal, pool-type light water materials testing research reactor. An external thermal neutron beam facility was recently built at the OSURR [120] for performing neutron based in-situ material characterization such as prompt-gamma activation analysis, neutron depth profiling, neutron radiography etc., and evaluation of novel neutron detectors. The facility provides a collimated, small-sized (<30 mm diameter), relatively clean thermal neutron beam to a workbench consisting of a large volume (178 L) stainless-steel high-vacuum chamber and an 8-channel 14-bit 100 MS/s

ADC digitizer (V1724, CAEN SpA.) based DAQ electronics [121]. The vacuum chamber houses an array of eight identical Si charged particle detectors positioned on independently adjustable detector mounts to obtain annular view of the sample with the same solid angle (Fig. 34). The specifications of the detectors are given in Table 8. The neutron beam passes through the center of the detector array and strikes the sample mounted on an Al sample holder. The detectors are connected separately to eight hybrid charge sensitive preamplifiers inside the chamber.

ORTEC ULTRA ion implanted Model (U016-300-100) Contact 500 Å boron implantation Active area 300 mm2 Minimum depletion 100 µm depth Resolution (FWHM) 16 keV @ 5.486 MeV α-energy Table 8. Specifications of the Si charged particle detectors used in the ICE measurements.

Fig. 34. Experimental setup inside a large stainless-steel high-vacuum chamber used to perform the Gd ICE measurements.

6.1.1.2. Results and discussion

The OSURR operated at 5 kWth for this measurement and delivered a thermal equivalent neutron flux of about 9.6  104 cm−2·s−1 and a gamma dose rate of about 27 mRad·hr-1 at the sample location. A schematic illustration of the setup is shown in Fig.

35.

Uncovered Si detector Si detector Sample covered by holder polyethylene

Gd foil Neutron beam Fiber-optic Charge-sensitive data preamplifiers transfer Host link PC

14 bit 100 MS/s digitizer ± 12 V power High-voltage supply to power supply preamplifiers to detectors

Fig. 35. Schematic illustration of the experimental setup and the DAQ electronics used in the ICE measurements.

A thin Gd foil (1.25 cm  1.25 cm  0.0025 cm) was mounted on the sample holder

(Fig. 34) inside the high-vacuum chamber. The multidetector setup provided an opportunity to acquire multiple spectra during one experiment. Therefore, two of the eight Si charged particle detectors were covered with 350 µm thick polyethylene caps to shield them from the ICEs, while the remaining six detectors were left unshielded. The detector signals were acquired from the eight independent detector channels using digitizer-based DAQ electronics. A lower level threshold of 23.3 keV was set on the detector pulses. A programmable trapezoidal energy filter was used to process the digitized pulses and enabled precise determination of the pulse height. Data were acquired in list mode, in which only the time stamp and pulse height corresponding to each detection event are recorded. The list data was stored on a host PC and were 76 analyzed offline to generate customized histograms of the detector pulse height with appropriate bin widths. A background measurement was also performed at the same instrument settings by removing the Gd foil from the chamber. The energy spectra of the six uncovered (i.e., unshielded from ICEs) Si detectors were identical. The energy spectra of the two covered (i.e., shielded from ICEs) Si detectors were also identical to each other but differed from that of the six uncovered detectors. Energy spectra representative of the set of uncovered detectors and the set of polyethylene covered detectors obtained after background subtraction and detector energy calibration (Appendix A: Detector calibration for identifying the ICE peak energies) are illustrated in Fig. 36. A highly intense and broad energy peak centered at ~71 keV is clearly seen from the bare detectors and two much less intense peaks at 131 keV and 173 keV are also visible. The origin of the 71 keV peak is due to the combined energy deposition of 29-88 keV ICEs and characteristic x-rays in the detector. The peak broadening is attributed in part to the limited energy resolution of the detector, but more to the fact that ICEs lose energy when escaping the Gd foil before reaching the Si detector. It is interesting to note that such peak broadening is indeed the electron depth profiling that could be potentially utilized to measure the thickness of a Gd thin film deposited on a substrate. The energy spectrum from the covered detectors indicates that the 350 µm thick polyethylene layer completely blocked the ICEs while still permitting some low energy gamma/x-rays into the detector.

An MCNP5 simulation confirmed that the 350 µm thick polyethylene cap used in this study can absorb gamma rays only up to ~15 keV (Appendix B: Attenuation of gamma rays in polyethylene).

2D Graph 1

1.2 ICE (71 keV) Uncovered Si detector Si detector covered by 1.0 polyethylene

0.8

0.6 ICE (173 keV) 0.4 ICE (131 keV)

0.2

Counts per bin per second per bin per Counts

0.0 0 50 100 150 200 250 300 Energy (keV)

Fig. 36. ICE energy spectrum measured using Si detectors during neutron activation of a thin Gd foil.

The experimental results indicate that the energy spectra from the bare detectors is attributed to mixed ICE and gamma ray response, whereas that from the polyethylene covered detectors is due only to gamma/x-rays. Thus, the neutron signal may be effectively separated from the gamma ray background by using the proposed gamma-ray rejection scheme of Fig. 25. The results also validate the hypothesis that the prompt gamma rays from Gd* are almost transparent to a small volume semiconductor detector

(e.g., Si), while the ICEs constitute the principal neutron signal in such a detector.

The Gd ICE measurement was repeated at a higher neutron flux by increasing the reactor power to 250 kWth. The thermal equivalent neutron flux and the gamma dose rate at the sample location were observed as 4.8 x 106 cm-2s-1 and 1.35 Radhr-1 respectively.

A lower level threshold of 20.1 keV was set on the detector pulses.

The energy spectra acquired from the uncovered and covered detectors and the experimental geometry in inset are shown in Fig. 37. As expected, due to higher neutron flux, the uncovered detector spectrum indicated much greater energy resolution with the three characteristic ICE peaks clearly resolved. An interesting feature is the small peak at

~48 keV on the slope of the main peak, which is due to the characteristic x-rays of Gd activated by neutrons as well as gamma rays. This peak is clearly identified in the energy spectrum of covered detector due to the separation of ICEs.

The results also indicate that the separation of gamma rays can be effective even at a much higher gamma dose rate. However, a fair judgment on the efficacy of the discrimination technique may only be possible with a measurement in a gamma abundant and neutron deficient radiation environment.

Fig. 37. ICE energy spectrum measured using Si detectors during neutron activation of a thin Gd foil at a higher neutron flux and gamma dose rate.

6.1.2. Experimental validation of optimal thickness of Gd

The experiments for evaluating the optimal thickness of Gd were performed at the cold neutron depth profiling (CNDP) facility of the National Institute of Standards and

Technology-Center for Neutron Research (NCNR). Experimental setup at the CNDP facility is discussed in detail by Downing et al [122] and is not presented here. The energy spectra of ICEs at three Gd thicknesses (5, 25, and 75 µm) were measured and are illustrated in Fig. 38, with the inset showing the experimental geometry. A thermal equivalent neutron flux of 1.1 × 109 cm-2·s-1 was maintained at the sample location during the experiments. The experimental setup facilitated simultaneous acquisition of ICE spectra in front- and back-illumination scenarios using Si detectors. This geometry represents the case in which neutrons are incident on the face of a semiconductor detector with a Gd coating layer. The testing detector, however, is placed at an angle to avoid direct neutron bombardment on Si. A real-world neutron sensor would be unaffected by this issue because the neutron field would generally be low and a radiation-hard semiconductor material (e.g., SiC or GaN) might have been used if the neutron field is high.

The results in Fig. 38 show no spectral difference for all three foils in back- illumination because neutron absorption saturates at ~15 µm (Fig. 17). In other words, a

25 µm thick Gd foil absorbs ~97.5% of the incident neutrons and deeper thicknesses become irrelevant; on the other hand, ICEs (29–71 keV) are self-blocked owing to their average maximum penetration depth in Gd being only 5 µm (section 2.4). The combined effect is identical ICE energy spectra at thicknesses ≥ 5 µm. This agrees well with the

80 numerical evaluation shown in Fig. 17, which indicates that the ICE escape efficiency is almost constant for thicknesses above ~5 µm. The energy spectra from front-illumination in Fig. 38 indicate negligible energy deposition of ICEs at 25 µm and 75 µm thickness, which is attributed to a very low ICE escape efficiency at these thicknesses; but, a much higher energy deposition of ICEs is seen at 5 µm due to the high ICE escape efficiency at

5 µm thickness. And the ICE spectral response at 5 µm in front-illumination is only slightly lower compared to that in back-illumination, which is also in agreement with the numerical evaluation (Fig. 17).

Fig. 38. ICE energy spectra obtained during neutron activation of 5, 25, and 75 µm thick Gd foils in front- and back-illumination.

Although the experimental results are in good agreement with the numerical evaluation, simulations were also performed using SWORD software package to corroborate the results. The simulations represented thermal neutron absorption in Gd

81 foils of varying thicknesses and subsequent energy deposition of ICEs in a Si detector in both front- and back-illumination cases. The simulation results for 5 µm and 25 µm agree well with the experimental results in both the cases, validating the simulation for thinner foils of 1 µm.

As shown in Fig. 39(a), in the back-illumination case, no difference is observed in the energy spectra for 5 and 25 µm foils, whereas in the front illumination case (Fig.

39(b)), the spectral response of ICEs for 25 µm foil is significantly lower than that for 5

µm, which also agree with the numerical evaluation. However, the 1 µm foil allows easy escape of ICEs from the Gd surface with minimal energy loss, giving rise to sharp energy peaks in the spectrum. Evidently, such a small thickness is not in favor of high neutron detection efficiency, but is appropriate for determining an ICE energy spectrum with high energy resolution and for Auger electron measurement.

2D Graph 1

100 1 m 5 m 80 25 m

Si detector 60 Gd foil

Counts per bin 0.0253 eV thermal neutron disk source 20

0 0 50 100 150 200 250 300 Energy (keV) (bin width = 1 keV) (a) continued Fig. 39. Energy spectrum of electrons in the Si detector obtained from 0.0253 eV thermal neutron absorption in Gd foils of different thicknesses in (a) back-illumination (b) front- illumination. 82

Fig. 39 continued 2D Graph 1

100 1 m 5 m 80 25 m Si detector 60

40 0.0253 eV thermal

Counts per bin Gd foil neutron disk source 20

0 0 50 100 150 200 250 300 Energy (keV) (bin width = 1 keV) (b)

6.1.3. Comparison of Gd, B and Li neutron reaction energy deposition rates

While the cross section of Gd is 64 and 680 times higher than that of B and Li, respectively (Table 1), it gives off low-energy ICEs with much lower stopping power compared to the heavy charged particles emitted from Li and B neutron reactions. These

ICEs present a challenge in charge generation and collection and also make the separation of the neutron-induced signal from the gamma-induced signal difficult. The objective of this study is to measure the energy spectrum of the three neutron converters with Si detector when the converters are exposed to the same neutron flux under the same geometry. The obtained characteristic energy spectra are useful in addressing the issues related to the use of Gd as a neutron converter.

A Gd foil, a boron carbide (B4C) sputtering target, and a lithium niobate (LiNbO3) thin sample were placed in the neutron beam, respectively, and the energy spectra of the

83 corresponding neutron capture reactions were measured using a Si detector at the CNDP facility of NCNR. Such measurements correspond to the response of a semiconductor neutron detector coated with thin films of Gd, B, or Li compounds (i.e., indirect- conversion). The energy spectra (Fig. 40) indicate the nature of energy loss suffered by the charged particles when they are emitted from the neutron converter films. Well- resolved ICE peak is seen in the Gd spectrum, whereas a step-shaped spectrum extending up to the maximum energy of the charged particles is observed in the B and Li spectra.

This is explained by the relatively longer neutron mean free path and increased self- absorption of charged particles in B and Li. The superior neutron cross section of Gd resulted in the high intensity of ICEs in the spectrum (relative to the background), whereas the low neutron cross sections of B and Li reflect the low counting rates in the respective spectra. Thus, the low energy of ICEs is compensated for by their high intensity as against the low intensity of high-energy charged particles in the case of B and

Li. However, a precise determination of the energy deposition rate from each reaction is essential for gaining a better understanding of the neutron detection efficiency of indirect- conversion semiconductor detectors based on Gd, B, or Li based thin films.

Fig. 40. A comparison of the energy spectra from Gd, B and Li neutron capture reactions in a Si detector.

The following conditions were observed in all the three experiments in order to ensure uniformity and unbiased evaluation of each neutron converter material.

 Constant neutron flux and geometry

 Same effective area of converter material exposed to the neutron beam (13 mm

diameter aperture)

 Identical instrument settings such as depletion depth of detector and pulse

processing parameters

 All energy spectra normalized to the counting time

Results from the calculation of energy deposition rates in each neutron capture reaction are presented in Table 9.

Neutron Converter material Gd B4C LiNbO3 Size (cm) 2.5 x 2.5 x 0.0005 Φ5.08 x 0.318 1.5 x 1.5 x 0.05 Density (g/cm3) 7.9004 2.52 4.65 Areal density (g/cm2) 0.0040 0.8014 0.2325 Areal density of the converter 0.0040 0.6272 0.0109 element (g/cm2) Atom density of the converter 1.513 x 1019 3.493 x 1022 9.470 x 1020 element (atoms/cm2) Thermal (0.0253 eV) neutron 1473.5 84.008 1.3561 absorptions* (cm-1) Neutron mean fee path (µm) 6.7865 119.04 7374 Energy deposition rate 2.6420 x 106 1.2427 x 107 2.9766 x 106 (keV·s-1) Energy deposition rate 1.7465 x 10-13 3.5572 x 10-16 3.1431 x 10-15 **(keV·s-1).(cm2·atom-1) *Normalized to the neutron flux **Normalized to the atom density of Gd, B and Li respectively. Table 9. List of specifications, characteristic parameters and energy deposition rates of Gd, B and Li compounds used in the experiments.

Integration of the energy spectra indicated that the energy deposition rate of B neutron reaction is about 4.7 and 4.2 times higher than that of Gd and Li neutron reactions, respectively. However, since this result is dependent on the size and composition of the converter films used, the energy deposition rates were normalized to the atom densities

(atoms/cm2) of respective elements. Consequently, the energy deposition rate for Gd is obtained to be 491 and 56 times higher than that for B and Li, respectively. Indeed, the energy deposition rate for Li exceeded that for B by a factor of 8.8. This is attributed to the significantly lower atom density but a much higher reaction energy of Li compared to that of B. Such a high energy deposition rate from Gd facilitates high efficiency neutron monitoring in current mode despite the low energy of ICEs, whereas the heavy charged

86 particles from B and Li neutron capture facilitate effective neutron counting by pulse mode.

From the above analysis, it is concluded that, despite the low energy of ICEs against that of the highly energetic charged particles of B and Li neutron reactions, Gd proves to be a competent neutron conversion material, particularly when high neutron field application is concerned. This is owing to the extremely large neutron cross section of

Gd. Nevertheless, it is imperative to note that the preceding analysis is valid only in the case of indirect-conversion semiconductor neutron detectors.

6.2. Evaluation of gamma ray sensitivity of gadolinium

As discussed in Chapter 3, the high gamma interaction probability of Gd (with Z=64) leads to inherent gamma ray sensitivity of Gd based solid-state neutron detectors due to the transduction of high/medium energy gamma rays into low energy K-X rays. It was also hypothesized that K-X rays from gamma activation of Gd interfere with ICEs in a thin film semiconductor. An experimental evaluation of Gd gamma ray sensitivity is highly essential in order to understand the extent of gamma ray interference caused to the neutron signal in such detectors.

A Gd foil (1.3 cm x 1.3 cm x 0.0025 cm), a Cd foil (1.3 cm x 1.3 cm x 0.1 cm), a Cd sheet (15 cm x 15 cm x 0.1 cm) and a Pb foil (2 cm x 2 cm x 0.1 cm) were used in the experiments to evaluate the gamma ray sensitivity of a Gd based semiconductor detector.

Measurements were performed at the OSURR using the experimental setup described in section 6.1.1.1 under various conditions for a comprehensive evaluation of gamma ray interference to the neutron signal (Table 10). The reactor delivered a thermal equivalent 87

neutron flux of approximately 9.6×104 cm-2·s-1 and a gamma dose rate of approximately

27 mRad·hr-1 at the sample location. A comparison of the energy spectra acquired from

the Si detector in different experimental scenarios is shown in Fig. 41.

Measurement Description i Gd foil in the neutron beam, inside the vacuum chamber Gd foil covered by a Cd foil in the neutron beam, inside the vacuum ii chamber Gd foil inside the vacuum chamber with a Cd sheet blocking the iii neutrons outside the vacuum chamber Gd foil covered by a Pb foil in the neutron beam, inside the vacuum iv chamber Gd foil covered by a Pb foil inside the vacuum chamber with a Cd sheet v blocking the neutrons outside the vacuum chamber vi Cd foil in the neutron beam, inside the vacuum chamber Table 10. List of different measurements performed to evaluate the gamma ray sensitivity of a Gd based semiconductor detector. 2D Graph 1

1 (i) Gd only (ii) Gd covered by Cd foil (iv) Gd covered by Pb foil

0.1 2D Graph 1 Si detector

2D Graph scale) 1 0.01 Gd foil

10 1.2 2D Graph 1 Gd only Neutron (log Gd with Cd sheet outside 1.2 beam GdGd onlycovered by Pb foil with 1.01.2 0.001 GdCd coveredsheet outside by Cd foil CdGd onlyonlyCd foil 1.0 Counts per bin per second Gd covered by Cd foil Gd covered by Pb foil

-1 0.81.0 Pb foil

-1

-1 0.8

.s 100 200 300 400 500

-1

-1 0.8

.s 0.6 Energy (keV) (bin width = 1.65 keV)

-1 0.6 (a) continued

Counts.bin 0.6 (a) Counts.bin Fig. 41. Energy spectrum of a Si detector obtained under different experimental 0.40.4

Counts.bin conditions during the evaluation of gamma ray sensitivity of Gd. Results are from 0.4 measurements (a) i, ii, and iv, (b) i, ii, and vi, and (c) i, iii, and v. 0.20.2 0.2 0.0 88 0.0 100 200 300 400 500 0.0 100 200 300 400 500 100 200 Energy (keV)300 400 500 Energy (keV) Energy (keV) 2D Graph 1 Fig. 41 continued

1 (i) Gd only (ii) Gd covered by Cd foil (vi) Cd only

0.1 2D Graph 1 Si 2D Graph scale) 0.01 1 Gd foil detector

10 1.2 2D Graph 1 Gd only (log Neutron Gd with Cd sheet outside 1.2 beam Gd onlycovered by Pb foil with 0.001 1.01.2 CdGd coveredsheet outside by Cd foil Counts per bin per second CdGd onlyC d foil 1.0 Gd covered by Cd foil Cd only

-1 0.81.0

-1

-1 0.8

.s 100 200 300 400 500

-1 -1 0.8 Energy (keV) (bin width = 1.65 keV)

.s 0.6

-1 0.6 (b) 0.6

Counts.bin

Counts.bin 0.40.4 2D Graph 1

Counts.bin 0.4 0.2 0.2 1 (i) Gd only 0.2 (iii) Gd with Cd sheet outside 0.0 0.0 (v) Gd covered by Pb foil with 100 200 300 400 500 Cd sheet outside 0.0 100 200 300 400 500 0.1 100 200 Energy (keV)300 400 500 2DEnergy Graph (keV) 1 Energy (keV) Si

scale) detector 2D Graph0.01 1Gd foil

10 1.2 2D Graph 1 Gd only Neutron (log Gd with Cd sheet outside beam 1.2 GdGd only covered by Pb foil with 1.0 0.001 GdCd withsheet Cd outsidesheet outside 1.2 Counts per bin per second Gd coveredonly by Pb foil with 1.0 GdCd sheetwith Cd outside sheet outside Gd covered by Pb foil with -1 0.8 1.0 Cd sheetPb outside foil Cd sheet

-1 -1 0.8

.s 100 200 300 400 500

-1 0.6-1 0.8 Energy (keV) (bin width = 1.65 keV)

-1 0.6 (c) Counts.bin 0.6

0.4Counts.bin 0.4

Counts.bin 0.4 0.2 0.2 The experimental results indicate that the neutron response of Gd is clearly 0.2 0.0 0.0 discernible100 and200 stronger300 than the gamma400 ray response500 in all cases. 0.0 100 200 300 400 500 Energy (keV) 100 200Energy (keV)300 400 89500 Energy (keV) Some important observations from Fig. 41 are:

 The response of the Si detector to the Gd foil covered by a Pb foil is slightly

higher compared to the Gd foil covered by a Cd foil. In the former case, the

detector response is predominantly due to the gamma rays from the Gd(n,γ)Gd*

reaction. In the latter case, the response of the detector is predominantly due to the

gamma rays from the Cd(n,γ)Cd* reaction. In both cases, ICEs are completely

blocked from reaching the detector. Although a large number of prompt gamma

rays are released in both cases, the presence of Pb (Z=82) leads to a relatively

higher gamma interaction probability.

 The response of the Si detector to the Gd foil covered by Cd and to the Cd foil

alone (no Gd at all) is nearly identical. In both cases, the detector response is

predominantly due to the gamma rays being released during the Cd(n,γ)Cd*

reaction. Again, the ICEs are completely blocked by the Cd foil.

 When a Cd sheet is placed outside the vacuum chamber, the response of the Si

detector to the Gd foil covered by Pb and the Gd foil alone is nearly identical.

Because thermal neutrons are blocked outside the chamber, the Si detector is

responsive mostly to the reactor gamma ray background, which is only slightly

enhanced in the presence of Pb.

The results thus indicate that, the neutron sensitivity of Gd is superior enough to surmount the gamma ray interference introduced from the reactor and the prompt gamma rays from Cd and Pb neutron activation. The gamma ray response is significantly low, particularly in the 30-80 keV energy region, where the ICEs are dominant but prone to

90 the interference from low energy gamma rays. For the energy region above 80 keV, the detector response is analogous to a typical small volume semiconductor's response to high energy gamma rays, i.e., it is mostly transparent to high energy gamma rays. It is worth mentioning that a fair comparison would only be possible when a controlled dose of pure gamma and neutron field are introduced.

6.3. Evaluation of the gamma rejection scheme

The proposed gamma ray rejection scheme supposes that the gamma ray response in both detectors is identical; hence, subtracting the two detector signals effectively cancels out the gamma ray interference in the final detector response. In order to substantiate this hypothesis and validate the simulation results, experiments were performed using two identical Si detectors8 and button-sized radioactive sources.

6.3.1. Experiments in gamma radiation field

The two Si detectors were separated by a 25 m thick Gd foil9 and a 350 µm thick polyethylene cap, and the gamma ray rejection scheme was reproduced. 57Co was used as the gamma ray source in the experiment. The experimental setup is as described in section 3.1.1. The instrument and geometry used in this experiment are shown in Fig. 42, and the vacuum cover is removed to display the setup.

8 Detector specifications are the same as in Table 8, but with a depletion depth ≥ 164 µm. 9 Gd foil of 5 µm thickness was unattainable for this study. 91

Si detector 1 Polyethylene cap

57Co source

Gd foil Si detector 2

Fig. 42. Experimental geometry reproducing the twin-detector scheme for rejection of external gamma rays; a 57Co source was used in the experiment.

The multichannel digitizer-based DAQ system was used for simultaneous signal acquisition from two independent detector channels. Energy spectra from both detectors were acquired using the digitizer with a trapezoidal energy filter. The response of the two detectors to 57Co gamma rays is illustrated in Fig. 43.

2D Graph 2

0.16 Si detector 1 0.14 Si detector 2

0.12

0.10

0.08

0.06

0.04

Counts per bin per second 0.02

0.00 0 25 50 75 100 125 150 175 200 Energy (keV) (bin size = 1.12 keV)

Fig. 43. Energy spectra of the two Si charged particle detectors obtained from the experiment studying the gamma ray rejection scheme using 57Co source.

The energy spectra encompass the resolved gamma peaks at 122 keV, a significant component from backscattered photons (~83 keV), and the characteristic K-X rays from

Gd. The detector 2’s gamma ray response is lower than that of detector 1 except in the low energy region. This attenuation in response is attributed to the point-source like geometry used in the experiment (Fig. 42); in other words, the detector that is closer to the source has a higher solid angle. This is in contrast to the collimated source used in the simulation, where both detectors have the same solid angles. Although the detector responses are not identical, both gamma spectra present the same shape and subtraction of the two energy spectra significantly reduces the gamma ray component of the resulting spectrum.

2D Graph 1

0.16 Si detector 1 0.14 Si detector 2

0.12

0.10

0.08

0.06

0.04

Counts per bin per second 0.02

0.00 0 50 100 150 200 250 300 350 Energy (keV) (bin size = 1.12 keV)

Fig. 44. Energy spectra of the two Si charged particle detectors obtained from the experiment studying the gamma ray rejection scheme using 133Ba source.

For further study, the experiment was repeated with 133Ba button-sized gamma source.

The response of the two detectors to 133Ba gamma rays is shown in Fig. 44. In analogy with the earlier case, detector 1 exhibited a higher response compared to detector 2, except in the low energy region. Nonetheless, both the spectra present identical shape and allow significant reduction in the gamma ray component of the differential spectrum.

6.3.2. Experiment in mixed beta-gamma (β-γ) radiation field

The practicability of the gamma rejection scheme was first studied by adopting a model of the twin-detector setup, which consisted of Si detectors10, Gd and polyethylene.

In this model, the ICEs and low energy gamma/x-rays were substituted with 14C pure beta

(β) and 57Co pure gamma-ray sources, respectively. The main motivation for this study was to develop insight into subsequent execution of the gamma rejection scheme in mixed n-γ radiation environment. The experimental geometry was arranged such that the gamma rejection scheme was faithfully reproduced. The two detectors were positioned back-to-back facing each other, separated by a 5 µm thick Gd foil and the polyethylene cap, but with the β-source intervening between Gd and polyethylene (Fig. 45). The 57Co source was positioned as shown in Fig. 45 in order to create external gamma ray background. The experimental setup for this measurement is as described in section 3.1.1.

The geometry illustrated in Fig. 45 is supposed to simulate the neutron absorption and subsequent isotropic emission11 of ICEs in a Gd foil. The 14C β-spectrum with an endpoint energy of 156.5 keV and an average energy of 49.5 keV is supposed to closely

10 Detector specifications are the same as in Table 8, but with a depletion depth ≥ 164 µm. 11 It is to be noted that the β-source used in the experiment is only semi-isotropic as the β-particles are fully absorbed in the plastic at the rear of the source. 94 represent the penetration and energy loss of the most prominent ICEs (29-78 keV) in Gd prior to reaching the detector. The 57Co source and the ensuing gamma activation of Gd are supposed to represent the low energy gamma ray interference to the neutron signal.

Fig. 45. Experimental geometry reproducing the twin-detector scheme for rejection of external gamma rays; 14C and 57Co sources were used in the experiment.

Energy spectra acquired from the two detectors are shown in Fig. 46. The Fig. 46 indicates that the detector 1 spectrum is due to mixed β-γ response, whereas that of detector 2 is due only to gamma/x-rays. The gamma only spectrum of detector 2 also shows a back-scattering peak (~83 keV) due to the 122 keV photons scattered from Al inside the chamber. Furthermore, spectrum subtraction yields a response attributed only to β-particles, analogous to the neutron signal from ICEs without the interference from low energy gamma/x-rays were Gd activated by neutrons.

2D Graph 1

1.0 Detector 1 Detector 2 Differential spectrum 0.8

0.6

0.4

0.2

Counts per bin per second

0.0 20 40 60 80 100 120 140 160 180 200 Energy (keV) (bin size = 1.087 keV) Fig. 46. Energy spectra of the two Si charged particle detectors and the differential spectrum obtained from the experiment to evaluate the gamma ray rejection scheme using 14C beta and 57Co gamma ray sources.

6.3.3. Experiment in mixed n-γ radiation field

In the previous section, a proof-of-concept of the gamma ray rejection scheme was discussed, in which the ICEs and gamma/x-rays were emulated by pure beta and pure gamma sources, respectively. The differential spectrum of the detectors was understood as almost exclusively due to β-particles with effective elimination of gamma ray interference. However, from section 3.1.2, it can be recalled that Si detector covered by

Gd foil shows higher response to 57Co gamma rays than the one covered by a polyethylene cap, owing to photoelectrons emitted from Gd. It is then expected that the mixed β-γ spectrum of detector 1 in Fig. 46 contains additional component from Gd photoelectrons; thus, the differential spectrum is not exclusively a beta response but is a mixed beta and photoelectrons response. However, this effect is not clearly reflected in

96 the above spectra because of the predominantly high β-response of the detector induced by strong β-radiation field. While the experiment provides only a proof-of-principle, it certainly is not representative of the real n-γ separation scenario in homeland security applications, which are typified by neutron starved and gamma intense radiation fields in contrast to the β-γ experiment.

In this section, the study of the gamma ray rejection scheme in mixed n-γ radiation field is presented. Experiments were performed at the OSURR in a low intensity neutron field, mixed with reactor gamma ray background in one measurement, and with reactor and external gamma ray background in the other measurement.

The twin detector based gamma ray rejection scheme was reproduced using two Si detectors12, a 5 µm thick Gd foil and 350 µm thick polyethylene cap on an Al apparatus.

This setup was mounted inside the stainless-steel high-vacuum chamber at nearly the center of the neutron beam such that the detector axes were parallel to the beam (see Fig.

47). The setup was oriented such that neutrons illuminated the rear surface of detector 1

(i.e., the detector on Gd side), thus, allowing for higher ICE escape into the detector. A

57Co source was also placed inside the chamber in order to enhance the gamma ray background. The detectors were connected separately to two hybrid charge sensitive preamplifiers inside the vacuum chamber.

12 Detector specifications are the same as in Table 8, but with a depletion depth ≥ 164 µm. 97

Fig. 47. Experimental setup inside the high-vacuum chamber at OSURR reproducing the twin-detector gamma rejection scheme; a 57Co source was used to enhance the gamma ray background in the measurement.

The reactor operated at 5 Wth to provide a low neutron field inside the vacuum chamber, delivering a thermal equivalent neutron flux of about 95 cm-2·s-1 at the detectors' location. However, it is to be noted that the whole apparatus, and the Gd foil in particular, was not perfectly aligned with the beam center. Thus, the thermal neutron flux striking the Gd foil is expected to be lower than the estimated value. Measurements were performed with and without the 57Co source inside the chamber. The data was acquired in both histogram and list modes from two detector channels of the multichannel digitizer using DPHA software. List mode recorded the time stamp and detector pulse height of each detection event and thus, provided an accurate measure of the acquisition time.

Histogram mode recorded the pulse height histogram of the detectors at the end of an acquisition period. Acquisition in both modes enabled normalization of histogram counts

98 to the counting time.

The pulse height in ADC-channels was converted to energy (keV) scale using 57Co energy peaks in the detector spectra. The energy peaks were fitted with Gaussian distribution for accurate estimation of the peak centroid and energy calibration. Energy spectra acquired from the detectors in both measurements are discussed in the following section.

Case 1 - Measurement without 57Co source:

2D Graph 1

0.020 Si detector 1 Si detector 2

0.015

0.010

0.005

Counts per bin per second

0.000 50 100 150 200 250 300 Energy (keV) (bin size = 0.92 keV)

Fig. 48. Energy spectra of Si detectors obtained during neutron irradiation of the twin detector setup; no external gamma source was used in the measurement.

In Fig. 48, the detector 1 energy spectrum indicates Gd ICE peaks at ~71 keV and

~131 keV. These energy peaks are easily distinguishable from the gamma ray background in the spectrum owing to their higher intensity. As expected, the detector 2

99 energy spectrum is devoid of the ICE response and represents a gamma-ray-only response. Subtraction of the two detector spectra effectively eliminates the gamma ray component in the resultant spectrum (i.e., the differential spectrum). Lastly, a noteworthy observation is the low count rate of ICE peaks, which was predictable, due to the low neutron flux in the beam.

Case 2 - Measurement with 57Co source:

A question remains about how effective the gamma ray rejection technique is in a gamma abundant but neutron deficient radiation environment. To investigate this scenario, measurements were repeated in a high gamma ray background created by 57Co source in addition to the reactor gamma background. Energy spectra of the detectors from this measurement are shown in Fig. 49. 2D Graph 1

Si detector 1 0.3 Si detector 2

0.2

0.1

Counts per bin per second

0.0 50 100 150 200 250 300 Energy (keV) (bin size = 0.92 keV)

Fig. 49. Energy spectra of Si detectors obtained during neutron irradiation of the twin detector setup; 57Co source was used in the measurement to enhance the gamma ray background.

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In the present case, not only is the gamma background much higher compared to the neutron flux, but also the Si detector sensitivity to low energy 57Co gamma rays (122 keV) is significant. Such gamma sensitivity of detectors is further enhanced by the significantly high photoelectric cross section of Gd at low photon energies. These are the primary reasons for higher count rate in the detector spectra, in which the neutron induced ICE response is completely overshadowed by the gamma response of detector 1.

The detector spectra also indicate much higher gamma response of detector 1 compared to that of detector 2, owing to the photo- and Compton-electrons from Gd. Consequently, the differential spectrum still includes a significant gamma ray component and separation of the low intensity neutron signal using the rejection scheme is not effective under such conditions observed in this experiment.

In conclusion, the gamma ray rejection scheme is effective when the gamma ray background is low, or when the gamma ray background is high, but caused mostly due to high energy (> 500 keV) gamma rays (this is because, the Compton and photoelectric cross sections of Gd at such energies are very low). However, the gamma ray rejection scheme is not very effective in a neutron deficient, but gamma abundant radiation environment induced solely by low energy (< 150 keV) gamma rays.

6.4. Further study

In this section, a basis for further research on the gamma ray rejection scheme is presented and a potential application of the Gd ICE spectrum is discussed.

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6.4.1. Experiments in a low intensity neutron field

As discussed in section 6.1.1, the experiments performed at the OSURR in mixed n-γ environment supported the feasibility of the gamma ray rejection technique. However, as stated earlier, precise evaluation of the efficacy of the method may only be possible with a measurement in gamma abundant and neutron deficient radiation field, which is also characteristic of the neutron detection scenario in homeland security. For this purpose, experiments were designed in a low neutron field mixed with gamma ray background, provided by a PuBe neutron howitzer (Fig. 50) at the NARS lab.

Fig. 50. The plutonium-beryllium (PuBe) neutron howitzer

6.4.1.1. Experimental setup

The experimental apparatus was built such that the neutron howitzer delivers a small-sized collimated neutron beam to a workbench housing the vacuum instrumentation system (section 3.1.1).

The PuBe neutron source is housed in a vertical channel at the center of a cylindrical

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Fig. 51. Top - SolidWorks 3D model of the PuBe source facility in NARS lab; Bottom - PuBe source station in its final form after construction.

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Al barrel, surrounded by a specially made paraffin wax moderator. The cylindrical barrel was mounted on an in-house built Al frame along with three high density polyethylene

(HDPE) slabs (48" × 48" × 1" each) on either side of the barrel for the purpose of shielding (Fig. 51).

Fig. 52. Graphite and sapphire rods and Al enclosure assembly built for the purpose of neutron collimation: Top - HDPE rings built for enclosing the graphite and sapphire rods. Bottom - final collimator assembly (components are separated for visibility).

A small collimator was built using a graphite rod (⌀1.5" × 3") and two single crystal sapphire rods (⌀1" × 7" total length). Graphite was selected for neutron moderation, while sapphire was considered for filtering fast neutrons owing to its effective neutron

104 scattering property. The graphite and sapphire rods were positioned next to each other and enclosed in a cylindrical shell made of HDPE rings, and the entire assembly was placed inside an Al enclosure (⌀2" × 12") (Fig. 52).

The Al enclosure housing the collimator can be inserted into the horizontal beam port 1 (⌀2.1" × 11") of the neutron howitzer, facing the source volume in the vertical channel of the barrel. A series of borated (12% B) HDPE (BPE) slabs (12" × 12" × 1" each) with ⌀1" apertures at the center was positioned outside the barrel tangential to its surface, such that the aperture was aligned with the beam port (Fig. 51). The BPE slabs were used for further neutron collimation while also functioning as neutron shielding.

Neutrons emerging from the exit of BPE collimator (BPE exit) can be used for neutron sensor evaluation.

For accurate evaluation of the gamma ray rejection scheme as well as neutron sensitivity of the twin detector, it was essential to understand the source neutron spectrum. The PuBe source characterization and neutron spectrum unfolding procedures are discussed in the next section.

6.4.1.2. PuBe source characterization

Attempts were made to characterize the source spectrum at the BPE exit using neutron foil activation method. For this purpose, the collimator was removed from inside the beam port. Gold (Au), indium (In), scandium (Sc), copper (Cu), titanium (Ti) and iron

(Fe) foils were considered for neutron irradiation (Fig. 53). Gamma ray decay from the activated foils was counted using a HPGe spectrometer at the OSURR, and the irradiated foil activity was determined using an activation analysis software. 105

Fig. 53. Neutron activation foils kit purchased from Shieldwerx (left), and the Au, In, Sc, Cu, Ti and Fe metal foils used in the activation study (right).

Au (197Au(n,γ)198Au) and In (115In(n,γ)116In) foils were irradiated at the BPE exit for sufficiently long periods of time, respectively, in order to attain saturation activity. The saturation activities were determined as 0.18 decays per second (dps) and 0.50 dps, respectively. Such low values could be attributed to the low thermal neutron flux at the

BPE exit. However, the thermal equivalent neutron flux using 198Au and 116In data was evaluated as 4.8 cm-2∙s-1 and 4.0 cm-2·s-1, respectively, using the number of target nuclei, thermal neutron cross section and saturation activity values. Remaining foils in the set didn't produce any measurable activity and were discarded from the evaluation.

Although the result may be absurd, an attempt was made to unfold the neutron spectrum using only the Au and In activity data. Spectrum unfolding was performed using SANDII code package [123], which employs iterative method to compute the neutron spectrum. Starting with a guess spectrum, the activity values are calculated and compared with the measured data, and corrective iterations are performed until the 106 calculated and measured activity values are in agreement with a stipulated degree of standard deviation. The neutron spectrum for which this level of accuracy is achieved is generated as the output spectrum from the code.

For SANDII computation, a guess neutron spectrum of PuBe consisting of 22 energy intervals was generated using Origen-Arp [124] sequence of SCALE6 software package

[125]. In this evaluation, a reference data from the year 1965 was used for the initial

PuBe nuclide concentration as the actual concentration data was unavailable. The initial and final nuclide concentrations from Origen-Arp are summarized in Table 11. The guess spectrum from Origen-Arp and the SANDII output neutron spectrum are shown in Fig.

54.

Initial, 48.3 years decay,* Nuclide i.e., 1965 i.e., 2013 (g) (g) 239Pu 72.79 72.69 240Pu 6.436 6.403 241Pu 0.6749 0.06534 242Pu 0.07037 0.07037 9Be 39.28 39.28 *obtained using OrigenArp Table 11. PuBe nuclide concentrations in grams used in Origen-Arp calculation.

Results from SAND-2 indicate that the neutron flux is dominated by the fast component with average energy of 7.38 MeV. An MCNP5 simulation of the PuBe source and BPE collimator detailed geometry was performed to test the validity of the result. In this simulation, Origen-Arp output spectrum was used as the source neutron distribution

107 and the neutron flux at the BPE exit was evaluated. The simulated and SAND-2 output spectra are compared with each other as shown in Fig. 55. The SAND-2 spectrum differs 2D Graph 3

1.2 Initial guess from OrigenArp SAND-2 output

1.0

0.8

0.6

0.4

Intensity (normalized) (neutrons per second) 0.2

0.0 0 2 4 6 8 10 12 14 16 18 Energy (MeV) Fig. 54. Initial guess spectrum obtained from Origen-Arp and the PuBe source neutron spectrum unfolded using SAND-2. 2D Graph 3

1.2 MCNP5 SAND-2

1.0

0.8

0.6

0.4

Intensity (normalized) (neutrons per second) 0.2

0.0 0 2 4 6 8 10 12 14 16 18 Energy (MeV) Fig. 55. Comparison of neutron spectra obtained from SAND-2 unfolding and MCNP5 simulation. 108 signficantly from that of the simulation, due to the possibly flawed computation of spectrum unfolding, besides the assumptions made in Origen-Arp evaluation.

A logical step forward in this investigation would be evaluating the neutron spectrum directly at the beam port exit instead of the BPE exit. Subsequently, the positions of graphite and sapphire inside the beam port may be optimized using the unfolded source spectrum in an MCNP5 simulation.

6.4.2. Depth profiling using ICEs

Neutron depth profiling is a nondestructive analytical technique for measuring the concentration of certain light elements as function of depth in the near surface region of solids. For example, neutron absorption in isotopes such as 10B, 6Li, 14N etc., releases charged particles with distinct energies. The energy lost by the charged particle before it exits the material is measured to determine the depth of its origin (i.e., the reaction site)

[126]. Residual energy of these particles is measured by a charged particle detector (e.g.,

Si ion-implanted), and the concentration profile of the element is derived from the energy spectrum. Accordingly, the ICE spectrum from Gd neutron capture provides a potential means for depth profiling of Gd based thin films and also estimating the thickness of such films up to a few microns.

A thin film containing Gd based (Gd2O3) powder in polydimethylsiloxane (PDMS), i.e., Gd2O3/PDMS was developed for neutron attenuation. In the following study, the thickness of Gd2O3/PDMS thin film has been estimated using the ICE energy spectrum resulting from thermal neutron irradiation of the film. The measurement was performed at the neutron beam facility at the OSURR and the energy spectrum was acquired during 109 thermal neutron activation of the thin film. Measurements were repeated with a bare substrate (not containing Gd2O3/PDMS) and with no film (i.e., background) in the neutron beam. Energy spectra obtained from a Si detector in all three cases are shown in

Fig. 56.

2D Graph 1

0.4 Gd 2O3/PDMS film Bare substrate Background 0.3

0.2

0.1

Counts per bin per second per bin Counts

0.0 50 100 150 200 250 300 350 400 Energy (keV) (bin size = 1.96 keV)

Fig. 56. Energy spectra obtained from a Si detector during neutron activation of Gd2O3/PDMS thin film and a bare substrate, and in a background measurement.

The energy spectrum obtained with the Gd2O3/PDMS film consists of a peak centered at 71 keV corresponding to the ICE energy. The energy spectra obtained with the bare substrate and the background measurement are almost identical and indicate the absence of a neutron signal. Thus, a comparison of the three spectra clearly indicates the sensitivity of Gd2O3 film to neutrons.

110

The energy spectrum of electrons after background subtraction was used to estimate the thickness of Gd2O3/PDMS thin film. Since the principal and most intense IC emission occurs at 71 keV, it is considered as a representative of the total range of 29-246 keV

ICEs. The count rate in the spectrum was factored into individual ICE contributions and only the 71 keV count rate was considered in the analysis. The energy scale of the '71 keV' electron spectrum was converted to depth scale indicating the depths of origin of 71 keV electrons in the thin film.

The energy of electrons escaping the thin film surface (escape energy) varies from a maximum of their initial kinetic energy (i.e., 71 keV) to a minimum of zero depending on the depth of their origin. Electrons originating at depth equal to their average MPD in the material may possess zero or negligible escape energy whereas those originating at or near the surface may escape with maximum kinetic energy. In other words, the deeper the origin of the electron, greater the electron energy loss in the material and smaller is its escape energy. This depth of origin can be understood as the electron average MPD corresponding to the energy lost in the material. The average MPD of all electrons in the energy range 0-71 keV (spaced by 1 keV; with 0 and 71 keV corresponding to 0 and 71 keV energy loss, respectively) in Gd2O3/PDMS was evaluated using the code CASINO.

In CASINO, the penetration of a narrow beam of electrons (radius 10 nm) of given energy in a Gd2O3/PDMS slab was simulated (Fig. 57). From the distribution of number of electrons vs. depth in the material, the average MPD was calculated as the average value of the distribution. The electron depths are then mapped to corresponding escape

111 energies in the energy spectrum and the count rate vs. depth relationship of 71 keV electrons was derived as shown in Fig. 58.

Fig. 57. 71 keV electron trajectories inside a slab of Gd2O3/PDMS. Trajectories in blue represent the transmitted electrons whereas those in red represent the back-scattered electrons. The zoomed portion of the figure (inset) shows the narrow electron beam (10 nm radius) penetrating the slab after striking it orthogonally.

0.12

0.1

0.08

0.06

0.04 Counts per second

0.02

0 0 2 4 6 8 10 12 Depth (m)

Fig. 58. Depth profiling analysis for the Gd2O3/PDMS film. Figure shows the histogram of count rate against the depth of origin of 71 keV ICEs in the thin film.

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Some level of uncertainty may creep into this estimation due to the following factors.

 It was assumed that the counts in the original spectrum (Fig. 56) are due only to

the ICEs of Gd excluding any contribution from gamma/x-rays.

 It was further assumed that the energy peak resulted from only the forward

emitted ICEs. However, due to the deflecting nature of electrons it is possible that

some of the backward emitted ICEs or even those that undergo very large angle

scattering may redirect their path, eventually escaping the surface of thin film and

reaching the detector.

 Since the electronic noise threshold in the measurement was about 30 keV, counts

below that energy were not considered in this evaluation. If these counts were to

be included, the depth scale would have extended by only a few more µm.

All such contributions however, are supposed to be minimal and thus, have only negligible effect on the overall calculation. From Fig. 58, the thickness of Gd2O3/PDMS film is estimated as 12.3 µm. The average of the distribution in Fig. 58 yielded an equivalent film thickness of 2.92 µm.

6.5. Summary

The ICE energy spectrum from Gd neutron capture was measured with high resolution using Si charged particle detectors during the thermal neutron activation of a thin Gd foil. The results confirmed that ICEs are the principal neutron signal carriers when using a small volume semiconductor for charge collection. Evaluation of optimal thickness of Gd conversion layer for a semiconductor neutron detector was validated by

113 experiments as well as simulations, considering both front- and back-illumination. The energy spectra of Gd, B, and Li neutron capture reactions were measured under the same neutron flux and geometry using thin-film samples of the converters. The characteristics of their energy deposition, as exemplified by the Si detector, were clearly demonstrated by the spectra, in that the ICEs stand out in the background region, although mostly of low energy 71 keV peak, and the high energy charged particles emitted after B and Li neutron capture extend far beyond the region of background interference, but with much lower emission rate. A quantitative comparison of the energy deposition rates considering the atomic density of each element confirmed the higher energy deposition rate of ICEs compared to that of the low intensity heavy charged particles from B and Li reactions.

The results favor Gd as a superior converter material for use in current mode neutron monitoring. A series of measurements were performed to evaluate the gamma ray sensitivity of Gd under different radiation exposures. The results indicated that the neutron sensitivity of Gd is characterized by the highly intense ICE peak, seemingly unaffected by gamma ray background introduced from both the reactor and Cd and Pb neutron activation.

The gamma ray rejection scheme proposed for a Gd based semiconductor neutron detector was evaluated extensively under different radiation exposures. In the experiments, the twin-detector scheme was reproduced using two identical Si charged particle detectors, a Gd foil and a polyethylene layer. The two detectors produced almost identical response in a gamma only radiation field, confirming the hypothesis of the rejection scheme. In a mixed β-γ radiation field that simulated the n-γ environment, the β-

114 only response was successfully separated from a mixed β-γ response by subtracting the two detector spectra. Thus, the results supported the feasibility of the gamma ray rejection scheme by providing a proof-of-concept of the method. Finally, the gamma ray rejection scheme was also proved practicable in mixed n-γ radiation field, when the neutron induced ICEs were effectively separated from gamma rays.

However, the performance of the method is limited in an abundance of low energy gamma rays due to the enhanced gamma interaction probability of Gd at such energies.

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Chapter 7. Conclusions

In this research, the feasibility of using Gd for neutron conversion and detection using a thin film semiconductor detector has been investigated. The issues pertaining to the application of Gd for neutron detection using a semiconductor detector such as, the low energy of ICEs, the ICEs range in Gd and gamma ray sensitivity of Gd are identified and discussed in detail. Extensive MC simulations and experiments were performed to quantify the neutron and gamma sensitivity of Gd. The investigations established Gd as a competent neutron conversion material despite its inherent limitations, and indicated superior neutron sensitivity of Gd compared to its gamma sensitivity.

The ICE spectrum of Gd neutron capture was measured with high energy resolution using a Si charged particle detector. The measurement confirmed that the ICEs are effectively detectable and represent the principal neutron information carriers in a small volume semiconductor. The optimal thickness of Gd for thin film coating on a semiconductor neutron detector was evaluated as 5 µm, and the result was validated by both experiments and simulations. Energy deposition spectra of Gd, B and Li neutron capture reaction products were all measured under same experimental conditions in a small volume Si detector. Quantitative comparison of the corresponding energy deposition rates indicated the suitability of Gd over B and Li for current mode neutron

116 monitoring, while the energetic charged particles of B and Li enable effective neutron pulse counting.

Even with the high neutron sensitivity of Gd, gamma ray discrimination was deemed indispensable for applications such as neutron detection in homeland security, which demand high detection efficiency with minimal false alarms. Toward this end, a gamma ray discrimination scheme consisting of two identical (twin) semiconductor detectors separated by Gd conversion layer and polyethylene electron separator was proposed. The proposed method of gamma ray discrimination based on differential response of the twin- detectors was demonstrated by extensive MC simulations. The simulations were validated by a multitude of experiments, which supported the feasibility of the gamma rejection method. The experimental results indicated effective separation of neutron induced ICEs from mixed ICEs and gamma rays in a thin semiconductor detector using

Gd and polyethylene.

Certain limitations of the gamma rejection method were also identified from this research. The proposed method is not very effective in a gamma abundant radiation environment, caused primarily by low energy gamma rays. The proposed scheme does not allow event-by-event classification of neutrons and gamma rays. In addition, gamma ray rejection with pulse-mode detector operation is not realizable in real-time, since pulse mode essentially involves subtraction of counts in the detector spectra. Nevertheless, real-time rejection is still attainable through current-mode detector operation.

The research further provided insight into a practical mode of gamma ray detection using Gd as a transducer to convert high/medium energy gamma rays into low energy K-

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X rays and photoelectrons. Such detection mode is sufficient for many applications where only the intensity of gamma rays is of interest.

For future work, it is recommended to complete the PuBe source characterization and evaluate the gamma rejection method in the low neutron field supplied by this source.

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Appendix A: Detector calibration for identifying the ICE peak energies

Energy calibration was performed to transform the detector pulse height (ADC channels) into energy (keV), and identify the ICE peaks accurately. The 59.5 keV and

122 keV gamma rays from americium-241 (241Am) and 57Co, respectively were considered for this purpose. They were deemed suitable for calibration as the two energies overlap with the region of interest (i.e., the ICE energy range of 29-246 keV) and correspond to the same radiation type (i.e., gamma rays).

The energy scale of the Si detectors and the spectroscopy system was carefully calibrated using button-sized 241Am and 57Co sources. The energy spectra of 241Am and

57Co obtained from one Si detector were overlaid, as shown in Fig. 59, for illustration.

The two peaks other than the 59.5 keV in the 241Am energy spectrum correspond, in part, to the 33.2 keV and 26.3 keV gamma rays. The energy calibration of the detectors yielded a conversion factor of 0.932 keV/ADC channel.

The interaction of 59.5 keV and 122 keV gamma rays with Si was also studied in order to corroborate the experimentally acquired spectra. While Si has greater interaction probability at gamma ray energies as low as 59.5 keV, it is supposed to be relatively transparent to higher energy gamma rays. It was thus essential to validate the experimental 57Co gamma ray spectrum used in the energy calibration. An analysis, which is supported by the following observations confirmed that the energy peak in 57Co

129 spectrum is indeed due to full energy deposition of 122 keV photons in the detector.

2D Graph 1

10 57 241 Co source Am (59.5 keV) 241Am source 8

2 57Co (122 keV)

Counts per bin per second

0 0 20 40 60 80 100 120 140 160 Energy (keV)

Fig. 59. Energy spectra of button-sized 241Am and 57Co sources used to calibrate the Si detectors. Measurements were made separately, but the spectra are overlaid.

 At reverse bias of 50 V, the Si detector is fully depleted, and attains a depletion

depth greater than its minimum value (i.e., 100 µm).

 For 122 keV gamma rays, the photoelectric (PE) and Compton scattering cross

sections of Si are about 0.64 barns and 6.44 barns, respectively. A large fraction

of counts in the resultant energy peak could be due to 122 keV PE absorption,

since the detector volume is largely insufficient for a Compton scatter followed by

PE absorption of the scattered photon. However, a small fraction of counts in the

peak could still be due to the second interaction mode, considering the higher

Compton scatter cross section of Si. Either of the two interaction modes leads to

130

complete energy deposition of the 122 keV gamma rays.

 As seen from the experimental spectra of 57Co and 241Am gamma rays (Fig. 59),

the intensity (counts per bin per second) of 122 keV peak i.e., 0.478 is only about

0.064 times that of the 59.5 peak i.e., 7.45, which is attributed to the much lower

cross section of Si at 122 keV (0.64 b for PE alone, 7.575 barns in total)

compared to that at 59.5 keV (6.2 b for PE alone, 15.25 barns in total).

A MCNP5 simulation comparing the energy spectra of 59.5 keV and 122 keV gamma rays in a 100 µm thick Si detector validated the preceding arguments (Fig. 60).

The ratio of peak intensities (122 keV to 59.5 keV) in the simulated spectra is obtained as

0.066, which is in close agreement with the experimental result. Thus, the experimentally measured spectra of 241Am and 57Co gamma rays are validated.

2D Graph 1

3.0x10-5 59.5 keV gamma rays 122 keV gamma rays 2.5x10-5

2.0x10-5

1.5x10-5

10-5

5.0x10-6

Counts per source particle per bin 0 0 20 40 60 80 100 120 140 Energy (keV) (bin size = 1 keV)

Fig. 60. Energy spectra of 59.5 keV and 122 keV gamma rays from a Si detector obtained using MCNP5 simulation.

131

Appendix B: Attenuation of gamma rays in polyethylene

The objective of this study is to confirm the validity of the ICE energy spectrum measured during thermal neutron activation of Gd (section 6.1.1). In order to validate the

ICE spectrum (Fig. 37), it is essential to prove that the bare detector's response and the corresponding 71 keV peak in the spectrum are due primarily to ICEs, with a minimal gamma/x-rays component. In order to prove the abovementioned, it is sufficient to show that the polyethylene cap on the covered detector fully absorbs the ICEs, with a negligible attenuation of gamma/x-rays, i.e., the polyethylene layer is transparent to gamma/x-rays but opaque only to ICEs.

It is known that the principal ICE energies occur in the range 29-246 keV. Using

PENELOPE, the range of 246 keV electrons in polyethylene is computed as 335 µm, which implies that the average MPD of these electrons in polyethylene is even lesser than

335 µm. Hence, a polyethylene layer of 350 µm completely absorbs the ICEs.

A MCNP5 simulation was also performed to understand the attenuation of gamma/x- rays by polyethylene. For this simulation, the source photon energy distribution was extracted from the SWORD simulated energy spectrum of Gd(n,γ)Gd* reaction products.

The source energies were confined to the range 0–300 keV, which overlaps with that of the experimental spectrum (Fig. 37). Energy spectra of gamma rays were tallied in a Si detector, with and without a 350 µm polyethylene layer covering the detector (Fig. 61,

132 left). Energy deposition of gamma rays in the polyethylene layer is also evaluated (Fig.

61, right ).

2D Graph 1 2D Graph 1

0.010 0.010 Si detector Si detector covered 0.008 by polyethylene 0.008

0.006 0.006

per bin per per bin per 0.004 0.004

0.002 0.002

Counts per source particle particle source Countsper

Counts per source particle particle source Countsper 0.000 0.000 0 20 40 60 80 100 0 20 40 60 80 100 Energy (keV) (bin size = 1 keV) Energy (keV) (bin size = 1 keV)

Fig. 61. Left - Energy spectra of 29-246 keV gamma rays from a Si detector obtained using MCNP5 simulation. Right - Energy deposition spectrum of the gamma rays in the polyethylene layer.

From Fig. 61, it can be clearly seen that the attenuation of gamma rays by polyethylene is negligible at energies above ~15 keV. It follows that, the gamma/x-ray component of the experimental Gd(n,γ)Gd* energy spectrum is not attenuated by a 350

µm thick polyethylene layer. In other words, the polyethylene layer is completely transparent to gamma rays at all energies above ~15 keV. Hence, the bare and polyethylene covered detectors are supposed to produce identical gamma ray response.

As exemplified by the gamma only response of the covered detector (Fig. 37), the gamma ray component of the bare detector spectrum is minimal compared to that of ICEs.

Thus, the ICE energy spectrum measured by the bare Si detector is validated.

133

Appendix C: Si PIN photodiode for Gd ICE measurement

The following study is performed for a preliminary investigation for using a Si PIN photodiode to measure the ICE energy spectrum of Gd neutron capture with high resolution.

A photodiode is a rugged detector type and offers several advantages, which include, but are not limited to, low noise (i.e., high signal-to-noise ratio), small device foot print, and high energy resolution to ionizing radiation. A photodiode is generally manufactured with an entrance glass window covering the photosensitive surface, in order to avoid surface contamination. However, the ICEs from Gd have less than 50 µm range in the window glass material. For this reason, photodiodes without entrance windows were specifically requested from the company OPTEK; samples provided by the company are used in the current application. The photodiode specifications, summarized in Table 12 demonstrate the low dark current and terminal capacitance of the diode detector.

Manufacturer OPTEK Model number OPF420 Active area 1 mm2 Depletion depth 300 µm Dark current 0.1 nA at 5 V reverse bias Terminal capacitance 3.0 pF at 20 V reverse bias Maximum reverse voltage 100 V Table 12. Specifications of the Si PIN photodiode

134

The photodiode is tested by measuring the energy spectra of 241Am, 14C and 57Co radioactive sources. In a first set of measurements with one of the photodiode samples

(PD 1), significant noise was observed in the low energy region of the detector spectra, which necessitated higher noise threshold on the detector pulses. Thus, the low energy measurement was not achievable. To investigate the noise issue, the reverse leakage current i.e., dark current of PD 1 was measured. As expected, the dark current was observed to be much higher than the specified value (Table 12). For example, at -50 V bias, the dark current was obtained as ~30 nA (Fig. 62). Such large leakage current can be partly attributed to detector surface contamination.

2D Graph 2

250

200

150

100

Dark currentDark (nA) 50

0 0 10 20 30 40 50 60 70 80 90 100 Reverse voltage (V) Fig. 62. Dark current vs. reverse voltage characteristics of PD 1

Before using a second photodiode sample (PD2) for energy measurements, the diode dark current was measured to understand the extent of potential noise in the detector energy spectrum. The dark current characteristics of PD 2 are shown in Fig. 63. At 50 V 135 reverse bias, the dark current is as low as ~0.27 nA, much lower compared to that in PD

1. Hence, subsequent energy measurements were performed with PD 2.

2D Graph 1

1.0

0.8

0.6

0.4

Dark currentDark (nA) 0.2

0.0 0 10 20 30 40 50 60 70 80 90 Reverse voltage (V)

Fig. 63. Dark current vs. reverse voltage characteristics of PD 2

C.1. Experimental setup

Measurements were performed using the experimental setup described in section

3.1.1. The detector and the source were mounted in the Al vacuum chamber using a breadboard and Al mount as shown in Fig. 64. The chamber was evacuated to a pressure of 20 mtorr using the dry vacuum pump. The detector was operated at 50 V reverse bias in all the measurements. Detector voltage pulses from the preamplifier were filtered and shaped by the trapezoidal energy filter in the digitizer. Pulse height spectra were acquired from the digitizer using the pulse height analysis software DPHA on the host PC. Digital pulse processing settings used in the measurements are summarized in Table 13. 136

241Am source

Si PIN photodiode

Fig. 64. Detector-source geometry used in the measurement of 241Am, 57Co and 14C energy spectra by the photodiode detector.

General Trapezoidal filter Trigger generation Parameter Value Parameter Value Parameter Value DC offset 1 Decay time 300 µs Trigger threshold 3.5 keV Decimation 4 Rise time 1.2 µs Smoothing factor 32 Flat top duration 2.5 µs Delay or rise time 0.8 µs Baseline mean 4096 Hold off 1 µs Trapezoid gain 4 Table 13. Pulse processing settings applied in the measurement of 241Am, 57Co and 14C energy spectra.

C.2. 241Am measurement

The energy spectrum of 241Am α-particles measured from the diode detector is shown in Fig. 65. A Gaussian fit was performed to the 5.486 MeV main energy peak, for which the energy resolution was obtained as 14.9 keV. The energy spectrum of 241Am gamma rays is presented in Fig. 66. In addition to the 59.5 keV gamma ray peak, other smaller peaks are observed at ~20 keV and ~40 keV. The peak at ~20 keV could be attributed to the 26 keV photons from 241Am. The origin of the 40 keV energy peak is not completely understood, however, it can be attributed to the collective contribution of 33 keV (from

241Am) and 48 keV (59.5 keV photon backscattered from Al) photons. It is interesting to

137 note the low intensity of the 59.5 keV peak, which is attributed to the very small active area of the detector. Results also indicate the low gamma ray sensitivity of the photodiode.

2D Graph 3

0.14

0.12

0.10

0.08

0.06

0.04

Counts per bin per second 0.02

0.00 5300 5350 5400 5450 5500 5550 5600 Energy (keV) (bin size = 1.025 keV)

Fig. 65. Energy spectrum of 241Am α-particles obtained using the photodiode.

2D Graph 2

0.25

0.20

0.15

0.10

0.05

Counts per bin per second

0.00 20 40 60 80 100 Energy (keV) (bin size = 1.117 keV)

Fig. 66. Energy spectrum of 241Am gamma rays obtained using the photodiode. 138

2D Graph 1

0.16 Si PIN photodiode 0.14 Si charged particle detector

0.12

0.10

0.08

0.06

0.04

Counts per bin per second 0.02

0.00 5300 5350 5400 5450 5500 5550 5600 Energy (keV) (bin size = 1.025 keV) Fig. 67. Comparison of the 241Am α-spectra measured using the photodiode and a large area Si charged particle detector.

The 241Am α-spectra obtained using the photodiode and a large area Si charged particle detector13 are compared, as illustrated in Fig. 67. There is only a slight improvement in the energy peak resolution by using the photodiode. This could be attributed to the geometry (Fig. 64), in which the source and the photodiode axes are not exactly parallel, unlike that in the large area Si detector measurement.

C.3. 57Co measurement

For gamma ray measurement, a button-sized 57Co gamma source was mounted inside the Al chamber with the same geometrical setup. The only prominent energy peak observed in the spectrum is due to the 14.4 keV photons (Fig. 68). No energy peaks

13 Detector specifications are the same as in Table 8, but with a depletion depth ≥ 164 µm. 139 corresponding to the 122.1 keV and 136.5 keV gamma rays were observed. This could be attributed to the very low gamma sensitivity of the detector.

2D Graph 4

0.20

0.15

0.10

0.05

Counts per bin per second

0.00 25 50 75 100 125 150 Energy (keV) (bin size = 1.117 keV)

Fig. 68. Energy spectrum of 57Co gamma rays measured using the photodiode.

C.4. 14C measurement

Energy spectra of 14C β-particles measured by the photodiode and the large area Si detector are shown in Fig. 69. The noise threshold for the large Si detector's pulses was set at ~25 keV in order to mitigate the low energy noise in the spectrum. However, in the case of photodiode, the lower dark current and terminal capacitance enabled much lower noise threshold on the detector pulses. This difference can be clearly noticed in Fig. 69.

This measurement demonstrates the reduction in detector noise achieved by using the photodiode.

140

2D Graph 4

0.25 Si PIN photodiode Si charged particle detector 0.20

0.15

0.10

0.05

Counts per bin per second

0.00 25 50 75 100 125 150 175 200 Energy (keV) (bin size = 1.117 keV)

Fig. 69. A comparison of 14C β-spectra measured using the photodiode and a large area Si detector.

C.5. Summary

Results obtained using the photodiode strongly support its application as a low noise alternative to the large area Si detector. It is supposed that the energy peaks of Gd ICEs can be better resolved with the photodiode detector than with the large area Si detector.

This indeed is explained by the high energy resolution of the photodiode demonstrated by the 241Am and 14C measurements. In addition, gamma ray interference to the ICE signal is expected to be minimal, owing to the low gamma sensitivity of the photodiode as exemplified by the 57Co measurement.

141

Appendix D: Evaluation of ICE escape efficiency

D.1. C++ program to generate MCNP5 input file

// this code generates an MCNP input file that simulates // the escape of Gd ICEs into detector

#include #include #include #include #include using namespace std;

#define DENSITY_GD 7.9004 // density of Gd in g/cm3 #define DENSITY_SI 2.33 // density of Si g/cm3 #define PLATE_UPPLANE 0.5 // film upper half thickness in cm #define PLATE_LOPLANE -0.5 // film lower half thickness in cm

// default parameter (programmable) values float t_Gd = 30; // thickness of Gd in um int n_layers = t_Gd; // number of layers in the Gd slab int runtime = 100; // simulation run time void mcnp_printer(); int main() { char ch; cout <<"Do you wish to change the maximum thickness of Gd from 30 um (y/n): ?"; cin >> ch; if(ch == 'y') { cout <<"Enter the thickness for Gd (in microns)"<< endl; cin >> t_Gd; cout << "\n"; } n_layers = t_Gd; cout <<"Do you wish to change the number of layers from "<< n_layers <<" (y/n): ?"; cin >> ch; if(ch == 'y') { cout <<"Enter the number of layers "<< endl;

142

cin >> n_layers; cout <<"\n"; } cout << "Do you wish to change simulation run time from 100 (y/n): "; cin >> ch; if(ch == 'y') { cout <<"Enter time for the simulation run\n"; cin >> runtime; cout <<"\n"; } cout << "\nYou've entered the following values"<< endl; cout << "Gd foil thickness: " << t_Gd <<" um" << endl; cout << "Number of layers: " << n_layers << endl; cout << "Simulation run time: " << runtime <<" min" << endl;

mcnp_printer(); cout << "\nPress 'q' and 'enter' to quit"<< endl; cin >> ch; getchar(); return 0; }

void mcnp_printer() { // PRINTING THE MCNP INPUT FILE

ofstream fmcp ("H:\\WORK\\MCNP\\PKdir2\\GdICeff\\input.i"); if(fmcp) { char importance [] = "imp:e = 1"; float step = (t_Gd*1e-4)/n_layers; int i; int k=0; // print description of the MCNP input file

fmcp <<" Tally the transmitted electrons from each layer of the Gd slab "<< endl; fmcp <<"c using f1 electron current tally"<< endl;

// print cell cards fmcp <<"c"<< endl << "c begin CELL CARDS"<< endl <<"c"<< endl; for(i=1;i<=n_layers;i++) { fmcp << 99+i <<" 1 "<< -DENSITY_GD <<" -"<< 99+i <<" "<< importance <<" $ Gd cell "<< i <<" of "<< step*1e4 <<" um" << endl; } fmcp <<"800 2 0.001205 -999 $ air medium inside "<< endl; for(i=1;i<=n_layers/5;i++) { fmcp <<" "; for(k=k+1;k<=i*5;k++) { 143

fmcp <<" "<<99+k; } if (k==n_layers+1){fmcp <<" "<< importance; } fmcp << endl; k--; } fmcp <<"900 0 999 imp:e = 0 $ void outside boundary "<< endl;

fmcp <<"c\nc end CELL CARDS\nc"<< endl; fmcp <<"\n"; // blank space between cell and surface cards

// print surface cards fmcp <<"c"<< endl << "c begin SURFACE CARDS"<< endl <<"c"<< endl; for(i=1;i<=n_layers;i++) { fmcp << 99+i <<" RPP "<< 5.00+(i-1)*step << " "<< 5.00+i*step << " -0.5 0.5 -0.5 0.5 $ Gd layer "<< i <<" of "<< step*1e4 << " um" << endl; } fmcp <<"999 SO 99 $ problem boundary" << endl; fmcp <<"c\nc end SURFACE CARDS\nc"<< endl;

fmcp <<"\n"; // insert blank space

// PRINT DATA and MATERIAL CARDS fmcp <<"c\nc begin DATA & MATERIAL CARDS\nc"<< endl; fmcp <<"mode e $ transport of electrons only"<< endl; fmcp <<"ctme "<< runtime << " $ simulation run time"<< endl; fmcp <<"c nps 1e6 $ number of histories to run"<< endl; fmcp <<"phys:e 100 0 0 0 0 1 1 1 1 0 $ electron physics"<< endl; fmcp <<"c\nc source definition and description\nc"<< endl; fmcp <<"c rectangular plane source centered on the origin and perpendicular "<< endl; fmcp <<"c to the x-axis. This uses a degenerate Cartesian volumetric source\nc"<< endl; fmcp <<"sdef par=3 erg=d1 pos=5 0 0 X=5 Y=d2 Z=d3 $ plane isotropic source "<< endl; fmcp <<"si1 L 0.029 0.039 0.071 0.078 0.081 $ the discrete particle energies"<< endl; fmcp <<" 0.088 0.131 0.149 0.173 0.180"<< endl; fmcp <<" 0.191 0.198 0.228 0.246\nc"<< endl; fmcp <<"sp1 0.0982 0.0419 0.2680 0.0617 0.0497 $ intensity of each energy"<< endl; fmcp <<" 0.0116 0.0341 0.0084 0.0146 0.0031"<< endl; fmcp <<" 0.0030 0.0006 0.0040 0.0002\nc"<< endl; fmcp <<"si2 -0.5 0.5 $ sampling range Ymin to Ymax"<< endl; fmcp <<"sp2 0 1 $ weighting for y sampling: here constant"<< endl; fmcp <<"si3 -0.5 0.5 $ sampling range Zmin to Zmax"<< endl; fmcp <<"sp3 0 1 $ weighting for z sampling: here constant"<< endl; fmcp <<"c\n"; // material cards fmcp <<"m1 64000 1.0000 $ Gadolinium "<< endl; fmcp <<"m2 06000 0.000151 $ Air\n 07014 0.784437\n 08016 0.210750\n 18000 0.004671"<< endl; // tally cards fmcp <<"c TALLY CARDS\nc "<< endl; fmcp <<"c electron current tally for electrons escaping the surface of each layer\nc "<< endl; fmcp <<"f1:e"; k=0; 144

for(i=1;i<=n_layers/5;i++) { fmcp <<" "; for(k=k+1;k<=i*5;k++) { fmcp <<" "<<99+k+0.1; } fmcp << endl; k--; } fmcp <<"c1 0 1 $ cosine bin limits"<< endl; fmcp <<"c\nc end DATA & MATERIAL CARDS\nc"<< endl;

fmcp.close(); } else cout <<"Unable to open file."<< endl; }

D.2. MCNP5 input file

Tally the transmitted electrons from each layer of the Gd slab c using f1 electron current tally c c begin CELL CARDS c 100 1 -7.9004 -100 imp:e = 1 $ Gd cell 1 of 0.5 um 101 1 -7.9004 -101 imp:e = 1 $ Gd cell 2 of 0.5 um 102 1 -7.9004 -102 imp:e = 1 $ Gd cell 3 of 0.5 um 103 1 -7.9004 -103 imp:e = 1 $ Gd cell 4 of 0.5 um 104 1 -7.9004 -104 imp:e = 1 $ Gd cell 5 of 0.5 um 105 1 -7.9004 -105 imp:e = 1 $ Gd cell 6 of 0.5 um 106 1 -7.9004 -106 imp:e = 1 $ Gd cell 7 of 0.5 um 107 1 -7.9004 -107 imp:e = 1 $ Gd cell 8 of 0.5 um 108 1 -7.9004 -108 imp:e = 1 $ Gd cell 9 of 0.5 um 109 1 -7.9004 -109 imp:e = 1 $ Gd cell 10 of 0.5 um 110 1 -7.9004 -110 imp:e = 1 $ Gd cell 11 of 0.5 um 111 1 -7.9004 -111 imp:e = 1 $ Gd cell 12 of 0.5 um 112 1 -7.9004 -112 imp:e = 1 $ Gd cell 13 of 0.5 um 113 1 -7.9004 -113 imp:e = 1 $ Gd cell 14 of 0.5 um 114 1 -7.9004 -114 imp:e = 1 $ Gd cell 15 of 0.5 um 115 1 -7.9004 -115 imp:e = 1 $ Gd cell 16 of 0.5 um 116 1 -7.9004 -116 imp:e = 1 $ Gd cell 17 of 0.5 um 117 1 -7.9004 -117 imp:e = 1 $ Gd cell 18 of 0.5 um 118 1 -7.9004 -118 imp:e = 1 $ Gd cell 19 of 0.5 um 119 1 -7.9004 -119 imp:e = 1 $ Gd cell 20 of 0.5 um 120 1 -7.9004 -120 imp:e = 1 $ Gd cell 21 of 0.5 um 121 1 -7.9004 -121 imp:e = 1 $ Gd cell 22 of 0.5 um 122 1 -7.9004 -122 imp:e = 1 $ Gd cell 23 of 0.5 um 123 1 -7.9004 -123 imp:e = 1 $ Gd cell 24 of 0.5 um 124 1 -7.9004 -124 imp:e = 1 $ Gd cell 25 of 0.5 um 145

125 1 -7.9004 -125 imp:e = 1 $ Gd cell 26 of 0.5 um 126 1 -7.9004 -126 imp:e = 1 $ Gd cell 27 of 0.5 um 127 1 -7.9004 -127 imp:e = 1 $ Gd cell 28 of 0.5 um 128 1 -7.9004 -128 imp:e = 1 $ Gd cell 29 of 0.5 um 129 1 -7.9004 -129 imp:e = 1 $ Gd cell 30 of 0.5 um 130 1 -7.9004 -130 imp:e = 1 $ Gd cell 31 of 0.5 um 131 1 -7.9004 -131 imp:e = 1 $ Gd cell 32 of 0.5 um 132 1 -7.9004 -132 imp:e = 1 $ Gd cell 33 of 0.5 um 133 1 -7.9004 -133 imp:e = 1 $ Gd cell 34 of 0.5 um 134 1 -7.9004 -134 imp:e = 1 $ Gd cell 35 of 0.5 um 135 1 -7.9004 -135 imp:e = 1 $ Gd cell 36 of 0.5 um 136 1 -7.9004 -136 imp:e = 1 $ Gd cell 37 of 0.5 um 137 1 -7.9004 -137 imp:e = 1 $ Gd cell 38 of 0.5 um 138 1 -7.9004 -138 imp:e = 1 $ Gd cell 39 of 0.5 um 139 1 -7.9004 -139 imp:e = 1 $ Gd cell 40 of 0.5 um 140 1 -7.9004 -140 imp:e = 1 $ Gd cell 41 of 0.5 um 141 1 -7.9004 -141 imp:e = 1 $ Gd cell 42 of 0.5 um 142 1 -7.9004 -142 imp:e = 1 $ Gd cell 43 of 0.5 um 143 1 -7.9004 -143 imp:e = 1 $ Gd cell 44 of 0.5 um 144 1 -7.9004 -144 imp:e = 1 $ Gd cell 45 of 0.5 um 145 1 -7.9004 -145 imp:e = 1 $ Gd cell 46 of 0.5 um 146 1 -7.9004 -146 imp:e = 1 $ Gd cell 47 of 0.5 um 147 1 -7.9004 -147 imp:e = 1 $ Gd cell 48 of 0.5 um 148 1 -7.9004 -148 imp:e = 1 $ Gd cell 49 of 0.5 um 149 1 -7.9004 -149 imp:e = 1 $ Gd cell 50 of 0.5 um 150 1 -7.9004 -150 imp:e = 1 $ Gd cell 51 of 0.5 um 151 1 -7.9004 -151 imp:e = 1 $ Gd cell 52 of 0.5 um 152 1 -7.9004 -152 imp:e = 1 $ Gd cell 53 of 0.5 um 153 1 -7.9004 -153 imp:e = 1 $ Gd cell 54 of 0.5 um 154 1 -7.9004 -154 imp:e = 1 $ Gd cell 55 of 0.5 um 155 1 -7.9004 -155 imp:e = 1 $ Gd cell 56 of 0.5 um 156 1 -7.9004 -156 imp:e = 1 $ Gd cell 57 of 0.5 um 157 1 -7.9004 -157 imp:e = 1 $ Gd cell 58 of 0.5 um 158 1 -7.9004 -158 imp:e = 1 $ Gd cell 59 of 0.5 um 159 1 -7.9004 -159 imp:e = 1 $ Gd cell 60 of 0.5 um 800 2 0.001205 -999 $ air medium inside 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 imp:e = 1 900 0 999 imp:e = 0 $ void outside boundary c c end CELL CARDS c

146 c c begin SURFACE CARDS c 100 RPP 5 5.00005 -0.5 0.5 -0.5 0.5 $ Gd layer 1 of 0.5 um 101 RPP 5.00005 5.0001 -0.5 0.5 -0.5 0.5 $ Gd layer 2 of 0.5 um 102 RPP 5.0001 5.00015 -0.5 0.5 -0.5 0.5 $ Gd layer 3 of 0.5 um 103 RPP 5.00015 5.0002 -0.5 0.5 -0.5 0.5 $ Gd layer 4 of 0.5 um 104 RPP 5.0002 5.00025 -0.5 0.5 -0.5 0.5 $ Gd layer 5 of 0.5 um 105 RPP 5.00025 5.0003 -0.5 0.5 -0.5 0.5 $ Gd layer 6 of 0.5 um 106 RPP 5.0003 5.00035 -0.5 0.5 -0.5 0.5 $ Gd layer 7 of 0.5 um 107 RPP 5.00035 5.0004 -0.5 0.5 -0.5 0.5 $ Gd layer 8 of 0.5 um 108 RPP 5.0004 5.00045 -0.5 0.5 -0.5 0.5 $ Gd layer 9 of 0.5 um 109 RPP 5.00045 5.0005 -0.5 0.5 -0.5 0.5 $ Gd layer 10 of 0.5 um 110 RPP 5.0005 5.00055 -0.5 0.5 -0.5 0.5 $ Gd layer 11 of 0.5 um 111 RPP 5.00055 5.0006 -0.5 0.5 -0.5 0.5 $ Gd layer 12 of 0.5 um 112 RPP 5.0006 5.00065 -0.5 0.5 -0.5 0.5 $ Gd layer 13 of 0.5 um 113 RPP 5.00065 5.0007 -0.5 0.5 -0.5 0.5 $ Gd layer 14 of 0.5 um 114 RPP 5.0007 5.00075 -0.5 0.5 -0.5 0.5 $ Gd layer 15 of 0.5 um 115 RPP 5.00075 5.0008 -0.5 0.5 -0.5 0.5 $ Gd layer 16 of 0.5 um 116 RPP 5.0008 5.00085 -0.5 0.5 -0.5 0.5 $ Gd layer 17 of 0.5 um 117 RPP 5.00085 5.0009 -0.5 0.5 -0.5 0.5 $ Gd layer 18 of 0.5 um 118 RPP 5.0009 5.00095 -0.5 0.5 -0.5 0.5 $ Gd layer 19 of 0.5 um 119 RPP 5.00095 5.001 -0.5 0.5 -0.5 0.5 $ Gd layer 20 of 0.5 um 120 RPP 5.001 5.00105 -0.5 0.5 -0.5 0.5 $ Gd layer 21 of 0.5 um 121 RPP 5.00105 5.0011 -0.5 0.5 -0.5 0.5 $ Gd layer 22 of 0.5 um 122 RPP 5.0011 5.00115 -0.5 0.5 -0.5 0.5 $ Gd layer 23 of 0.5 um 123 RPP 5.00115 5.0012 -0.5 0.5 -0.5 0.5 $ Gd layer 24 of 0.5 um 124 RPP 5.0012 5.00125 -0.5 0.5 -0.5 0.5 $ Gd layer 25 of 0.5 um 125 RPP 5.00125 5.0013 -0.5 0.5 -0.5 0.5 $ Gd layer 26 of 0.5 um 126 RPP 5.0013 5.00135 -0.5 0.5 -0.5 0.5 $ Gd layer 27 of 0.5 um 127 RPP 5.00135 5.0014 -0.5 0.5 -0.5 0.5 $ Gd layer 28 of 0.5 um 128 RPP 5.0014 5.00145 -0.5 0.5 -0.5 0.5 $ Gd layer 29 of 0.5 um 129 RPP 5.00145 5.0015 -0.5 0.5 -0.5 0.5 $ Gd layer 30 of 0.5 um 130 RPP 5.0015 5.00155 -0.5 0.5 -0.5 0.5 $ Gd layer 31 of 0.5 um 131 RPP 5.00155 5.0016 -0.5 0.5 -0.5 0.5 $ Gd layer 32 of 0.5 um 132 RPP 5.0016 5.00165 -0.5 0.5 -0.5 0.5 $ Gd layer 33 of 0.5 um 133 RPP 5.00165 5.0017 -0.5 0.5 -0.5 0.5 $ Gd layer 34 of 0.5 um 134 RPP 5.0017 5.00175 -0.5 0.5 -0.5 0.5 $ Gd layer 35 of 0.5 um 135 RPP 5.00175 5.0018 -0.5 0.5 -0.5 0.5 $ Gd layer 36 of 0.5 um 136 RPP 5.0018 5.00185 -0.5 0.5 -0.5 0.5 $ Gd layer 37 of 0.5 um 137 RPP 5.00185 5.0019 -0.5 0.5 -0.5 0.5 $ Gd layer 38 of 0.5 um 138 RPP 5.0019 5.00195 -0.5 0.5 -0.5 0.5 $ Gd layer 39 of 0.5 um 139 RPP 5.00195 5.002 -0.5 0.5 -0.5 0.5 $ Gd layer 40 of 0.5 um 140 RPP 5.002 5.00205 -0.5 0.5 -0.5 0.5 $ Gd layer 41 of 0.5 um 141 RPP 5.00205 5.0021 -0.5 0.5 -0.5 0.5 $ Gd layer 42 of 0.5 um 142 RPP 5.0021 5.00215 -0.5 0.5 -0.5 0.5 $ Gd layer 43 of 0.5 um 143 RPP 5.00215 5.0022 -0.5 0.5 -0.5 0.5 $ Gd layer 44 of 0.5 um 144 RPP 5.0022 5.00225 -0.5 0.5 -0.5 0.5 $ Gd layer 45 of 0.5 um 145 RPP 5.00225 5.0023 -0.5 0.5 -0.5 0.5 $ Gd layer 46 of 0.5 um 146 RPP 5.0023 5.00235 -0.5 0.5 -0.5 0.5 $ Gd layer 47 of 0.5 um 147 RPP 5.00235 5.0024 -0.5 0.5 -0.5 0.5 $ Gd layer 48 of 0.5 um 148 RPP 5.0024 5.00245 -0.5 0.5 -0.5 0.5 $ Gd layer 49 of 0.5 um 149 RPP 5.00245 5.0025 -0.5 0.5 -0.5 0.5 $ Gd layer 50 of 0.5 um 147

150 RPP 5.0025 5.00255 -0.5 0.5 -0.5 0.5 $ Gd layer 51 of 0.5 um 151 RPP 5.00255 5.0026 -0.5 0.5 -0.5 0.5 $ Gd layer 52 of 0.5 um 152 RPP 5.0026 5.00265 -0.5 0.5 -0.5 0.5 $ Gd layer 53 of 0.5 um 153 RPP 5.00265 5.0027 -0.5 0.5 -0.5 0.5 $ Gd layer 54 of 0.5 um 154 RPP 5.0027 5.00275 -0.5 0.5 -0.5 0.5 $ Gd layer 55 of 0.5 um 155 RPP 5.00275 5.0028 -0.5 0.5 -0.5 0.5 $ Gd layer 56 of 0.5 um 156 RPP 5.0028 5.00285 -0.5 0.5 -0.5 0.5 $ Gd layer 57 of 0.5 um 157 RPP 5.00285 5.0029 -0.5 0.5 -0.5 0.5 $ Gd layer 58 of 0.5 um 158 RPP 5.0029 5.00295 -0.5 0.5 -0.5 0.5 $ Gd layer 59 of 0.5 um 159 RPP 5.00295 5.003 -0.5 0.5 -0.5 0.5 $ Gd layer 60 of 0.5 um 999 SO 99 $ problem boundary c c end SURFACE CARDS c c c begin DATA & MATERIAL CARDS c mode e $ transport of electrons only ctme 100 $ simulation run time c nps 1e6 $ number of histories to run phys:e 100 0 0 0 0 1 1 1 1 0 $ electron physics c c source definition and description c c rectangular plane source centered at (5,0,0) and perpendicular c to the x-axis. This uses a degenerate Cartesian volumetric source c sdef par=3 erg=d1 pos=5 0 0 X=5 Y=d2 Z=d3 $ plane isotropic source si1 L 0.029 0.039 0.071 0.078 0.081 $ the discrete particle energies 0.088 0.131 0.149 0.173 0.180 0.191 0.198 0.228 0.246 c sp1 0.0982 0.0419 0.2680 0.0617 0.0497 $ intensity of each energy 0.0116 0.0341 0.0084 0.0146 0.0031 0.0030 0.0006 0.0040 0.0002 c si2 -0.5 0.5 $ sampling range Ymin to Ymax sp2 0 1 $ weighting for y sampling: here constant si3 -0.5 0.5 $ sampling range Zmin to Zmax sp3 0 1 $ weighting for z sampling: here constant c m1 64000 1.0000 $ Gadolinium m2 06000 0.000151 $ Air 07014 0.784437 08016 0.210750 18000 0.004671 c TALLY CARDS c c electron current tally for electrons escaping the surface of each layer c f1:e 100.1 101.1 102.1 103.1 104.1 105.1 106.1 107.1 108.1 109.1 148

110.1 111.1 112.1 113.1 114.1 115.1 116.1 117.1 118.1 119.1 120.1 121.1 122.1 123.1 124.1 125.1 126.1 127.1 128.1 129.1 130.1 131.1 132.1 133.1 134.1 135.1 136.1 137.1 138.1 139.1 140.1 141.1 142.1 143.1 144.1 145.1 146.1 147.1 148.1 149.1 150.1 151.1 152.1 153.1 154.1 155.1 156.1 157.1 158.1 159.1 c1 0 1 $ cosine bin limits c c end DATA & MATERIAL CARDS c

D.3. Matlab script for evaluating the optimal thickness

% matlab script used to evaluate the escape efficiency of ICEs using the % results from MCNP5 simulation clc; clear all; close all;

N = 100; % total number of incident neutrons T = input('Please enter the Gd slab thickness\n'); n_layers = input('\nPlease enter the number of layers\n'); t = (0:T/n_layers:T)'; n_layers = size(t,1)-1; Abs_eff = 1-exp(-1476.2*1e-4*t); % intrinsic neutron absorption efficiency for i=1:n_layers dAbs_eff(i,1) = Abs_eff(i+1)-Abs_eff(i); % number of neutrons absorbed in layer 'i' N_e(i,1) = 0.6*N*dAbs_eff(i); % number of IC electrons emitted in layer 'i' end dir = 'H:\WORK\MCNP\PKdir2\GdICeff\'; ftally = fopen( [dir esc_eff.o'],'r'); while ~feof(ftally) curr_line = fgetl(ftally); if length(curr_line) > 6 && strcmp(curr_line(1:6),'1tally') for i=1:2 % skip the first 2 lines; curr_line = fgetl(ftally); end k = 1; while k<=n_layers for i=1:7 % skip 7 lines for each surface tally curr_line = fgetl(ftally); end curr_line = fgetl(ftally); % read tally val = curr_line(18:28); N_esc(k,1) = sscanf(val,'%f'); k=k+1; 149

end break end end

% example - 3 um thickness-> N_esc(3)*N_e(1) + N_esc(2)*N_e(2) + N_esc(1)*N_e(3) Esc_E_forw(1,1) = 0; Esc_E_back(1,1) = 0; for n=1:n_layers sum_forw = 0; sum_back = 0; for i=1:n sum_forw = sum_forw + N_esc(n-i+1)*N_e(i); sum_back = sum_back + N_esc(i)*N_e(i); end Esc_E_forw(n+1,1) = sum_forw; Esc_E_back(n+1,1) = sum_back; end X = t(:,ones(3,1)); Y = [Esc_E_forw Esc_E_back Esc_E_back+Esc_E_forw]; plotter(X,Y,'.-','Gd thickness (\mum)',{'IC e_{-} escape efficiency';'(per 100 incident neutrons)'},'Optimizing Gd thickness for neutron convertor coating', [0 30],{'Escape efficiency front; 'Escape efficiency back';' Total escape efficiency'});

150

Appendix E: MCNP5 input for gamma rejection simulation

Neutron gamma separation in a Gd based semiconductor neutron detector c Detectors' response to gamma rays from U-235 alpha decay c c Cell Cards c 600 2 -2.33 61 -62 66 -65 64 -63 $ Si detector 1 (100 um) 700 1 -7.9004 71 -72 76 -75 74 -73 $ Gd (5 um) thin film 800 3 -0.94 81 -82 86 -85 84 -83 $ polyethylene layer (350 um) 900 2 -2.33 91 -92 96 -95 94 -93 $ Si detector 2 (100 um) c 909 4 -0.001205 -99 #600 #700 #800 #900 $ world (air) c c kill particles outside the region of interest c 999 0 99 $ outside world (void) c c end of cell cards c c c Surface Cards c c Si detector 1 surfaces c 61 px 5.00 $ Si det1 left x plane 62 px 5.01 $ Si det1 right x plane 63 pz 0.50 $ Si det1 upper z plane 64 pz -0.50 $ Si det1 lower z plane 65 py 0.50 $ Si det1 front y plane 66 py -0.50 $ Si det1 rear y plane c c Gadolinium layer surfaces c 71 px 5.01000 $ Gd foil left x plane 72 px 5.01050 $ Gd foil right x plane 73 pz 0.50 $ Gd foil upper z plane 74 pz -0.50 $ Gd foil lower z plane 75 py 0.50 $ Gd foil front y plane 76 py -0.50 $ Gd foil rear y plane c c polyethylene layer c 81 px 5.01050 $ poly layer left x plane 82 px 5.04550 $ poly layer right x plane 151

83 pz 0.50 $ poly layer upper z plane 84 pz -0.50 $ poly layer lower z plane 85 py 0.50 $ poly layer front y plane 86 py -0.50 $ poly layer rear y plane c c Si detector 2 surfaces c 91 px 5.04550 $ Si det2 left x plane 92 px 5.05550 $ Si det2 right x plane 93 pz 0.50 $ Si det2 upper z plane 94 pz -0.50 $ Si det2 lower z plane 95 py 0.50 $ Si det2 front y plane 96 py -0.50 $ Si det2 rear y plane c c outside world c 99 so 50 $ problem boundary c c end of surface cards c c c Data cards c mode p e $ photon and electron transport ctme 400 $ run time phys:p 100 0 0 0 0 $ photon physics phys:e 100 0 0 0 0 1 1 1 1 0 $ electron physics c c source definition and description c c sdef erg=d1 par=2 pos=0 0 0 axs=1 0 0 vec=1 0 0 c ara=3.14159 ext=0 dir=1 rad=d2 $ disk source sdef erg=d1 par=2 pos=10.0555 0 0 $ point isotropic source c c source energy distribution c si1 l 31.60e-3 34.7e-3 41.4e-3 41.96e-3 51.22e-3 54.1e-3 54.25e-3 60.5e-3 64.37e-3 72.7e-3 73.72e-3 75.02e-3 94e-3 95.7e-3 96.09e-3 109.16e-3 115.45e-3 120.35e-3 136.55e-3 140.76e-3 142.40e-3 143.76e-3 147.0e-3 150.93e-3 163.33e-3 173.3e-3 182.1e-3 182.61e-3 185.715e-3 194.94e-3 198.90e-3 202.11e-3 205.311e-3 215.28e-3 221.38e-3 228.78e-3 233.50e-3 240.87e-3 246.84e-3 251.5e-3 266.45e-3 275.129e-3 275.43e-3 279.50e-3 281.42e-3 282.92e-3 289.56e-3 291.2e-3 291.65e-3 301.7e-3 310.69e-3 317.10e-3 325.80e-3 343.5e-3 345.90e-3 356.03e-3 387.82e-3 390.3e-3 410.29e-3 433.0e-3 448.40e-3 455.1e-3 517.2e-3 742.5e-3 794.7e-3 c sp1 0.016 0.037 0.03 0.06 0.020 0.002 0.03 0.0 0.04 0.11 0.01 0.06 0.0 0.0 0.086 1.54 0.07 0.026 0.012 0.22 0.005 10.96 0.0 0.076 5.08 0.010 0.0 0.34 57.2 0.63 0.042 1.08 5.01 0.027 0.12 0.008 0.029 0.075 0.053 0.04 152

0.006 0.042 0.007 0.27 0.006 0.005 0.007 0.0 0.038 0.005 0.004 0.001 0.0004 0.003 0.038 0.005 0.038 0.04 0.003 0.004 0.001 0.008 0.0004 0.0004 0.0006 c c si2 0 0.5 $ radius sampling limits c sp2 -21 1 $ sampling weights for radius c imp:p,e 1 1 1 1 1 0 c c Material Definitions c m1 64152 0.002 $ Gadolinium 64154 0.0218 $ natural abundance 64155 0.1480 64156 0.2047 64157 0.1565 64158 0.2484 64160 0.2186 c m2 14000 1.0000 $ Silicon c m3 01001 0.667 $ high density poly 06012 0.333 c m4 6000 -0.000124 $ C - Air 7000 -0.755268 $ N 8000 -0.231781 $ O 18000 -0.012827 $ Ar c c tallies for photons c f4:p 600 900 $ photon fluence in det1 and det2 e4 1e-3 799i 0.8 c c pulse height tally c f8:p 600 900 $ pulse height distribution in Si detectors e8 0.0 1e-5 1e-3 798i 0.8 c c energy deposition tally c *f18:p 600 900 $ energy deposition in Si detectors e18 0.0 1e-5 1e-3 798i 0.8 c

153