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 By 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 Copyright by 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. iii Dedication This document is dedicated to my parents and my beloved sister. iv 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. v 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. vi 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. vii 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). viii Fields of Study Major Field: Nuclear Engineering ix 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