A Proof-of-Principle Investigation for a Neutron-Gamma Discrimination Technique in a Semiconductor Neutron Detector THESIS Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By Praneeth Kandlakunta, B.E. (Hons.) Graduate Program in Nuclear Engineering The Ohio State University 2012 Thesis Committee: Dr. Lei Cao, Advisor Dr. Don Miller Copyright by Praneeth Kandlakunta 2012 Abstract Gadolinium (Gd) is an efficient thermal neutron conversion material due to the superior thermal neutron absorption cross-section predominantly composed of 157Gd, which is also 15.65% abundant in natural Gd. A thermal neutron capture by 157Gd results in an excited 158Gd nucleus. The de-excitation of 158Gd* involves the emission of prompt gamma rays with competing internal conversion (IC) electrons. Following the expulsion of conversion electrons from the atomic shells of 158Gd, the excitation energy of the atom is released through the emission of Auger electrons and characteristic x-rays. The low energy conversion electrons and Auger electrons are considered the principal component of neutron-induced signal in a Gd-based thin film semiconductor. Besides possessing a high thermal neutron absorption cross-section, Gd also has a good interaction probability for high/medium energy gamma rays, owing to its high Z (64) number. A Gd atom activated by an external high energy gamma ray leads to the emission of characteristic x- rays that come primarily from the K-shell. These x-rays have a fairly low energy (43.0 keV, 42.3 keV for Kα1, Kα2 respectively) compared to those of the prompt gamma rays that are emitted following a neutron capture. Thin film semiconductors, although transparent to high energy gamma rays, are comparatively sensitive to low energy gamma rays and x-rays. Hence, it is supposed that a thin film semiconductor neutron detector using Gd as a neutron convertor receives ii greater interference from low energy x-rays that are emitted following gamma ray activation in Gd, than that from high energy background gamma rays. Thus, due to the presence of an inherent gamma ray background, separation of the neutron-induced signal from a gamma/x-ray induced signal is central to a semiconductor neutron detector employing Gd as the conversion material. A method of separation of these two signals by means of a current subtraction technique has been proposed. This gamma ray rejection scheme presents two identical semiconductor detectors separated by a thin Gd foil and a polyethylene thin layer. In the presence of a mixed neutron and gamma ray environment, a subtraction of signals resulting from these two detectors generates a ‘neutron only’ induced signal. Nevertheless, an experimental validation will reinforce the abovementioned supposition and provide substantiation of the same. The objective of this research is thus to validate the principle proposed for gamma ray rejection in a thin film semiconductor neutron detector based on Gd. As the first stage, an experimental setup was designed and constructed to perform the required radiation measurements. In the second phase, preliminary measurements were performed to calibrate the instrumentation system and to gain expertise on using the signal processing electronics. In the final phase, a mixed beta- gamma measurement using two silicon detectors was performed in order to simulate a neutron-gamma discrimination scenario in a Gd based semiconductor detector. The output energy spectra encompassed a mixed beta-gamma spectrum from an unshielded silicon detector and a gamma ray only spectrum from a shielded silicon detector. iii Subtraction of the two spectra generated a beta-only spectrum representing a detector’s response to the IC and Auger electrons from Gd. iv Acknowledgments I sincerely thank my research advisor, Dr. Lei Cao, for having given me the opportunity to work on this research endeavor. I’d also like to thank Dr. Cao for his guidance, support and constant encouragement, without which the investigation wouldn’t have been complete. I’m thankful to my research colleague James Ralston for sharing his inputs with me and for the several discussions we had on the subject matter. I would like to express my gratitude to my fellow graduate students Danyal Turkoglu, Padhraic Mulligan and Jinghui Wang for their assistance and support. I’m also very thankful to the technical support team from CAEN SpA, for their periodic feedback, support and guidance in using the digital data processing module and the pulse processing software. v Vita December 2008………………....B.E.(Hons.), Birla Institute of Technology and Science (BITS) – Pilani, India. September 2010 to present……. Graduate Research Associate, Nuclear Engineering Graduate Program, Department of Mechanical and Aerospace Engineering, The Ohio State University. Publications 1. 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. 2. 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 American Nuclear Society. Vol. 104. Hollywood, FL, USA. (June 2011): 227-229. 3. 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: The Transaction of America Nuclear Society. Vol. 104. Hollywood, FL, USA. (June 2011): 207-209. 4. D. Turkoglua, J. Straha, P. Kandlakuntab, L. Cao. "Development of an External Neutron Beam Facility at The Ohio State University." In: The Transaction of America Nuclear Society. Vol. 102. Las Vegas, NV, U.S.A. 5. Praneeth Kandlakunta, Lei Cao*. "GAMMA RAY REJECTION, OR DETECTION, WITH GADOLINIUMAS A CONVERTER." Radiation Protection Dosimetry. (February 2012) (IF: 0.966) (Formally Accepted). vi Fields of Study Major Field: Nuclear Engineering vii Table of Contents Abstract ............................................................................................................................... ii Acknowledgments............................................................................................................... v Vita ..................................................................................................................................... vi Table of Contents ............................................................................................................. viii List of Tables ...................................................................................................................... x List of Figures .................................................................................................................... xi 1. Introduction .................................................................................................................... 1 2. Background .................................................................................................................... 3 2.1 Neutron Detectors ................................................................................................ 3 2.2 Solid-state neutron detectors ................................................................................ 5 2.3 Neutron converter materials ................................................................................. 6 2.4 Interaction of neutrons with gadolinium ............................................................ 10 2.4.1 Intrinsic thermal neutron detection efficiency of a thin Gd foil ................. 16 2.5 Interaction of gamma rays with gadolinium ...................................................... 18 2.6 Gamma ray discrimination techniques in neutron detectors .............................. 20 2.6.1 A literature review of some gamma ray discrimination techniques ........... 21 2.6.2 Proposed gamma ray discrimination scheme .............................................. 25 2.7 Theory of radiation measurement systems and digital pulse processing (DPP) techniques ...................................................................................................................... 26 2.7.1 A review of DAQ systems used in radiation measurement ........................ 27 2.7.2 Trapezoidal energy filter ............................................................................. 32 2.7.3 Operation of a digitizer built by CAEN SpA .............................................. 34 2.7.4 Digital treatment of the signal – advantages and disadvantages ................. 36 3. Experimental approach ................................................................................................. 38 3.1 Objective and the Experiment ............................................................................ 38 3.2 Radioactive sources for a ‘proof of principle investigation’ .............................. 38 viii 3.3 Instrumentation system for the measurements ................................................... 40 3.3.1 In-house Al vacuum chamber ..................................................................... 41 3.3.2 DAQ System ............................................................................................... 44 3.3.3 Different acquisition modes with the digitizer............................................ 47 4. Experimental setup and instrument calibration ...........................................................