On Neutron Spectroscopy Using Gas Proportional Detectors Optimized by Transport Theory

On Neutron Spectroscopy Using Gas Proportional Detectors Optimized by Transport Theory

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RADIATION MEASUREMENTS AND INSTRUMENTATION KEYWORDS: radiation transport, Monte Carlo, neutron detection ON NEUTRON SPECTROSCOPY USING GAS PROPORTIONAL DETECTORS OPTIMIZED BY TRANSPORT THEORY GABRIEL GHITA,* GLENN SJODEN, and JAMES BACIAK University of Florida, Nuclear and Radiological Engineering Department Florida Institute of Nuclear Detection and Safety (FINDS), 202 NSB, POB 118300 Gainesville, Florida 32611-8300 Received April 17, 2008 Accepted for Publication July 16, 2009 We explore in this study the practical limits in de- signing a neutron detector array to resolve the spectra from special nuclear material (SNM) neutron sources using 3 He detectors. We demonstrate that radiation trans- port analysis yielded a spectrum unfolding strategy based on the energy structure of the BUGLE-96 cross-section library, with 47 neutron energy groups. The initial com- putational model used is an isotropic planar source in- cident on a block of high-density polyethylene moderator. Assuming 3 He is diluted throughout the moderator, the 3 He(n,p) reaction rate energy group matrix in the block was computed using a completely “flat” neutron source spectrum. Analyzing the energy group matrix, there are neutrons from specific collections of energy groups (en- ergy “bands”) that induce a maximum reaction rate in specific locations; we determined that these groups can- not be further differentiated within the energy band using 3 He detectors. It was determined that optimal spectral fidelity for SNM detection and characterization is achiev- able using four spectral bands spanning among groups 1 through 29 (31.8 keV to 17.3 MeV). Using ideal-filter materials to remove the neutrons from different regions of the spectrum, we predicted the maximum neutron spec- tral resolution obtainable using this approach. To dem- onstrate our method, we present the optimally detected spectral differences between SNM materials (plutonium and uranium), metal and oxide, using ideal-filter mate- rials. We have also selected a number of candidate fil- tering materials and, by replacing the ideal filters with real materials, we exemplified our approach with a de- sign of a neutron detector array capable of resolving the spectra from SNM neutron sources using 3 He detectors. I. INTRODUCTION Continuing threats to national security have placed renewed emphasis on the development of efficient, ac- curate nuclear material interrogation methods, with em- phasis on the need to detect special nuclear materials ~ SNM ! . Therefore, active and passive interrogation tech- niques for the detection of SNM are under investigation for many applications in the areas of nuclear safeguards, nuclear nonproliferation, and homeland security. 1–3
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On Neutron Spectroscopy Using Gas Proportional Detectors Optimized by Transport Theory

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