Feasibility of creating a neutron detector array for the detection of special nuclear materials

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Benson, Bixler C.
Hertel, Nolan E.
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The purpose of this research is to determine the feasibility of creating an energy-dependent neutron detector array that could be used to detect special nuclear materials. The proposed detector array is composed of idealized thermal neutron detectors covered by moderators of differing size, namely thickness and shape. Through a series of Monte Carlo radiation transport calculations, the size, shape, and location of the moderators and detectors were configured to determine the efficacy of obtaining a neutron energy spectrum or spectral information in a single measurement. This approach expands on the concepts of the Bonner Sphere Spectrometer (BSS) which relies on multiple measurements using differing size moderating spheres. Although a proven approach, the traditional BSS method is not practicable for military applications. The alternative approach to the spectrum measurement is the use of detector sums or ratios to distinguish fission neutron spectra from other neutron spectra that might be encountered. Several 5x5 detector/moderator arrays were modeled in MCNP to determine which models would have the best responses to monoenergetic neutron sources. Five models of 5x5 detector arrays were chosen that best optimize the detectors reaction rates over certain energy ranges. This 5x25 detector array was then simulated with monoenergetic neutron sources and fission sources. A least squares fitting method was attempted to find coefficients that would generate a weighted sum of detectors that yield a flat fluence response. The response function did not have a flat response. The 5x25 detector system did produce characteristic curves for moderated and unmoderated fission sources. A 5x30 detector array that contained a 5x5 detector array that had a beryllium layer was also simulated. The 5x30 detector array had a higher response to high energy neutrons, but not a significant increase for low energy neutrons. The resulting 5x25 system that was designed is capable of being vehicle mounted so it would be mobile. This research shows that it is feasible to create a neutron detector array that is sensitive to neutron energy spectra.
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