Imaging of special nuclear materials using monochromatic gamma rays from low-energy nuclear reactions
Rose, Paul Brian
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The pursuit of shielded special nuclear material in cargo can be facilitated by active interrogation employing discrete energy photons. A variety of detectors can be used for imaging cargo contingent upon the energies and fluence used in the interrogation. If the gamma ray energies are sufficiently well separated, as the case in 11B(d,n-γ)12C reaction resulting in photons of numerous energies up to 15.1 MeV, spectral analysis is possible with a variety of detector technologies. Identifying and characterizing a source like this one is crucial to produce a fieldable active interrogation system capable of imaging and identification of special nuclear material. Spectral analysis of the image can be used to confirm the presence of high-Z materials by analyzing the relative transmission of the two main energies emitted by exploiting the large difference in Compton scatter and pair production cross sections. Cherenkov detectors offer a low-cost solution but require a special approach to design and energy calibration due to the lack of resolution in order for spectral analysis to take place. High-density scintillators or semiconductor detectors can yield much better spectral information, but are considerably more expensive. Intermixing detector technologies in a compact array, such as silicon photomultipliers, can lead to data fusion techniques when coupled with the proper electronics. This thesis addresses the source characterization as well as imaging with a variety of detectors for active interrogation with monoenergetic photons and compares the imaging and material identification performance.