Modeling and characterization of a monoenergetic gamma-ray imaging system for active interrogation applications
Harms, Joseph M.
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The goal of this thesis is to develop an understanding of the benefits of using a monoenergetic photon source for imaging of cargo containers, as opposed to polyenergetic bremsstrahlung beams which are currently used. Monoenergetic beams can reduce dose to both the cargo and any potential stowaways within a container while increasing penetration and image contrast. In this work, imaging beams are tested for beam penetration, dose, and scatter within the container. It is found that higher energy beams scatter less widely, and can offer dose reductions to the cargo on the order of 40-60% while enhancing image quality. This is confirmed through simulation studies in Geant4 on both small-scale and full-scale cargo containers. Radiation detectors specifically targeted for imaging in this high-intensity environment are optimized and characterized, Cherenkov-based quartz detectors and LYSO scintillating detectors are used in the final imaging system. Imaging simulations include fully validated quartz detector response models. Dual-energy acquisition techniques, based on differences in attenuation coefficient as a function of energy, are developed and characterized for material-specific radiography. It is found that the monoenergetic sources offer better material specificity and higher contrast. Finally, tomographic image reconstruction algorithms are developed to take advantage of the isotropic nature of nuclear-reaction driven imaging beams. The incorporation of images taken at multiple views of the container can allow for unfolding of the cargo composition in 3D, enhancing operator safety if a container was found to hold suspicious material.