Thermal Neutron Point Source Imaging Using a Rotating Modulation Collimator

This thesis demonstrates a previously untested capability of the Rotating Modulation Collimator (RMC) to image a point-like neutron source. The encouraging results, achieved using low-energy neutrons, provide motivation for further refinement and continued research with higher-energy neutrons. The detector and the masks on an existing RMC imaging system were exchanged to function with neutrons. The source in this research produced a poly-energetic spectrum of neutrons through the reaction. The source of alpha particles was a 72.7 mCi 239Pu source. The RMC detector was located 250 cm from the bare source and operated for three hours to generate a modulation profile: The number of particles detected at each rotation angle of the masks -- it is unique for each source location. The measured modulation profiles were used in a Maximum-Likelihood-Expectation-Maximization algorithm to reconstruct the images, and a Bootstrap resampling technique was used to determine uncertainty. The reconstructed images exhibited high contrast but low precision. The resampled image locations were widely distributed, but the most frequent value was very accurate. The uncertainty originated from an expectation model that did not account for fast neutron downscatter into the thermal neutron region as well as the fast neutrons streaming through the masks and being detected.

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