Quantification of Fast-Neutron Sources with Coded Aperture Imaging

Quantification of the mass of plutonium in facilities that process plutonium is important for both nuclear safeguards concerns and safety concerns, and multiple methods to nondestructively quantify plutonium sample characteristics have been proposed, particularly when the sample is located directly adjacent to or within the measurement device. In prior work, coded-aperture fast neutron imaging has been developed to demonstrate the imaging of neutron emitting radiation sources in a qualitative fashion, where the sources may be located meters to tens of meters away. Building upon prior work, this work develops the use of a Maximum Likelihood Expectation Maximization (MLEM) reconstruction technique to simultaneously reconstruct neutron sources measured from different detector positions. Moreover, a modified system response model is developed to accurately but quickly perform forward projections in order to accurately reconstruct and quantify neutron source characteristics including source intensity and location. The system response model incorporates mask transmission, a heterogeneous detector pixel array, scattering within the mask, and scattering within the detector, allowing for the expected detector data from a single source position to be generated in less than a second. The behavior of the MLEM reconstruction technique is discussed, and measurements of Cf-252 sources, acting as a surrogate Pu material, are reconstructed and analyzed. Using the methods developed here, a single 74 µCi Cf-252 point source placed at a distance of 200 cm is reconstructed within 2% of the known position and within 3% of known intensity at distances up to 300 cm. Measurements of more than one source and implications for Pu measurements in facilities are also discussed