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Development of Superconducting High-Resolution Gamma-Ray Spectrometers for Nuclear Safeguards

Abstract

Superconducting high-resolution gamma-ray spectrometers based on molybdenum/ copper transition edge sensors (TES) with tin absorbers have been developed for nuclear safeguard applications. This dissertation focuses on plutonium analysis, specifically the direct measurement of the 242Pu gamma-ray signature at 44.915keV. As existing nondestructive analysis methods cannot directly measure this or any other 242Pu signature, the feasibility of making such a measurement using a TES based system is presented. Analysis from of Monte Carlo simulations and analytical noise models shows that the direct detection of this gamma-ray line of is possible and can be quantified in the presence of a 240Pu gamma-ray line with a line separation of 324eV, even if the emission from the 240Pu is several orders of magnitude stronger. Spectroscopic measurements conducted in a liquid cryogen system offered an energy resolution of 180eV, adequate for the measurement of 242Pu; however, TES operation in a liquid-cryogen-free pulse tube refrigerator degraded sensor performance such that this measurement was no longer possible. The numerical noise model indicates that the energy resolution of this device is adequate to demonstrate a direct measurement of 242Pu if the noise pickup from the mechanical cooler can be suppressed. This work shows that the precise measurement of low-intensity gamma-ray signatures, such as the 44.915keV gamma ray from 242Pu, will require arrays of low-noise TES sensors and that such a system would offer invaluable information in the analysis of plutonium bearing materials.

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