Advancements in coincidence counting experimental and analytical techniques for the quantitative detection of low-level isotopes relevant to non-proliferation activities in high dead-time applications

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Yoho, Michael Duncan

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This thesis develops experimental and analytical procedures to detect trace amounts of isotopes of interest relevant to activities related to nuclear non-proliferation via γ-γ coincidence counting. By using multiple-detector systems, the time-correlated nature of γ-ray emissions of isotopes with complex decay schemes can be utilized to reduce background levels and interferences. This thesis is divided into five major sections. After a review of the state-of-the-art, system capability is presented by documenting the quantitative determination of selenium in neutron-activated fly ash. The complex fly-ash γ spectrum containing multiple interferences and high background levels constitutes a rigorous quality-assurance test of system performance and further simulates the complex and difficult nature of fission-product spectra relevant to non-proliferation activities. Next, this thesis maps out system dead-time performance and presents a methodology to correct for dead-time in coincidence systems. This thesis then presents the experimental analysis of fission-product spectra generated via neutron activation in the TRIGA Mark II reactor at the University of Texas at Austin. Analytical procedures are presented to determine coincidence signatures and intensities, necessary for quantitative determination of any fission product via Monte Carlo techniques. Finally, the system’s capability to quantify trace plutonium is examined, and a criterion is developed in order to determine the benefit of coincidence counting methodology for a specific isotope over standard nuclear counting techniques.


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