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Searching for the Decay of 229m Th


The existence of a low energy excited state in 229<\super>Th was first postulated in 1975. The energy of the state was revised over the years from less than 100 eV in 1975, to 3.5 ± 1.0 eV in 1994, to 7.8 ± 0.5 eV in 2009, among others. Despite this progress, the decay of the state has never been observed, and the half life is unknown. While no measurement described in this dissertation was successful in these experiments either, many areas were searched and exclusion plots were produced. Measurements included looking for the internal conversion electron from the isomer, as well as any possible γ emission. 229m<\super>Th was produced via the α decay of 233<\super>U, and the recoiling 229<\super>Th was collected on a catcher. Short time scales were probed using an α coincidence measurement that was sensitive from tens of nanoseconds to almost a millisecond. Intermediate time scales used a mechanical shutter that was sensitive from 2 milliseconds to a few seconds. Longer time scale measurements moved the catcher from the source to the detector, and were sensitive from a few seconds to a few days. The proper function of the system was confirmed by measuring 235m<\super>U, which generated new results for the variation in its half life based on catcher material.

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