Theses and Dissertations from UMD

Permanent URI for this communityhttp://hdl.handle.net/1903/2

New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM

More information is available at Theses and Dissertations at University of Maryland Libraries.

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2013 - 20142

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    A Search for the Neutrinoless Double Beta Decay of Xenon-136 with Improved Sensitivity from Denoising
    (2014) Davis, Clayton G.; Hall, Carter; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The EXO-200 detector is designed to search for the neutrinoless double beta decay of 136Xe. ββ0ν decay, if it occurs in nature, would demonstrate the fundamental nature of neutrino mass; set the mass scale of the neutrino sector; and demonstrate lepton number non-conservation. Since the ββ0ν decay produces a monoenergetic peak, the energy resolution of the detector is of fundamental importance for the sensitivity of the experiment. The present work describes a new analysis technique which improves the energy resolution of EXO-200 through a combination of waveform denoising and weighting of waveform components based on their expected signal-to-noise ratio. With this method, the energy resolution of the detector is improved by 21% and the expected background in the 2σ region of interest is reduced by 32%. Applying this technique to 99.8 kg*years of exposure collected by EXO-200 between October 5, 2011 and September 1, 2013, we find no statistically significant evidence for the presence of ββ0ν in the data. We set a half-life limit T1/2 > 1.1 × 1025 years at 90% confidence. We also describe further improvements which could impact the energy resolution of EXO-200, and consider implications for the planned nEXO experiment.
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    A Search for the Double-Beta Decay of Xe-136 to an Excited State of Ba-136 with EXO-200
    (2013) Yen, Yung-Ruey; Hall, Carter R; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This thesis presents a search for the two neutrino double beta decay (ββ2ν) of 136Xe to the 0+1 excited state of 136Ba using data from the EXO-200 detector collected between 2011 and 2012. The ββ2ν decay to the excited state is a process that have been observed for other double beta decay nuclei. An observation of this decay would shed some light on the validities of the various nuclear physics models. Located at the Waste Isolation Pilot Plant (WIPP) near Carlsbad, NM, EXO-200 is a liquid xenon time projection chamber filled with 200 kg of 80.6% isotopically enriched 136Xe. The liquid xenon serves both as the decay source and the detection medium. Maximum likelihood fits of the sum energy spectra based on Monte Carlo simulations are used to constrain the number of ββ2ν decay to the 0+1 excited state of 136Ba. A half-life lower limit on this decay of 1.2 · 1023 year at 90% C.L is set, still a couple orders of magnitude from our expected theoretical rate of 2.5 · 1025 year from the applying the calculated phase space factor and the nuclear matrix element suppressions on the measured ββ2ν decay to the ground state. A developing analysis using a new energy variable designed specifically for the search of the decay to the excited state is also discussed.