Theses and Dissertations from UMD

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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

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Subject: search.filters.subject.Nuclear physics

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Now showing 1 - 7 of 7
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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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    New insights from large Nc QCD
    (2013) Krejcirik, Vojtech; Cohen, Thomas D; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    In this dissertation, three problems--one in meson physics, one in baryon physics, and one in nuclear physics--are analyzed in the large Nc limit of quantum chromodynamics. It is shown that a Hagedorn spectrum emerges naturally for mesons, i.e. that the number of mesons grows exponentially with mass; a new relation for the electromagnetic form factors of nucleon is derived in the combined large Nc and chiral limit of QCD; Nc scaling of the nucleon-nucleon scattering S-matrix is deduced as well as an exact formula for the nucleon-nucleon cross section in the leading order in 1/Nc expansion is derived.
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    Measurement of the Surface and Underground Fast Neutron Spectra with the UMD/NIST Fast Neutron Spectrometers
    (2013) Langford, Thomas Jonathan; Beise, Elizabeth J; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The typical fast neutron detector falls into one of two categories, Bonner sphere spectrometers and liquid scintillator proton recoil detectors. These two detector types have traditionally been used to measure fast neutrons at the surface and in low background environments. The cosmogenic neutron spectrum and flux is an important parameter for a number of experimental efforts, including procurement of low background materials and the prediction of electronic device faults. Fast neutrons can also cause problems for underground low-background experiments, through material activation or signals that mimic rare events. Current detector technology is not sufficient to properly characterize these backgrounds. To this end, the University of Maryland and the National Institute of Standards and Technology designed, developed, and deployed two Fast Neutron Spectrometers (FaNS) comprised of plastic scintillator and 3He proportional counters. The detectors are based upon capture-gated spectroscopy, a technique that demands a delayed coincidence between a neutron scatter and the resulting neutron capture after thermalization. This technique provides both particle identification and knowledge that the detected neutron fully thermalized. This improves background rejection capabilities and energy resolution. Presented are the design, development, and deployment of FaNS-1 and FaNS-2. Both detectors were characterized using standard fields at NIST, including calibrated 252Cf neutron sources and two monoenergetic neutron generators. Measurements of the surface fast neutron spectrum and flux have been made with both detectors, which are compared with previous measurements by traditional detectors. Additionally, FaNS-1 was deployed at the Kimballton Underground Research Facility (KURF) in Ripplemead, VA. A measurement of the fast neutron spectrum and flux at KURF is presented as well. FaNS-2 is currently installed in a shallow underground laboratory where it is measuring the muon-induced neutron flux and spectrum.
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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.
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    A Search for Neutrinoless Double-Beta Decay with EXO-200
    (2013) Slutsky, Simon; Hall, Carter R.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This work presents a search for neutrinoless double beta decay of 136Xe using data from the EXO-200 detector collected between 2011 and 2012. Neutrinoless double beta decay (ββ0ν) is a hypothetical nuclear decay possible only if the neutrino is massive and is a Majorana particle. Observation of this process would constitute a measurement of the absolute neutrino mass scale, which is known to be non-zero from neutrino oscillation experiments. EXO-200 is a liquid xenon time projection chamber located at the Waste Isolation Pilot Plant (WIPP) in Carlsbad, NM. The detector is filled with 200 kg of liquid xenon isotopically enriched to 80.6%, used as both detection medium and decay source. Spectral fits based on detailed Monte Carlo simulations are used to constrain the number of events in the data. The analysis finds no evidence for ββ0ν in 136Xe, placing a lower limit on the half-life of 1.6 ·1025 yr at 90% confidence level. This implies an upper limit on the effective Majorana neutrino mass between 0.14-0.38 eV, one of the most stringent limits ever set on ββ0ν.
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    Some Novel Phenomena at High Density
    (2013) Berkowitz, Evan; Bedaque, Paulo; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Astrophysical environments probe matter in ways impossible on Earth. In particular, matter in compact objects are extraordinarily dense. In this thesis we discuss two phenomena that may occur at high density. First, we study toroidal topological solitons called vortons, which can occur in the kaon-condensed color-flavor-locked phase of high-density quark matter, a candidate phase for the core of some neutron stars. We show that vortons have a large radius compared to their thickness if their electrical charge is on the order of 104 times the fundamental charge. We show that shielding of electric fields by electrons dramatically reduces the size of a vorton. Second, we study an unusual phase of degenerate electrons and nonrelativistic Bose-condensed helium nuclei that may exist in helium white dwarfs. We show that this phase supports a previously-unknown gapless mode, known as the half-sound, that radically alters the material's specific heat, and can annihilate into neutrinos. We provide evidence that this neutrino radiation is negligible compared to the star's surface photoemission.
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    Elliptic flow at forward rapidity in sqrt(s_(NN)) = 200 GeV Au+Au collisions
    (2012) Richardson, Eric; Mignerey, Alice C.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Forward rapidity elliptic flow (v2) of both unidentified charged hadrons and decay muons has been measured from √sNN = 200 GeV Au+Au collisions as a function of pseudorapidity (η), transverse momentum, and number of nucleon collision participants. The measurements were performed at Brookhaven National Laboratory's Relativistic Heavy Ion Collider using the PHENIX experiment's Muon Arm spectrometers, located at 1.2 < |η| 􏰁 2.4. To identify hadrons, which consist mostly of pions, kaons, and protons, a longitudinal momentum cut was applied to tracks stopping in the shallow steel layers of the Muon Arms. Those particles traversing completely through the Muon Arms consist of mostly muons from pion and kaon decays. The standard event plane (EP) method was used to measure v2, whose accuracy was improved ∼20-25% by combining the measured EP angles of several detectors, instead of using the measured EP from a single detector. Additionally, a hit swapping technique was devised to optimize track cuts, estimate background, and apply a background correction. To investigate the ability of the Muon Arms to accurately measure unidentified hadron v2, a GEANT simulation was also undertaken. The forward rapidity v2 results show good agreement with mid-rapidity measurements for central collisions (􏰁 20-30% centrality), indicating a longitudinally extended thermalized medium with similar eccentricity, at least out to the Muon Arm η region. Only when compared to very forward BRAHMS measurements (η ≈ 3) is a v2 suppression seen for central collisions. For increasingly peripheral collisions, a growing suppression in v2 is observed for the Muon Arm measurements compared to mid-rapidity, indicating increased changes in the medium properties of ever smaller systems. For peripheral collisions of the same/similar centralities, an increased suppression is observed toward forward η.