Browsing by Subject "Xenon"
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Item A Search for Dark Matter with the ZEPLIN II Detector(2010-01-14) Gao, JiantingGalaxies and clusters of galaxies are believed to be dominated by non-luminous non-baryonic dark matter. A favored candidate is a new type of Weakly Interacting Massive Particle (WIMP) with a mass of order 100 GeV/c^2. The ZEPLIN II experiment is a WIMP search experiment that attempts to directly detect WIMP interactions using the two-phase xenon approach. The detector measures both scintillation and ionization generated by interactions in a 31 kg liquid xenon target. This approach provides a powerful discrimination between nuclear recoils, as expected from WIMPs, and background electron recoils. In this work, we develop a new X^2 approach to determine the three dimensional event positions in an attempt to improve the background rejection. The optical properties of the PTFE reflectors and the grids of the detector were determined using the Geant4 simulation, and event positions were obtained by finding the best match to the amount of light in each photomultiplier. This was found to greatly improve the position resolution. The approach was then applied to the WIMP search data. It was found that one of the dominating background sources was events from the gas above the anode grid and not from the PTFE walls caused by the small signals as previously thought. WIMP search results were then obtained from the first 31 days of stable ZEPLIN II data using two methods. Although the X^2 method greatly improved position resolution, the number of background events was not significantly altered and the new limit agreed well with the limit published by the collaboration.Item Low Energy Nuclear Recoil Response in Xenon Gas for Low Mass Dark Matter WIMP Search(2014-04-16) Sofka, Clement JamesOver 80 years of astrophysical observations suggest that the observable luminous matter makes up ? 5% of the total energy density in the Universe. The remaining ~ 95% comes from matter and energy that has not been observed directly. Discovering these "dark" sources of matter/energy is the single most important concern in the modern quest for understanding Nature. We live in an epoch that is almost certainly characterized by a at, expanding Universe. Coupling this with the wealth of astrophysical surveys, we are able to probe the vastness of space, and develop theories of space-time evolution, going back in time several billions of years. The evidence suggests that the Universe began in a Big Bang, underwent a brief moment of Inflation, then cooled and began forming the structures (atoms, molecules, stars, galaxies, etc.) we observe plainly today. An integral part of this consistent story of the Universe's birth and cosmic evolution is the existence of cold dark matter in the form of Weakly Interacting Massive Particles (WIMPs) and dark energy. Initial cosmological considerations suggested that WIMPs were some type of Standard Model (SM) particle, but even the best-case estimates lead to matter energy densities that come up well short without a significant modification of the underlying theory of gravity. The best proposed WIMP candidate has surfaced from efforts motivated by particle physics. A new type of WIMP arises out of Supersymmetry (SUSY). The Lightest Supersymmetric Particle (LSP), a neutralino, seems to fit perfectly into both particle physics and cosmology. First estimates from a Minimal Supersymmetric Standard Model (MSSM) placed the WIMP in the mass range of O(10) - O(10^(3)) GeV/c^(2). However, there is mounting evidence in recent years that suggests the existence of a low mass WIMP as a suitable dark matter candidate. Some of the most sensitive detectors to low mass WIMPs employ noble liquids as a target medium. Groups using noble liquid detectors are currently limited to the detection of relatively higher mass WIMPs because of detector threshold limits, background effects, or a lack of fundamental understanding of very low energy nuclear recoils (< 3 keVnr). This work is aimed at studying these very low nuclear recoil energies in xenon to improve noble element detector sensitivities and develop a fundamental understanding of nuclear stopping power theories originally studied by Lindhard et al. in the 1960's. We present the nuclear recoil results from measurements using a nearly mono-energetic beam of neutrons aimed at high-pressure gaseous xenon (HPXe) in a time projection chamber (TPC). This work demonstrates the viability of future low mass dark matter WIMP and other rare event searches (e.g. Neutrinoless Double Beta Decay, 0 ) using high pressure noble gases.Item Mitigation of the radioxenon memory effect in beta-gamma detector systems by deposition of thin film diffusion barriers on plastic scintillator(2010-12) Fay, Alexander Gary; Biegalski, Steven R.; Haas, DerekThe significance of the radioxenon memory effect in the context of the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty is introduced as motivation for the project. Existing work regarding xenon memory effect reduction and thin film diffusion barriers is surveyed. Experimental techniques for radioxenon production and exposure, as well as for thin film deposition on plastic by plasma enhanced chemical vapor deposition (PECVD), are detailed. A deposition rate of 76.5 nm min⁻¹ of SiO₂ is measured for specific PECVD parameters. Relative activity calculations show agreement within 5% between identically exposed samples counted on parallel detectors. Memory effect reductions of up to 59±1.8% for 900 nm SiO₂ films produced by plasma enhanced chemical vapor deposition and of up to 77±3.7% for 50 nm Al₂O₃ films produced by atomic layer deposition are shown. Future work is suggested for production of more effective diffusion barriers and expansion to testing in operational monitoring stations.Item Radioactive xenon and argon production and transport in the environment(2016-12) Johnson, Christine Michelle; Biegalski, Steven R.; Haas, Derek; Landsberger, Sheldon; Lowrey, Justin; Schneider, ErichVerification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) includes both atmospheric and on-site sampling and detection of radioactive noble gases that are produced in a nuclear explosion, particularly radioactive isotopes of xenon and argon. However, other sources of these radioactive noble gases can complicate the analysis by introducing backgrounds which may either mask a signal or may alter the isotopic ratios that are used to distinguish the origin of the gases. A series of field experiments were conducted in order to better understand background sources of these noble gases and their potential impact on CTBT verification activities. Two sampling campaigns were conducted near the Chalk River Laboratories (CRL) medical isotope production facility. During these sampling campaigns air samples were collected from both the atmosphere and the shallow subsurface (1-2 m) using proposed on-site inspection sampling techniques. The Subsurface Transport over Multiple Phases (STOMP) simulator was then used to make predictions about the expected imprinting during the experimental campaign and the results were compared to the experimental results. The combined results of these experiments provided first of their kind measurements of imprinted radioxenon and provided clear evidence that imprinting of atmospheric radioxenon gas into the subsurface does occur as predicted by transport models. Two potential sources of background 37Ar were also examined. First, the results of 37Ar and 41Ar concentration measurements from around The University of Texas at Austin's research reactor were used to make a more general estimate of the 37Ar release rates from the reactor and atmospheric transport modeling was used to estimate the impact of such releases. Atmospheric transport modeling of release rates predicted by this work indicates that research reactors do not release 37Ar in concentrations measurable by proposed OSI argon detection equipment. Second, natural production of 37Ar was examined as a potential subsurface background source by analyzing samples of subsurface air from a high calcium limestone geology for their 37Ar concentration. The final results of two, one-day samples taken from an 18-in deep hole which was tarped at the surface found 37Ar concentrations of 1.6 +- 1 mBq/m3 on the first day, and 0.98 +- 0.17 mBq/m3 on the second day. From the experience gained during this experiment, recommendations were made to improve future sampling in hard geologies.