Bulletin of the American Physical Society
APS April Meeting 2019
Volume 64, Number 3
Saturday–Tuesday, April 13–16, 2019; Denver, Colorado
Session Q10: WIMP Dark Matter III |
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Sponsoring Units: DPF Chair: Ray Bunker, PNNL Room: Sheraton Governor's Square 12 |
Monday, April 15, 2019 10:45AM - 10:57AM |
Q10.00001: Determination of Radiogenic Backgrounds in the LUX Detector Kelsey Colleen Oliver-Mallory LUX (Large Underground Xenon) is a retired, liquid xenon, direct dark matter detection experiment that published its third and best limit on the spin-independent cross section for WIMP-nucleon scattering in the fall of 2016. Subsequently, the collaborations efforts have shifted to completing searches for new physics at energies greater than the standard WIMP-nucleon scattering range. In this new regime, accurate determination of the background rate from radiogenic sources is exceedingly important due to the complicated shape of the energy spectrum from these events. This talk presents the latest analyses of α, β, and γ backgrounds intrinsic to the liquid xenon and originating from the detector materials. This work refines previous radio-contamination measurements, and attempts to better understand the spatial variation of specific background-generating isotopes. The impact of the revised background model on the detector sensitivity to new high energy physics searches is discussed. |
Monday, April 15, 2019 10:57AM - 11:09AM |
Q10.00002: ABSTRACT WITHDRAWN
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Monday, April 15, 2019 11:09AM - 11:21AM |
Q10.00003: Lead-210 in SuperCDMS Copper Raymond A Bunker High-purity copper is an important structural material for the Super Cryogenic Dark Matter Search (SuperCDMS) at SNOLAB; it will be used to fabricate low-background detector housings for each of the experiment's germanium and silicon crystals, as well as a large cryostat. Owing to the high chemical purity available with commercially produced copper, specific activities of radiocontaminants (e.g. U and Th isotopes) in the bulk copper material are generally modest. However, recent studies have demonstrated surprisingly high levels of Pb-210 in the bulk material, far out of secular equilibrium with the U-238 parent. Further, exposure to atmospheric radon can result in radon daughters plating out onto the surfaces of fabricated copper parts, ultimately leading to the implantation of Pb-210. I will describe experiments performed to test copper-surface treatments, together with low-level radiopurity assay measurements, to assess Pb-210 mitigation effectiveness and to directly verify Pb-210 levels for the copper that SuperCDMS SNOLAB will use in its detector payload. |
Monday, April 15, 2019 11:21AM - 11:33AM |
Q10.00004: Background Explorer: A Toolkit for Modeling Radioactive Backgrounds in Low Background Detectors Ben Loer Background contributions for the upcoming SuperCDMS SNOLAB experiment are estimated by simulating decay chains of naturally-occurring radioactive elements in the cryostat, readout system, and shielding. Absolute background rates are predicted by normalizing to contamination levels determined by material assay. The current model includes 374 components and assemblies and 864 simulation datasets. We have developed a tool, called Background Explorer, to manage this complex model, which includes a web interface intended to provide insights into relative background contribution levels and help guide detector design decisions related to background control. I will describe the design and use of this system for SuperCDMS, and present version 2, which has been significantly redesigned to facilitate use by a wider variety of low-background experiments. |
Monday, April 15, 2019 11:33AM - 11:45AM |
Q10.00005: Mitigation of Radon Daughter Plate-out in the LUX-ZEPLIN Experiment Eric S Morrison Growing theoretical and empirical evidence for the existence of dark matter has driven a search in the field of physics for its elusive elementary particle. A wide variety of experiments have spawned in order to discover the leading candidate, the WIMP. The primary goal of the LUX-ZEPLIN (LZ) experiment is to detect an interaction between a WIMP and a xenon nucleus, allowing physicists to begin understanding this exotic type of matter. An important background arises from charged radon daughters that attach to PTFE during construction and upon decay generate (α,n) reactions and recoiling 206Pb atoms in the detector. PTFE is of particular interest because it is used for the walls of the inner detector and its radon daughter plate-out rate can be orders of magnitude larger than the plate-out rate onto other materials. To mitigate radon contamination and meet LZ cleanliness goals, filtration systems have been constructed to provide low-radon air to locations where materials are exposed during detector assembly. Results on the measured PTFE radon daughter plate-out rate and the effectiveness of the radon reduction air filtration systems will be discussed. |
Monday, April 15, 2019 11:45AM - 11:57AM |
Q10.00006: Radon reduction in gases for rare event search experiments Maris Arthurs, Wolfgang B Lorenzon Radon-222 provides one of the most significant contaminants for rare-event search experiments. It is an inevitable product of trace amounts of natural uranium that is continuously supplied from detector components. Adsorption characteristics of radon in nitrogen, argon, and xenon carrier gases on various types of charcoals have been studied in the temperature range of 190-295 K at flow rates of 0.5 and 2 slpm. Studied charcoals have different adsorbing properties and intrinsic radioactive purities. R&D studies of the radon reduction system for the LZ experiment performed at The University of Michigan will be presented. |
Monday, April 15, 2019 11:57AM - 12:09PM |
Q10.00007: Emanation and Diffusion of Radon Through Gasket Materials for SuperCDMS SNOLAB Brandon M DeVries, Michael Bowles, Richard William Schnee The SuperCDMS SNOLAB experiment, currently under construction, will attempt to directly detect dark matter particles. Shielding surrounding the experiment’s detectors will reduce interactions of particles from radioactivity and cosmic rays. A gas purge will remove radon from gaps in the shielding to reduce backgrounds further. Gaskets used to seal this purge volume must allow sufficiently low radon diffusion through them while emanating little radon into the purge volume. I will describe measurements of radon diffusion through gaskets made of EPDM, Zip-A-Way, and Silicone, inferred from the time dependence of radon concentration in a volume separated from a high-radon volume by the gasket in question. Results of these diffusion measurements and of the radon emanation measurements will be given in the presentation. |
Monday, April 15, 2019 12:09PM - 12:21PM |
Q10.00008: Radon Background Control for the SuperCDMS SNOLAB Dark Matter Experiment Joseph Street The Super Cryogenic Dark Matter Search (SuperCDMS) experiment at SNOLAB will use solid-state germanium and silicon cylindrical detectors to measure ionization and phonons produced by the scattering of dark matter particles. The dominant expected background at low energy for SuperCDMS SNOLAB is from radon daughters that have plated out onto detector surfaces. Therefore, understanding and mitigating plate-out rates during detector fabrication, assembly, and installation is critical. A study of radon-daughter plate-out during detector polishing and assays of plate-out onto detector hardware provide limits on backgrounds. I will describe the construction and commissioning of the SuperCDMS SNOLAB radon mitigation system, which is built upon the design of the SD Mines prototype radon mitigation system that has achieved a 4000× reduction of radon to a cleanroom activity of ∼20 mBq/m3, and show the resulting expected background from radon daughters for the experiment. Recent results from an etch to reduce otherwise dominant sidewall backgrounds without damaging sensors on the detector faces will also be described. |
Monday, April 15, 2019 12:21PM - 12:33PM |
Q10.00009: Surface Background Rejection in Liquid Argon Dark Matter Detectors using Layered Wavelength-Shifting and Scintillating Thin Films Mark Boulay A technique using layered wavelength shifting, scintillating and non-scintillating films is presented to achieve discrimination of surface alpha events from low-energy nuclear recoils in liquid argon detectors. A discrimination power greater than 10^{8}, similar to the discrimination possible for electronic recoils in argon, can be achieved by adding a thin layer of scintillator with a suitably slow decay time to a wavelength-shifter coated surface. The technique allows suppression of surface alpha events in a very large next-generation argon dark matter experiment (with hundreds of square meters of surface area) without the requirement for position reconstruction, and could also be used to suppress surface backgrounds in compact argon detectors of low-energy nuclear recoils, for example in measurements of coherent neutrino-nucleus scattering or for sensitive measurements of neutron fluxes. |
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