Years of astrophysical observations suggest that dark matter comprises more than ~80 % of all matter in the universe. Particle physics theories favor a weakly-interacting particle that could be directly detected in terrestrial experiments. The Super Cryogenic Dark Matter Search (SuperCDMS) Collaboration operates world-leading experiments to directly detect dark matter interacting with ordinary matter. The SuperCDMS Soudan experiment searched for weakly interacting massive particles (WIMPs) via their elastic-scattering interactions with nuclei in low-temperature germanium detectors.
During the operation of the SuperCDMS Soudan experiment, 210Pb sources were installed to study background rejection of the Ge detectors. Data from these sources were used to investigate energy loss associated with Frenkel defect formation in germanium crystals at mK temperatures. The spectrum of 206Pb nuclear recoils was examined near its expected 103 keV endpoint energy to extract the first experimentally determined average displacement threshold energy of 19.7 +/- 0.5 (stat) +/- 0.1 (syst) eV for germanium. This has implications for the sensitivity of future germanium-based dark matter searches including the SuperCDMS SNOLAB experiment.
The SuperCDMS SNOLAB experiment will employ germanium and silicon detectors to improve current WIMP-search results by at least one order of magnitude for masses <= 10 GeV/c2. This will require substantial shielding against cosmogenic and radiogenic backgrounds. The SuperCDMS SNOLAB passive shield will be permanent for the duration of the experiment so extensive simulations were undertaken to optimize the shield design. This resulted in a design of an outer layer of 60 cm of water, a middle layer of 20 cm of lead, and 30 cm of polyethylene which limits the background rate to that required for the primary physics goals of the experiments.
The experiment will begin operations in 2020 and care must be taken during the construction phase to limit exposure to the ~135$ Bq/m3 radon activity in the laboratory. The daughter products of 222Rn can attach to nearby surfaces leaving long-lived 210Pb in place for the duration of the experiment. For non-line-of-sight surfaces of the polyethylene shield, the maximum allowable 210Pb activity is 10,000 nBq/cm2. A study was conducted to experimentally determine the contamination rate of polyethylene and copper by exposing samples for 83~days at SNOLAB. From the resulting surface activities, obtained from high-sensitivity measurements of alpha emissivity using the XIA UltraLo-1800 spectrometer, the average 210Pb plate-out rate was determined to be 249 and 423 atoms/day/cm2 for polyethylene and copper, respectively. A time-dependent model of alpha activity was developed leading to a maximum exposure time of 39 days in the SNOLAB environment.
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Stein, Matthew, "An Analysis of Frenkel Defects and Backgrounds Modeling for SuperCDMS Dark Matter Searches" (2018). Physics Theses and Dissertations. 3.
Available for download on Tuesday, May 14, 2019