Radon Background Reduction in DEAP-1 and DEAP-3600

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O'Dwyer, Eoin
dark matter
The Dark Matter Experiment with Liquid Argon Using Pulse Shape Discrim- ination (DEAP) is a dark matter experiment based in the SNOLAB facility in Sudbury, Ontario. Its aim is to detect WIMPs, Weakly Interacting Massive Par- ticles, that may make up the missing component of the matter in our universe by the scintillation of liquid argon from nuclear recoils. A 7 kg prototype, DEAP-1, is currently in operation with work underway to scale up to a 1 tonne detector, DEAP-3600, by 2012. For DEAP-3600 to be a competitive dark matter search, a limit of 0.2 fiducial surface alpha events is required in the energy region of interest for three years of run time, or 150 total surface events. Of particular concern to the DEAP experiment is 222Rn and its daughter prod- ucts, as the alpha decays of these isotopes may create events in the detector that mimic a WIMP signature. The first half of this thesis concerns the testing and suc- cessful use of an activated carbon trap to eliminate 222Rn from the argon gas source in DEAP-1. The Carbo-Act F2/F3 grain activated carbon brand was tested as a potential ultra-low activity candidate for a DEAP-3600 filtration system and was found to have an upper limit for its 222Rn emanation rate of 284 atoms/day/kg. A temperature swing system is proposed. If operated at 110 K, an upper limit of five atoms of 222Rn can be expected to enter the detector from the trap. An indirect relationship between the number of low energy nuclear recoil events in the DEAP WIMP region of interest and the number of radon alpha decays was found. The ratio between the low energy events in the WIMP region of interest and the high energy alphas was found to be 0.18 ± 0.03 in the detector. From this, the upper limit of the contribution from the proposed radon trap to the WIMP background in DEAP-3600 will be be ten events for three years of run time, which is within acceptable limits.
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