Alpha backgrounds in the DEAP dark matter search experiment.
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One of the pressing concerns in Dark Matter detection experiments is ensuring that the potential signal from exceedingly rare Dark Matter interactions is not obscured by background from interactions with more common particles. This work focuses on the ways in which alpha particles from primordial isotopes in the DEAP detector components can cause background events in the region of interest for Dark Matter search, based on both Monte Carlo simulations and data from the DEAP-1 prototype detector. The DEAP experiment uses liquid argon as a target for Dark Matter interactions and relies on the organic electroluminescent dye tetraphenyl butadiene (TPB) to shift the UV argon scintillation light to the visible range. The light yield and pulse shape of alpha particle induced scintillation of TPB, which is an essential input parameter for the simulations, was experimentally determined. An initial mismatch between simulated and measured background spectra could be explained by a model of geometric background events, which was experimentally confirmed and informed the design of certain parts of the DEAP-3600 detector that is under construction at the moment. Modification of the DEAP-1 detector geometry based on this model led to improved background rates. The remaining background was well described by the simulated spectra, and competitive limits on the contamination of acrylic with primordial isotopes were obtained. Purity requirements for the DEAP-3600 detector components were based on this work. The design and testing of a novel large area TPB deposition source, which will be used to make TPB coatings for the DEAP-3600 detector, is described.