Identification and Reduction of the Alpha Background, From 238U and 232Th Decay Chains, in the DEAP-3600 Experiment.

Abstract

Different cosmological and astronomical observations from the past century indicate that non-baryonic matter, commonly referred to as dark matter, dominates the composition of the universe. Discovering the nature and properties of dark matter has been one of the most pursued goals of modern physics. The DEAP collaboration developed a single-phase 3600 kg liquid argon experiment, DEAP-3600, to measure possible nuclear recoils generated by the interactions between the target nuclei and WIMP particles from the surrounding galactic dark matter halo. DEAP-3600 utilizes the powerful pulse-shape discrimination method to distinguish between the aforementioned interaction and its different types of background. The work presented in this thesis focuses on the identification and reduction of the background from alpha particle decays, from the $^{238}$U and $^{232}$Th decay chains. This was first undertaken with the development and deployment of a custom designed sanding robot, aimed at reducing radiogenic surface contamination, which removed 506$\pm$50 $\mu$m of acrylic from the inner-most portion of the detector surface. Furthermore, different analysis tools were developed and used during different phases of the experiment to measure the remaining radiogenic activities carefully. Said analysis estimated a liquid argon bulk activity of 0.540$\pm$0.0016 mBq for the $^{222}$Rn chain and 0.0127$\pm$0.0022 mBq for the $^{220}$Rn chain, as well as an overall surface activity of 3.87$\pm$0.41 mBq. This thesis also reports on the first WIMP analysis performed with the DEAP-3600 experiment. With a total fiducial exposure of 14226$\pm$1384 kg-days, no WIMP candidate events were identified in the dataset allowing to set an upper limit on the WIMP-nucleon cross-section of $1.77\times10^{-44}$ cm$^{2}$, at a WIMP mass of 100 GeV.