Characterization of Alpha Decays and Detector Response and Search for 5.5 MeV Solar Axions in DEAP-3600

DEAP-3600 is a liquid argon (LAr) based spin-independent direct dark matter search experiment. It is designed to detect nuclear recoils induced by the elastic scattering of weakly interacting massive particles (WIMPs) on argon nuclei. In 2019, DEAP-3600 reported its second physics result, which included the best reported upper limit on the WIMP-nucleon spin-independent cross section on a LAr target of 3.9 × 10^{-45} cm^2 for a 100 GeV/c^2 WIMP mass at 90% CL. An essential component of this result involved measuring the rates of alpha-decays within the detector and determining their impact on the expected background rate in the WIMP search region. The techniques used to measure and characterize these rates in-situ, and their results, will be discussed here in detail. Like α decays, neutron scatters off LAr nuclei also produce nuclear-recoils, potentially mimicking a WIMP interaction. A good estimate of the expected number of neutron scatters in the WIMP region of interest is, therefore, essential for performing a search for WIMP interactions. This estimate was revised and improved for the upcoming DEAP-3600 WIMP search result, which will include approximately 800 live-years of data. The details and results of this analysis are included in this report. In addition, DEAP-3600's large target mass and excellent ability to distinguish between electronic and nuclear recoils makes it well-suited for the detection of 5.5 MeV solar axions, which would produce electronic recoils in the LAr, at higher energy than most backgrounds. The 5.5 MeV solar axion search analysis, including the calibration of the energy response function on AmBe neutron calibration data, development of the Monte Carlo based background and signal models, the algorithm developed to fit the MC model to the data using Bayesian techniques, corrections made to the GEANT4 neutron capture spectrum, and the approach that will be used to calculate the final result will be discussed in detail.