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The DEAP-3600 detector searches for the scintillation signal from dark matter particles scattering on a 3.3 tonne liquid argon target. The largest background comes from $$^{39}\text{ Ar }$$ 39 Ar beta decays and is suppressed using pulse-shape discrimination (PSD). We use two types of PSD estimator: the prompt-fraction, which considers the fraction of the scintillation signal in a narrow and a wide time window around the event peak, and the log-likelihood-ratio, which compares the observed photon arrival times to a signal and a background model. We furthermore use two algorithms to determine the number of photons detected at a given time: (1) simply dividing the charge of each PMT pulse by the mean single-photoelectron charge, and (2) a likelihood analysis that considers the probability to detect a certain number of photons at a given time, based on a model for the scintillation pulse shape and for afterpulsing in the light detectors. The prompt-fraction performs approximately as well as the log-likelihood-ratio PSD algorithm if the photon detection times are not biased by detector effects. We explain this result using a model for the information carried by scintillation photons as a function of the time when they are detected.

DEAP-3600 is a dark matter experiment using 3.3 tonnes of liquid argon as a scintillation target to directly detect Weakly Interacting Massive Particles (WIMPs), a dark matter candidate. Mitigating background sources is crucial to dark matter searches. A large background model contribution comes from attenuated alphas originating from 210 Po decays within the acrylic vessel surfaces. Alphas from decays within the acrylic inner vessel and from the acrylic neck flowguide are analyzed. The activity of the inner vessel is separated into surface and bulk components, and determined to be 0.22 ± 0.02 mBq/m 2 and 3.68 ± 0.06 mBq. An event rate of 53.5 +30 −4.6 µHz is found for alphas originating from the neck flowguide. An optimized event selection is obtained, making use of machine-learning algorithms to reject neck flowguide alphas and maximize WIMP sensitivity. In 802 live-days of DEAP-3600 data, the expected upper limit on the spin-independent WIMP-nucleon interaction cross-section is 1.9×10 −45 cm 2 (90% C.L.) for a 100 GeV/c 2 WIMP mass. D&D nights and mid-day Catan sessions kept me alive during this entire process, and this thesis would likely sound like the words of a madman without all three of you.Most importantly, I want to thank my parents. I am incredibly lucky to have such supportive and caring parents, who were there for me through this whole process. You taught me how to put my full effort into my work, never give up, and be proud of my accomplishments. That knowledge is the most important lesson I have learned in my life. I will always be grateful for your unconditional love. And to Shawna. It is incredible to me that you can somehow manage your own Masters, a full time job, and all of my nonsense, but you do, and I am grateful every day that you are with me. You are the rock in my life, and the first person I go to when I am excited or nervous. Now that we're both through, I can't wait to see what our life has in store.

No abstract