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

Abstract

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 the best 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 α-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. I would like to thank Kevin Graham for his guidance and mentorship during my journey through graduate school. He taught me how to communicate, work, and think like a scientist. He also made sure to prioritize the person before the project, always considering it of utmost importance that I enjoy the work that I do, and taking a strong interest in my personal life and well-being. He formed me into a scientist and I could not have asked for a better supervisor! I would like to thank the DEAP Collaboration, particularly the people at Carleton, for their friendship and support of my work. Everyone I met was always willing to answer questions and lend a hand. I would like to thank Matt Dunford for getting me started with DEAP, and for sharing his advice and knowledge when I first joined, and Andrew Erlandson and David Gallacher for their help and advice as I ran into issues with the software or when I just needed good ideas. I want to thank Tina for helping me learn how to use the DEAP DB and for supporting me as DB co-admin, Simon Viel and Marcin Kuźniak for their support and advice, Björn Lehnert and Alexander Derbin for lending their expertise to the axion work, and Hamid Moradi for excellent discussions that slowed down my PhD work. I would especially like to thank Rob Stainforth and Shawn Westerdale. They both put so much of their time and effort into helping me with the α and axion work that is described in this thesis. Without their time, extensive knowledge, advice, and constant support, I wouldn't have been able to do it. Both were always ready and iv available to answer questions, and frequently took the time to provide very detailed feedback on my work. My parents worked very hard to provide me and my siblings with a quality education growing up, and always encouraged me to pursue a career I was passionate about. My parents and siblings, Natasha, Rebecca, and Joseph, made me the person I am today through their constant love and support throughout my entire life. And lastly, I owe a deep debt of gratitude to my wife, Gloria. She put up with the often long hours of work I had to put in to accomplish this, and her constant love and support got me through it. My wife and children, Christopher and Veronica, have all been constant sources of joy and have been me going, especially during the most difficult ti...

Recent results from DEAP-3600

“…For events at 18 keV 𝑒𝑒 , so about 110 N 𝑠𝑐 , the leakage probability of electron recoils entering the nuclear recoil F 𝑝𝑟 𝑜𝑚 𝑝𝑡 band is 10 −10 for a nuclear recoil acceptance of 50 % [3]. The electron recoil background was fully modeled up to 8 MeV, including the 𝛽s and 𝛾s released by radioactive traces in the inner detector, which mainly extends up to about 3 MeV [4], and the neutron capture 𝛾s (n, 𝛾), which are the dominant background in the electron recoil band up to about 10 MeV [5]. On the other hand, one of the most insidious backgrounds in the last dark matter search has been 𝛼s from the neck of the detector.…”

Recent results from DEAP-3600

“…18 keV ee , so about 110 N sc , the leakage probability of electron recoils entering the nuclear recoil F prompt band is 10 −10 for a nuclear recoil acceptance of 50% [3]. The electron recoil background was fully modeled up to 8 MeV, including the βs and γs released by radioactive traces in the inner detector, which mainly extends up to about 3 MeV [4], and the neutron capture γs (n, γ), which are the dominant background in the electron recoil band up to about 10 MeV [5]. On the other hand, one of the most insidious backgrounds in the last dark matter search has been αs from the neck of the detector.…”

Neck Alpha Background Rejection in DEAP-3600 using Pyrene and the Search for Superheavy Dark Matter

“…Table 5.4: Assumed rates for the dominant neutron capture γ sources in DEAP-3600. These rates are obtained from the AmBe calibration fits performed in [99].…”

“…The light yield, up to 10 MeV, has been measured in situ by fitting a Gaussian response function to neutron capture gamma lines, as described in [99]. A scale factor C is defined to match the qPE predicted by RAT (the simulation) to the qPE predicted by this Gaussian response function (the data).…”