Radon Detection and 241Am-7Li Source Development for the Lux-Zeplin Dark Matter Experiment

Dark matter presents a conundrum at the intersection of particle physics and cosmology. Observational data from astronomy and precision cosmology have allowed scientists to calculate the relic abundance of dark matter with considerable accuracy. Yet, the exact nature of the dark matter particle remains a mystery. Theoretical models and hypotheses suggest that the mass of dark matter particle candidates ranges from as low as $10⁻²² eV/c² to as high as 10³ M_⊙(10⁷⁰ eV/c², spanning an astounding ~90 orders of magnitude! Of the numerous candidate particles, those that also resolve issues in the highly successful standard model of particle physics, are of particular interest to experimentalists. One such candidate is the Weakly Interacting Massive Particle (WIMP), which if it exists, could also resolve the gauge hierarchy problem in particle physics. WIMPs are theorized to interact with certain detector materials such as liquid xenon (LXe), via the weak force, active on all other known particles, and produce nuclear recoil (NR). However, this detection scheme meets several challenges. Firstly, the LXe must be sufficiently purified of radioactive impurities like ⁸⁵Kr and ²²²Rn to not overwhelm the rare NR signal events in terms of interaction rate. Secondly, the detector requires precise calibration of its response to NR-type interactions, specifically in the energy region where WIMP interactions are expected. In this thesis, I describe my contributions to the ongoing direct dark matter search experiment called LUX-ZEPLIN (LZ). Most of my work relates to the preparation of the experiment, namely the measurement of the release of trace amounts of radon gas from detector materials, forming the dominant background component. This work was important to ensure a low detector background and, with it, high sensitivity to dark matter. As a second contribution, I participated in all phases of the development, manufacture, testing and calibration of AmLi neutron sources important for mapping out the NR band for WIMP detection. Both subjects have been documented in publications with me as the first author; the work was performed at UA. The thesis, furthermore, describes my data analysis effort, utilizing AmLi source calibration data to help establish the WIMP event parameter range in LZ.