Studies on Radioactive Background Mitigation for the PICO-500 Dark Matter Search Experiment

Abstract

The PICO-500 dark matter search experiment features the next-generation bubble chamber detector designed by the PICO collaboration. The sensitivity and live-time of ultra-low background detectors, such as PICO-500, are crucially dependent on the radio-pure materials used in the construction. The PICO-500 detector is anticipated to achieve world-leading sensitivity in the spin-dependent dark matter regime, necessitating radioactive background control during the construction and assembly. This thesis focuses on strategies and measurements related to mitigating radioactive backgrounds, specifically focusing on Radon-222, a gaseous and radioactive product of the Uranium-238 decay chain, and its progeny. The diffusive nature of radon prompts investigation into radon emanation from detector materials and radon permeation through a Parker PTFE Prädifa series NAE, FlexiSeal®, which is to be used in the PICO-500 inner vessel. The Parker PTFE seal permeation coefficient was determined using a noble gas extrapolation method and direct measurement using a radon emanation chamber detector, yielding 5.10 (+0.92, -1.10) x 10^-10 cm^3(STP)·cm / (s·cm^2·cm-Hg) and 5.37 ± 0.49 x 10^-10 cm^3(STP)·cm / (s·cm^2·cm-Hg), respectively. The radon activity contribution from the PTFE seals in the PICO-500 detection volume was estimated to be 30.5 Radons/Day. To prepare for the deployment of PICO-500, studies establishing cleaning strategies for the synthetic quartz vessels were developed. Full-sized PICO-500 natural quartz vessels were subjected to tests using ultra-pure water soap solutions to remove dust and particulates, which can also act as carriers for radon progeny. Using a 0.05-micron filtration and a customized cleaning system built at the UofA, surfaces of the vessels were documented to be cleaned to the IEST-STD-CC1246D-25 standard. Techniques to reach this standard consistently were developed for the synthetic quartz cleaning of PICO-500, which will be implemented at the SNOLAB facility.