Evaluating 238U external background for SNO+ experiment using radon Aasays and 214Bi analysis

Abstract

SNO+ is large multipurpose detector located at SNOLAB filled with liquid scintillator. The scintillator will then be loaded with Te isotope, allowing to look for neutrino-less double beta decay which is extremely rare. This will determine if the neutrino is its own antiparticle. One of the main concerns for these rare event experiments is the presence of backgrounds, which could mask the signals of interest. This thesis will focus on 222Rn, one of the most common backgrounds due to its excessive prevalence in the mine environment. Radon decays into daughter nuclei where the energies lie within the region of interest for neutrino-less double beta decay. The detector is housed in a large cavity that is filled with ultrapure water and has a nitrogen cover-gas in order to avoid contamination. Radon Assay is a technique that was developed for the original SNO experiment to keep track of the radon content within the cavity and the covergas systems. The Assay system itself is well calibrated with low backgrounds. Assays are performed frequently at different positions of the cavity and cover-gas to monitor the radon levels. During a radon assay, radon is cryogenically trapped, concentrated, and shared into a ZnS coated Lucas cell for a period of time and known amount of flow. This Lucas cell is then connected to a PMT, which detects the decayed alphas that are used to calculate the number of radon atoms in the assay. This technique is a crucial part of measuring and monitoring the low backgrounds for the experiment which is then verified from the in-situ Bi214 analysis for accuracy.