Environmental background in a dual-phase xenon dark matter experiment

There is an abundance of cosmological and astrophysical evidence for dark matter. The best-motivated candidate for particle dark matter is the Weakly Interacting Massive Particle (WIMP). The search for WIMPs and the hypothesised neutrinoless double beta (0νββ) decay requires detectors with exceptionally low background levels. Next-generation rare-event searches must be shielded from various backgrounds, including gamma-rays from rock. This thesis describes simulations of gamma-ray transport through water shielding and assesses the thickness needed to suppress rock-induced background to a negligible level. The study focuses on a next-generation xenon observatory targeting both WIMP and 0νββ searches in Xe-136. For rock radioactivity of 1 Bq / kg, results show that 3.5 m of water plus 0.5 m of liquid scintillator reduces background in the 0 – 20 keV WIMP range to negligible levels. For 0νββ, fewer than 1 background event in 10 years can be achieved with a fiducial mass of 39.3 tonnes. Reducing the water shielding by 1 m lowers the 0νββ fiducial mass to 19.1 tonnes, while still ensuring negligible WIMP background.

This thesis also presents the measurements of radioactivity in rock in the Boulby Mine, a potential site for a future dual-phase xenon experiment. The measurements normalise simulation results and assess Boulby’s suitability as host. Polyhalite, the Boulby rock type likely to surround a future detector, shows very low Th-232 and U-238 concentrations. At these levels, reducing the water shielding by 1 m remains viable for WIMP search, yet decreases the 0νββ decay fiducial mass due to 238 U. Additionally, an overview of the new radon emanation facility at Boulby is provided, along with preliminary results from polyhalite.

Finally, this thesis includes a study of the muon flux at SURF using LZ data and simulations, identifying a 20 % discrepancy due to rock density assumptions. This contributes to improved modelling of cosmogenic backgrounds for LZ and future experiments hosted at SURF.