Background studies for LUX and LZ: activation, gammas from rock and krypton removal

Although there is considerable evidence for the existence of dark matter, which makes up the majority of matter in the Universe, a fundamental question still remains unanswered; what is dark matter made of? A new theorised elementary particle called Weakly Interacting Massive Particle (WIMP) is a promising candidate for dark matter. Dual-phase xenon time projection chamber (TPC) technology can search for low energy scattering signals produced by WIMP interactions with atomic nuclei.

Utilising this technology the Large Underground Xenon (LUX) experiment, located at the Sanford Underground Research Facility (South Dakota, USA), provided stringent limits on the spin-independent WIMP-nucleon cross-section. Its successor the LUX-ZEPLIN (LZ) experiment, a multi-tonne dual-phase xenon TPC, aims at considerably increasing the sensitivity to WIMP dark matter. In order to achieve high sensitivity in any rare event search experiment, characterising and suppressing backgrounds is fundamental.

This work characterises a number of important backgrounds and explores their mitigation strategies for the LUX and LZ experiments. The LZ gas charcoal chromatography system removes problematic noble radioactive impurities (primarily 85Kr) in the xenon. An overview of this system along with xenon impurity measurements, taken with the LZ sampling system, is presented. The cavern walls surrounding the detector emit high energy gamma rays from the naturally occurring radiation in the rock. High statistics event biasing simulations have been performed to determine the background contribution in the LUX and LZ detectors, which are located in a large water tank to provide shielding. LZ commissioning data is also analysed to evaluate the current cavern wall gamma ray shielding. In addition, simulations of the activation of xenon and surroundings in LZ caused by neutron calibration sources, have been carried out to determine the background due to the delayed decay of radioactive isotopes.