Measurement of Dissolved Beta Decay Sources in the LUX-ZEPLIN Experiment

Abstract

Weakly Interacting Massive Particles (WIMPs) are one of the most well-motivated dark matter candidates. Liquid Xe (LXe) time projection chambers (TPC) are the leading technology for probing the WIMP mass range. The LUX-ZEPLIN (LZ) experiment is a 10 tonne LXe TPC searching for the elusive WIMP dark matter. As detector masses increase to maximize sensitivity, background sources in the Xe, such as $^{85}$Kr and $^{39}$Ar, must be reduced to meet background requirements. These species are more difficult to quantify and require more stringent monitoring compared to previous generations of experiments. In this work, the techniques used to monitor $^{85}$Kr and $^{39}$Ar during commissioning and through LZ's WIMP Search 2024 science run (WS2024) are discussed. A mass spectrometry technique is used to measure $182 \pm 26$ ppq (g/g) ($^{nat}$Kr/Xe) during WS2024. A complementary measurement of an excited state decay of $^{85}$Kr found a $^{nat}$Kr equivalent concentration of $124_{-37}^{+44}$ ppq (g/g) ($^{nat}$Kr/Xe). The measured WS2024 Ar concentration is $1.10 \pm 0.18$ ppb (g/g) (Ar/Xe). LZ set a world leading limit on the interaction cross section across the expected WIMP mass range, with a minimum of $2.1 * 10^{-48}$ cm$^2$ at 36 GeV/c$^2$.