# Measurement of the Gamma Ray Background in the Davis Cavern at the Sanford Underground Research Facility

## TL;DR

This study measures gamma-ray backgrounds in the Davis Cavern at Sanford Underground Research Facility to inform low-background experiments like LUX-ZEPLIN, revealing specific activity levels of natural isotopes and gamma fluxes.

## Contribution

It provides the first in-situ gamma-ray background characterization of the Davis Cavern, including activity levels of key isotopes and gamma flux measurements for underground physics experiments.

## Key findings

- Gamma flux in the cavern ranges from 596 Hz to 1355 Hz.
- Activities of $^{40}$K, $^{238}$U, and $^{232}$Th were quantified.
- Total gamma flux is approximately 1.9 gamma/cm²/s.

## Abstract

Deep underground environments are ideal for low background searches due to the attenuation of cosmic rays by passage through the earth. However, they are affected by backgrounds from $\gamma$-rays emitted by $^{40}$K and the $^{238}$U and $^{232}$Th decay chains in the surrounding rock. The LUX-ZEPLIN (LZ) experiment will search for dark matter particle interactions with a liquid xenon TPC located within the Davis campus at the Sanford Underground Research Facility, Lead, South Dakota, at the 4,850-foot level. In order to characterise the cavern background, in-situ $\gamma$-ray measurements were taken with a sodium iodide detector in various locations and with lead shielding. The integral count rates (0--3300~keV) varied from 596~Hz to 1355~Hz for unshielded measurements, corresponding to a total flux in the cavern of $1.9\pm0.4$~$\gamma~$cm$^{-2}$s$^{-1}$. The resulting activity in the walls of the cavern can be characterised as $220\pm60$~Bq/kg of $^{40}$K, $29\pm15$~Bq/kg of $^{238}$U, and $13\pm3$~Bq/kg of $^{232}$Th.