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
$1.9pm0.4$~$gamma~$cm$^{-2}$s$^{-1}$. The resulting activity in the walls of
the cavern can be characterised as $220pm60$~Bq/kg of $^{40}$K,
$29pm15$~Bq/kg of $^{238}$U, and $13pm3$~Bq/kg of $^{232}$Th. |