Measuring Backgrounds from ⁸⁵Kr and ²¹ºBi to Improve Sensitivity of Dark Matter Detectors

Abstract

Understanding background events in a low-energy search for the WIMP dark matter candidate is critical to enable a viable search as these events must be reduced to avoid overwhelming the rare dark matter signal. The βˉ decay spectra from ²¹ºBi and from ⁸⁵Kr have energies in the dark matter search region of ≤100 keV. This work identifies 38.0 ± 6.2 ⁸⁵Kr events via its rare β-γ coincidence decay path in the LUX detector's 332-day dark matter search. Furthermore, the activity of ²¹ºBi which can be tolerated in the detector before it is discernible in the low-energy region of the radioactive background spectrum is determined for a variety of radial cuts to study how the mobile ²¹ºBi infiltrates a dark matter detector's fiducial volume. This sets a limit on acceptable ²¹ºBi activities which will help future low-energy experiments determine the steps they must take to build competitively sensitive detectors.