Development of a tagged source of Pb-206 nuclei

Abstract

There is a particular class of unavoidable backgrounds that plague low-background experiments and rare event searches, particularly those searching for nuclear recoil event signatures: decaying daughters of the 238U nuclear decay chain, which result from radon plate-out on detector materials. One such daughter isotope, 210Po, undergoes α-decay and produces a recoiling 103 keV 206Pb nucleus. To characterize this important background in the context of noble element detectors, we have implemented a triggered source for these 206Pb recoils in a dual-phase xenon time projection chamber (Xe TPC) within the Davis Xenon R&D testbed system (DAX). By adhering 210Po to the surface of a PIN diode and electrically floating the diode on the cathode of the TPC, we tag the α signals produced in the PIN diode and trigger on the correlated nuclear recoils in the liquid xenon (LXe). We discuss our methods for 210Po deposition, electronic readout of the PIN diode signals at high voltage, and analysis methods for event selection.