Phonon-Based Position Determination in SuperCDMS iZIP Detectors

Abstract

SuperCDMS is currently operating a 10-kg array of cryogenic germanium detectors in the Soudan Underground Laboratory to search for weakly interacting massive particles, a leading dark matter candidate. These detectors, known as iZIPs, measure ionization and athermal phonons from particle interactions with sensors on both sides of a Ge crystal. The ionization signal can be used to efficiently tag events at high radius and near the top and bottoms surfaces, where diminished charge collection can cause events to mimic WIMP-induced nuclear recoils. Using calibration data taken with a $^{206}$Pb source underground at Soudan, we demonstrate rejection of surface events of $(4.5 \pm 0.9) \times 10^{-4}$ with 46 % acceptance of nuclear recoils using the phonon signal only. We also show with $^{133}$Ba calibration data underground that the phonon channels can efficiently identify events near the sidewall. This phonon-based approach can also be extended to lower energies than the ionization-based position reconstruction.