Methodologies Used to Remove Effects of Uranium Surface Contamination

Abstract

In the production phase of the SuperCDMS detectors, Transition-Edge Sensors containing tungsten were deposited photolithographically onto the surface of detectors. Afterward, most of this tungsten was etched away. However, the remaining tungsten naturally contains a certain amount of uranium. Through the uranium decay chain an alpha particle gets released into a coincident detector, and a recoiling nucleus gets sent into the detector which emitted this alpha particle. After analyzing data from simulations, a general observation was reached that a large majority of the coincident hit pairs contain one hit above 2 MeV from U238 and U234 decays, and this is a result of the uranium contamination. There are a few regions of interest in terms of energy ranges which are observed when finding a number of Weakly Interacting Massive Particles (WIMP) detections, and the recoiling nucleus can have an energy deposition within one of these ranges. After applying a cut which removes these hits, the effectiveness of this cut within the HV Region of Interest (ROI), 3 eV to 2 keV, is around 90 to 100%, and this effectiveness within the iZIP ROI, 1 keV to 50 keV, is around 83%-95%.