Results from a calibration of XENON100 using a source of dissolved radon-220

Abstract

A Rn220 source is deployed on the XENON100 dark matter detector in order to address the challenges in calibration of tonne-scale liquid noble element detectors. We show that the Pb212 beta emission can be used for low-energy electronic recoil calibration in searches for dark matter. The isotope spreads throughout the entire active region of the detector, and its activity naturally decays below background level within a week after the source is closed. We find no increase in the activity of the troublesome Rn222 background after calibration. Alpha emitters are also distributed throughout the detector and facilitate calibration of its response to Rn222. Using the delayed coincidence of Rn220-Po216, we map for the first time the convective motion of particles in the XENON100 detector. Additionally, we make a competitive measurement of the half-life of Po212, t1/2=(293.9±(1.0)stat±(0.6)sys) ns.