A ${}^{220}\mathrm{Rn}$ 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 ${}^{212}\mathrm{Pb}$ 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 ${}^{222}\mathrm{Rn}$ background after calibration. Alpha emitters are also distributed throughout the detector and facilitate calibration of its response to ${}^{222}\mathrm{Rn}$. Using the delayed coincidence of ${}^{220}\mathrm{Rn}\text{−}{}^{216}\mathrm{Po}$, 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 ${}^{212}\mathrm{Po}$, $t_{1/2}=(293.9\pm (1.0{)}_{\mathrm{stat}}\pm (0.6{)}_{\mathrm{sys}})\mathrm{ns}$.

• Received 10 November 2016

DOI:https://doi.org/10.1103/PhysRevD.95.072008