Characterization of a ${}^{220}$Rn source for low-energy electronic recoil calibration of the XENONnT detector

Abstract

Low-background liquid xenon detectors are utilized in the investigation of rare events, including dark matter and neutrinoless double beta decay. For their calibration, gaseous ${}^{220}$Rn can be used. After being introduced into the xenon, its progeny isotope ${}^{212}$Pb induces homogeneously distributed, low-energy ($<30$ keV) electronic recoil interactions. We report on the characterization of such a source for use in the XENONnT experiment. It consists of four commercially available ${}^{228}$Th sources with an activity of 55 kBq. These sources provide a high ${}^{220}$Rn emanation rate of about 8 kBq. We find no indication for the release of the long-lived ${}^{228}$Th above 1.7 mBq. Though an unexpected ${}^{222}$Rn emanation rate of about 3.6 mBq is observed, this source is still in line with the requirements for the XENONnT experiment.

9 pages, 5 figures