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

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.