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 Rn-220 can be used. After being introduced into the xenon, its progeny isotope Pb-212 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 Th-228 sources with an activity of 55 kBq. These sources provide a high Rn-220 emanation rate of about 8 kBq. We find no indication for the release of the long-lived Th-228 above 1.7 mBq. Though an unexpected Rn-222 emanation rate of about 3.6 mBq is observed, this source is still in line with the requirements for the XENONnT experiment.