Measurement of ²¹⁸Po Ion Fraction and Mobility in Liquid Xenon

The EXO-200 detector is a single-phase liquid xenon time projection chamber containing 200 kg of enriched ¹³⁶Xe that was used to search for neutrinoless double beta decay. At the end of its run, EXO-200 conducted an internal source calibration campaign by injecting ²²²Rn into the detector. Using this dataset, we have performed an analysis of the ion fraction and mobility of the ²¹⁸Po daughters of ²²²Rn alpha decays in liquid xenon. This data was collected under a higher electric field (567 V/cm) than previous EXO-200 alpha ion analyses (380 V/cm). The results from this study will inform background modeling and event identification for future liquid xenon time projection chambers. Next-generation ¹³⁶Xe neutrinoless double beta decay experiments plan to “tag” the ¹³⁶Ba daughters of double beta decays to eliminate all backgrounds except for two-neutrino double beta decay. Successful ¹³⁶Ba tagging will rely on studies of alpha decay ion fractions and ion drift in xenon.