Measurements of the ion fraction and mobility of α− and β -decay products in liquid xenon using the EXO-200 detector

Alpha decays in the EXO-200 detector are used to measure the fraction of charged Po218 and Bi214 daughters created from α and β decays, respectively. Rn222 α decays in liquid xenon (LXe) are found to produce Po+218 ions 50.3±3.0% of the time, while the remainder of the Po218 atoms are neutral. The fraction of Bi+214 from Pb214 β decays in LXe is found to be 76.4±5.7%, inferred from the relative rates of Po218 and Po214 α decays in the LXe. The average velocity of Po218 ions is observed to decrease for longer drift times. Initially the ions have a mobility of 0.390±0.006cm2/(kVs), and at long drift times the mobility is 0.219±0.004cm2/(kVs). Time constants associated with the change in mobility during drift of the Po+218 ions are found to be proportional to the electron lifetime in the LXe.