Measurements of the ion fraction and mobility of alpha and beta decay products in liquid xenon using EXO-200

Abstract

Alpha decays in the EXO-200 detector are used to measure the fraction of charged ${}^{218}\mathrm{Po}$ and ${}^{214}\mathrm{Bi}$ daughters created from alpha and beta decays, respectively. ${}^{222}\mathrm{Rn}$ alpha decays in liquid xenon (LXe) are found to produce ${}^{218}{\mathrm{Po}}^{+}$ ions $50.3\pm 3.0\%$ of the time, while the remainder of the ${}^{218}\mathrm{Po}$ atoms are neutral. The fraction of ${}^{214}{\mathrm{Bi}}^{+}$ from ${}^{214}\mathrm{Pb}$ beta decays in LXe is found to be $76.4\pm 5.7\%$, inferred from the relative rates of ${}^{218}\mathrm{Po}$ and ${}^{214}\mathrm{Po}$ alpha decays in the LXe. The average velocity of ${}^{218}\mathrm{Po}$ ions is observed to decrease for longer drift times. Initially the ions have a mobility of $0.390\pm 0.006{\mathrm{cm}}^2/(\mathrm{kV}\mathrm{s})$, and at long drift times the mobility is $0.219\pm 0.004{\mathrm{cm}}^2/(\mathrm{kV}\mathrm{s})$. Time constants associated with the change in mobility during drift of the ${}^{218}{\mathrm{Po}}^{+}$ ions are found to be proportional to the electron lifetime in the LXe.