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

TL;DR

This study measures the ion fraction and mobility of alpha and beta decay products in liquid xenon using the EXO-200 detector, revealing ionization probabilities and mobility changes over drift times.

Contribution

It provides the first detailed measurements of ion fractions and mobility of decay products in liquid xenon, including their dependence on drift time and correlation with electron lifetime.

Findings

$^{218}$Po ions are produced 50.3% of the time from $^{222}$Rn alpha decays.

$^{214}$Bi ions are produced 76.4% of the time from $^{214}$Pb beta decays.

Ion mobility decreases from 0.390 to 0.219 cm$^2$/V·s over longer drift times.

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.