Ba+ neutralization fraction during capture from LXe into SXe

Determine the relative fractions of Ba+ ions and neutral Ba atoms that result when Ba+ produced in liquid xenon (LXe) is captured into solid xenon (SXe) at thermal energy, including the neutralization pathway via electron capture, to inform barium tagging strategies in nEXO.

Background

In the nEXO barium-tagging approach, Ba++ daughters produced in liquid xenon are expected to recombine to Ba+. When extracting and freezing the decay daughter into solid xenon on a cryogenic probe, the final charge state (Ba+ versus neutral Ba) impacts which optical transitions can be used for single-atom detection and thus affects tagging efficiency.

The authors note that, while an analogue measurement exists in LXe for other species, the specific fraction that remains ionic versus neutral after capture into SXe at thermal energy is not established, making this an important unresolved parameter for the method.