Imaging of single barium atoms in a second matrix site in solid xenon for barium tagging in a $^{136}$Xe double beta decay experiment

TL;DR

This study advances single-atom imaging of barium in solid xenon, crucial for detecting double beta decay events, by exploring different matrix sites and recovery techniques to improve signal stability.

Contribution

It extends previous work by identifying Ba atoms in hexa-vacancy sites and demonstrates methods to mitigate photobleaching, enhancing barium tagging in xenon matrices.

Findings

Successful imaging of Ba in hexa-vacancy sites despite photobleaching.

Recovery of fluorescence signals through annealing processes.

Evidence of matrix site transfer upon laser excitation.

Abstract

Neutrinoless double beta decay is one of the most sensitive probes for new physics beyond the Standard Model of particle physics. One of the isotopes under investigation is $^{136}$Xe, which would double beta decay into $^{136}$Ba. Detecting the single $^{136}$Ba daughter provides a sort of ultimate tool in the discrimination against backgrounds. Previous work demonstrated the ability to perform single atom imaging of Ba atoms in a single-vacancy site of a solid xenon matrix. In this paper, the effort to identify signal from individual barium atoms is extended to Ba atoms in a hexa-vacancy site in the matrix and is achieved despite increased photobleaching in this site. Abrupt fluorescence turn-off of a single Ba atom is also observed. Significant recovery of fluorescence signal lost through photobleaching is demonstrated upon annealing of Ba deposits in the Xe ice. Following annealing, it is observed that Ba atoms in the hexa-vacancy site exhibit antibleaching while Ba atoms in the tetra-vacancy site exhibit bleaching. This may be evidence for a matrix site transfer upon laser excitation. Our findings offer a path of continued research toward tagging of Ba daughters in all significant sites in solid xenon.