Precision measurement of positron decay modes of Xe-125 in the LUX-ZEPLIN experiment

TL;DR

This study measures the positron decay modes of Xe-125 in the LUX-ZEPLIN experiment, confirming positron emission with high significance and providing the first constraints on individual branching ratios.

Contribution

It provides the first direct measurement and constraint of positron emission branching ratios of Xe-125 decay modes in a dark matter detection experiment.

Findings

Positron emission confirmed with 5.5σ significance.

Total branching ratio of positron emission measured as 0.29±0.08(stat.)±0.04(sys.).

First constraints on individual decay branching levels of Xe-125.

Abstract

The radioisotope $^{125}\text{Xe}$ is a short-lived ($T_{1/2}\sim16.9 h$) activation product of the neutron calibrations performed in the LUX-ZEPLIN experiment. Subsequently, $^{125}$Xe decays primarily ($>99\%$) via electron capture, but positron emission has been confirmed by direct measurement to at least the 243 keV level of $^{125}\text{I}$. An additional decay to the 188keV level is expected from triple-coincident measurements of the annihilation and relaxation $\gamma$ rays, but has not been directly confirmed. By utilizing multiple-scatter event analysis and the pre-activation data to constrain backgrounds, this work reports positron emission with a statistical significance of 5.5$\sigma$. This corresponds to a total branching ratio of $0.29\pm0.08_{\text{stat.}}\pm0.04_{\text{sys.}}$ %, and is the first constraint to the individual branching levels of $^{125}\text{I}$.