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

The radioisotope ${}^{125}\text{Xe}$ is a short-lived ($T_{1\mathrm{/}2}\sim 16.9h$) activation product of the neutron calibrations performed in the LUX-ZEPLIN experiment. Subsequently, ${}^{125}$Xe decays primarily ($>99\mathrm{\%}$) 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 $0.29\pm {0.08}_{\text{stat.}}\pm {0.04}_{\text{sys.}}$ %, and is the first constraint to the individual branching levels of ${}^{125}\text{I}$.