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

The radioisotope 125Xe is a short-lived (T1/2∼16.9h) activation product of the neutron calibrations performed in the LUX-ZEPLIN experiment. Subsequently, 125Xe decays primarily (>99%) via electron capture, but positron emission has been confirmed by direct measurement to at least the 243 keV level of 125I. An additional decay to the 188keV level is expected from triple-coincident measurements of the annihilation and relaxation γ 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 5.5σ. This corresponds to a total branching ratio of 0.29±0.08stat.±0.04sys. %, and is the first constraint to the individual branching levels of 125I.