Towards a Cluster-wise Energy Calibration in EXO-200 from Pair Producing Gamma Rays

EXO-200 is a liquid xenon time projection chamber (TPC) that operated between 2010 and 2018 to search for neutrinoless double beta decay of Xe-136. As part of a neutron calibration campaign with an AmBe source placed in the vicinity of the TPC, in 2020 the EXO-200 collaboration measured the beta spectrum for the decay of Xe-137 (J^π = 7/2-) to the ground state of Cs-137 (7/2+).

A second part of this study is to measure the beta spectrum of Xe-137 beta decay to the lowest excited state of Cs-137 (J^π = 5/2+). This decay is followed by a prompt 455 keV gamma de-excitation, which together with the beta electron, produces spatially distributed energy depositions in the EXO-200 TPC, which are reconstructed as “multi-site” events. This study requires a dedicated energy calibration on individual charge clusters of multi-site events to extract the beta energy alone. This talk will discuss the use of pair-producing gamma ray events present in EXO-200 calibration data and reproduced in simulations to identify a controlled set of beta clusters in multi-set events over a range of energies.