Improving Kr-83m Calibration and Energy Resolution in NEXT Neutrinoless Double Beta Decay Detectors

Abstract

The most promising experimental proposal for detecting Majorana neutrinos is searching for neutrinoless double beta decay, an extremely rare lepton number violating process. Many experiments are currently searching for this decay using a variety of techniques. The Neutrino Experiment with a Xenon Time Projection Chamber (NEXT) is one such effort. NEXT uses a high pressure gaseous xenon time projection chamber to search for neutrinoless double beta decay in $^{136}$Xe. This technology gives NEXT exquisite resolution, topological event discrimination, and low background, all of which have been achieved in the NEXT-White demonstrator detector. In this work, we discuss the calibration of NEXT with gaseous $^{83m}$Kr, which gives pointlike, low energy (41.5~keV) events uniformly throughout the detector. NEXT is shown capable of achieving energy resolutions $ 4%$ at 41.5~keV for $^{83m}$Kr decays, extrapolating to $ 0.6%$ at the $Q_{\beta \beta}$ of $^{136}$Xe. A novel method of assigning $z$ position in time projection chambers using $^{83m}$Kr is presented, and shown to achieve position resolution of ${\sim}$1~cm, with minimal impact on energy resolution. $^{83m}$Kr decays are leveraged to determine the structure of the electroluminescence gap in NEXT-White, which is found to have a 1~mm planar tilt and moderate curvature. Finally, $^{83m}$Kr is used to investigate the potential for detector imperfections to degrade energy resolution in the upcoming NEXT-100 detector. The detector is shown to maintain excellent performance with energy resolution $ 1%$ at $Q_{\beta \beta}$ even for lifetimes as low as 3~ms, only 20% of the statistics used in NEXT-White, and significant spatial inhomogeneities, althought it is more sensitive to the electroluminescence light yield.