Event Reconstruction in a Liquid Xenon Time Projection Chamber with an Optically-Open Field Cage

Abstract nEXO is a proposed tonne-scale neutrinoless double beta decay (0 ν β β ) experiment using liquid 136 Xe (LXe) in a Time Projection Chamber (TPC) to read out ionization and scintillation signals. Between the field cage and the LXe vessel, a layer of LXe (“skin” LXe) is present, where no ionization signal is collected. Only scintillation photons are detected, owing to the lack of optical barrier around the field cage. In this work, we show that the light originating in the skin LXe region can be used to improve background discrimination by 5% over previous published estimates. This improvement comes from two elements. First, a fraction of the γ -ray background is removed by identifying light from interactions with an energy deposition in the skin LXe. Second, background from 222 Rn dissolved in the skin LXe can be efficiently rejected by tagging the α decay in the 214 Bi- 214 Po chain in the skin LXe.