The nEXO light detection system and photo-detector characterization efforts

The nEXO experiment is a planned 5-tonne LXe TPC designed for the search for the neutrinoless double beta decay of ¹³⁶Xe. In order to reach the prospected half-life of up to 10²⁸ years a good energy resolution of 1% (sigma) at the Q-value (2457.8 keV) and a good background suppression is crucial resulting in strong requirements on the collection efficiency of the VUV scintillation photons produced by any events in the LXe. The light detection system needs to be designed accordingly. nEXO will use large area VUV-sensitive SiPMs as photo detectors covering the lateral surface of the cylindric TPC. Their instrinsic properties such as the photo detection efficieny (PDE) or number of events suffering correlated avalanches (such as Crosstalk and Afterpulsing) as well as the general layout of the nEXO TPC needs to be optimized to meet the requirements on the light detection system. For example, the PDE of any SiPM candidates needs to be at least 15% for LXe scintillation light (λ = 175 nm) to detect at least 7% of the overall scintillation light of a given event. We report current aspects of the nEXO light detection system and characterization results of trending VUV-sensitive SiPMs examined by different collaboration members focussing on their performance at LXe temperatures.