Measuring Intrinsic Correlated Avalanches: SiPM Testing in Liquid Xenon

The nEXO experiment aims to search for neutrinoless double beta decay of Xe-136 with a half-life sensitivity >10^28 years. The nEXO detector is a 5-tonne, single-phase liquid xenon time projection chamber (LXe TPC) enriched to 90% in the mass 136 isotope. nEXO collects both ionization charge and scintillation light and uses ~ 4.6 m^2 of vacuum ultraviolet (VUV) silicon photomultipliers (SiPMs) to detect 178 nm scintillation photons. The intrinsic properties of the SiPMs, such as charge gain, breakdown voltage, correlated avalanche (CA) probability, and photon detection efficiency (PDE) play an important role in achieving the best possible energy resolution and need to be measured in the 167K LXe environment. This talk will report the measurements of the gain, breakdown voltage, and CA probability in a kg-scale LXe setup at UMass Amherst. A method using a pulsed, UV LED in low illumination mode to extract the single photoelectron (SPE) signal beneath the otherwise overwhelming scintillation glow in the cell due to radioactivity and cosmic rays is presented. The results are compared with measurements of CA probability taken in vacuum and gaseous nitrogen, in which scintillation is absent and systematic effects are quantified. A comparison across various cryogenic temperatures and media will also be presented.