Formalisation of the number of photons detected by a silicon photomultiplier in liquid xenon

Evidence of 0νββ decay would imply physics beyond the Standard Model. The nEXO experiment will search for 0νββ decay of 136Xe with an expected sensitivity in excess of 10²⁸ years using a single-phase, liquid xenon time projection chamber (TPC) enriched to 90% in the mass 136 isotope. The detector will be instrumented with vacuum ultraviolet (VUV)-sensitive silicon photomultipliers (SiPMs) to sense the 178 nm scintillation light. Key SiPM parameters that determine the energy resolution of the scintillation signal include the gain, breakdown voltage, correlated avalanches (CA), and photon detection efficiency (PDE). This talk will report on the measurement of the number of LXe scintillation photons detected by a FBK VUVHD3 SiPM as a function of over-voltage using a kg-scale LXe cell at UMass Amherst equipped with a 6.2 kBq 241Am alpha source. The measurement requires the determination of the over-voltage, gain and CA probability of the SiPM at the operating temperature. To extract the number of detected photons for given LXe cell configurations, we correct the measured pulses by the CA probability. Measurements carried out with a variety of reflectors and source-to-SiPM separations to determine the PDE of the SiPM will be presented.