Reflectance of VUV-sensitive SiPM surfaces in liquid xenon

The nEXO experiment will operate an ultra-low background time projection chamber filled with $5\text{t}$ of isotopically enriched liquid xenon (LXe) for the search for the neutrinoless double beta decay in Xe-136. The detector will use $4{\text{m}}^2$ of silicon photomultipliers (SiPMs) operated at about $-104{}^{\circ }\text{C}$ to collect the scintillation light. There are strong requirements on the background levels, light collection efficiency and SiPM performance to set the most stringent limits on the half-life. Detecting the $0\nu \beta \beta$-decay would have wide-spread implications for neutrino physics. The SiPMs need to be sensitive to the vacuum-ultraviolet scintillation light of LXe at $178\text{nm}$ which is not given for off-the-shelf SiPMs. The SiPM performance requirements adopted by nEXO are (among others) a photon detection efficiency of at least 15% at $178\text{nm}$, a correlated avalanche probability of less than 20% and a low dark-rate. Additionally, the VUV-reflectance of the SiPM surface in LXe is an important parameter for optimal light collection and thus the energy resolution of nEXO. We present measurements with state-of-the-art VUV-sensitive SiPMs focusing on the surface VUV-reflectance in liquid xenon.