A study on the VUV reflectivity of detector materials for the nEXO experiment

nEXO is a tonne-scale experiment that will search for neutrinoless double beta decay of $^{136}\text{Xe}$ in an ultra-low background, single-phase liquid xenon (LXe) time projection chamber equipped with a readout of scintillation light. The projected sensitivity for the half-life of this decay is $>10^{28}$ years over 10 years of run time. Scintillation photons of 178 nm wavelength from interactions in the LXe will be collected by a large-array ($\sim$ 4.6 $\text{m}^2$ area ~\cite{precodr}) of VUV-sensitive silicon photomultipliers (SiPMs). To maximise the light collection efficiency, the detector materials need to be highly VUV reflective. This presentation reports the study of VUV optical properties of materials considered for the nEXO detector in an LXe test bench at UMass Amherst housing an FBK VUV HD3 SiPM ($\sim$ 1 $\text{cm}^2$), an $^{241}\text{Am}$ alpha source, and reflectors under test. A GPU-based ray tracing software called Chroma was used to simulate experimental configurations. In addition to the reflector's refractive index and the SiPM reflectivity in LXe, the photon transport efficiency (PTE) of the system depends on multiple parameters of the system, including the LXe refractive index, scintillation wavelength, and absorption and scattering lengths. Using the best estimates of these factors from literature \cite{lxe,lxescin,lxescatter}, simulated PTE values are compared to the measurements of the same configurations illuminated by alpha scintillation pulses. The estimates of the diffusivity and specularity of the reflectors along with their systematic errors are reported. Future work entails the construction of a multivariate matrix to span the broad parameter space and understand its correlations. This will ultimately become an important part of the scintillation event reconstruction algorithm in nEXO.