Chroma-based optical simulations for the UMass liquid xenon test apparatus

nEXO is a tonne-scale neutrinoless double beta decay experiment that will search this nuclear decay in ¹³⁶Xe using a single-phase time projection chamber (TPC) filled with 5 tonnes of isotopically enriched liquid xenon (LXe). A large array of VUV-sensitive silicon photomultipliers (SiPMs) detect 178 nm scintillation photons originating from ionizing events. A kg-scale LXe setup at UMass is operated to characterize the response of prototype SiPMs for nEXO and study VUV optics in LXe. We use a GPU-based optical ray tracing simulation called Chroma to simulate the experimental setup under a range of optical parameters (refractive indices of components, absorption and scattering lengths of LXe, LXe scintillation wavelength, diffusive and specular reflectivity of surfaces including the SiPM), varied within ranges suggested by values found in literature. Simulations are compared with measurements performed with different layouts (e.g. SiPM-source separation, presence of reflectors, etc.) to break degeneracies and control systematic uncertainties in the extraction of the photon transport efficiency (PTE) of each configuration. This presentation will showcase the progress made towards the determination of the optical properties of interest.