Angle-Resolved Reflectance Measurements in the Vacuum Ultraviolet for Metallic Samples

Abstract

The light yield of noble-element scintillation detectors depends strongly on the reflective properties of detector materials in the vacuum ultraviolet (VUV), where optical measurements are scarce. For this reason, we built a custom rotary setup at IFIC, equipped with a monochromator and operated in a gaseous argon atmosphere, to perform angle-resolved reflectance measurements in the specular reflection plane. We describe the setup and its extensive characterization, together with the procedure used to measure the bidirectional reflectance distribution function (BRDF) of materials at various angles of incidence, for wavelengths from 128 to 500 nm, covering both the argon (128 nm) and xenon (175 nm) scintillation wavelengths. To illustrate the capabilities of the setup, we present results for rough aluminum and stainless-steel surfaces as reference materials. Fitting the measured angular profiles with the analytical GGX microfacet model allows us to extract the real and imaginary parts of the refractive index and the surface roughness parameter of each sample. These parameters are then used to extrapolate the in-plane measurements to the full hemisphere and from gaseous argon to liquid media, enabling the inference of the hemispherical reflectance in gaseous argon, liquid argon and liquid xenon.

arXiv comment18 pages, 19 figures, 1 table