LArRI: a new setup for Liquid Argon Refractive index measurement

Liquid Argon (LAr), widely used as active target in neutrino and dark matter experiments, is a scintillator with a light yield of about 40 photons/keV, an attenuation length of the order of meters and a scintillation peak at 128 nm. Adding small amounts of dopant, i.e. xenon, (around 10 ppm) allows to shift the scintillation center to 178 nm without spoiling the light yield. The longer wavelength simplifies the light detection thereby impacting the development of imaging systems. A precise knowledge of LAr optical properties in the VUV range is essential to improve the performance of liquid argon-based experiments. Besides, the refractive index is a crucial parameter for the development of imaging systems. LArRI (Liquid Argon Refractive Index) aims at a direct measurement of LAr refractive index in the VUV spectrum using an interferometric technique. This approach relies on the comparison of two interference patterns, created in vacuum and in liquid, respectively, and measured by cryogenic silicon photomultipliers. We obtain a source of monochromatic, coherent light using a mercury lamp with emission peaks at 254 and 184 nm. In this Article, we present the results of the commissioning stage of the experiment, including our first measurements with cryogenic liquids.