XII International Workshop on Ring Imaging Cherenkov Detectors - RICH2025

The High Energy Light Isotope eXperiment (HELIX), a balloon-borne detector targeting 3% mass resolution for cosmic-ray isotope composition, uses a Ring Imaging Cherenkov (RICH) with aerogel radiators (n=1.155, 1 cm thick) for velocity measurements above 1 GeV/n. Achieving the mass resolution requires a velocity resolution of 0.1%, necessitating characterization of the aerogel radiator's refractive index and thickness. We introduce an independent method to characterize the refractive index and thickness of aerogel radiators simultaneously: Spectral Domain Optical Coherence Tomography (SD-OCT). SD-OCT relies on changes in optical path length to simultaneously extract the group refractive index and thickness. This work demonstrates the feasibility of OCT for these direct group refractive index and thickness measurements, from which the phase refractive index at 405 nm is then derived. The aerogel thickness varies between 9.45 mm and 9.65 mm, with refractive index values at 405 nm ranging from 1.155 to 1.159, each determined to within an order of $10^{-3}$ mm and $10^{-4}$ respectively.