Radiation Hardness Assurance of Optical Coatings for the COMPASSO Iodine Reference

Abstract

The COMPASSO payload demonstrates compact iodine-based optical frequency references on the International Space Station (ISS), requiring optical windows and antireflective coatings that remain stable in the Low Earth Orbit (LEO) radiation environment. Radiation analysis was performed to quantify how trapped electrons, protons, and ions affect the fused-silica iodine cell and its coatings under mission-relevant shielding and exposure scenarios. The aim was to determine whether selected coatings meet the COMPASSO optical requirements after realistic LEO irradiation, identify degradation mechanisms linked to deposition technology and materials, and provide selection guidance for coatings used in spaceborne iodine spectroscopy and optical-clock payloads. Nominal and worst-case corpuscular environments for the ISS orbit were derived using AE8/AP8, CREME96, ESP, and ISO15390 models. Eight coating stacks produced by different deposition methods were exposed to proton and electron irradiation corresponding to the nominal and worst-case mission loads. Pre- and post-irradiation characterization included reflectance and transmittance measurements, together with visual and microscopic inspection. The tests led to the selection of a four-layer $\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5/Si{\mathrm{O}}_2$ stack for the iodine cell optical window. Total ionizing dose levels for all layers of the selected coating were derived with GRAS, showing that the coating will not be affected by corpuscular radiation for years in the LEO ISS environment, including the COMPASSO mission at the ISS Bartolomeo platform.

arXiv commentAccepted for publication in Acta Astronautica