Radiation effects on second surface mirrors

Second surface mirrors used for spacecraft temperature control have recently been evaluated in various space radiation environments simulated in the laboratory. The spectral reflectance and solar absorptance properties of these fused silica/silver mirrors were investigated extensively in a number of tests. Radiation exposures included several combination of protons between 3keV and 80keV energy, electrons between 20keV and 230keV energy, and ultraviolet radiation between one and 16 suns in intensity and up to 20,000 equivalent sun hours. Particle fluences ranged from 1014 to more than 1016 per square centimeter. Important new and confirmatory information has been learned about (1) the radiation resistance of fused silica; (2) the significance of bonding agents (adhesives) in introducing degradation, self-contamination, and cross-contamination; and (3) the necessity for test chamber cleanliness. Several tests showed that RTV silicone adhesive will outgas from hot mirrors and creep from underneath cold mirrors onto adjacent reflective surfaces, contaminating them. Repolymerization of the silicone also occurs under the influence of incident radiation, altering the thermophysical properties of test samples in ways that may or may not represent an adequate simulation of space condition. Quantitative values and qualitative effects will be included in the paper. It has been concluded that fused silica itself is quite stable during exposure to various combinations of natural space radiation, but that use of organic materials on a spacecraft in the vicinity of fused silica mirrors may significantly compromise their stability and performance