Radiation enhanced degradation of SiO overcoated aluminium mirrors

The reflectivity of mirrors required for diagnostic and remote handling applications in ITER may be degraded by many different mechanisms ranging from simple contamination, to sputtering erosion due to particle bombardment, and radiation effects. Reflectivity changes from 250 to 2500 nm for high quality SiO overcoated aluminium on Pyrex glass mirrors irradiated up to 10 MGy at 25 °C in dry air, nitrogen, and high vacuum (10−6 mbar) have been determined. Irradiation in nitrogen and vacuum caused no measurable change in reflectivity. But irradiation in dry air produced a marked decrease in reflectivity in the visible and particularly in the UV range. X-ray photoelectron spectroscopy (XPS) analysis of the chemical species formed on the surface indicates that radiation enhanced oxidation of the SiO protective layer occurs. As this layer oxidizes the thickness is modified hence degrading the optimised optical interference conditions.