Catastrophic damage has been observed in ZERODUR mirrors used as first mirrors in two beam lines at the National Synchrotron Light Source. ZERODUR was selected as a substrate for these uncooled, grazing incidence mirrors because of its superior thermal expansion properties near room temperature. Despite the high reflectivity of the coatings used on these mirrors, a significant flux of high energy photons penetrates below the coating and is absorbed mainly in a thin layer at the top of the substrate. Over a long time period the absorbed flux causes the glass material to compact, leading to a build-up of surface stress, gross figure changes, and the growth of fractures. The total dose of absorbed radiation for these mirrors is estimated to be in the range of 106 MRads, i.e. 1012 Rads, which is orders of magnitude greater than the dose used in conventional radiation damage studies for space-borne optics and deep UV lithography systems.
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