Comprehensive review of the effects of ionizing radiations on the ZERODUR® glass ceramic

Abstract

The low thermal expansion glass ceramic ZERODUR ® has a long and successful history in spaceborne optical applications. This material is especially used where precise shape invariance is required, i.e., for the mirror substrates dimensional stability when subject to temperature gradients and transients. In space, temperature may not be the exclusive driving force impacting the form stability; influence from ionizing radiations require considerations. The real impact of ionizing radiation on ZERODUR ® has become a matter of reconciling, on the one hand, in situ experience, e.g., that the secondary mirror of low Earth orbit (LEO) Hubble Space Telescope crossing the South Atlantic Anomaly or the overall optics of geosynchronous equatorial orbit (GEO) Chandra X-ray Observatory are not reporting any specific problems related to dimensional stability at the optical form level. On the other hand, finite element simulation based on early lab experiments of ZERODUR ® compaction are suggesting the opposite. This debate was brought to the forefront with the SILEX mission, where radiative ageing models were significantly overestimating the deformation experimentally observed on the lab replicas and were in even stronger disagreement with the observations collected over the mission. It has been speculated that an erroneous form factor in the physical model used to derive the phenomenological compaction law was responsible for these discrepancies. Following this hypothesis, we readdressed the effect of ionizing radiation induced by γ, electron, and proton fluences on ZERODUR ® compaction. For each of these, we present and discuss the irradiation source, the experimental setup, the sample design, and the measurement procedure as well as the observations. Consistent with the feedback gathered over many different space missions, we confirm that the compaction observed is significantly smaller than the estimations available in the prior literature.