Optical coating behavior under y irradiation for space applications

Optical coatings are widely used in optical instrumentation for astronomical and space applications. The required optical components are similar to the ones necessary for ground-based instrumentation: antireflection coatings, mirrors, transmission filters. In addition, depending on the specific application, resistance to the radiation induced damage is needed. The analysis here is limited to optical components for instruments to be used for Earth observation from the polar sun-synchronous orbit, where they will withstand a total radiation dose of 50 Gy accumulated during a typical lifetime of 5 years. In this work a set of optical coating materials were submitted to γ irradiation at the 60Co radioisotope source (ENEA Research Center) in order to simulate the hostile radiation environment in which they will be employed. The selected coatings are made with a multilayer structure that contains oxide layers and/or metal layers. Before the coating irradiation, the behavior of single-layer materials was investigated by comparing their reflectance and transmittance before and after the γ-ray exposure. Results are reported for a number of oxide single layers (SiO2, HfO2, Y2O3), metal layers (Ag, Al) and multilayer coatings made with these materials, by the physical vapor deposition (PVD) technique. The behavior of different substrates (glass, quartz) is also compared.