High-Dose Neutron Irradiation Performance of Dielectric Mirrors

The study presents the high-dose behavior of dielectric mirrors specifically engineered for radiation tolerance. Alternating layers of Al₂O₃/SiO₂ and HfO₂/SiO₂ were grown on sapphire substrates and exposed to neutron doses of 1 and 4 displacements per atom (dpa) at 458 ± 10 K in the High Flux Isotope Reactor (HFIR). In comparison to previously reported results, these higher doses of 1 and 4 dpa result in a drastic drop in optical reflectance, caused by a failure of the multilayer coating. HfO₂/SiO₂ mirrors failed completely when exposed to 1 dpa, whereas the reflectance of Al₂O₃/SiO₂ mirrors reduced to 44%, eventually failing at 4 dpa. Transmission electron microscopy (TEM) observation of the Al₂O₃/SiO₂ specimens showed SiO₂ layer defects, which increase in size with irradiation dose. The typical size of each defect was ≈8 nm in 1-dpa specimens and ≈42 nm in 4-dpa specimens. Buckling-type delamination of the interface between the substrate and first layer was typically observed in both 1- and 4-dpa HfO₂/SiO₂ specimens. Composition changes across the layers were measured in high-resolution-scanning–TEM mode using energy dispersive spectroscopy. A significant interdiffusion between the film layers was observed in the Al₂O₃/SiO₂ mirror, although it was less evident in the HfO₂/SiO₂ system. The ultimate goal of this work is to provide insight into the radiation-induced failure mechanisms of these mirrors.