Radiation tests of optical glasses for BepiColombo/MMO imaging camera

Irradiation tests of 9.1MeV and 70MeV protons have been made for optical glasses which are planned for the usage in an imaging camera proposed for one of science instruments on board the next ESA/ISAS collaborative Mercury mission BepiColombo/MMO. It is generally known that ordinary optical glasses are colored particularly in UV and VIS ranges when they are exposed to high-energy radiation because of adsorption of light by ionized electrons and holes generated in lattice defects of glasses. The total dose for BepiColombo/MMO is estimated to be about 200krad for the whole mission, which is 5 to 11 times larger than that of earth orbiting spacecraft. A feasibility study of optical elements on board BepiColombo/MMO is therefore quite important for realizing an effective imaging camera system. In the irradiation tests, totally nine kinds of glasses were tested; i.e., two kinds of fluorites (UV and SO grades), six kinds of non-browning glasses, and one normal glass (BK7). As the results, the transmission loss caused by the irradiation was within 10% at the wave length of 600nm and roughly 80% transmission was assured for CaF2(SO grade) and non-browning glasses with the thickness of 5mm. On the other hand, the transmission loss was more than 35% at the wave length of 600nm for CaF2(UV grade) and BK7 with the thickness of 5mm. In this study, it is also confirmed that a model of transmission values for irradiated glasses, whose effectiveness has been already confirmed for the irradiations of gamma-ray, is almost applicable to proton irradiation. (author)