Growth and Decay Kinetics of Radiation-Induced Attenuation in Bulk Optical Materials

Using a dedicated setup, the growth and decay kinetics of the radiation-induced attenuation (RIA) have been characterized during and after steady-state 40 keV Xray exposures of bulk (1 cm thickness) optical materials. Such materials served to design the lenses of radiation hardened cameras of FURHIS project intended to withstand MGy(SiO2) dose levels associated with ITER remote handling operations. RIA is monitored in situ in the 400 to 800 nm spectral domain for glass types ranging from radiation sensitive (BK7) to radiation hardened ones: puresilica, BK7G18, K5G20 and SF6G05 from SCHOTT. In addition to the detailed characterization of these various glasses, our study highlights the systematic underestimation of the RIA measured by the usual qualification approach consisting in post irradiation RIA measurements after dose steps. The error due to this underestimation is shown to strongly depend on the glass type, very large for the BK7G18 glass. This material is associated with high transient RIA, its loss levels exceed the ones of its non radhard counterpart (BK7) at low dose levels. For the development of MGy radiation-hardened systems using these glasses, a vulnerability study of the in situ response of glass lenses appears mandatory. By improving our knowledge on the origin of these transient losses, appropriate hardening techniques to maintain the camera photometry budget will be tested in the future.