Darkening of UV optics irradiated at a CW 1-MeV linear electron accelerator

Darkening of SiO2 glasses, CaF2, MgF2 and Al2O3 crystals commonly used for e-beam-pumped KrF laser windows or coatings was investigated at CW 1-MeV linear electron accelerator. Absorbed doses in excess of 20 MGy were close to expected for the Inertial Fusion Energy laser driver. Comparison with pulsed 280-keV e-beam source and CW 400-keV linac-based gamma source shows that induced absorption saturates with a dose while a saturation level depends on the kind of radiation and dose rate. The best resistance to both scattered fast electrons and bremsstrahlung gamma-rays exhibits a dry fused silica glass KS-4V with the lowest hydroxyl content.