Comparative study of pulsed X-ray and γ-ray radiation-induced effects in pure-silica-core optical fibers

We have investigated the variation of the optical absorption induced by pulsed (dose rate ≫ 10⁸ rad/s) and continuous (≪ 50 rad/s) γ-ray exposures in pure-silica-core optical fibers. Tested multimode waveguides, designed with well-defined concentrations of hydroxyl groups and chlorine impurity, are possible candidates for integration in the plasma diagnostics of LMJ and ITER facilities. We evaluated their radiation-tolerance to low dose levels (≪ 5×10⁴ rad) in the visible and near-infrared parts of the spectrum. We measure the time dependent changes (10^-9 -10^-1 s) of the radiation-induced attenuation (RIA) at fixed wavelengths and we complete these measurements with a spectral (450-1100 nm) analysis of these losses in the time range from 10^-2 to 10³ s. We compared the responses obtained under transient exposures with measurements done at same dose levels (5×10³ rad) with lower dose rate (≪10² rad/s). We showed that the radiation-induced changes are strongly dose rate dependent for this kind of optical fibers. Depending on the hydroxyl group concentration in the silica-glass, different point defects are shown to alter the fiber transmission after both irradiation types. The self-trapped holes play a particular role in the transient responses of optical fibers whereas non-bridging oxygen hole centers are mainly responsible for γ-ray radiation-induced losses. The consequences of these different behaviors for the integration of optical fibers in the LMJ are discussed.