GAMMA-RAY-INDUCED OPTICAL ATTENUATION IN GE-DOPED-SILICA FIBER IMAGE GUIDES

Optical attenuation in fiber optic image guides comprising germanium-doped-silica-core/ pure-silica-clad picture elements was investigated in the range 420-1100 nm during and after room-temperature Co-60 gamma irradiation. The induced absorption spectra of the picture elements were successfully fitted by linear combinations of three known defect bands in irradiated Ge-doped SiO2: the Ge(1) center, the GeX center, and the nonbridging oxygen hole center (peak positions of 4.4, 2.6, and 2.0 eV, respectively). Dose for dose, the relative strengths of these three bands differed greatly between two nominally identical image guide samples from the same manufacturer. Defect production kinetics were followed at four dose rates ranging from 1.1 to 180 Gy/h. These kinetics and the post-irradiation anneal behaviors are interpreted in terms of the previously reported tendency of the Ge(1) center to undergo a thermal conversion to GeX. The net effect on absorption at 550 nm is a dose-rate-independent linear growth up to similar to 1000 Gy(Si) followed by saturation, with essentially no post-irradiation thermal decay at 22 C. At 650 nm, the kinetics are similar except that there is an ''inverted'' dose-rate dependence (lower losses at higher dose rates) and a small post-irradiation increase in absorption with time at 22 C. Irrespective of dose rate, these image guides are unsuitable for use in multimeter lengths if there is a likelihood of a lifetime radiation exposure of similar to 100 Gy (worst sample) or similar to 1000 Gy (best sample).