Comparison between fast neutron and gamma irradiation of optical fibres

A variety of undoped multimode step-index fibres and Gedoped single mode and graded index fibres is irradiated by 14 MeV neutrons an Co60 gamma rays up to the same total dose. Radiation-induced loss and breaking stress are measured. The loss induced during gamma irradiation of Ge-doped fibres is about 2.5 times higher than during neutron irradiation with the same dose rate, up to a fluence of about 3X10E12 1/cmE2 (14 MeV). This loss ratio decreases to <2.0 after about 10E13 1/cmE2 (14 MeV). Monte Carlo calculations of the dose enhancement as caused by fast recoil protons out of the H-containing acrylate coating are confirmed experimentally. 14 MeV neutron irradiation up to <10E13 1/cmE2 seems to decrease fibre breaking stress by 3 - 4 per cent, whereas Co60 gamma irradiation up to the same total dose has no effect.