The radiation-induced absorption band at 600 nm and its impact in fibroscopy

Pure silica core multimode step-index fibres with high OH and low OH contents were irradiated up to /spl ap/2.5 MGy in order to test their gamma-radiation resistance in the visible range. To achieve this dose, two successive irradiations at dose rates of /spl ap/0.3 kGy/h and /spl ap/24.0 kGy/h were conducted in the CMF gamma irradiation facility of SCK/spl middot/CEN, Mol, Belgium. The radiation-induced attenuation was measured continuously during and after irradiations from 400 nm to 1200 nm. The radiation-induced absorption band at 600 nm was observed in both types of fibre. A saturation of its growth occurred during the first irradiation and persisted during the next irradiation at higher dose rate. This might reveal a hardening phenomenon due to the first irradiation. Radiation-induced attenuation maximum values of 0.5 dB/m and 1.2 dB/m were found at 600 nm for the high-OH and low-OH types respectively. In fibroscopy this radiation-induced absorption band at 600 nm will be a limiting factor. However the radiation resistance improvement after a first irradiation would bring potential benefits.