On-line characterization of gamma radiation effects on single-ended raman based distributed fiber optic sensor

We report distributed temperature measurements based on Raman scattering performed during steady state $\gamma $-ray irradiation at a dose rate of 1 kGy($\rm SiO-2$)/h and up to a total ionizing dose (TID) of $\sim 0.1\ \hboxMGy$. We characterize on-line the evolution of the performances of a single-ended Raman distributed temperature sensor (RDTS) during the $\gamma $-ray exposure of different classes of commercial multimode fibers (MMFs) acting as the sensing element. RDTS is influenced by the radiation-induced attenuation (RIA) phenomena leading to both large errors in the temperature measurements and a diminution of the useful sensing length. The amplitude of the radiation-induced temperature error strongly depends on the fiber choice and on the irradiation conditions. For the single-ended RDTS operation in the targeted Cigéo application the selection of a radiation tolerant sensing fiber will be mandatory, but not sufficient, to overcome the expected severe ambient conditions around radioactive wastes. For efficient temperature sensing up to an accumulated dose of 0.1 MGy, pre-irradiation of the selected radiation resistant (RR) fibers appears also necessary to improve the sensor performances.