Radiation response behavior of high phosphorous doped step-index multimode optical fibers under low dose gamma irradiation

This paper revealed the role of various parameters on the radiation response behavior of P2O5 doped step-index multimode (SIMM) fibers having different content of P2O5 (12–6 mol%) at room temperature. Their suitability for use as a radiation sensitive fiber in fiber optic dosimeter was studied under 60Co-gamma irradiation at different dose rates 0.01–0 Gy/h. The suitable wavelength region is found to be 500–600 nm where the fibers shows the maximum radiation sensitivity due to formation of phosphorous–oxygen hole centers identified from their radiation induced absorption spectra. The influence of each parameter like the doping levels of P2O5, dose rates, doping region and energy of the radiation source on their sensitivities was examined. The radiation sensitivities recorded at 502, 540 and 560 nm depends strongly on P2O5 content of the fiber at low dose rates within 0.01–1 Gy/h. However, the fibers shows almost dose rate independent behavior at high dose rates (>1.0 Gy/h) under 60Co-gamma radiation source of energy 1.25 MeV with respect to all the three wavelengths. The fiber shows almost linear relation to the total dose up to saturation levels of 4.0 Gy at all dose rates. Their sensitivities could be explained through analyses of such type of glass as well as behavior of the radiation induced P-related defects generated in the light guiding core region. At low dose rates the fibers becomes more radiation sensitive compared to the high dose rates due to conversion of POHC centers to P1 (phosphorous E′) defect centers as observed from their induced loss curves during gamma irradiation. At low dose rates 0.1–1 Gy/h under 502 nm transmission wavelength the fibers shows an excellent linear relation with respect to Cs-137 radiation source of energy 0.662 MeV having average sensitivity of 1.0452 ± 0.0346 dB/m/Gy and very low fading behavior at room temperature. The results suggest that P-doped SIMM fiber (40 μm core diameter) containing 16 mol% P2O5 exhibit an excellent linear radiation response property of high sensitivity around 0.89 ± 0.09 dB/m/Gy at 502 nm wavelength with very low recovery and little dose rate dependence within 0.5–10 Gy/h region that makes them a very promising candidate as radiation sensor for use in fiber optic personal dosimeter to detect low dose gamma radiation of 0.002 Gy for human safety purpose.