In situ gamma radiation-induced attenuation in sapphire optical fibers heated to 1000°C

The purpose of this work was to determine the suitability of using instrumentation utilizing sapphire optical fibers in a high-temperature gamma radiation environment. In this work, the broadband (500-2200 nm, or 0.56-2.48 eV) optical attenuation of commercially available sapphire optical fibers was monitored in situ during continuous gamma irradiation from room temperature up to 1000°C. The gamma dose rate of the irradiation facility was measured to be 370 rad/h (dose in sapphire). Results show rapid growth of an absorption band centered below 500 nm (the minimum detectable wavelength in this work) and extending as far as ∼1000 nm that reached a dynamic equilibrium after approximately 4 h of irradiation at room temperature. Increasing temperature generally reduced the added attenuation, although the added attenuation came to a nonzero equilibrium at temperatures of 1000°C and below. No peaks in the added attenuation were observed above 1000 nm, and the added attenuation at 1300 and 1550 nm remained less than 0.7 dB throughout the entire experiment. Fitting the attenuation spectra to Gaussian functions at each temperature revealed a large absorption band near 3 eV that monotonically decreased both in magnitude and width with increasing temperature. A second smaller band near 1.73 eV monotonically increased in both magnitude and width with increasing temperature, suggesting that some interconversion between defects may be occurring at higher temperatures.