Suitability of type-II fiber Bragg gratings in sapphire optical fiber for temperature sensing in treat

Sapphire optical fiber, with an internal cladding and an array of type-II Bragg gratings inscribed in it, was tested in-situ in the Ohio State Research Reactor (OSURR) to determine the viability of using sapphire as a sensor in an ionizing radiation environment. The sapphire fiber was attached to an optical frequency domain reflectometer (OFDR), which recorded the temperature of the fiber, at the locations of the gratings along the fiber, during the irradiations in the OSURR. The sapphire Bragg gratings survived the irradiations in the OSURR and produced reasonable temperature measurements for ~2.1 Equivalent Full Power Hours (EFPHs) of irradiation, corresponding to a neutron fluence of ~1.7 x 10¹⁷ n/cm². The lead-in silica fiber, and perhaps the sapphire fiber itself, exhibited darkening, which affected sensing during the third day of irradiation; but adjusting the sensitivity of the OFDR corrected for this. During the reactor irradiations on the following day, the fiber produced reasonable temperature measurements to a four day total irradiation of ~8.8 EFPH, corresponding to a neutron fluence of ~7.3 x 10¹⁷ n/cm². In summary, the sapphire sensors survived to fluences that are larger than those that they must withstand for testing in TREAT (~1 x 10¹⁷ n/cm²). The accuracy and precision of these sensors still needs to be determined. Also, it is must be acknowledge that fiber darkening in silica and sapphire may be flux dependent.