We have studied the gamma dose-rate effect on the radiation sensitivity of a phosphorous doped optical fiber. The radiation sensitivity significantly increases with decreasing dose-rates.
KEYWORDS: Optical fibers, Temperature metrology, Single mode fibers, Environmental sensing, Environmental monitoring, Multimode fibers, Standards development, Reflectometry, Signal attenuation, Signal to noise ratio
Optical fibers stimulate much interest since many years for their potential use in various nuclear environments, both for radiation tolerant and EMI-free data communication as well as for distributed sensing. Besides monitoring temperature and stress, measuring ionizing doses with optical fibers is particularly essential in applications such as long-term nuclear waste disposal monitoring, and for real-time aging monitoring of power and signal cables installed inside a reactor containment building. Two distinct options exist to perform optical fiber dosimetry. First, find an accurate model for a restricted application field that accounts for all the parameters that influence the radiation response of a standard fiber, or second, develop a dedicated fiber with a response that will solely depend on the deposited energy. Using various models presented in literature, we evaluate both standard commercially available and custom-made optical fibers under gamma radiation, particularly for distributed dosimetry applications with an optical time domain reflectometer (OTDR). We therefore present the radiation induced attenuation at near-infrared telecom wavelengths up to MGy total dose levels, with dose rates ranging from about 1 Gy/h up to 1 kGy/h, whereas temperature was raised step-wise from 25 °C to 85 °C. Our results allow to determine and compare the practical limitations of distributed dose measurements with both fiber types in terms of temperature sensitivity, dose estimation accuracy and spatial resolution.
Over the past years, various attempts have been made to accurately model the radiation response of the optical attenuation in optical fibres for nuclear environments. In this paper we present the results of a study on pure silica fibres where both the spectral and kinetic behaviour are explored during and after irradiation in spent fuel gamma facilities. Basic first- and second-order kinetic models are considered, as they provide insight into fundamental dependencies on temperature and dose-rate. Other popular models, like the power-law and stretched exponential forms of the basic kinetic paradigms, are investigated with emphasis on the spectral dependence of the constituting parameters. Since the radiation induced attenuation is in general the sum of contributions from different absorption bands, related to underlying radiation induced defects, the spectral dependencies are also tackled by Gaussian resolution. With this method, the recorded absorption spectra in the range from 450 to 1600 nm are decomposed into individual absorption bands, with fixed positions and widths in the photon-energy domain for the course of the experiments. The resulting amplitudes from the non-linear estimation process are then evaluated with respect to the same models used as for the radiation induced attenuation at single wavelengths.
The potential for intrinsic dosimetry with three different fibre types through the gamma ray radiation induced attenuation is investigated with respect to annealing, temperature dependencies, photo-bleaching and, to a limited extent, dose-rate. The radiation sensitivity of these three fibres covers an effective dose range of 0.1 Gy to 100 kGy which makes them suitable for a broad range of nuclear applications. With detailed analysis in both the spectral and temporal domain, it is shown that the long-term annealing effects can be suppressed by exploiting the stability of certain colour centres in the fibre core and adequate interrogation of the induced attenuation at a discrete set of wavelengths. Photo-bleaching is observed by employing a low power white light source (a few microwatts in the fibre) with Erbium doped fibres and when Germanium is added as a co-dopant with Phosphorous fibres. Temperature dependencies during irradiation and annealing are addressed from room temperature to 80 ?C. The Phosphorous and Erbium doped samples exhibit a reverse behaviour: the optical absorption increases with temperature.
Nuclear industry may benefit from the implementation of Optical Fiber Sensor (OFS) technology. This is obvious if the OFS integrates features that meet specific nuclear sensing needs, such as distributed dose monitoring in underground radioactive waste disposals. The use of radiation sensitive fibers for dosimetry has already been proposed in literature. In this paper, we report on our gamma irradiation of such dedicated Ge-P co-doped and pure P-doped optical fiber, fabricated by the Fiber Optic Research Center in Moscow. We show that after a careful selection of the wavelength at which the radiation-induced attenuation is measured, it is possible to reconstruct the dose within 20% accuracy by means of a linear model or a second order polynomial, depending on the temperature.
Fiber optics is now widely used for data communication, sensing and vision applications. In the nuclear industry, however, its use is still quite limited. This paper reviews two particular areas where the optical fibers could bring substantial advantages: the decommissioning of shut- down nuclear installations, and the long term storage of nuclear waste. It summarizes some expected potential benefits, but points out also the challenges to be met for wider applications of fiber optics in a nuclear environment.
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