Distributed fiber optic radiation sensors

The international research efforts focused on the development of radiation sensors based on optic fibers have their origins in the 1970s (Evans et al., 1978). Generally, the lightweight fiber optic sensors are immune to electromagnetic field interference and high voltages making them deployable in harsh environments at hard to reach areas where conventional sensors usually will not work at all. A further advantage of such radiation sensors is the possibility of remote and real-time monitoring (Huston et al., 2001). In this work, we present our results achieved in several research activities for development of fiber optic dosimeters. The findings show that both the measurement of the radiation-induced attenuation (RIA) along the entire sensing fiber and the accompanying change in the refractive index of the fiber core can be used for distributed radiation monitoring in the kGy and MGy range, respectively. Depending on the fiber type and material the RIA shows varying response to dose rates, environmental temperatures and the wavelength of the laser source used. Thereby, an operation with visible laser light provides most favorable performance in terms of high radiation sensitivity. Operating at these wavelengths, RIA monitoring could yield high-sensitivity dose measurement with sub-gray resolution and accuracy (Stajanca and Krebber, 2017b); however, conventional optical time-domain reflectometry (OTDR) systems for RIA measurements operating in the visible range suffer from low-spatial resolution, long measurement times and poor signal-to-noise (SNR) ratio. The limitations of the OTDR performance can be overcome by the incoherent optical frequency domain reflectometry (I-OFDR) developed by the Federal Institute of Materials Research and Testing (BAM, Liehr et al., 2009) with potential for dynamic real-time measurement. Over the years, several highly radiation sensitive fibers, such as perfluorinated polymer optical fibers (PF-POF, Stajanca and Krebber, 2017a), phosphorous-doped silica optical fibers (SOF, Paul et al., 2009), aluminium-doped SOF (Faustov et al., 2013) and erbium-doped SOF (Wosniok et al., 2016) have been identified and are commercially available. As mentioned before, the radiation-induced RIA increase is associated with an increase in the refractive index leading also to material compaction in the fiber core. The latter two effects can be used for measuring radiation distribution based on Brillouin scattering in the range of high radiation doses of several MGy (Phéron et al., 2012; Wosniok et al., 2016). When using fiber optic sensors for radiation monitoring, the existing post-irradiation annealing behavior of the optical fiber sensors must also be considered.