First steps towards a distributed optical fiber radiation sensing system
Ionizing radiations affect installed electronics, limit equipment lifetimes and eventually alter materials both in space applications as well as high energy accelerators and physics experiments. In order to monitor radiation levels and predict equipment and material lifetimes, an accurate radiation dosimetry is highly important and very challenging often due to two main reasons: (i) large areas and (ii) extended dose range of interest. In this respect a radiation sensor based on a distributed optical fiber sensing system could be a promising solution. Such sensor systems are immune to electromagnetic field interference, have a light weight and small size making them suitable for a large number of applications including possible future space missions. In addition, optical fiber sensors have reached a very high accuracy in measuring physical quantities as a function of distance very accurately with metre or sub-metre-scale spatial resolutions. Their distributed nature in fact, is particularly suitable for real-time monitoring of long distances as particles accelerator tunnels like the one of the Large Hadron Collider (LHC) or large space stations as the International Space Station (ISS) which, to the best of our knowledge, only holds punctual radiation sensors as passive radiation dosimeters (PRD) among many others. The first step of the feasibility study of a distributed optical fiber radiation sensing system consists in characterizing and selecting the most suitable optical fiber which represents the sensing mean of the system. The effect of ionizing radiation on optical fibers has been well documented in literature and already several decades ago it was known that an irradiated fiber will suffer from radiation induced attenuation (RIA). Depending on the fiber's type and composition the RIA shows varying response to total dose, dose rate, temperature, wavelength and light power. In the case of P-doped fibers, the radiation response is independent from the dose rate the fiber is exposed to, which makes it suitable for sensing a wide range of different radiation environments. Moreover, P-doped fibers have a low annealing behavior and their RIA doesn't show any strong temperature dependence for room temperature environments. All the above mentioned parameters affecting the RIA are important in order to fully characterize fiber candidates to be used for dosimetry and will be detailed in the next section. Based on the above considerations, in this paper we investigate the response to ionizing radiation of three different P-doped optical fibers which could be suitable candidates for the radiation sensing system. Two of the fibers were single mode (SM) and the RIA has been studied at 1312 nm and 1570 nm as a function of the total dose up to about 18 kGy and with a dose rate of 46.7 mGy/s in one case and almost 60 kGy at 153 mGy/s in the other. The third fiber which was a multimode (MM) one has been tested at 830 nm and 1312 nm and has been irradiated with dose rates ranging from 0.5 mGy/s up to almost 1.7 Gy/s reaching almost 60 kGy. The characterization of the three fibers has been carried out at Fraunhofer INT using a 60Co source. Finally, distributed measurements have been carried out by means of an Optical Time Domain Reflectometer (OTDR) aiming for a first application test setup.
European Organization for Nuclear Research (CERN), Switzerland / Scuola Superiore Sant'Anna (SSSA), Italy