Radiation tolerant fiber optic humidity sensors for High Energy Physics applications

This work is devoted to the development of fiber optic humidity sensors to be applied in high-energy physics applications and in particular in experiments currently running at CERN. The high radiation level resulting from the operation of the accelerator at full luminosity can cause serious performance deterioration of the silicon sensors which are responsible for the particle tracking. To increase their lifetime, the sensors must be kept cold at temperatures below 0 C. At such low temperatures, any condensation risk has to be prevented and a precise thermal and hygrometric control of the air filling and surrounding the tracker detector cold volumes is mandatory. The technologies proposed at CERN for relative humidity monitoring are mainly based on capacitive sensing elements which are not designed with radiation resistance characteristic. In this scenario, fiber optic sensors seem to be perfectly suitable. Indeed, the fiber itself, if properly selected, can tolerate a very high level of radiation, optical fiber transmission is insensitive to magnetic field and electromagnetic noise and it is perfectly suited for read-out over very long distances. Although many different approaches to humidity measurements through optical fibers have been reported in literature, no commercial sensor of this family is available on the market, and only a few approaches have proved really conclusive performances for a practical application out of the laboratory environment. In 2011, our multidisciplinary research group has been involved in the development of new generation of relative humidity fiber opticbased sensors at CERN. Investigations were originally concentrated on polyimide-coated Fiber Bragg Gratings and gave for the first time the experimental demonstration of the possibility to use them at low temperatures as well as in presence of strong ionizing radiations. For this reason, this platform has been selected for the hygrometric control of the air in critical areas of the Compact Muon Solenoid experiment: 72 fiber optic-based thermo-hygrometers, organized in arrays, have been installed in the detector and are currently providing a full map of temperature and humidity in front of the tracker volume. Furthermore, a second generation of high-sensitivity titanium dioxide-coated Long Period Grating-based humidity sensors has been recently proposed. Preliminary results are very encouraging and are evidencing the strong potentialities of the proposed technology in light of its application in the particles tracking detectors of the future.