Radiation hard polyimide-coated FBG optical sensors for relative humidity monitoring in the CMS experiment at CERN

This work investigates the performance and the radiationhardness capability of optical thermo-hygrometers based on FibreBragg Gratings (FBG) for humidity monitoring in the Compact MuonSolenoid (CMS), one of the four experiments running at CERN inGeneva. A thorough campaign of characterization was performed on80 specially produced Polyimide-coated RH FBG sensors and 80commercial temperature FBG sensors. Sensitivity, repeatability andaccuracy were studied on the whole batch, putting in evidence thelimits of the sensors, but also showing that they can be used invery dry conditions. In order to extract the humidity measurementsfrom the sensor readings, commercial temperature FBG sensors werecharacterized in the range of interest. Irradiation campaigns withionizing radiation (γ-rays from a Co60 source) atincremental absorbed doses (up to 210 kGy for the T sensors and upto 90 kGy for the RH sensors) were performed on sample of T andRH-Sensors. The results show that the sensitivity of the sensors isunchanged up to the level attained of the absorbed dose, while thenatural wavelength peak of each sensor exhibits a radiation-inducedshift (signal offset). The saturation properties of this shift arediscussed