Radiation study of Lead Fluoride crystals for the Crilin calorimeter

Lead fluoride ($PbF_{2}$) crystals represent an excellent and relatively innovative choice for high resolution electromagnetic calorimeters with high granularity and fast timing. During the R\&D stages of the Crilin calorimeter, three \pbfd crystals sized $5\times 5 \times 40 $ mm$^3$ were irradiated with $^{60}$Co photons up to $\sim 4$ Mrad and with 14 MeV neutrons up to a $10^{13}$ n/cm$^2$ total fluence. Their loss in transmittance was evaluated at different steps of the photon and neutron irradiation campaign, and two optical absorption bands associated with the formation of colour centres were observed at $\sim 270$ nm and $\sim 400$ nm. Natural and thermal annealing in the dark, along with optical bleaching with 400 nm light, were performed on the irradiated specimens resulting in a partial recovery of their original optical characteristics.