Measurements of lead tungstate crystals behaviour under irradiation for the CMS electromagnetic calorimeter at the LHC

This brief article presents the issue of the radiation hardness of the crystal that will fit the electromagnetic calorimeter (ECAL) of the compact muon solenoid (CMS). CMS is one of the 2 detectors that are foreseen to study hard proton-proton collisions at a center-of-mass energy of 14 TeV in the LHC. At the highest design performance of the LHC, one expects maximum dose rates of electromagnetic radiation around 0.18 Gy/h in the ECAL barrel, it means that the integrated dose over 10 years of service will near 2.5 kGy. The hadron flux in the calorimeter is almost entirely due to neutrons, and is expected to reach 2*1013 n/cm2 in the ECAL barrel after 10 years of running at LHC. However no damage by neutrons is expected while an effect is observed with photon irradiation. Electromagnetic radiation causes the formation of colour centers, which reduce the crystal transparency. The crystal producers are working systematically to reduce the number of defects in the crystal lattice