Radiation damage measurements of undoped lead tungstate crystals for the CMS electromagnetic calorimeter at LHC

A measurement method is described which allows to accurately track the scintillation light yield changes of lead tungstate crystals. The method has been used to study crystals to which post-growth high-temperature annealing treatments in various atmospheres were applied; it has allowed to select the most radiation resistant crystals and thus to optimize the annealing process. We describe the irradiation facility, irradiation procedure and results obtained. This work has led to the production of radiation resistant 50 mm long crystals and the process is now being extended to 230 mm long crystals.