Maintaining low radiation damage of lead tungstate scintillation crystals operating in high dose rate radiation environment

Scintillation crystals of the lead tungstate family - PWO, PWO-II - became widely used in electromagnetic calorimeters in high energy physics experiments at high luminosity accelerator facilities. During the operation of electromagnetic calorimeters a damage of the optical transmission of the crystals occurs due to creation of color centers. In addition to the recharge of a priori in the crystals existing defects by γ-radiation additional damage of the crystal matrix occurs due to hadrons. Therefore, radiation induced optical absorption can limit the energy resolution of the calorimeter. We have minimized radiation damage effects by technological efforts to obtain perfect crystals with a minimal concentration of the defects creating metastable color centers as well as the recoven´ of color centers by stimulation with infrared light. In this paper we discuss the method of stimulated recovery of radiation induced absorption which can be even applied in situ. The mechanisms of the damage under γ- and hadron irradiation are described.