Physical origin of coloration and radiation hardness of lead tungstate scintillation crystals

The prevailing role of defect clusters, WO3−x, of variable composition in the coloration of scintillation crystals PbWO4 (PWO) and mechanisms underlying this process are revealed. Comparative studies of the influence of irradiation and reductive annealing on the optical properties of the material show that both treatments cause variation of tungsten valency in the WO3−x clusters that results in a variation of the optical absorption spectrum and, consequently, in the color of the entire PWO crystal. Irradiation causes reversible redistribution of electrons inside the cluster, while the reductive annealing initiates migration of oxygen atoms out of the crystal which makes the coloration unrecoverable. The negative influence of intentionally introduced variable-valency element (niobium) on the radiation hardness as well as possibilities for improvement of the radiation hardness by growth optimization based on the model proposed are demonstrated.