Influence of variable tungsten valency on optical transmittance and radiation hardness of lead tungstate (PWO) scintillation crystals

A new approach to interpret the radiation hardness of PbWO4 (PWO) scintillators is developed by revealing importance of the inclusions of tungsten oxides WO3−x with variable valency. It is demonstrated that the influence of the ionizing radiation on PWO is, in many aspects, similar to the effect of the high-temperature annealing in oxygenless ambient. In both cases, a valency change of the tungsten oxides is initiated and results in induced absorption and, consequently, in crystal coloration. In the PWO crystals doped with L2O3 (L=Y,La,Gd), the radiation hardness and the optical properties are mainly affected by inclusions of W1−yLyO3−x (0<x<0.3) instead of inclusions of WO3−x prevailing in the undoped samples. It is demonstrated that the radiation-induced bleaching and the photochromic effect of PWO are caused by phase transitions in the inclusions of tungsten oxide. Thermodynamic conditions for the phase transitions are discussed and the optimal oxidation values for the growth of radiation-hard PWO are determined. The optimization is implemented on an industrial scale to grow PWO crystals with improved, stable and reproducible characteristics for the ALICE experiment at CERN.