Lead tungstate crystals for the ALICE/CERN experiment

Technological conditions favorable for fabrication of PbWO4 (PWO) single crystals to be used as scintillators in high-energy physics experiments is under study. Importance of phase transformations in the PbWO4 crystal at all stages of fabrication, including preparation of raw materials, crystal growth, intentional doping, and after-growth annealing is revealed. We put forward a suggestion that scintillation properties and their variations in dependence of fabrication conditions are mainly caused by formation of inclusions of different tungsten oxides (WO3, intermediate stable oxides WO2.96, WO2.8−2.88, WO2.7−2.75, WO2) or their combinations in the PWO crystal. Doping of the melt for growth of PWO monocrystals with L2O3-type oxides of L=Y, La, Gd replaces WO3−x (x ranging from 0 to 0.3) by a phase W1−yLyO3−x, where L bounds oxygen and prevents its escape. With these results taken into account, the technology of fabrication of PWO scintillators with reproducible and stable characteristics is implemented on industrial scale at the “North Crystals” company. The PWO scintillator crystals fabricated by this company for the CERN project ALICE have dimensions of 22×22×180 mm3 and the following characteristics: light yield of >8.5 phe/MeV, inhomogeneity of light yield along the crystal of <5%, and longitudinal transmission of 10%, 55%, and 70% at wavelengths of 350, 420 and 600 nm, respectively.