Luminescence characteristics of lead tungstate (PbWO4) scintillation crystal doped with fluorine anions

Lead tungstate (PbWO4, shorten as PWO) crystal has attracted much interest as a very dense, fast and radiation hard scintillator. However, its application is strongly hampered by its poor light output. At present, great efforts have been devoted to improve the scintillating efficiency of PWO crystals. In this paper, fluorine anions were used as dopant to improve the light output of PWO crystals. The crystals were grown by modified Bridgman method. The charge was obtained from PbO and WO3 powder with purity of 99.99% and 99.999% respectively. The transmittance spectra measured with a spectrophotometer Shimadzu UV-2501PC reveal that the transmission of PWO: F is much higher than that of undoped PWO, especially in the short wavelength region (330-500 nm). The most significant characteristic of the F-doped PWO is that two emission components can be identified in their photoluminescence spectra, a fast component related to the blue emission (419 nm) and a slow component related to the green emission (553 nm). The measured light yield of PWO:F, based on the pulse height spectra stimulated by Cs-137, is as high as 2 to 3 times of the undoped PWO and meanwhile will be increased with the integration of time gate. This means that the significant contribution to the light yield of PWO may come from the green luminescence. The decay constants excited by Na-22 gamma ray can be fitted into three components, they are tau(1) = 2.68 [57.5%], tau(2) = 47.6 [31.0%] and tau(3) = 183 [11.5%]. 88.5% of total scintillating light decays within 50 ns. However, the distribution of light yield along the crystal axis is not uniform, i. e. higher at the tail end than that in the seed end. It is suggested that the blue emission is ascribed to the regular lattice centers, namely [WO4](2-) and the green one to a defect [WO3 + F] cluster. The significant defects in PWO lattice induced by F-anion should be responsible for the increase of the light yield.