Optical anisotropy effects in lead tungstate crystals

The anisotropic crystal structure of lead tungstate leads to strong consequences in its optical properties and their characterization. Beyond the variation of surface reflections due to the birefringence of the material, we report the observation that part of the bulk light absorption is sensitive to the light polarization direction. This variable part varies with the quality of the crystal, and is clearly related to internal structural defects. Irradiation experiments with gamma rays or fast neutrons confirm the recent improvement of the short term resistance of this scintillator. Its long term behaviour remains identical, and acceptable for the use in high energy electromagnetic calorimetry. A polarization direction dependant induced absorption is observed, specially after high levels of radiation. Ray tracing programs in development for the simulation of the light yield in lead tungstate should take into account all these anisotropic effects.