Observation of scattering and absorption centers in lead fluoride crystals

For the first time, lead fluoride is used as a fast and compact material in electromagnetic calorimetry. Excellent optical and mechanical properties of the pure Cherenkov crystals are necessary for the A4 Collaboration to perform a measurement of the nucleon's strange form factors. Visible scattering and absorption centers as well as surface damages have been investigated to characterize the quality of more than one thousand crystals. Besides, transmittance measurements have been performed on all crystals to reveal absorption bands produced by intrinsic or impurity related point-structure defects. As a consequence, 89 crystals had to be replaced by the Chinese manufacturer SICCAS.