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 o f the pure Cherenkov crystals are necessary for the A4 collaboration to per form a measurement of the nucleon's strange form factors. Visible scatterin g and absorption centers as well as surface damages have been investigated to characterize the quality of more than one thousand crystals. Besides, tr ansmittance measurements have been performed on all crystals to reveal abso rption bands produced by intrinsic or impurity related point-structure defe cts. As a consequence, 89 crystals had to be replaced by the Chinese manufa cturer SICCAS.