Bench tests with La-doped PbWO4 crystals

Bench tests of 20 lanthanum-doped PbWO4 crystals with dimensions 23.8×23.8×200mm3, delivered in 1997 from BTCP, Bogoroditsk, demonstrated excellent qualities. Together with an optimized optical readout 1240 photoelectrons/GeV were achieved. More than 99% of the scintillation light signal was emitted within 100ns. Online measurements during low dose rate irradiations showed 20% transmittance loss through 20cm length after only a few Gray but at this value the damage was already saturated. Annealing times were long enough to allow a reasonable calibration correction of crystals in calorimeters, when irradiation rates were constant. However, sharp changes in irradiation rates require a corresponding rate increase in the re-calibration of the calorimeter. An observed correlation between the radiation-induced absorbance and the transmittance before irradiation at 380nm could help to find radiation stable crystals without copious investigations. High dose rate irradiations confirmed the low dose rate results. Although the properties of the 20 crystals differ relatively strongly from one another they fulfil, in contrast to prior crystals, the stringent demands for a small compact calorimeter at ZEUS/DESY which in the meantime has been confirmed by beam tests. Although the 1997 crystals showed a marked improvement over previous batches, newer crystals may be better still.