High-energy proton induced damage in PbWO4 calorimeter crystals

Eight production quality PbWO4 crystals of CMS have been irradiated in a 20 GeV / c proton beam up to fluences of 5.4 × 10 13 cm - 2 . The damage recovery in these crystals has been followed for over a year. Comparative irradiations with 60Co photons have been performed on seven other crystals using a dose rate of 1 kGy/h. In proton-irradiated crystals the light transmission band-edge shifts and the induced absorption length μ IND ( λ ) ∝ λ - 4 . In γ -irradiated crystals the band-edge does not shift but formation of absorption bands is seen clearly. The absorption length induced by γ -radiation in crystals verified to have excellent radiation hardness, saturates at a level below 0.5 m - 1 . In the case of protons, we observe no correlation with the pre-characterised radiation hardness of the crystals and the induced absorption increases linearly with fluence. After a fluence of 5 × 10 13 cm - 2 , an induced absorption length of ∼ 15 m - 1 is seen with no sign of saturation. These observations provide strong evidence that high-energy protons create damage that cannot be reproduced with γ -irradiation. However, these hadronic effects manifest themselves only at integral fluences well beyond 10 12 cm - 2 and most likely would escape undetected at lower fluences. A large fraction of the damage, both in proton- and γ -irradiated crystals, is either stable or recovers very slowly.