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 5.4×1013 cm. The damage recovery in these crystals has been followed for over a year. Comparative irradiations with Co 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 μIND(λ) ∝ λ. 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. 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 5×1013 cm, an induced absorption length of ∼15 m 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 cm and most likely would escape undetected at lower fluences. A large fraction of the damage, both in protonand γirradiated crystals, is either stable or recovers very slowly. Submitted to Elsevier Science