High-energy proton induced damage in Lead Tungstate calorimeter crystals

Eight production quality PbWO4 crystals of CMS have been irradiated in a 20 GeV/c proton beam up to fluences of 5.4E13cm-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 is proportional to the inverse of the 4th power of the wavelength. In gamma-irradiated crystals the band-edge does not shift but formation of absorption bands is seen clearly. The absorption length induced by gamma-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 5E13 cm-2, an induced absorption length of approx. 15m-1 is seen with no sign of saturation. These observations provide strong evidence that high-energy protons create damage that cannot be reproduced with gamma-irradiation. However, these hadronic effects manifest themselves only at integral fluences well beyond 1E12cm-2 and most likely would escape undetected at lower fluences. A large fraction of the damage, both in proton- and gamma-irradiated crystals, is either stable or recovers very slowly.