Comparison of Radiation Damage Effects in PWO Crystals Under 150 MeV and 24 GeV High Fluence Proton Irradiation

Radiation damage effects induced by the hadronic part of ionizing radiation in experiments at high-energy and high-luminosity accelerators will play a significant role as limiting factor of the long term stability when operating an experimental setup. Measurements of the deterioration of the optical transmission of lead tungstate (PbWO 4, PWO) scintillation crystals were performed after irradiation with high and low energy protons. One sample with CMS specification was irradiated in 2010 with a 24 GeV/c proton beam at the CERN Proton Synchrotron (PS) with a flux of about 10⁹ p/(cm²s) up to an accumulated fluence of 3.0 10³ p/cm². Eight more crystals of PWO-II quality with dimensions of 2×2×5 cm³ were selected from the set of crystals produced for the PANDA electromagnetic calorimeter at the future FAIR facility at Darmstadt. Four crystals were produced by the Bogoroditsk Technical Chemical Plant (Russia) and the others by the Shanghai Institute of Ceramics (China). These samples were irradiated in 2012 with a 150 MeV proton beam at the AGOR accelerator at KVI (Groningen, The Netherlands) with a flux of 3 10⁹ up to an integral fluence of about 1.0 10¹² p/cm² and 1.8 10¹³ p/cm², respectively. Due to the proton induced activation of long-lived radioactive secondaries the optical inspection of all samples had to be performed several months after irradiation for safety reasons. However, the samples were kept continuously cold to minimize thermal recovery. Both irradiations produced a similar set of induced absorption bands. Moreover, a shift of the fundamental absorption edge at short wavelength appears even after irradiation with low energy protons. The contribution will show in detail the set of experimental data and discuss a possible mechanism for an understanding and interpretation of the observed effects.