Radiation damage and recovery mechanisms in cooled PbWO4 crystals

The Electromagnetic Calorimeter (EMC) of the future PANDA detector relies on a stabile performance of the PbWO₄ crystals of enhanced PWO-II quality. In spite of the expected moderate radiation dose imposed by hadrons and electromagnetic probes during operation, the interplay between damaging and statistical recovery processes is strongly influenced by the low operating temperature of T=-25° C. Previous investigations have shown that relaxation mechanisms become extremely slow and the limit on the asymptotically reached light output depends on the intrinsic concentration of defect centers in the crystal. The process of stimulated recovery leads in case of damage due to γ-rays at room temperature to an almost complete recovery using even infrared light. At low temperature, the mechanism leads to a substantial recovery but on significantly longer time scales. The paper will discuss a series of detailed measurements in order to understand recovery mechanisms at low temperature and the sensitivity to different external light sources. Part of the program is EPR-based measurements to identify the population or extinction of well-known color centers. A second experimental set-up allows measuring the change of the optical transmission spectrum of a permanently cooled crystal in situ using a photo spectrometer. The transmittance over the relevant region of wavelength is controlled before and after irradiation as well as during the recovery with different external light sources. The studies of the damage due to a high flux of γ-rays are compared complementary to the impact of 90 MeV protons performed at the cyclotron at KVI (Groningen Netherlands). Up to a fluence of 6.75×10¹¹ protons/cm² protons will impinge and be completely stopped in the test samples.