Recovery and dose rate dependence of radiation damage in scintillators, wavelength shifters and light guides

The radiation damage induced by γ-rays in scintillators (mainly SCSN-38 with a polystyrene base), wavelength shifters (Y7 and K27 in PMMA) and different PMMA-base light guides and its dose rate dependence has been studied under different experimental conditions (doses D=103-105 Gy, dose rates D=(30-1000) Gy/h, gas atmospheres: Ar, N2, air). The production of new absorption centers and the deterioration of fluor molecules are the main effects caused by radiation, leading to a reduction of the primary light yield and of the attenuation length. The post-irradiation behaviour after short-term irradiation was compared with the behaviour during long-term irradiations. In the polystyrene base plastic scintillator SCSN-38 the radiation induced absorption recovers rapidly during and after irradiation, if the samples are in contact with oxygen. In different materials (light guides GS 218, GS 233, wavelength shifters Y7 and K27) with a PMMA matrix we find strong recovery in air after irradiation, but surprisingly no recovery during long-term irradiations at medium dose rates. In all investigated polymers the observed radiation damage is dose rate dependent. In addition it is shown that the PMMA-base scintillator used in the AFS calorimeter behaves completely different from all other materials studied in the present experiment. (orig.)