RECOVERY AND PERMANENT RADIATION DAMAGE OF PLASTIC SCINTILLATORS AT DIFFERENT DOSE RATES

The radiation stability of plastic scintillators and wavelength shifters for the calorimeter of the ZEUS detector has been investigated by irradiating them with protons, a s°Cc+source, and depleted uranium. Changes in light yield, absorption length and absorption coefficient have been measured for storage in inert and oxygen atmospheres during and after irradiation. Radiation doses up to 40 kGy with dose rates of 30 up to 2000 Gy/h have been applied. The polystyrene based scintillator SCSN-38 and the wavelength shifters Y-7 and K-27 in PMMA show a strong additional absorption but a recovery in air to a low permanent damage (at 10 IrGy) which is proportional to the applied dose. Series investigations on samples of all production cycles of the ZEUS scintillators with high dose rates show only minor differences in radiation hardness. The recovery can be well described by a simple oxygen diffusion model for high and medium dose rates down to 30 Gy/h. During long term irradiations at low dose rates (< 100 Gy/h) of 3 mm thick SCSN-38 in air the radiation damage recovers to a permanent damage which does not depend on the dose rate. On the other hand the radiation damage at very low dose rates (17 Gy/a) seems to be higher than expected for the accumulated dose.