Untersuchungen zur Strahlungsresistenz polymerer Materialien für den Einsatz in Experimenten der Hocheneriephysik

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Radiation stability of scintillators and light guides is of fundamental importance for experiments in high energy physics, because detectors have to provide highty reliable and precise measurements over several years. This dissertation descnbes changes due to irradiation in optica) properties of wave length shifter tnaterials based on polymethylmethacrylate (PMMA) and ofscintillator materials based on polystyrene (PS). These polymers are investigated after irradiation in $\gamma^-$ and neutron fields under different conditions of absorbed dose, dose rate and surrounding atmosphere. This work presents studies of the dependencies of optical damage such as creation of absorption Centers or destruction of colour dyes by the absorbed radiation dose In much of the applied dose range from l kGy to 100kGy a linear dependence of damage levels on the absorbed dose is observed. The influence of additives in the materials is also reported. The presence of oxygen before, during and after irradiation plays an important role in optical damage intensity and development. On the one band, higher degradations can be detected in some cases if oxygen is present during irradiation compared to damage levels without oxygen. On the other hand, the presence of oxygen after an irradiation leads to a clear decrease of the initial damage level, which is observed directly after the end of irradiation, to a final damage level constant in time. The duration of the recovery process from the inital to the final damage heigth is determined by thediffusion of oxygen into the material. For approximately 2mm thick plates of PS based materials the recovery process takes some days at Standard temperature and pressure, for PMMA based materials of the same thickness some 100 days of recovery time is observed under the same conditions. By means of a diffusion model for both materials a dose rate can be calculated below which complete recovery to the final damage level occurs during irradiation. For PS based scintillators (2 6mm thick) the dose rate for complete recovery is 57Gy/h, for PMMA based wave length shifters {2mm thick) it is 0.44Gy/h. For the ZEUS calorimeter operating at the electron proton collider HERA at DESY in Hamburg in an estimated dose rate level of 0.04 Gy/h in air atmosphere, a recovery to final damage values during Operation can be concluded. A specially developed algorithm and Computer code allows to simulate light propagation throughout rectilinear parallelpipeds of scintillators and light guides. Results obtained from relatively compact lest samples can be applied to the langer geometries of optical components in the ZEUS calorimeter. Calculations for the ZEUS calorimeter which will absorb 3 kGy dose during an operating time of 10 years, show that calorimeter performance will not be significantly affected by radiation damage

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