Impairment of optical properties and change of fluorescence of plastic scintillators by X-rays

Radiation resistance of plastic scintillators becomes more and more important in high energy physics, especially calorimeters close to beampipes are exposed to a high radiation background. During irradiation the used polymers form color centers absorbing visible and ultraviolet light and so reducing the light yield. This leads to a decrease in energy resolution. The work on hand investigates the formation and decay of such absorption centers. The absorption behaviour of two lightguide and four scintillator fibers was observed during and after irradiation with 100 keV X-rays in air by means of transmission spectroscopy. The light guides BCF-98 (Bicron) made of polystyrene and FL-51 (Hoechst) made of polymethylmethacrylate (PMMA) as well as the polystyrene scintillators SCSF-38, SCSF-81, SCSF-81(Y7) (Kuraray) und BCF-60 (Bicron) were investigated. A permanent radiation damage induced by stable absorption centers is well known. In the presented investigations, it rises linearly with dose in BCF-98 up to doses of 6 kGy, but the radiation sensitivity is larger by a factor of 13 than in independent experiments with higher irradiation doses. According to this, a linear extrapolation of radiation damages at high doses to radiation sensitivity of polymers at low doses is impossible. (orig.)