Modelisation and simulation of the light collection in the CMS lead tungstate crystals

The ageing of scintillating crystals under radiation leads to a deteriorati on of their transparency, thus of their effective light yield and their cal ibration. The link between the transparency variation, as measured by a fib re optic monitoring system, and the calibration variation is not trivial. A model describing the light collection process is proposed. It helps to und erstand the parameters which govern this correlation, mainly absorption len gth and light back-reflection or diffusion characteristics. This model desc ribes well the results of numerical qualitative simulations of light collec tion in lead tungstate crystals. It also explains the differences between c alibration and monitoring variations observed in test beam. (C) 2000 Elsevi er Science B.V. All rights reserved.