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

The ageing of scintillating crystals under radiation leads to a deterioration of their transparency, thus of their effective light yield and their calibration. The link between the transparency variation, as measured by a fibre optic monitoring system, and the calibration variation is not trivial. A model describing the light collection process is proposed. It helps to understand the parameters which govern this correlation, mainly absorption length and light back-reflection or diffusion characteristics. This model describes well the results of numerical qualitative simulations of light collection in lead tungstate crystals. It also explains the differences between calibration and monitoring variations observed in test beam.