|Light collection in the CMS lead tungstate crystals : relation between monitoring and calibration variations|
Abstract: ========= 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. Important slopes could be obtained when the crystal ends have a different behaviour for monitoring and scintillation lights, like in the case of bare surfaces. As anticipated, the correlation slope tends toward one, when absorption is low and both monitoring and scintillation lights are well contained in the crystal, and thus independently of the geometry (front or back) of the monitoring light injection.