Résumé du preprint DAPNIA/SED/99-02

Light collection in the CMS lead tungstate crystals : relation between monitoring and calibration variations
R. Chipaux
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


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