Transparency Variations and Calibration in CMS Electromagnetic Calorimeter Crystals: Monte Carlo Studies

The aging of CMS electronic calorimeter scintillating crystals under radiation leads to a deterioration of their transparency, and thus of their effective light yield and their calibration. The correlation between transparency variation, as measured by the fiber-optic monitoring system, and the calibration variation depends on the optical properties of the scintillating material itself and of its environment. In previous studies, the possibility of light scattering within the bulk of the crystals was not taken into account. We present here the results of the simulation of this correlation for different types of CMS electronic calorimeter crystals using the program Litrani improved to allow light scattering in bulk. They show that scattering could explain the variation of correlation coefficients observed between CMS/ECAL endcap and barrel crystals.