The accuracy of the lead tungstate electromagnetic calorimeter of the CMS experiment under
construction at CERN relies, among other things, on the correction of the calibration parameter from
variations dues to crystal ageing. This ageing will be measured by a so-called monitoring system, but
the relation between monitoring and calibration parameter variations is not so trivial, and depends
much on the overall optical characteristics of crystal and photodetector. We present here simulations
done with the program Litrani based on real ageing data for a realistic CMS crystal with a defined
surface quality (optically polished, with or without one lateral face slightly depolished), covered by
coatings of various characteristics, from totally absorbing to nominal aluminum or diffusing medium.
The correlation coefficient between monitoring and scintillation signals depends greatly on these
characteristics, and varies between about 1.3 and more than 10 (the optimum being one). |
