Analysis of the response evolution of the CMS electromegnetic calorimeter under electron and pion irradiation

The electromagnetic calorimeter of the CMS experiment consists of about 75000 PbWO_4 scintillating crystals. A monitoring system using laser light injection into the crystals is foreseen to follow variations in the light transmission through the crystals due to radiation exposure. The reliability of the monitoring system was demonstrated in the past on a restricted number of PbWO_4 crystals all belonging to the same production batch. To get more insight in the crystal behaviour under different irradiation conditions, during year 2003 additional PbWO_4 crystals in their final configuration were exposed to pion and electron beams providing a radiation dose comparable to the one expected in the ECAL barrel electromagnetic calorimeter at the LHC. The evolution of their response was monitored with a reference electron beam of 120 GeV/c momentum and compared to the response of the laser monitoring system. Here we report the results of the data analysis, which corroborates the reliability of the laser monitoring showing that the response loss observed under pion and electron irradiation is adequately followed by the monitoring system by means of an universal relation.