The Compact Muon Solenoid experiment at the CERN LHC features a high precision PbWO4 crystal electromagnetic calorimeter. Each crystal is first precalibrated with a radioactive source and by means of optical measurements. After the assembly, each supermodule (1700 crystals) is exposed to comics rays.

The comparison between intercalibration obtained from cosmic muons and electrons from test beam was performed at the end of 2004 for an initial set of 130 channels and showed that a precalibration with a statistical precision of 1 to 2% can be achieved within approximately one week. An important aspect of the comics muons analysis is that it is entirely based on the calorimeter data, without using any external tracking device.

We will present the setup and results from the 2004 test as well as recent data recorded on many supermodule.

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