The electromagnetic calorimeter of CMS

The electromagnetic calorimeter of CMS at the Large Hadron Collider (LHC) has been designed to measure the energy electrons and photons with high resolution over a wide energy range. The detector consists of 61200 PBWO4 crystals for the barrel and 14648 crystals for the endcaps, with a pseudo-rapidity coverage of vertical bar eta vertical bar <= 3.0. Preshower detectors are located in front of each endcap. The calorimeter has a pseudo-projective geometry. Great care has been taken in its design to minimize the effects of gaps and cracks in order to ensure the required energy resolution. The detector will operate in a magnetic field of 4 Tesla, and with a bunch crossing rate of 40 MHz. This requires fast and radiation hard crystals, photo-detectors and read-out electronics. An overview of the design, and the status of the construction, will be presented with special consideration given to the challenging goals.