The Electromagnetic Calorimeter ( ECAL ) is a large device that can very precisely measure the energies of electrons, positrons, and high-energy photons. The design is complete and construction is underway. To detect electrons efficiently a material with a high atomic number is used; in ECAL specialized crystals of lead-tungstate. Lead ( Pb : #84 ) and tungsten ( W : #74 ) form an extremely dense clear crystal with oxygen ( PbW0₄ ) which scintillates, meaning the crystal will flash with light whenever some energy passes through it. The flash is rapid and faint, but specialized photo sensors called avalanche photo diodes can detect the light and produce an electronic pulse proportional to that light's energy.

The flash in the crystal is created by a charged particle or a neutral particle creating an electromagnetic shower passing through it. For example, a muon passes straight through and produces a very small pulse. An electron or positron, on the other hand, will shower and deposit a considerable amount of energy in the crystal, producing a detectable amount of light; a proton will also create an electromagnetic shower and a bright flash. The avalanche photon diodes then translate the light energy again, this time into a proportional current.

CMS Detector