OPAL's electromagnetic calorimeter system measures the energies and positions of electrons, positrons and photons. It is a total absorption calorimeter mounted between the coil and return yoke of the magnet. The barrel and endcap lead glass systems together cover 98% of the solid angle. Since particles must traverse about 2 radiation lengths of material (mostly due to the magnet coil and pressure vessel) before reaching the calorimeters, most electromagnetic showers start before reaching the lead glass. The presamplers are used to measure the amount of showering in the coil to improve energy resolution. In addition, their high granularity helps improve the neutral pion / photon and the electron / hadron discrimination.
Information about the OPAL detector
Information about the central tracking chambers
Information about the outer detectors