The Electromagnetic Calorimeter (ECAL)

The Electromagnetic Calorimeter (ECAL), see Figure 1, has three-dimensional imaging capability with a depth of 17 radiation lengths.  This allows for an accurate measurement of the electron-positron energy $E$ and shower shape.  The ECAL consists of a multilayer sandwich of 98 lead foils and 50,000 scintillating fibers.  The calorimeter is composed of nine superlayers, each 18.5 mm thick and made of 11 grooved, 1 mm thick lead foils interleaved with ten layers of 1 mm diameter scintillating fibers (the last foil of the last superlayer is made of aluminum).  In each superlayer, the fibers run in one direction only. The 3D imaging capability of the detector is obtained by stacking alternating superlayers with fibers parallel to the x and y axes (five and four superlayers, respectively).  The fibers are read out on one end by 1296 photosensors with a linearity of 1 in 105 per sensor.  The energy resolution has been measured to be $\sigma(E)/E=((0.104)^{2}/E+(0.014)_{2}))^{1/2}$ where $E$ is in GeV.

Figure 1. The construction and energy resolution of the ECAL.