Electromagnetic Calorimetry, ECL

The main purpose of the electromagnetic calorimeter is the detection of photons from $B$-meson decays with high efficiency and good resolutions in energy and position. Since most of these photons are end-products of cascade decays, they have relatively low energies and, thus, good performance below 500 MeV is especially important. Important two-body decay modes such as $B \rightarrow K^*\gamma$ and $B^0 \rightarrow \pi^0\pi^0$ produce photons energies up to 4 GeV and good high resolution is needed to reduce backgrounds for these modes. Electron identification in Belle relies primarily on a comparison of the charged particle momentum and the energy deposits in the electromagnetic calorimeter. Good electromagnetic energy resolution results in better hadron rejection. High momentum $\pi^0$ detection requires the separation of two nearby photons and a precise determination of their opening angle. This requires a fine-grained segmentation in the calorimeter.
In order to satisfy the above requirements, we have decided to use a highly segmented array of CsI($Tl$) crystals with silicon photodiode readout installed in a magnetic field of 1.5 T inside a super-conducting solenoid magnet [4]. CsI($Tl$) crystals have various nice features such as a large photon yield, weak hygroscopicity, mechanical stability and moderate price.