The crystals are forming a sphere of 960 mm outer diameter that covers 77% of the 4 p sr solid angle. Ten of the 240 crystals, at each side, enclose the beam entrance and its symmetric counterpart . They are used to identify lateral shower losses and therefore referred to as veto crystals. CsI offers a low Molière radius of 3.8 cm and thus small lateral distribution of showers. With a high Z and relatively high density its radiation length, which amounts 1.85 cm, is small and therefore the calorimeter provides a good stopping power for both positrons and photons. Undoped CsI was used due to its fast response time.
Due to the importance of the shower calorimeter its geometry, as well as the properties of the CsI crystals, are described in the following chapter.