The electromagnetic calorimeter of the BELLE II detector for experiments at SuperKEKB $e^+e^-$ collider is described. A core part of the calorimeter, 8736 counters based on CsI(Tl) crystals read out by PIN photodiodes, is reused from the Belle detector which operated at the KEKB asymmetric energy collider from 1999 to 2010. Since the project luminosity of the SuperKEKB, $8\times 10^{35}$, is 40 times higher than that of the previous collider, much more severe background conditions are expected. Therefore all readout electronics will be replaced to a new one that will be able to cope with high event rate. A second step of the upgrade when the crystals in the end caps are replaced to the fast pure CsI crystals is under study. Since the photon emission of undoped CsI crystal is roughly 10 times smaller than that of doped one, the photosensors with amplification should be used. Current baseline option is vacuum photopenthods, their characteristics and performance study is presented. An option of a read out with the large area APD is also under study.

Budker Institute of Nuclear Physics, Novosibirsk State University