First results on the performance of the PADME electromagnetic calorimeter

The PADME experiment, hosted at the Laboratori Nazionali di Frascati in Italy, is dedicated to the dark photon search. It looks for the reaction e(+) e(-) -> A' gamma, where A' indicates the dark photon. The 550 MeV positrons beam that impinges on electrons, coming from an active diamond target, allows to scan A' masses up to 23.7MeV. In this context, the segmented electromagnetic calorimeter plays a fundamental role, since it measures the final photon fourmomentum. It consists of 616 BGO crystals displaced in a cylindrical shape of approximate to 29 cm radius with a central square hole of 5 crystals side, to let Bremsstrahlung radiation pass, to limit the calorimeter trigger rate. Each crystal is read by a HZC XP1911 type B photomultipliers and their signal is digitised by means of a CAEN V1742 board, taking 1024 samples at 1 GS/s. Here we report in detail the solutions adopted for the calorimeter together with the results obtained in tests performed on a small prototype and on single scintillating units.