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 $e^{+}\,e^{-}\rightarrow A'\,\gamma$, where $A'$ indicates the dark photon. The $550\,\text{MeV}$ positrons beam that impinges on electrons, coming from an active diamond target, allows to scan $A'$ masses up to $23.7\,\text{MeV}$. In this context, the segmented electromagnetic calorimeter plays a fundamental role, since it measures the final photon four-momentum. It consists of $616$ BGO crystals displaced in a cylindrical shape of $\approx29\,\text{cm}$ 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 $1\,\text{GS/s}$. 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.