Performance of the PADME calorimeter prototype at the DA$Phi$NE BTF

The PADME experiment at the DA$Phi$NE Beam-Test Facility (BTF) aims at searching for invisible decays of the dark photon by measuring the final state missing mass in the process $e^+e^- to gamma+ A$, with $A$ undetected. The measurement requires the determination of the 4-momentum of the recoil photon, performed using a homogeneous, highly segmented BGO crystals calorimeter. We report the results of the test of a 5$times$5 crystals prototype performed with an electron beam at the BTF in July 2016.