Electron beams of Scanning Electron Microscope (SEM) and dual-beam Focused Ion Beam (FIB) have been used to characterize a Geiger-mode Avalanche Photodiode (GAPD) array. The main advantage of those electron beams is the spot-size unprecedent resolution compared to the laser beams typically used for the characterization of optical imagers. Operation mode is also different, as instead of generating electron-hole pairs due to photon absorption, electrons are generated by the high energy injected electrons. The GAPDs used have been fabricated with a conventional CMOS process. They are excellent for the purpose of this experiment because of their very high intrinsic gain and speed, giving rise, when properly biased, to a large output signal as a consequence of detected photons or ionizing particles. The detected counts due to injected particles have been correlated with Montecarlo simulations (Penelope) of the injected charge in the interaction region.