A radiation sensor device is disclosed for use with a radiation source, capable of emitting radiation with photon energies larger than the work function of the target comprising a target plate to be impacted by the radiation to generate photo-electrons, the target plate being electrically isolated from a shielding electrode. The shielding electrode is arranged to collect energy-filtered photo-electrons from the target plate, using an electrostatic barrier for the filtering. The target plate is constructed of a carbon material. A current measurement device is operative to keep the target plate at a preset voltage difference with respect to the shielding electrode and measure a photo-electron deficit current as a result of radiation impact on the target plate.