A sensor is adapted to sense the intensity of an electron beam generated by an electron beam generator and exited from the generator through an exit window along a path towards a target within a target area. The sensor comprises at least one area of at least one conductive layer located within the path and connected to a current detector. The area, or areas, of the at least one conductive layer are shielded from the surrounding environment and from the exit window (and from one another when there are more than one area) by a shield. The shield is formed on the exit window. The sensor forms a part of a sensing system.