A silicon wafer has a plurality of spaced diode junctions formed in its upper surface and through openings in a silicon dioxide layer. A conductive grid is disposed over the silicon dioxide layer and has openings exposing the elemental diode surfaces. The top of the exposed diode surfaces are cesiated. Each of the diodes is reverse biased and the cesiated surfaces of the diodes emit electrons, appropriately adjusting their potential in response to radiation applied to the adjacent rear surface of the wafer.