Shielded focusing electrode assembly for a photomultiplier tube

Abstract

An improved photomultiplier tube comprises an evacuated envelope having a faceplate extending across one end thereof. A photoemissive cathode is disposed on the interior surface of the faceplate. A support electrode having a centrally located aperture therethrough is spaced from the faceplate. An electron multiplier assembly is attached to the support electrode. A focusing assembly is disposed about the centrally located aperture in the support electrode on a side of the support electrode opposite the electron multiplier assembly. The focusing electrode assembly comprises an insulating member having a generally tubular body with an interior surface and an exterior surface and having a proximal end and a distal end. A top-cap, having a substantially U-shaped cross-section including a flat central base and two mutually parallel projections at the ends of the base, is attached to the distal end of the insulating member. The parallel projections are directed toward the support electrode and extend along at least a portion of the interior and exterior surfaces of the insulating member. At least one antimony evaporator for forming the photocathode is disposed adjacent to the focusing assembly. One of the projections at the end of the top-cap shields the exterior surface of the insulating member from antimony deposition and thus prevents electrical shorting of the focusing assembly to the support electrode. The other projection shields the interior of the insulating member from impingement by photoelectrons from the photocathode and thereby prevents electrical charging of the insulating member. A focus potential is applied to the top-cap of the focusing electrode assembly to focus the photoelectrons into the electron multiplier assembly.