Scanning Electron Microscopy


A new type of high brightness thermionic cathode has been developed. The cathode utilises emission from a small flat surface (generally less than 50 μm diameter) prepared parallel to a specific crystal plane on a single crystal of Lanthanum Hexaboride. Emission from the rest of the single crystal is suppressed by the use of a high work function coating of pyrolytic graphite. When used in a conventional triode gun, the maximum total electron emission is controlled by the area of the micro-flat, the work function and the temperature of the emitter. The Wehnelt potential serves a minor role in controlling the divergence of the beam. At certain emitter height settings, the gun produces the maximum axial brightness at zero bias. The field at the surface of the micro-flat is higher than that for pointed emitter s in a conventional configuration and no longer limits the gun brightness. As the emitting region is now parallel to a specific crystallographic surface, the emission anisotropy of LaB6 can now be utilized in developing emitters of optimum brightness. The new sources reduce Wehnelt aperture contamination and offer long lifetimes under favourable vacuum conditions.

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