ELECTROSTATIC MICROFOCUSING LIMITS FOR POSITRON BEAMS

Electrostatic condensing lenses of very short focal length were invest igated via trajectory calculations with the goal of determining the mi nimum spot diameter that could be formed by focusing positrons of nega tive work function emitted from a 100 mu m diameter portion of a Ni(10 0) foil re-moderator. The specific applications for which the calculat ions were carried out were brightness enhancement and transmission-mod e positron re-emission microscopy, in which positrons are accelerated to 5 keV onto the target, which also serves to terminate the electric field of the condensing lens. The minimum spot size that could be form ed was 3 mu m using a 1 mm diameter two-cylinder lens with a tapered s econd cylindrical element.