The point cathode as an electron source

Abstract

The electron source usually used in the electron microscope and other electron optical apparatus is the hair-pin cathode, producing a beam by thermionic emission. The use of a cold point cathode, as employed in the field emission microscope [Müller (1) (22)], promises certain advantages [Marton (2); Pattee (3)] owing to its very high specific emission. As compared with the value of about 1 Amp/cm² given by a hot cathode in normal operating conditions, the point cathode can give 10⁴ Amp/cm² continuously and as much as 10⁸ Amp/cm² in pulsed operation [Dyke and coworkers (7)], but only in very high vacuum. If it is heated to a temperature of 1500°–2000° C, the point can be used in poor vacuum; the emission is then of the Schottky type, intermediate between field and thermionic emission, and has come to be termed T-F emission [Dolan and Dyke (4)]. The original estimates by Marton and Pattee of the advantage to be gained from cold emission have been critically discussed by Cosslett and Haine (5), who showed that the full benefit of the high specific emission was not in practice available, owing primarily to the effects of spherical aberration in the electron lenses. At that time it was only possible to make a rough estimate of there limitations and especially of the working conditions in which the cold or T-F cathode would be preferable to the hot cathode. As more detailed evidence is now available of the performance of these emitters, especially from the measurement of Drechsler and Henkel (6) and of Dyke and his school [Dyke and Dolan (7)], it is opportune to review the position, with special reference to point focus tubes. Maenehile some evidence has also been provided by electron microscopists, in the course of photographing Fresnel frings, of the practical value of the point cathode [Hibi (8); Sakaki and Mölienstedt (9); Sakaki et al. (10)]. Attenpts to use it in electron probe work have so far prove disappointing [Marton, Schrack and Placious (11); Wittry (12)].