Elements of a Transmission Electron Microscope

Abstract

Not only does the electron gun of an electron microscope emit electrons into the vacuum and accelerate them between cathode and anode, but it is also required to produce an electron beam of high brightness and high temporal and spatial coherence. The conventional thermionic emission from a tungsten wire is limited in temporal coherence by an energy spread of the emitted electrons of the order of a few electronvolts and in spatial coherence by the gun brightness. Schottky-emission and field-emission guns are newer alternatives, for which the energy spread is less and the gun brightness higher.