TABLE OF CONTENTS The electron gun produces a beam of electrons whose kinetic energy is high enough to enable them to pass through thin areas of the TEM specimen. The gun consists of an electron source, also known as the cathode because it is at a high negative potential, and an electron-accelerating chamber. There are several types of electron source, operating on different physical principles, which we now discuss.
Figure 3-1 shows a common form of electron gun. The electron source is a V-shaped ("hairpin") filament made of tungsten (W) wire, spot-welded to straight-wire leads that are mounted in a ceramic or glass socket, allowing the filament assembly to be exchanged easily when the filament eventually "burns out." A direct (dc) current heats the filament to about 2700 K, at which temperature tungsten emits electrons into the surrounding vacuum by the process known as thermionic emission.
The process of thermionic emission can be illustrated using an electron-energy diagram (Fig. 3-2) in which the vertical axis represents...
