Transmission Electron Microscope:

Instrumental Part:

Sources (Electron Guns): Thermionic guns:

Figure 12.04: Schematic of electron gun in TEM.

• 100 μm diameter tungsten cathode or LaB6 cathode is used in the form of fine wire in hairpin shape (V-shape).

• Passing high current generates a temperature of nearly 2400°C.

• The electrons potential distribution of 0 to 2 volts. With a bias voltage (0 to 500V), electrons can be accelerated toward the anode. The typical lifetime is ~ 50 hours.

• A high voltage is placed between the cathode and the anode, modified by a potential on the Wehnelt which acts as the grid in a triode system. The electric field from the Wehnelt focuses the electrons into a crossover.