Gun

Electron Source (GUN)

All Electron Microscopes utilize an electron source of some kind with the majority using a Themionic Gun as shown below:

gun

A Thermionic Electron Gun functions in the following manner:

  1. An positive electrical potential is applied to the anode
  2. The filament (cathode) is heated until a stream of electrons is produced
  3. The electrons are then accelerated by the positive potential down the column
  4. A negative electrical potential (~ 500 V) is applied to the Whenelt Cap
  5. As the electrons move toward the anode any ones emitted from the filament's side are repelled by the Whenelt Cap toward the optic axis (horizontal center)
  6. A collection of electrons occurs in the space between the filament tip and Whenelt Cap. This collection is called a space charge
  7. Those electrons at the bottom of the space charge (nearest to the anode) can exit the gun area through the small (< 1 mm) hole in the Whenelt Cap
  8. These electrons then move down the column to be later used in imaging

This process ensures several things:

  • That the electrons later used for imaging will be emitted from a nearly perfect point source (the space charge)
  • The electrons later used for imaging will all have similar energies (monchromatic)
  • Only electrons nearly parallel to the optic axis will be allowed out of the gun area