Electron Source (GUN)
All Electron Microscopes utilize an electron source of some kind
with the majority using a Themionic Gun as shown below:
A Thermionic Electron Gun functions in the following manner
- An positive electrical potential is applied to the anode
- The filament (cathode) is heated until a stream of electrons
is produced
- The electrons are then accelerated by the positive potential
down the column
- A negative electrical potential (~500 V) is applied to the
Whenelt Cap
- 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)
- A collection of electrons occurs in the space between the
filament tip and Whenelt Cap. This collection is called a space
charge
- 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
- These electrons then move down the column to be later used
in imaging
This process insures 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