At the top of the TEM column is the electron gun, which is the source of electrons. The electrons are accelerated to high energies (typically 100-400 keV) and then focussed towards the sample by a set of condenser lenses and apertures.
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Source
The source is chosen so that the rates of electrons incident on the sample per unit area and leaving the source per unit solid angle (brightness) are maximised. This is so that the maximum amount of information can be extracted from each feature of the sample.
There are two major types of electron source. Guns of the cheaper and more common of the two generate electrons by thermionic emission. If enough thermal energy is added to a material its electrons may overcome the energy barrier of the work function and escape. Unfortunately, to avoid the source melting, the material used must either have a very high melting point (such as W) or an exceptionally low work function (certain rare-earth boride crystals such as LaB6 are widely used).
Another way of extracting electrons from a material is by applying a very large electric field. By drawing tungsten wire to a very fine point (<0.1 μm), application of a potential of 1 kV gives an electric field of 1010 A m-1 which is large enough to allow electrons to tunnel out of the sample. This is called field emission.
Field emission guns are around twice as expensive as thermionic electron guns, and must be used under ultra-high vacuum conditions. They are favourable for applications in which a high brightness and low energy-spread of incident electrons is needed. (eg. HRTEM, FEG.)
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