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Electron

  CBED     8 of 8
 
Conventional electron diffraction techniques use a parallel beam of incident radiation, these techniques are called Selected Area Diffraction (SAD). In contrast Convergent Beam Electron Diffraction (CBED) uses a convergent beam of electrons to limit the area of the specimen which contributes to the diffraction pattern.

Each spot then becomes a disc within which variations in intensity can usually be seen. Such patterns initially seem more difficult to interpret but they contain a wealth of information about the symmetry and thickness of the crystal and are widely used in TEM.

The big advantage of CBED over SAD techniques is that most of the information is generated from small regions beyond the reach of other techniques.

Have a go imageIn this simulation the relationship between a spot pattern and a convergent beam pattern can be explored by altering the beam convergence angle.

A convergent beam of electrons creates a
Fresnel (near field) diffraction pattern.

 

 
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