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TEM Basics

Probe size

The actual beam diameter results from the diameter of the original beam leaving the electron gun, dg broadened by the effect of spherical aberation in the lenses ds and diffraction at the aperture, dd. These depend on the current, i, convergence angle, a, brightness, b, spherical aberation coefficient, Cs, and wavelength, l, via

The total beam diameter is found by adding these three effects in quadrature i.e.

For best resolution in many applications we need to use the smallest beam diameter.

In what situation would the diffraction limited beam diameter dd become more important than the gun-limited diameter dg ?

Use the simulation to investigate the effect of the key parameters on the minimum beam diameter. Unfortunately, in a real microscope you cannot change Cs, nor can you reduce l below the minimum set by the maximum available kV.




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  Electron-atom interactions | Electron gun | Lenses

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