beam diameter in in situ microanalysis

Diameter of the beam between which 68% of the electrons fall. For Gaussian beam shapes this corresponds to the 2 $σ$ -value of the intensity distribution of the beam. Measurement can be carried out by scanning a beam across a sufficiently sharp edge and recording the transmitted electrons.

Source:

PAC, 1983, 55, 2023 (Nomenclature, symbols and units recommended for in situ microanalysis (Provisional)) on page 2027