Diameter of the beam between which 68% of the electrons fall. For Gaussian beam shapes this corresponds to the 2-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.
PAC, 1983, 55, 2023
(Nomenclature, symbols and units recommended for in situ microanalysis (Provisional))
on page 2027
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook