All primary electrons which are scattered out of the original direction and retransmitted through the surface of the solid. In practice, electrons emitted from the surface of a solid under electron bombardment which have a kinetic energy in the range between 50 eV and excitation energy (E0).
Source:
PAC, 1983, 55, 2023. 'Nomenclature, symbols and units recommended for in situ microanalysis (Provisional)' on page 2026 (https://doi.org/10.1351/pac198355122023)