Number of back scattered electrons (BSEs) generated per primary electron for a given specimen and experimental condition. It depends on (mean) atomic number of the excited area of the sample, angle between electron beam and sample surface, primary electron energy and thickness of the sample.
Source:
PAC, 1983, 55, 2023. 'Nomenclature, symbols and units recommended for in situ microanalysis (Provisional)' on page 2026 (https://doi.org/10.1351/pac198355122023)