primary electrons

in in situ microanalysis
https://doi.org/10.1351/goldbook.P04829
Electrons generated by a thermal or field emission source used to bombard the specimen for generation of the analytical signals.
Source:
PAC, 1983, 55, 2023. (Nomenclature, symbols and units recommended for in situ microanalysis (Provisional)) on page 2026 [Terms] [Paper]