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The primary electrons in EMs are high energy electrons in the electron beam which interact with and pass through the specimen (for TEM and STEM) or interact with the specimen only (e.g. for SEM).

Constant-loss approximation [1 – 3] suggested that the energy dissipation of primary electrons (PE) within the material is approximately constant, the number of primary electrons would decrease linearly to a depth R. The number of SEs produced per unit path length would be a constant, given by,

            ----------------- [4470]

where ξ -- Energy required to excite one SE inside the solid
R -- Maximum range of the PE.

[1] Joy DC (1987) J Microsc 147(1):51
[2] Dionne GF (1973) J Appl Phys 44(12):5361
[3] Lye RG, Dekker AJ (1957) Phys Rev 107(4):977