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The emission depth of secondary electrons (SEs) is different from materials to materials, depending on escape depth of SEs. The emission depths for metals and insulators are 5 nm and 50 nm, respectively.
Constantloss 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,
 [4800]
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
