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The removal rate (sputtering yield, Y) of atoms caused by electron irradiation in EMs (electron microscopes) is related to the sputtering cross section. The number of surface atoms removed per incident electron is given by Y = σ_dN_s. Here, N_s is the number of surface atoms per unit area. Since N_s is of the order of 10¹⁵ cm^-2 and a sputtering cross section is of the order of 10^-22 cm², the removal rate of atoms is of the order of 10^-7.