Practical Electron Microscopy and Database

When an electron in an accelerating electron beam irradiates on materials, a Frenkel pair, consisting of an interstitial and a vacancy can be produced. This process is called ballistic knock-on process. In this process, the energy transferred in an elastic collision between the electron and the nucleus exceeds the displacement energy, that varies from ~ 0.1 eV for Van der Waals bonding to tens eV for ionics. That is, this process requires a minimum electron beam energy (called threshold energy) before it can occur. The value varies with the atomic number of the sample. For instance, Table 4663 lists the knock-on threshold energies of some typical bulk materials at room temperature. In a crystal, in addition to channelling effects, the threshold energy also depends on the direction of transferred momentum, being the highest in the direction normal to close-packed crystal planes and the smallest for atoms knocked-on into close-packed directions.

Table 4663. Knock-on threshold energies of typical bulk materials due to electron irradiation at room temperature.

Note that this is not currently a problem with SEMs due to low voltages (≤30 keV). However, the knock-on thresholds for thin films, e.g. in TEM specimens, can be much lower than that for bulk materials.