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Ionization cross section has the form [1],
 [4553a]
where E_{c} is the critical ionization energy and C_{0} is a constant that can be determined from
other fundamental atomic data that depend on the particular transition.
Bethe went a step further [2] to include the effects of virtual photon exchange
(i.e. the Breit interaction) in a perturbation expansion, retaining terms up to β(v^{2}/c^{2}).
The resulting ionization cross section is given by,
 [4553b]
contains two additional terms that do not appear in Equation [4553a]. These terms are sometimes
referred to as the “relativistic rise", and it is straightforward to verify that they provide
an increasingly positive contribution to the cross section with increasing impact energy.
As a result of Equation 4553a, it is expected to be
valid if the incident ionizing electron energy is suficiently
high, e.g. at accelerating voltages (E_{0}) of the incident electrons in TEM and SEM. The total ionization cross section derived by Bethe can be given by,
 [4553c]
n_{i} – The number of electrons in a shell or subshell (e.g., n_{i} = 2 for a Kshell, n_{i} = 8 for an Lshell, and n_{i} = 18 for an Mshell
b_{s}, c _{s} – The constants for a particular innershell and from leastsquares fitting to ionization data
E_{c} – Ionization energy of given element A in the K, L, or Mshell (keV)
If we apply overvoltage into Equation [4791a], and then, we can have,
 [4553d]
Here, the dimensions are ionizations/e/(atom/cm^{2}).
Combining Equations 4553b and 4553c, Bethe cross section equation becomes,
 [4553e]
[1] An Overview of Relativistic DistortedWave Cross Sections Christopher J. Fontes, Hong Lin Zhang and Joseph Abdallah, Jr., CP730, Atomic Processes in Plasmas: 14th APS Topical Conference on Atomic Processes in Plasmas, edited by J. S. Cohen, S. Mazevet, and D. P. Kilcrease.
[2] H. Bethe, Z. Physik 76, 293 (1932).
