Physical Principles of Electron Microscopy: An Introduction to TEM ...

166 Chapter 6
to balance the units in our equation, there must be an additional factor whose
dimensions are m 2 . This factor is the ionization cross section �A for creating
a vacancy in an inner shell of element A. Such a cross section can be
interpreted as a target area for inelastic scattering of an incident electron by
the atom, as discussed Section 4.3. The value of �A depends on the type of
inner shell (K, L, etc.) as well as on the atomic number of the element.
Still missing from our equation is a factor known as the fluorescence
yield that allows for the fact that not every inner-shell vacancy gives rise to
the emission of an x-ray photon during the de-excitation of the atom. An
alternative process allows the excited atom to return to its ground state by
donating energy to another electron within the atom, which is ejected as an
Auger electron (named after its discoverer, Pierre Auger). For elements of
low atomic number, this Auger process is the more probable outcome and
their x-ray fluorescence yield � is considerably less than one (�