X-ray emission is the most important secondary signal generated in the specimen by an incident electron beam. Once atomic electrons are promoted to excited states, they will inevitably dispose of this excess energy. When the excited atom has electrons ejected from its inner core states, the subsequent filling of the core hole is accompanied by either X-ray fluorescence or Auger electron emission. Two types of X-rays are induced by the electron beam: characteristic X-rays and Bremsstrahlung X-rays. X-rays can be emitted in a specimen by different accelerated particles such as electrons, photons, and positive ions. The most frequently used types of excitation are by electrons in electron microscopes and by photons in X-ray tubes.
Figure 4691 shows an example of the ionization processes and generations of X-rays. A high-energy electron (incident electron) must penetrate through the outer conduction/valence bands and interact with the inner-shell (or core) electrons. If the high-energy electron transfers more than a critical amount of energy to an inner-shell electron (K electron here), that electron is ejected into the vacuum, that is, it escapes (step 2b in the figure) the attractive field of the nucleus, leaving a hole in the inner shell (K shell in the figure) and escapes above the Fermi level into the unfilled states. In this case, the atom is ionized. The excited atom can return almost to its ground state (lowest energy) by filling in the hole with an electron from an outer shell (step 3). This transition is accompanied by the emission of an X-ray in Figure 4691. The energy of the X-ray emission is characteristic of the difference in energy between the two electron shells involved (L3 → K in the figure) and this energy difference is unique to the specific atom.
Figure 4691. Example of the ionization processes and generations of X-rays. The numbers indicates the process sequence. In this process, an inner (K) shell electron is ejected
from the atom by a high-energy electron (incident electron). In step 3, the hole in the K shell
is filled by an electron from the L shell (L3 here), characteristic (Kα) X-ray emission
occurs (step 4). The beam electron loses energy but continues on through the
specimen (step 2a). Steps 2a and 2b almost occur at the same time.
Furthermore, X-ray emission also depends on the energy of incident beam. For incident electrons at low energies, it is very important to know that a useful X-ray signal can only be generated with a beam energy which is at least 1.3 x the ionization energy for the relevant characteristic X-rays. X-ray yields, in this condition, are also relative low and depend strongly on overvoltage (U<3) and the X-ray signals are relatively weak.