2.4.2 Production of characteristic emission

Ionization of the atom can occur with certain probability (cross section) when photons (either gamma or x-rays) reach the atom and have enough energy as to expel one electron from the inner shells. Ionization can also be produced by electrons, fast protons or other positive ions.

The main mechanisms for the atom to return to its stable state are the emission of characteristic x-ray radiation, the emission of satellite lines and the ejection of Auger electrons. Characteristic x-rays are emitted when an electron from an outer shell takes the vacancy created by the ionization, and the excess of energy is released in the form of an x-ray. As the number of probable transitions is ruled by the principle of Pauli exclusion, each chemical element has different sets of probable emission energies. When more than one electron is ejected, multiple transitions can occur and the excess of energy be released as an x-ray with energy higher than that of characteristic lines. This emission of satellite lines has a very low probability to occur and has no high relevance in x-ray spectroscopy.

When the characteristic radiation is produced by photons, the phenomenon is called x-ray fluorescence, otherwise is called x-ray emission.