Introduction to Characteristic X-rays
Characteristic X-rays are produced when an electron in an atom transitions between different energy levels (shells) to fill a vacancy created by ionization or excitation. This transition results in the emission of a photon, with energy characteristic of the difference in binding energies between the two energy levels. The energy of the emitted photon is unique to the atom, and this process is referred to as characteristic (or fluorescence) radiation.
Process of Characteristic X-ray Production
When an atom's inner shell electron (usually from the K-shell or L-shell) is ejected due to ionization or excitation (for example, by interaction with an incident high-energy photon), a vacancy is created in that shell. An electron from a higher energy level then transitions down to fill this vacancy. During this transition, energy is released in the form of a photon, the energy of which is equal to the difference in binding energies of the two shells involved.
For example, if an electron from the L-shell moves to the K-shell to fill a vacancy, the energy of the emitted X-ray photon corresponds to the energy difference between the K-shell and L-shell binding energies. This energy is characteristic of the atom and is specific to the element.
Instead of emitting a photon, the energy may be transferred to another orbital electron, ejecting it from the atom with kinetic energy equal to the transition energy minus the binding energy of the ejected electron. This ejected electron is known as an Auger electron.
Auger Effect
In some cases, the energy released during the transition between shells may be transferred to another electron, ejecting it from the atom. This process results in the emission of an Auger electron. The kinetic energy of the Auger electron is the energy of the transition minus the binding energy of the Auger electron's initial shell.
For example, if the energy from the L-shell to K-shell transition in copper (with energy 8.05 keV) is transferred to an electron in the M-shell (with a binding energy of 0.28 keV), the Auger electron will have a kinetic energy of:
Eₐ = Eₖₐ - Eₘ = 8.05 keV - 0.28 keV = 7.77 keV
This Auger electron is then emitted with the kinetic energy calculated above. The Auger effect can provide valuable information about the elemental composition of materials, especially in electron spectroscopy.
Applications of Characteristic X-rays
Characteristic X-rays are widely used in both analytical and diagnostic applications, particularly in X-ray spectroscopy and medical imaging:
- X-ray Fluorescence (XRF) Spectroscopy: Used for elemental analysis, where characteristic X-ray emissions provide a "fingerprint" of the elements present in a sample.
- Medical Imaging: Used in X-ray imaging and CT scans, where characteristic X-ray photons interact with detectors to form an image of the internal structures of the body.
- Material Characterization: Used in materials science for the study of thin films, coatings, and surface layers.