Characteristic X-rays are emitted when outer-shell electrons fill a vacancy in the inner shell of an atom, releasing X-rays in a pattern that is "characteristic" to each element. Characteristic X-rays were discovered by Charles Glover Barkla in 1909, who later won the Nobel Prize in Physics for his discovery in 1917.
Characteristic X-rays are produced when an element is bombarded with high-energy particles, which can be photons, electrons or ions (such as protons). When the incident particle strikes a bound electron (the target electron) in an atom, the target electron is ejected from the inner shell of the atom. After the electron has been ejected, the atom is left with a vacant energy level, also known as a core hole. Outer-shell electrons then fall into the inner shell, emitting quantized photons with an energy level equivalent to the energy difference between the higher and lower states. Each element has a unique set of energy levels, and thus the transition from higher to lower energy levels produces X-rays with frequencies that are characteristic to each element.
The different electron states which exist in an atom are usually described by atomic orbital notation, as is used in chemistry and general physics. However, X-ray science has special terminology to describe the transition of electrons from upper to lower energy levels: traditional Siegbahn notation, or alternatively, simplified X-ray notation.
In Siegbahn notation, When an electron falls from the L shell to the K shell, the X-ray emitted is called a K-alpha X-ray. Similarly, when an electron falls from the M shell to the K shell, the X-ray emitted is called a K-beta X-ray.
Sometimes, however, instead of releasing the energy in the form of an X-ray, the energy can be transferred to another electron, which is then ejected from the atom. This is called the Auger effect.
Characteristic X-rays can be used to identify the particular element from which they are emitted. This property is used in various techniques, including X-ray fluorescence spectroscopy, particle-induced X-ray emission, energy-dispersive X-ray spectroscopy, and wavelength-dispersive X-ray spectroscopy.