Characteristic radiation

Last revised by Lachlan McKay on 20 Feb 2024

Characteristic radiation is a type of energy emission relevant for X-ray production. As opposed to the continuous spectrum of bremsstrahlung radiation, characteristic radiation is represented by a line spectrum. As each element has a specific arrangement of electrons at discrete energy levels (dependent on the composition of the nucleus). Therefore, it can be appreciated that the radiation produced from such interactions is 'characteristic' of the element involved.

Characteristic radiation never exists in isolation and the line spectra is usually superimposed on the continuous spectra of bremsstrahlung radiation.


This energy emission happens when a fast-moving electron collides with a K-shell electron, the electron in the K-shell is ejected (provided the energy of the incident electron is greater than the binding energy of K-shell electron) leaving behind a 'hole'. An outer shell electron fills this hole (from the L-shell, M-shell, etc. ) with an emission of a single x-ray photon, sometimes called a characteristic photon. The energy level of this characteristic photon is equivalent to the energy level difference between the outer and inner shell electron involved in the transition. Using tungsten as an example:

  • K-shell energy = 69.5keV

  • L-shell energy = 11.5kEV

    • Therefore an electron moving from the L-shell to fill a K-shell vacancy in tungsten would produce a photon with energy approx. 58.0keV (69.5-11.5)

When an electron falls (cascades) 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-ray1. However, it is possible to have M-L transitions and so on but their likelihood is so low they can be safely ignored. Besides, the energy of L-shell characterisitc rays are too low for diagnostic X-ray use 3.

In a tungsten target, electron transitions from the L-shell to the K-shell to produce x-rays photons with energies of of 57.98 and 59.32 keV. These two energy levels are due to the Pauli exclusion principle which states that no two particles of half-integer spin (such as electrons) in an atom can occupy exactly the same energy state at the same time; therefore the K-shell represents two different energy states, the L-shell eight states and so on.


If kV of the X-ray tube is less than 69.5 keV, the binding energy of the tungsten anode, no characteristic radiation is produced. When the kV exceeds 69.5 keV, characteristic radiation may contribute up to 25% of total X-rays produced 3.

The efficiency of X-ray production is governed by major factors such as kVp, current, exposure time, atomic number of the anode material, and beam filtration 3.

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