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Characteristic radiation is discrete X-ray emission that occurs when an incident high-speed electron ejects an inner-shell (K or L) electron from the target atom.

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• The vacancy is filled by an outer-shell electron dropping to a lower energy level, releasing an X-ray photon with energy equal to the difference in binding energies of the two shells.
• The emitted photon has a characteristic energy specific to the target element (hence the name).

![Generation of a characteristic x-ray in a target atom occurs in the following sequence: (1) The incident electron interacts with the K-shell electron via a repulsive electrical force. (2) The K-shell electron is removed (only if the energy of the incident electron is greater than the K-shell binding energy), leaving a vacancy in the K-shell. (3) An electron from the adjacent L-shell fills the vacancy. (4) A Kα characteristic x-ray photon is emitted with energy equal to the difference between the binding energies of the two shells. In this case, a 59.3-keV photon is emitted.

McCollough CH. The AAPM/RSNA physics tutorial for residents. X-ray production. Radiographics. 1997;17(4):967-984. doi:10.1148/radiographics.17.4.9225393 X-ray production, tubes, and generators. Radiology Key. Published May 16, 2021. https://radiologykey.com/x-ray-production-tubes-and-generators/](attachment:55f6640f-a3b0-499d-a032-4fe2c35fb95b:image.png)

Generation of a characteristic x-ray in a target atom occurs in the following sequence: (1) The incident electron interacts with the K-shell electron via a repulsive electrical force. (2) The K-shell electron is removed (only if the energy of the incident electron is greater than the K-shell binding energy), leaving a vacancy in the K-shell. (3) An electron from the adjacent L-shell fills the vacancy. (4) A Kα characteristic x-ray photon is emitted with energy equal to the difference between the binding energies of the two shells. In this case, a 59.3-keV photon is emitted.

McCollough CH. The AAPM/RSNA physics tutorial for residents. X-ray production. Radiographics. 1997;17(4):967-984. doi:10.1148/radiographics.17.4.9225393 X-ray production, tubes, and generators. Radiology Key. Published May 16, 2021. https://radiologykey.com/x-ray-production-tubes-and-generators/

Mechanism

1. Incident cathode electron collides with a bound inner-shell electron of tungsten (or other target).
2. If incoming electron’s kinetic energy > binding energy of that shell → inner-shell electron is ejected (ionization).
3. An outer-shell electron drops into the vacancy.
4. Energy difference = X-ray photon emitted.

• For tungsten (Z = 74):
  • K-shell binding energy ≈ 69.5 keV.
  • So, incident electron must have ≥70 kVp for K-characteristic X-rays to be produced.

Spectrum Characteristics

• Discrete peaks on the X-ray emission spectrum, superimposed on the continuous Bremsstrahlung background.
• Example (tungsten target): prominent Kα (~59 keV) and Kβ (~67 keV) peaks.
• Energy values are unique to target material (used in X-ray fluorescence analysis).

Comparison: Bremsstrahlung vs Characteristic

Clinical/Imaging Relevance