What is characteristic radiation?
Characteristic radiation is produced when an incoming high-energy electron ejects an inner-shell electron from a target atom, creating an electron vacancy. When an electron from a higher energy shell falls into this vacancy, the energy difference between the shells is released as an X-ray photon.
Characteristic radiation occurs when inner-shell electron vacancies are filled by outer-shell electrons, producing X-ray photons with discrete energies specific to the target material.
Because atomic energy levels are discrete, the emitted photons have specific energies characteristic of the target material. This is why the process is called characteristic radiation.
Understanding the physics
When high-speed electrons strike the tungsten anode, they may collide directly with the orbital electrons of the target atoms. If the incoming electron has sufficient kinetic energy, it can eject an electron from one of the atom’s inner shells, typically the K-shell.
Removing this electron leaves the atom in an unstable state with a vacancy in the inner electron shell. To restore stability, an electron from a higher energy shell (such as the L-shell or M-shell) moves down to fill the vacancy.
During this transition, energy must be released because the inner shell has a lower energy level than the outer shell. This energy difference is emitted as an X-ray photon.
The energy of the emitted photon is determined by the difference between the two electron binding energies:
Ephoton = Einner − Eouter
Because these energy levels are fixed for each element, the emitted photon energies are also fixed. This produces discrete peaks in the X-ray spectrum rather than a continuous distribution.
In tungsten targets, the most important characteristic transitions involve electrons falling into the K-shell. The binding energy of the tungsten K-shell is approximately 69.5 keV, meaning the incoming electron must have at least this energy to eject a K-shell electron. Therefore, K-shell characteristic radiation only appears when tube voltage exceeds roughly 70 kVp.
Once this threshold is exceeded, characteristic photons appear at specific energies (for tungsten, approximately 59 keV and 67 keV).
Although characteristic radiation contributes fewer photons than Bremsstrahlung in most diagnostic X-ray beams, it still forms an important component of the X-ray spectrum.
Where this matters clinically
Characteristic radiation contributes to the overall X-ray spectrum and therefore influences beam quality and penetration.
In general radiography using tungsten targets, Bremsstrahlung dominates the spectrum. However, characteristic radiation becomes more prominent at higher kVp values.
In mammography, target materials such as molybdenum and rhodium are specifically chosen because their characteristic photon energies fall within an optimal range for breast imaging.