In an X-ray tube, electrons from a heated cathode are accelerated by a high voltage (kV) into a metal target. Most of their energy becomes heat; a small fraction becomes X-rays, as a continuous bremsstrahlung spectrum whose maximum photon energy in keV equals the peak tube voltage in kVp, plus characteristic lines of the target element.
Reviewed 2026-09-24
Inside the tube
Inside an X-ray tube
- The heated cathode filament emits electrons (tube current, mA).
- The high voltage (kV) between cathode and anode accelerates them.
- The electron beam strikes the anode target.
- X-rays are produced at the target; about 99% of the energy becomes heat.
- X-rays leave through the window; filtration removes low-energy photons.
Bremsstrahlung and characteristic radiation
- Bremsstrahlung (“braking radiation”): electrons decelerate near target nuclei and emit a continuous spectrum up to the kVp value.
- Characteristic radiation: electrons knock out inner-shell electrons of the target; the vacancies are filled and emit lines at fixed energies, about 59 keV and 67 keV for tungsten K lines.
- In diagnostic tubes roughly 99% of the electron energy becomes heat, which is why anode cooling matters.
What kV, mA and filtration change
| Setting | Effect on the beam |
|---|---|
| kV / kVp | Shapes the spectrum and maximum photon energy; higher kV penetrates further |
| mA × s (mAs) | Number of photons produced; does not change spectral shape |
| Filtration (e.g. aluminium) | Removes low-energy photons and “hardens” the beam |
Typical operating ranges differ widely: general radiography commonly uses about 50–150 kV, and X-ray security scanners about 40–180 kV. The Radbar food scanner curtain catalog lists 90 / 120 kV; the security curtain series are listed for ≤120 kV or ≤160 kV.
References
- Bushberg et al., The Essential Physics of Medical Imaging, 3rd ed., 2012
- IAEA — Radiation, People and the Environment (2004)
- 新莱福防辐射产品手册（2024-09 V1）科普章节
This encyclopedia is for general and engineering reference and is not medical advice. Consult radiation protection or medical professionals for individual exposure assessment.