Overview
This lecture covers Bremsstrahlung radiation as the primary process for X-ray photon production in X-ray tubes, focusing on its physical mechanism and energy characteristics.
X-ray Photon Production Requirements
- Creating X-ray photons requires free electrons, electron acceleration, and electron deceleration.
- Free electrons are generated at the cathode filament via thermionic emission.
- Electrons are accelerated across the tube by applied voltage (kVp).
- Deceleration of electrons occurs when they strike the tungsten anode.
Bremsstrahlung Radiation Mechanism
- Bremsstrahlung radiation is produced when high-energy electrons interact with the tungsten atom nucleus.
- The electron is attracted and slowed by the positive nucleus, causing it to change direction (deflection).
- Lost kinetic energy from the electron is emitted as an X-ray photon.
- The process is called "Bremsstrahlung," meaning "braking radiation" in German.
Determinants of Bremsstrahlung Photon Energy
- The maximum photon energy equals the incoming electron's energy, determined by kVp.
- Most photons have less than maximum energy since electrons rarely lose all their energy in one interaction.
- Photon energy varies with how closely electrons pass to the nucleus—closer passes yield higher-energy photons.
- If an electron slows from 90 keV to 60 keV, a photon of 30 keV is emitted.
Bremsstrahlung X-ray Spectrum
- The X-ray energy spectrum is a bell curve, with most photons having moderate energies.
- Few photons are at very low or very high energies.
- Low-energy photons are filtered out, as they cannot penetrate tissue.
- Almost all photons in the X-ray beam are Bremsstrahlung X-rays.
Key Terms & Definitions
- Bremsstrahlung Radiation — X-ray photons produced by the deceleration of electrons interacting with atomic nuclei.
- kVp (kilovolt peak) — The maximum voltage applied to the X-ray tube, setting the maximum photon energy.
- Thermionic Emission — Release of electrons from the heated cathode.
- Deflection — Change in the direction of an electron due to interaction with a nucleus.
Action Items / Next Steps
- Review the X-ray photon energy spectrum and identify main features.
- Study the role of kVp in controlling photon energy.
- Prepare for questions on how electron-nucleus proximity affects Bremsstrahlung photon energy.