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X-rays are produced by high-energy electrons bombarding a target, especially targets that have a high proton number (Z). When bombarding electrons penetrate into the target, some electrons travel close to the nucleus due to the attraction of its positive charge and are subsequently influenced by its electric field. The course of these electrons would be deflected, and a portion or all of their kinetic energy would be lost. The principle of the conservation of energy states that in producing the X-ray photon, the electron has lost some of its kinetic energy (KE):
final KE of electron = initial KE of electron - energy of X-ray photon
The 'lost' energy is emitted as X-ray photons, specifically bremsstrahlung radiation (bremsstrahlung is German for 'braking radiation'). Bremsstrahlung can have any energy ranging from zero to the maximum KE of the bombarding electrons (i.e., 0 to Emax), depending on how much the electrons are influenced by the electric field, therefore forming a continuous spectrum. The 'peak' of the filtered spectrum typically occurs at approximately one-third of Emax so for a bremsstrahlung spectra with an Emax value of say 120 keV, the peak of the spectrum would be at approximately 40 keV. The unfiltered bremsstrahlung spectrum demonstrates a ramp shape with the probability of photon production being inversely linear with KE.
The intensity of bremsstrahlung radiation is proportional to the square of the atomic number of the target (Z), the number of unit charges of the bombarding particle (z) and inversely with the mass of the bombarding particle (m): Z² z / m. It follows that light particles such as electrons and positrons bombarding targets of high atomic number are more efficient producers of bremsstrahlung radiation than heavier particles such as alpha particles or neutrons (which can also cause X-rays to be produced through bremsstrahlung, though it's much more unlikely than with electrons).
Most of the X-ray beam consists of Bremsstrahlung interactions in the diagnostic range of 30 to 150 kVp. When the applied voltage is less than 70kVp, Bremsstrahlung interactions account for ~100% of the X-ray beam. If the applied voltage is more than 70kVp, the Bremsstrahlung interactions. accounts for ~85% of the X-ray beam 3. Most of the X-rays produced are of low energy because electron beams have higher chances of striking outer zones of the target atom that produces low energy photons than inner zones of the target atom that produces high energy photons 4.
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 4.
- 1. Allisy-Roberts P, Williams J. Farr's Physics for Medical Imaging. W.B. Saunders Company. (2007) ISBN:0702028444. Read it at Google Books - Find it at Amazon
- 2. Jerrold T. Bushberg, John M. Boone. The Essential Physics of Medical Imaging. (2011) ISBN: 9780781780575 - Google Books
- 3. The X-Ray Beam. Radiology Key.
- 4. Perry Sprawls. Physical Principles of Medical Imaging. (1993) ISBN: 9780834203099 - Google Books X-Ray Production