Mechanism of production
Energetic charged particles, whatever their source, interact through electromagnetic forces with atoms. As a charged particle passes near a nucleus, its speed and/or direction may change rapidly due to the electrostatic force. This change in velocity means the charged particle is experiencing acceleration, and an accelerating charged particle releases radiation.
Figure 1 shows a production of a bremsstrahlung photon from the interaction between an electron and a nucleus. The resulting bremsstrahlung photon has energy equal to the difference between the electron's energy before and after the interaction.
The radiation released is in the form of photons with a continuous spectrum of energies that extend as high as the initial charged particle's energy. Bremsstrahlung production is more likely for high-energy charged particles and high atomic number (Z) materials. This is significant for shielding and safety considerations related to beta-emitting nuclides and x-ray machines.
Sources of bremsstrahlung
Three commonly encountered sources of bremsstrahlung are
beta-emitting nuclides, x-ray machines, and high-voltage electronic equipment.
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Beta-emitting nuclides release high-energy electrons or positrons as their nucleus attempts to reach a more stable configuration. These charged particles interact with shielding materials to produce bremsstrahlung photons. These nuclides are used in a wide variety of medical, industrial, and research contexts.
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X-ray machines use a high voltage to accelerate electrons across a vacuum tube. The accelerated electrons collide with a metal target, producing bremsstrahlung photons. While x-ray machines do not emit radiation when power is off, radionuclides continuously emit radiation, and the associated hazard must be controlled.
- Electrons produced in
high-voltage electronic equipment can be accelerated to energies where bremsstrahlung production can be significant.
Radiation protection considerations
An understanding of bremsstrahlung is essential to the responsible use of x-ray machines and beta-emitting nuclides.
- High Z material, such as lead, is ideal for absorbing and blocking photon radiation, but it is generally not appropriate as the only shield for charged particles due to the potential for bremsstrahlung.
- For relatively low energies or small quantities, it may be sufficient to shield with a low Z shield such as a sheet of plastic.
- For higher energies or greater quantities, it may be necessary to use two layers of shielding (a layer of low Z material thick enough to completely attenuate the energetic electrons, followed by a high Z material) to absorb any bremsstrahlung produced in the low Z shield.
References and further reading
U.S. Department of Commerce, National Bureau of Standards. 1983. NBSIR 82-2550A, Stopping Powers and Ranges of Electrons and Positrons,
prepared by Berger MJ and SM Selzer, Washington D.C.
Martin, James E. ed. 2006.
Physics for Radiation Protection—A Handbook 2nd edition, Wiley-VCH, Weinheim.
Turner, James E. ed. 2007.
Atoms, Radiation, and Radiation Protection 3rd edition, Wiley-VCH, Weinheim.
