Coherent scattering

Last revised by Mohammadtaghi Niknejad on 23 Feb 2024

Coherent scattering (also known as Rayleigh, unmodifiedclassical or elastic scattering) is one of three forms of photon interaction which occurs when the energy of the x-ray or gamma photon is small in relation to the ionization energy of the atom (for instance, during mammography) 5. Such low-energy radiation triggers all the electrons in an atom to vibrate at a similar frequency to the incident photon 3. Thus, the whole atom acts as a unit and recoils when bombarded by a low-energy photon 4.

Upon interacting with the attenuating medium, the photon does not have enough energy to liberate the electron from its bound state (i.e. the photon energy is well below the binding energy of the electron) so no energy transfer occurs. There is no energy deposition and thus no dose resulting from coherent scattering. The only change is a change of direction (scatter) of the photon, hence 'unmodified' scatter. Coherent scattering is not a major interaction process encountered in radiography at the energies normally used.

Coherent scattering varies with the atomic number of the absorber (Z) and incident photon energy (E) by Z/E2.

History and etymology

In 1871, Lord Rayleigh explained the origin of blue color of the sky is due to electromagnetic scattering of sunlight by small water droplets in the sky, where the size of the water droplets should be 10 times smaller than the wavelength of a photon 2.

See also

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Cases and figures

  • Figure 1
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