Coherent scattering (also known as unmodified, classical or elastic scattering) is divided into two main types namely: Rayleigh and Thompson scattering. Rayleigh scattering occurs when the energy of the x-ray or gamma photon is small in relation to the ionisation 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.
Meanwhile, Thompson scattering is the scattering of photon by free electrons 6.
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 colour 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.
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