Attenuation Mechanisms

PhotoelectricAbsorption is one of the principal forms of interaction of x-ray and gamma photons with matter. A low energy photon interacts with an electron in the atom and removes it from its shell.

https://en.wikipedia.org/wiki/Photoelectric_effect https://radiopaedia.org/articles/photoelectric-effect

Incoherent scatter (Compton effect or Compton scatter) is one of principle forms of photon interaction. It is the main cause of scattered radiation in a material. It occurs due to the interaction of the photon (x-ray or gamma) with free electrons (unattached to atoms) or loosely bound valence shell (outer shell) electrons. The resultant incident photon is scattered (changes direction) and imparts energy to the electron (recoil electron). The scattered photon will have a different wavelength (observed phenomenon) and thus a different energy (E=hc/λ). Energy and momentum are conserved in this process. The Compton effect is a partial absorption process and as the original photon has lost energy, known as Compton shift (i.e. a shift of wavelength/frequency). The wavelength change of the scattered photon can be determined by 0.024*(1- cosθ), where θ is scattered photon angle. Thus, the energy of the scattered photon decreases with increasing scattered photon angle.

https://en.wikipedia.org/wiki/Compton_scattering https://radiopaedia.org/articles/compton-effect?lang=us

Coherent scattering (also known as unmodified, Rayleigh, classical 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. It therefore occurs with low energy radiation.

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.

https://radiopaedia.org/articles/coherent-scattering