QUIZ 4 Solution
The X-ray beam attenuates due to photons either being absorbed by
the atoms of the material, or being scattered away from their original
directions of travel.
For the range of photon energies most commonly encountered for
diagnostic imaging (from 20 to 150 keV), the mechanisms responsible for
these two contributions to attenuation are the photoelectric and the Compton
effects.
Photoelectric absorption consists of an x-ray photon
imparting all its energy to a tightly bound inner electron in an atom. The
electron uses some of this acquired energy to overcome the binding energy
within its shell, the rest appearing as the kinetic energy of the thus freed
electron.
The Compton scattering, on the other hand, consists of the
interaction of the x-ray photon with either a free electron, or one that is only
loosely bound in one of the outer shells of an atom. As a result of this
interaction, the x-ray photon is deflected from its original direction of travel
with some loss of energy, which is gained by the electron.
As a consequence, the X-ray photons lose all or some of their energy due to photoelectric
absorption and Compton scattering respectively, which results in attenuation.