(c) X-ray attenuation mechanisms; simple scatter, photoelectric effect,…

The idea

As an X-ray beam passes through matter, photons are removed by several mechanisms whose relative importance depends on photon energy and the atomic number Z of the absorber. Simple (coherent) scatter redirects a photon with negligible energy change. In the photoelectric effect a photon is fully absorbed and ejects a tightly bound inner-shell electron; the probability rises strongly with Z and falls rapidly as photon energy increases. In the Compton effect a photon scatters off a loosely bound or free electron, transferring some energy and producing a recoil electron and a lower-energy scattered photon. Pair production occurs only when photon energy exceeds about 1.02 MeV, creating an electron–positron pair near a nucleus.

What earns the marks

• Simple (coherent) scatter: photon is redirected with essentially no energy loss, contributing little to attenuation.
• Photoelectric effect: photon is completely absorbed and ejects a bound inner-shell electron; probability increases with atomic number Z and decreases as photon energy rises.
• Compton effect: photon scatters off a loosely bound electron, transferring energy; the scattered photon has lower energy and a recoil electron is produced.
• Pair production: requires photon energy greater than about 1.02 MeV (twice the electron rest energy) and occurs in the field of a nucleus, producing an electron–positron pair.
• The dominant attenuation mechanism depends on photon energy and absorber atomic number, which is why contrast media use high-Z elements such as iodine or barium.
• Attenuation reduces beam intensity exponentially with absorber thickness, governed by the attenuation coefficient.

Common mistakes and how to put them right

• Treating simple scatter as a major attenuation mechanism.

  Put it right: It changes direction but removes little energy, so it contributes little to attenuation.

• Stating that the photoelectric effect dominates at high photon energies.

  Put it right: Its probability falls rapidly as photon energy increases and rises with Z.

• Confusing Compton scattering with the photoelectric effect.

  Put it right: Compton involves a loosely bound electron and partial energy transfer, photoelectric involves full absorption and a bound electron.

• Believing pair production can occur at any photon energy.

  Put it right: It requires photon energy above about 1.02 MeV.

Exam tips

• For each mechanism, state what happens to the photon (absorbed, scattered, converted) and what particle is ejected.
• Link the dominance of each mechanism to photon energy and atomic number Z, using contrast media as a concrete example.
• Quote the pair production threshold as approximately 1.02 MeV and explain it as twice the electron rest energy.
• Use precise vocabulary: coherent scatter, photoelectric absorption, Compton scattering, pair production, recoil electron, positron.