Which results in total absorption of an incident x-ray photon?

Which results in total absorption of an incident x-ray photon?

• A. Photoelectric interaction
• B. Compton interaction
• C. Coherent scatter
• D. Pair production

Answer: (A)

The main idea is that total absorption of an x-ray photon happens through the photoelectric interaction. In this process the photon is completely taken up by an atom: its energy goes into overcoming the binding energy of an inner-shell electron and into the kinetic energy of the ejected electron. There’s no scattered photon left—the original photon’s energy has been fully absorbed by the atom. Other interactions don’t absorb the entire photon energy. Compton scattering transfers only part of the energy to an electron and leaves a scattered photon with reduced energy, so some energy exits as radiation. Coherent (Rayleigh) scatter is elastic, so the photon’s energy remains essentially unchanged. Pair production converts the photon into a particle–antiparticle pair near a nucleus, but this occurs at higher energies and is not the typical diagnostic X-ray absorption pathway; it also ends up removing the photon’s energy from the beam, but it’s a different process and requires threshold energy. So the photoelectric interaction is the mechanism that results in the photon’s total absorption.

The main idea is that total absorption of an x-ray photon happens through the photoelectric interaction. In this process the photon is completely taken up by an atom: its energy goes into overcoming the binding energy of an inner-shell electron and into the kinetic energy of the ejected electron. There’s no scattered photon left—the original photon’s energy has been fully absorbed by the atom.

Other interactions don’t absorb the entire photon energy. Compton scattering transfers only part of the energy to an electron and leaves a scattered photon with reduced energy, so some energy exits as radiation. Coherent (Rayleigh) scatter is elastic, so the photon’s energy remains essentially unchanged. Pair production converts the photon into a particle–antiparticle pair near a nucleus, but this occurs at higher energies and is not the typical diagnostic X-ray absorption pathway; it also ends up removing the photon’s energy from the beam, but it’s a different process and requires threshold energy.

So the photoelectric interaction is the mechanism that results in the photon’s total absorption.