Which x-ray interaction results in complete absorption of the incident photon by an orbital electron?

Which x-ray interaction results in complete absorption of the incident photon by an orbital electron?

• A. Photoelectric effect.
• B. Pair production.
• C. Classical scattering.
• D. Compton effect.

Answer: (A)

The key idea is that the photon is fully absorbed by a bound electron in an atom. In the photoelectric effect, the incident x-ray photon transfers all its energy to that electron, provided the photon’s energy exceeds the electron’s binding energy. The electron is ejected from the atom as a photoelectron, and any leftover energy becomes its kinetic energy. Since the photon ceases to exist, this interaction appears as complete absorption of the photon by the orbital electron. This effect is more likely at lower photon energies and with higher atomic numbers, which is why it plays a major role in diagnostic radiography and image formation. Other interactions don’t wipe out the photon in this way. Compton scattering involves the photon being deflected and losing part of its energy, with the recoil electron carrying away energy; Rayleigh (classical) scattering is elastic and the photon scatters with essentially no energy change; pair production needs photon energy above 1.022 MeV and results in a photon transforming into a particle–antiparticle pair near a nucleus, not absorption by an orbital electron.

The key idea is that the photon is fully absorbed by a bound electron in an atom. In the photoelectric effect, the incident x-ray photon transfers all its energy to that electron, provided the photon’s energy exceeds the electron’s binding energy. The electron is ejected from the atom as a photoelectron, and any leftover energy becomes its kinetic energy. Since the photon ceases to exist, this interaction appears as complete absorption of the photon by the orbital electron. This effect is more likely at lower photon energies and with higher atomic numbers, which is why it plays a major role in diagnostic radiography and image formation.

Other interactions don’t wipe out the photon in this way. Compton scattering involves the photon being deflected and losing part of its energy, with the recoil electron carrying away energy; Rayleigh (classical) scattering is elastic and the photon scatters with essentially no energy change; pair production needs photon energy above 1.022 MeV and results in a photon transforming into a particle–antiparticle pair near a nucleus, not absorption by an orbital electron.