Photoelectric absorption is likely to occur when the incident x-ray photon interacts with a:

• A. atomic nucleus
• B. whole atom
• C. inner shell electron
• D. outer shell electron

Answer: (C)

Photoelectric absorption happens when an incoming x-ray photon is absorbed by an atom’s tightly bound inner-shell electron, typically the K-shell. The photon must have enough energy to overcome that electron’s binding energy, and its entire energy goes into ejecting the electron from the atom. The nucleus isn’t directly involved, and the interaction is not with the whole atom or with outer-shell electrons. Inner-shell electrons have the highest binding energies and the strongest interaction with the photon at diagnostic energies, so they dominate the photoelectric process. After the inner-shell vacancy is created, electrons from higher levels drop down to fill it, emitting characteristic x-rays or Auger electrons. The probability of this absorption is higher in high‑Z materials and at photon energies just above the inner-shell binding energies; as energy increases, this process becomes less likely compared to others such as Compton scattering.