Compton scattering is most likely to occur when an x-ray photon interacts with which type of electron?

• A. Inner shell electron
• B. Outer shell electron
• C. Atomic nucleus
• D. Whole atom

Answer: (B)

Compton scattering happens when an x-ray photon collides with a relatively free or very loosely bound electron and transfers some of its energy to that electron, producing a photon with a longer wavelength and a recoil electron. Outer-shell electrons are only loosely bound, so they behave almost like free electrons at typical x-ray energies. This makes Compton scattering far more likely from outer-shell electrons than from tightly bound inner-shell electrons, where the interaction is more often the photoelectric effect rather than scattering. Scattering off the atomic nucleus is negligible at x-ray energies, and treating the whole atom as the scatterer doesn’t reflect how the momentum transfer to an individual electron drives the Compton process. So, outer-shell electrons provide the right conditions for Compton scattering to occur.