Which interaction involves an outer-shell electron ejection, energy loss, and photon deflection?

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

Answer: (D)

This question tests how to recognize photon interactions by energy transfer and electron involvement. The described interaction is Compton scattering: a photon collides with a loosely bound outer-shell electron, transfers part of its energy to that electron causing its ejection, and the photon loses energy and changes direction. In the photoelectric effect, the photon is fully absorbed by an electron and only an ejected electron results—there isn’t a scattered photon with reduced energy. Coherent (Rayleigh) scattering deflects the photon without any energy loss, so no electron is ejected. Pair production involves the photon converting into an electron-positron pair in the vicinity of a nucleus, not ejecting an outer-shell electron or producing a deflected, lower-energy photon. So the interaction that matches all three features—outer-shell electron ejection, energy loss, and photon deflection—is Compton scattering.