Energy of scattered radiation relative to the primary beam?

• A. More energetic
• B. Equally energetic
• C. Less energetic
• D. Varies widely

Answer: (C)

Scattered photons in diagnostic radiology are generally less energetic than the primary beam because the main interaction—Compton scattering—is inelastic. In Compton scattering, part of the photon’s energy is transferred to a recoil electron, so the scattered photon emerges with reduced energy. The amount of energy lost depends on the scattering angle: larger angles transfer more energy to the electron, producing a lower-energy scattered photon, while smaller angles leave the scattered photon energy closer to the original but still typically lower than the primary. Because most scatter at diagnostic energies comes from Compton interactions, the scattered radiation is, on average, less energetic than the primary beam. (Elastic Rayleigh scattering can occur and leaves energy unchanged, but it is a smaller contributor at these energies.)