Which of the following interactions is least likely to occur with increasing photon energy?

Which of the following interactions is least likely to occur with increasing photon energy?

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

Answer: (A)

The photoelectric effect is most likely to decrease in occurrence as photon energy increases. This is because the photoelectric effect involves the complete absorption of a photon by an inner-shell electron, which is more probable at lower photon energies. As the energy of the photons increases, the likelihood of this interaction diminishes significantly because higher-energy photons are more likely to pass through materials or undergo other interactions.

In contrast, Compton scatter, pair production, and Rayleigh scatter are more relevant at higher photon energies. Compton scattering occurs when a photon interacts with a loosely bound outer-shell electron, which can happen across a wide range of photon energies, including higher levels. Pair production occurs at high energies (at least 1.022 MeV), when photon energy exceeds the combined rest mass energy of an electron and positron, and this process becomes increasingly likely as photon energy rises. Rayleigh scatter, although frequency-dependent and more related to lower energies, does not experience the same level of dependency on energy as the photoelectric effect does.

Understanding the interaction decreases with increasing photon energy helps in predicting the behavior of radiation in various materials and contributes to effective imaging techniques in radiography.