Which interaction is least probable among the following in diagnostic radiography?

• A. Photoelectric Absorption
• B. Pair Production
• C. Coherent Scatter
• D. Compton Scatter

Answer: (C)

At diagnostic radiography energies, how often a photon interacts depends on energy and the material. Coherent (Rayleigh) scattering is an elastic process with a very small cross‑section in the diagnostic range, so its probability is much lower than the other common interactions you see in imaging. Photoelectric absorption tends to be more likely at lower energies and in higher‑Z tissues, and Compton scattering is the dominant interaction at the higher end of diagnostic energies since it involves outer‑shell electrons and is relatively probable across soft tissue.

Pair production, on the other hand, needs photon energies above 1.02 MeV, which are not used in standard diagnostic beams, so it doesn’t occur in typical clinical settings. Given these factors, coherent scattering is the least probable interaction among the options in diagnostic radiography.