Which interaction is least probable in diagnostic radiography?

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

Answer: (C)

In diagnostic radiography, photon energies are typically tens of keV. The likelihood of different photon–matter interactions inside tissues depends on energy and atomic structure.

Coherent (Rayleigh) scatter is an elastic process off the entire atom and has a very small cross-section at diagnostic energies, meaning it occurs very rarely compared with other interactions. Photoelectric absorption, especially in higher‑Z tissues, and Compton scattering both occur with higher probability in this energy range, with Compton scattering being particularly common in soft tissues.

Pair production, on the other hand, requires photon energy above about 1.022 MeV, which is not reached in standard diagnostic beams, so it does not occur in practice for diagnostic radiography.

Thus, among the options, coherent scatter has the smallest probability in the diagnostic energy range.