Coherent scattering events are most likely when the incident photon energy is:

• A. Low: <10 keV
• B. Moderate: 10 to 30 keV
• C. High: 30 to 100 keV
• D. Very High: >100 keV

Answer: (A)

Coherent scattering (Rayleigh scattering) is an elastic interaction where the photon is scattered by the whole atom without a change in energy. This happens most readily when the photon’s wavelength is long compared with the atomic dimensions, so the electrons respond collectively and interfere constructively. In this regime, the elastic scattering cross-section is larger, and the likelihood of coherent scattering is higher than for other processes. As energy rises, the wavelength shortens and interactions with individual electrons become more important, causing the elastic (coherent) component to diminish while inelastic processes like Compton scattering become more probable. Therefore, coherent scattering is most likely at the lowest energies in the x-ray range, typically well below about 10 keV.