Photoelectric effect is most likely to occur under which conditions?

• A. High energy photons with low-Z absorber
• B. Low energy photons with low-Z absorber
• C. High energy photons with high-Z absorber
• D. Low energy photons with high-Z absorber

Answer: (D)

Photoelectric absorption is most probable when a photon has just enough energy to eject an inner-shell electron, and its likelihood scales strongly with the absorber’s atomic number while decreasing as photon energy rises. In high-Z materials, inner electrons are more tightly bound and there are more electrons that can be ejected, so the cross-section for photoelectric absorption is much larger. When you pair this with low-energy photons (near the binding energy, not too high), the probability is at its highest. If the photon energy is too high, the cross-section drops; if the absorber is low-Z, the cross-section is also smaller. So the scenario with low-energy photons and a high-Z absorber yields the greatest chance of a photoelectric interaction.