The photoelectric effect is more likely with which conditions?

• A. High energy photons and high atomic number materials
• B. Low energy photons and high atomic number materials
• C. Low energy photons and low atomic number materials
• D. High energy photons and low atomic number materials

Answer: (B)

The photoelectric effect is most likely when photons are relatively low in energy and the absorbing atom has a high atomic number. The cross-section for photoelectric absorption increases strongly with atomic number, because heavier atoms have more tightly bound electrons and a stronger electromagnetic field that makes it easier for a bound electron to absorb the photon and be ejected. At the same time, the cross-section falls off rapidly as photon energy increases (roughly inversely with the cube of the energy). So low-energy photons interacting with high-Z materials maximize the probability of photoelectric emission. The other scenarios either raise the energy (which lowers the likelihood) or use low-Z materials (which also lowers the probability), so the combination of low energy and high Z is the most favorable.