Which interaction describes a low-energy gamma photon ejecting an electron from the atom?

• A. Compton Scatter
• B. Photoelectric Effect
• C. Elastic Scatter
• D. Pair Production

Answer: (B)

Photoelectric effect is when a photon is absorbed by an atom and a bound electron is ejected. For low-energy gamma photons, the energy can be just enough to overcome the electron’s binding energy, and the entire photon energy goes into liberating the electron, which leaves the atom as a photoelectron. The photon is absorbed rather than scattered in this process. This mechanism is especially probable in high‑Z materials at lower photon energies, making it the best description for a low-energy gamma photon ejecting an electron.

Compton scattering, by contrast, involves the photon transferring only part of its energy to a loosely bound or free electron and continuing with reduced energy. Elastic (Rayleigh) scattering transfers essentially no energy to the electron. Pair production requires the photon to have energy above 1.022 MeV and results in an electron-positron pair in the field of the nucleus, not a single bound-electron ejection.