Which interaction occurs for high-energy gamma rays (>1.022 MeV) producing an electron-positron pair in the field of a nucleus?

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

Answer: (A)

When a gamma ray has energy above 1.022 MeV, it can transform into an electron and a positron in the electromagnetic field of a nucleus. The energy threshold equals twice the rest-mass energy of an electron, because you must create both the electron and the positron. The nucleus is essential here to conserve momentum; without a nearby heavy charged body to take up some momentum, pair creation cannot satisfy both energy and momentum conservation in free space.

In this process, the gamma’s energy becomes the rest-mass energy of the two new particles plus their kinetic energy. The nucleus recoils slightly to balance momentum. The likelihood of this happening increases with photon energy and is higher in materials with larger atomic number, since the nuclear field that enables the interaction is stronger.

Other interactions describe different ways photons interact with matter. The photoelectric effect involves absorbing the photon to eject an electron from an atom, which is more common at lower photon energies. Compton scattering is a collision with a loosely bound electron that transfers part of the energy to the electron and scatters the photon at a lower energy. Elastic (Rayleigh) scattering is the photon bouncing off the entire atom with little energy transfer.