Which interaction occurs when a medium-energy gamma uses part of its energy to eject an electron and continues as a lower-energy gamma?

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

Answer: (B)

When a medium-energy gamma ray interacts with matter, it can collide with an electron and transfer part of its energy to that electron. The photon does not disappear; instead, it is scattered with lower energy and a change in direction. This exact process is Compton scattering.

The key idea is that energy is shared: the ejected electron takes some kinetic energy, and the remaining energy stays with the photon, now as a lower-energy gamma ray. The amount of energy the photon loses depends on the scattering angle—the larger the angle, the more energy is transferred to the electron, and the lower the energy of the scattered photon.

This differs from elastic scatter, where the photon would bounce off without losing energy; from the photoelectric effect, where the photon is absorbed and no scattered photon remains; and from pair production, where the photon energy is used to create an electron-positron pair (and there’s no scattered gamma ray in the simplest description). The described situation—part of the energy goes to eject an electron and a lower-energy gamma continues—fits Compton scattering precisely.