Which interaction is described as gamma rays scattering off a loosely bound electron and changing direction with reduced energy?

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

Answer: (C)

This shows Compton scattering: a gamma photon interacts with an electron and scatters in a new direction while transferring some energy to the electron, so the photon comes out with reduced energy. When the electron is loosely bound, it behaves like a nearly free electron, so the photon’s energy decreases and its path bends in proportion to the scattering angle. The bigger the angle, the more energy is given to the electron and the greater the change in the photon’s energy (its wavelength increases).

Photoelectric effect would involve the photon being absorbed and an electron ejected, with no scattered photon remaining. Pair production requires the photon to create an electron-positron pair, which happens at higher energies and once again doesn’t describe a single scattered photon. Elastic scatter would imply little to no energy change in the photon, which doesn’t match the reduced energy described here.