Which interaction is characterized by photons interacting with matter and causing ionization via recoil electrons?

• A. Compton scatter
• B. Photoelectric effect
• C. Coherent scatter
• D. Pair production

Answer: (A)

The interaction characterized by photons interacting with matter and causing ionization via recoil electrons is Compton scatter. In this process, an incoming photon collides with a loosely bound outer-shell electron in an atom. As a result, the photon transfers part of its energy to the electron, which is then ejected from its atom, achieving ionization in the process. The scattered photon, now with reduced energy, continues on a different path. This phenomenon is vital in radiography and medical imaging since Compton scattering contributes to the image formation while also affecting the dose received by the patient.

In contrast, the photoelectric effect involves complete energy absorption of the incoming photon by an electron, leading to its ejection and the overall ionization of the atom, but does not result in recoil photons. Coherent scatter (or Rayleigh scatter) occurs when low-energy photons interact with matter and scatter without ionization, preserving the photon's energy. Pair production involves the conversion of a photon into a matter-antimatter pair (electron and positron) when the photon energy exceeds 1.022 MeV, which is not characterized by the ejection of a recoil electron. Thus, Compton scatter distinctly fits the description of the interaction that causes ionization via recoil electrons