Which interaction between x-ray photons and matter involves a recoil electron?

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

Answer: (C)

The interaction between x-ray photons and matter that involves a recoil electron is known as Compton scatter. In this process, an incoming x-ray photon interacts with a loosely bound outer shell electron of an atom. When the photon strikes the electron, it transfers part of its energy to the electron, which is then ejected from the atom, resulting in a recoil. The photon loses energy and changes direction, leading to a lower-energy scattered photon.

This phenomenon is significant in medical imaging and radiation therapy, as it contributes to image formation and the dose distribution of x-ray beams through tissues. Compton scattering primarily occurs at intermediate energies of x-ray photons, making it a crucial interaction mechanism within the diagnostic x-ray range.

In contrast, other interactions do not involve the ejection of an electron from the atom in the same way. Coherent scatter primarily involves the scattering of photons without energy loss, while the photoelectric effect results in complete absorption of the photon leading to the ejection of an electron from an inner shell, and pair production occurs at very high photon energies where an x-ray photon can transform into an electron-positron pair near a nucleus, both of which do not involve recoil in the same sense as Compton scattering.