The photoelectric effect contributes to patient dose primarily through which process?

• A. Excitation
• B. Divergence
• C. Scattering
• D. Ionization

Answer: (D)

When a photon undergoes photoelectric absorption, it is completely absorbed by an atom and an inner-shell electron is ejected. The energy that goes into overcoming the electron’s binding energy becomes the kinetic energy of the ejected photoelectron, which travels through tissue and loses energy by causing further ionizations and excitations along its path. This ionization deposits energy locally, which is what contributes to patient dose.

Excitation would leave the electron in a higher bound state rather than ejecting it, so it doesn’t produce the same local ionization and dose. Divergence is about where the beam goes, not how energy is deposited. Scattering (Compton) is a separate interaction that also deposits energy, but it’s not the mechanism described by the photoelectric effect.