Which interaction is involved in absorbing diagnostic x-ray photons by the phosphor layer in radiographic imaging, among the options?

• A. Classical
• B. Compton
• C. Photoelectric
• D. Characteristic

Answer: (C)

The primary interaction at diagnostic X-ray energies that leads to the absorption of photons by the phosphor layer is the photoelectric effect. This happens because the phosphor material used in radiographic imaging contains relatively high atomic number elements, which increases the likelihood of photoelectric absorption compared with other interactions at typical clinical energies. In the photoelectric process, an incoming photon is completely absorbed and transfers its energy to an inner-shell electron, ejecting that electron from its orbit. The absorbed energy creates electron traps in the phosphor lattice, storing energy that constitutes the latent image. Later, when the plate is read with a stimulating light source, those trapped electrons release energy as visible light, producing the image signal.

Compton scattering can occur and contributes to scatter and image noise, but it does not preferentially absorb the photon energy in the phosphor as efficiently as photoelectric absorption. Characteristic radiation is a consequence of the transitions that follow certain inner-shell vacancies and is not the mechanism by which the photon is initially absorbed by the phosphor. Classical concepts don’t describe a specific interaction type for this absorption in radiographic phosphors.