What interaction is primarily responsible for x-ray absorption in the phosphor layer at diagnostic energies?

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

Answer: (A)

Photoelectric absorption is the key interaction responsible for X-ray energy deposition in a high‑Z phosphor at diagnostic energies. In materials like cesium iodide, the probability of photoelectric absorption is high in the 20–100 keV range and increases with atomic number, while it falls off as the photon energy rises. When a photon is absorbed this way, an inner-shell electron is ejected and the absorbed energy is converted into visible light by the phosphor’s scintillation process, producing the image signal. Compton scattering does occur, but it mainly transfers some energy to a recoil electron and scatters photons away, rather than fully absorbing the photon to generate light, so it’s not the primary mechanism for absorption in the phosphor. The so‑called characteristic radiation results from subsequent de‑excitation after a photoelectric event, not the initial absorption itself, and is not the primary absorber.