Which interaction is responsible for x-ray absorption in the phosphor layer of a CR plate, among the listed options?

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

Answer: (B)

Absorption in the CR phosphor layer at diagnostic x-ray energies occurs mainly through the photoelectric effect. In high‑Z phosphor materials, an incident x-ray can eject a tightly bound inner-shell electron, and the energy released when that vacancy is filled produces electron-hole excitations that the europium activator converts into visible light. This absorption pathway is efficient in this energy range, so the deposited energy forms the latent image where x-rays hit. Compton scattering, by contrast, mostly involves photons being scattered and transferring only part of their energy to recoil electrons, so it contributes to scatter and fog rather than the stored energy needed for luminescence. Characteristic interactions happen after inner-shell ionization and emit secondary x-rays; they are a follow-up effect, not the primary mechanism generating the stored energy. Classical isn’t a recognized interaction in this context.