The reasoning
| Interaction | Electron involved | Energy range | Outcome | Imaging effect |
|---|---|---|---|---|
| Coherent (classical): Thomson (single electron), Rayleigh (whole atom) | whole atom / loosely bound | very low | photon changes direction, no energy lost | minor fog |
| Photoelectric | inner shell (K/L) | low (diagnostic, esp. < ~30 keV) | photon totally absorbed, photoelectron + characteristic radiation | contrast; no scatter; but dose to patient |
| Compton | outer shell (loosely bound) | medium (dominant above ~30 keV in soft tissue) | photon loses part of its energy and scatters | scatter: fog, reduced contrast, staff dose |
| Pair production | nuclear field | > 1.02 MeV | electron + positron (then annihilation, 511 keV × 2) | radiotherapy, PET physics; not diagnostic X‑ray |
| Photodisintegration | nucleus | > ~10 MeV | nucleus emits a particle | high‑energy radiotherapy only |