X-Ray Ionization: Photoelectric Effect, Compton Scattering & Pair Production

X-ray interactions with matter cause ionization via three primary mechanisms:

1. Photoelectric Effect (Low Energy, High Z)

• Photon transfers all energy to inner-shell electron → ejected (photoelectron).
• Atom becomes positive ion; vacancy filled by outer electron → characteristic X-ray/Auger electron.
• Dominant <50 keV; probability ∝ Z³/E³.
• Basis of radiographic contrast (bone vs. soft tissue).

2. Compton Scattering (Intermediate Energy)

• Photon collides with loosely bound outer electron → shares energy.
• Scattered photon (lower energy) + recoil electron.
• Dominant 50 keV–5 MeV; probability ∝ Z/E.
• Major source of scatter radiation (fog on images, staff dose).

3. Pair Production (High Energy >1.022 MeV)

• Photon near nucleus → electron-positron pair.
• Positron annihilates → two 511 keV annihilation photons.
• Dominant >10 MeV; probability ∝ Z².
• Relevant in radiotherapy (linacs), not diagnostic imaging.

Applications

• Medical imaging (contrast via photoelectric effect).
• Radiotherapy (ionization kills tumor cells).
• Industrial NDT, X-ray crystallography.

Understanding interactions optimizes image quality, dose, and safety.