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Compton scattering is an interaction between an incident X-ray photon and a loosely bound outer-shell electron, in which the photon is deflected and loses part of its energy, while the electron is ejected (Compton or recoil electron).

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• It is the dominant interaction in soft tissue at diagnostic X-ray energies (~30–150 keV).

![Illustrative summary of x-ray and γ-ray interactions. (A) Primary, unattenuated beam does not interact with material. (B) Photoelectric absorption results in total removal of incident x-ray photon with energy greater than binding energy of electron in its shell, with excess energy distributed to kinetic energy of photoelectron. (C) Rayleigh scattering is interaction with electron (or whole atom) in which no energy is exchanged and incident x-ray energy equals scattered x-ray energy with small angular change in direction. (D) Compton scattering interactions occur with essentially unbound electrons, with transfer of energy shared between recoil electron and scattered photon, with energy exchange described by Klein–Nishina formula.

Seibert JA, Boone JM. X-Ray Imaging Physics for Nuclear medicine Technologists. Part 2: X-Ray Interactions and Image Formation. Journal of Nuclear Medicine Technology. Published March 1, 2005.](attachment:9038dd57-0643-414e-b937-bffec2706b74:image.png)

Illustrative summary of x-ray and γ-ray interactions. (A) Primary, unattenuated beam does not interact with material. (B) Photoelectric absorption results in total removal of incident x-ray photon with energy greater than binding energy of electron in its shell, with excess energy distributed to kinetic energy of photoelectron. (C) Rayleigh scattering is interaction with electron (or whole atom) in which no energy is exchanged and incident x-ray energy equals scattered x-ray energy with small angular change in direction. (D) Compton scattering interactions occur with essentially unbound electrons, with transfer of energy shared between recoil electron and scattered photon, with energy exchange described by Klein–Nishina formula.

Seibert JA, Boone JM. X-Ray Imaging Physics for Nuclear medicine Technologists. Part 2: X-Ray Interactions and Image Formation. Journal of Nuclear Medicine Technology. Published March 1, 2005.

Mechanism

1. Incoming photon strikes an outer orbital electron.
2. Electron absorbs part of the photon energy → ejected (Compton electron).
3. Photon continues with reduced energy and altered direction.
4. Energy conservation:
   • Ephoton=Ephoton′+Eelectron+EbindingE_{photon} = E'{photon} + E{electron} + E_{binding}
   • Since binding energy of outer electron is small, nearly all energy split between recoil electron and scattered photon.

Key Features

• Independent of atomic number (Z): depends only on electron density → why Compton dominates in soft tissue.
• Probability increases with:
  • Higher photon energy (up to ~150 keV, then decreases).
  • Higher electron density (more tissue → more scatter).
• Scattered photon angle:
  • Forward scatter more common at higher energies.
  • Backscatter more likely at lower energies.

Radiological Importance

1. Image quality
   • Scatter photons reaching detector cause image fog, loss of contrast.
2. Radiation dose
   • Major contributor to staff occupational exposure (scattered radiation around patient).
3. Patient dose
   • Compton electrons deposit energy locally → contributes to absorbed dose.

Comparison with Other Interactions