<aside>

The photoelectric effect is an x-ray interaction with matter in which an incident x-ray photon transfers all its energy to an inner shell electron (usually K- or L-shell), causing its ejection from the atom.

</aside>

It is one of the primary mechanisms by which x-rays are attenuated in tissue and is the dominant interaction at low photon energies (diagnostic radiology range ~20–100 keV in high-Z materials like bone).

![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. Incident photon strikes an inner shell electron.

2. Photon energy (Eγ) must be greater than or equal to the binding energy (Eb) of that electron.

3. Photon is completely absorbed → no scattered photon.

4. Electron is ejected as a photoelectron with kinetic energy:

   $$ Epe=Eγ−EbE_{pe} = E_{\gamma} - E_{b} $$

5. Vacancy in inner shell is filled by outer shell electron → release of characteristic x-rays or Auger electrons.

Probability & Dependence

• Atomic number (Z): Probability ∝ Z³ → higher in high-Z materials (bone, contrast media like iodine, barium).
• Photon energy (E): Probability ∝ 1/(E³) → decreases rapidly with increasing energy.
• Dominant at low energies and high-Z tissues, hence contributes to image contrast in diagnostic radiology.

Radiological Importance

• Image Contrast:
  • In soft tissue vs bone → bone attenuates more due to higher Z → white on X-ray.
  • In contrast studies (iodine, barium) → high-Z contrast agents enhance attenuation.
• Absorbed Dose:
  • Photoelectric absorption contributes significantly to patient dose, as all energy is deposited locally.
• K-edge effect:
  • Sudden increase in absorption just above the binding energy of inner shell electrons (e.g., K-edge of iodine ~33 keV, barium ~37 keV).
  • Utilized in contrast imaging.