Introduction
Summary
- The notes detail the interactions of ionizing radiation with matter, emphasizing the key processes of photoelectric absorption, Compton scattering, and electron-positron pair formation, alongside the principles governing radiation intensity attenuation. Each process is analyzed in terms of its dependency on factors such as photon energy, atomic number, material density, and incident angle, highlighting their roles in medical imaging and radiation therapy.
Photoelectric Absorption
- Process Description: Occurs when an X-ray photon transfers all its energy to an electron, ejecting it from the atom.
- Key Dependencies:
- Incident Photon Energy: Must be high enough to overcome electron binding energy.
- Atomic Number (Z): Higher Z increases absorption probability due to more tightly bound electrons.
- Material Density (ρ): Denser materials increase interaction likelihood due to higher atom density.
Compton Scattering
- Mechanism Overview: Moderate-energy X-rays scatter off loosely bound electrons, transferring part of their energy and causing electron ejection.
- Key Aspects:
- Photon Scattering: Reduced energy photon scatters at an angle dependent on energy transfer.
- Material Density Influence: Affects scattering probability and intensity of scattered X-rays.
Electron-Positron Pair Formation
- Interaction Mechanism: High-energy photons convert into an electron and a positron upon interacting with the nuclear electromagnetic field.
- Energy Requirements:
- Threshold Energy: Minimum of 1.022 MeV, corresponding to twice the electron rest mass.
- Energy Distribution: Excess photon energy is divided between the kinetic energies of the newly formed electron and positron.
Intensity Attenuation Law and General Linear Attenuation Coefficient
- Attenuation Law:
- Equation:
- Variables:
- Linear Attenuation Coefficient (μ):
- Dependency: Influenced by material properties such as density, atomic number, and X-ray energy.
- Contribution: Sum of attenuation effects from photoelectric absorption, Compton scattering, and pair production.
- Total linear attenuation coefficient (μ) for X-ray equation :
- μ=τ+σ+κ
- μ (mu) is the total linear attenuation coefficient
- τ (tau) is the photoelectric absorption coefficient
- σ (sigma) is the Compton scattering coefficient
- κ (kappa) is the pair production coefficient