Lecture on Photon Interactions with Matter

Introduction

• Focus on photon interactions and shielding calculations.
• Correcting previous errors in energy calculations of Compton scattering.

Compton Scattering

• Energy & Momentum Balance: Essential for understanding photon interactions.
  • Photons have no mass, rely on energy and momentum.
  • Relationships between incoming photon energy, outgoing photon energy, and electron recoil energy.
• Wavelength Shift: Independent of photon energy; depends on scattering angle and electron mass.
• Recoil Energy: Dependent on both photon energy and angle.
• Limiting Cases:
  • Low-energy photons transfer minimal energy.
  • High-energy photons can transfer maximum energy.
  • Maximum energy transfer occurs at backscatter (theta = PI).
• Practical Applications:
  • Photon detection and measurement of recoil energy of electrons.

Compton Camera

• Detection: Using two detectors to pinpoint radiation source location.
• Mechanism:
  • Determine angle and energy of scattered photons.
  • Different angles lead to different energy distributions.
  • Helps locate sources in complex environments like cargo ships.

Cross Sections and Photon Interactions

• Cross Sections in Photon Matter Interactions
  • Photoelectric Effect: Dominant at low energies and high Z materials.
  • Compton Scattering: Intermediate range.
  • Pair Production: Dominant at high energies and high Z materials.
  • Klein-Nishina Formula: Describes Compton scattering probability as a function of angle.

Differential Cross Sections

• Interpretation: Shows probability of scattering at a specific angle.
• Applications: Used in Compton cameras to deduce source locations.

Mass Attenuation Coefficients

• Purpose: Describes the removal of photons in a beam by a material.
• Mass Attenuation Coefficient: Expressed in cm²/g, helps in shielding calculations.
• Dependence: Density and thickness of the material.

Shielding Calculations

• Example: Comparing different states of water (0°C vs 100°C) in photon shielding.
• Effect of Temperature: Density changes affect shielding efficiency.

Photon Energy and Material Effects

• Material and Energy Dependency: Photon interactions vary with energy level and material (Z value).
• Empirical Cross Section Data: Available for practical calculations.

Conclusion

• Photon interactions are complex and require understanding of quantum physics.
• Applications in shielding, detection, and energy measurement.
• Useful for practical applications in industry and safety.