Optimised geometry to calculate dose rate conversion coefficient for external exposure to photons

Area of Science:

• Radiological Physics
• Environmental Dosimetry
• Computational Geophysics

Background:

• Accurate calculation of absorbed dose rate in air is crucial for environmental radiation monitoring.
• Monte Carlo methods are standard for radiation transport but computationally intensive, especially for large volumes like soil.
• Natural radionuclides in soil are a significant source of environmental radiation exposure.

Purpose of the Study:

• To develop a single-parameter, optimized soil geometry for efficient Monte Carlo calculations of absorbed dose rate in air.
• To reduce computational time in simulations of photon emitters from natural radionuclides in soil.
• To validate the optimized geometry's effectiveness in determining dose rate conversion coefficients.

Main Methods:

• Developed a simplified soil geometry based on physical assumptions regarding radiation reach to the detector.
• Implemented the optimized geometry within the Geant4 simulation toolkit.
• Conducted simulation tests using uniform and exponential radioactivity distributions to assess dose contribution from different soil volumes.

Main Results:

• The optimized geometry significantly reduced computation time.
• For a cylindrical soil model (40m radius, 1m deep), only 6% of the volume contributed >98% of the dose rate conversion coefficients (uniform distribution).
• The optimized geometry achieved 99% of conversion coefficients with ~1m depth and 100% with ~2m depth.

Conclusions:

• A single-parameter optimized geometry effectively describes soil for Monte Carlo absorbed dose rate calculations.
• This approach drastically improves computational efficiency while maintaining high accuracy for dose rate conversion coefficients.
• The findings enable faster and more resource-efficient environmental radiation assessment from natural soil radioactivity.