Evaluation of Multi-Phase Microstructural Soil Properties on (Rn-222) Emanation Kinetics: A Geant4 Monte Carlo Approach

Backgroug: Traditional macroscopic environmental models frequently smooth over micro scale interfaces, failing to capture the highly localized, multi-phase boundary dynamics that govern natural radon gas escape from solid mineral structures into mobile pore spaces. Quantifying this phenomenon defined by the emanation coefficient(ϵ) is critical for defining accurate radiological source terms before large-scale transport occurs. Methods: In this work, a high-fidelity microscopic simulation framework was constructed using the Geant4 Monte Carlo toolkit (v11.4.1) within a native to model the two-body conservation kinematics of Radium-226 alpha decay. The simulation systematically tracked the resulting 86.2 keV Radon-222 heavy recoil ion through multi-phase micro scale soil configurations consisting of solid quartz mineral grains (ρ = 2.65 ց/cm3 ), concentric liquid water coating layers (ρ= 1.00ց/cm3), and air-filled interstitial pore channels under standard temperature and pressure (STP) conditions. Tracking loops were processed in optimized batch configurations (105 primary histories per run) with trajectory storage disabled to ensure computational efficiency.Results: The simulation isolated a sharp sigmoidal crossover boundary occurring near 40% volumetric moisture content. Below this critical threshold, attenuated recoil ranges caused adjacent grain implantation to dominate, trapping up to 85.0% of the recoiling ions inside neighboring crystal lattices. At 40% moisture, the fluid layer acted as an optimized kinetic "brake pad," compressing the track length to 88.1 ± 3.7 nm and maximizing loose pore accumulation ϵ = 0.237±0.003. Beyond 50% moisture, the total emanation fraction plateaus at its maximum value ( 60%), while the phase-specific Net Pore-Air Emanation Coefficient drops sharply (ϵair → 0.032) due to fluid-phase displacement and moisture-induced pore blockage. Furthermore, geometric grain scaling evaluation validated that radon escape is a surface-layer boundary phenomenon adhering strictly to an inverse power-law relationship, with the primary escape fraction collapsing from 88.4% in sub-micron matrices down to 1.8% in fine sand fractions (50.0 µm).Conclusion: These microstructural physics profiles replace empirical macroscopic adjustments with mathematically rigorous, physics-based data arrays. By defining the exact physical limitations governing initial gas release, this framework establishes the precise source-term boundary conditions required to improve the accuracy of macroscopic environmental transport codes and downstream public health risk assessments.