Verification of Geant4-DNA step-by-step-reaction-diffusion master equation model for long-term radiolysis simulation

This paper describes the verification of the step-by-step-reaction-diffusion Master Equation (SBS-RDME) model, implemented in Geant4-DNA, for long-term radiolysis simulations in the Fricke dosimeter.

Scaling for reaction rate constants due to the high acidity of the Fricke solution was applied. The secondary electrons generated by gamma irradiation from Co-60 were used as the radiation source for the simulations. Model parameters were optimized by observing changes in output and computation time in response to variations in the starting time t and initial voxel resolution ℎ of the compartment-based simulation. The yields of ferric ion G(Fe³⁺) and chemical species influencing its formation were calculated using the SBS-RDME model and using the IRT method for comparison. The time evolution of the yields of Fe³⁺, OH, H, HO₂, H₂O₂, and H₂ were compared, and the reactions affecting the yield of each chemical species were analyzed.

The model parameters were set to t = 5 ns and h = 12.5 nm. The yield trends over time for chemical species were consistent between the SBS-RDME model and the IRT method. At 50 s, the G(Fe³⁺) from the two calculations agreed within 3.2 %. Contribution analysis of the reactions affecting the generation/removal of each chemical species indicated that the main reason for this discrepancy between the two calculations might be the inability of the SBS method to consider specific reaction types during simulation.

The SBS-RDME model was verified for long-term simulations by comparing its results to those obtained from the IRT method.