Implementation of temperature–pH coupling in the Geant4-DNA chemistry module for water radiolysis simulations

Purpose: The Geant4-DNA source code was locally modified by users to enable water radiolysis at various temperatures and pH by previous work, however, without linking those two parameters. This study aimed to integrate a quantitative correlation between temperature and pH in a user-modified local version of the Geant4-DNA. Methods: We quantified the temperature-pH relationship via polynomial regression and integrated it into the Geant4-DNA chemistry constructor. The integration was validated using a user-modified local version of Geant4-DNA (version 11.3.0), simulating isotropic irradiation of a 1.0 km 3 water cube (pH 7) by a central point source across 25-150 °C. Results were compared against literature data. Results: The effects of temperature and pH became observable around 100 ns (the end of the non-homogeneous stage), as pH mainly affects reactions between generated and background species. While pure water pH is 7.0 at 25 °C, it decreases at higher temperatures, shifting away from neutrality. When one accounts for this temperature–pH interdependence, higher temperatures amplify the deviation from pH 7, thereby enhancing ionic reactions after the intra-spur and non-homogeneous stages. In our simulations of water radiolysis, the resulting G-values agree with experimental measurements within 15.33% and with other simulated data within 11.99%, with uncertainties below ± 0.60%. The trends we observe in these G-values also closely mirror those reported in the literature. Conclusions: The successful integration of the temperature-pH correlation in a custom version of Geant4-DNA chemistry module was demonstrated. This integration improves the physical realism of water radiolysis and contributes to various scientific fields.