Advancements in Monte Carlo simulations with gMicroMC: reactive species build-up promotes radical-radical reactions at Flash dose rates

Ultra-high dose rate irradiations to water indicate an enhancement of radical-radical reactions, which could potentially correlate with the Flash effect. The purpose of this work was to extend gMicroMC to support multiple pulse simulations and Flash dose rates, and to investigate, in a pure water model, the mechanisms underlying the enhancement of radical-radical reactions under Flash conditions. gMicroMC, a GPU-based Monte Carlo track-structure algorithm, was extended to simulate multiple pulses. Pure water was exposed to multiple 70 MeV protons pulses delivering up to 20 Gy. The pulse dose rate was set to 2*10^5 and 10^6 Gy/s, while the average dose rate ranged from 0.01 to 100000 Gy/s. The G-values of H2O2 were used to monitor the influence of dose rate on radical-radical reactions. The multiple pulse extension of gMicroMC was validated against Kinetiscope. Multiple pulse simulations indicated an average dose rate threshold. Below it, complete radical depletion occurred within the pulses, leading to constant G-values. Above it, reactive species accumulated throughout the irradiation, resulting in an increase of radical-radical reactions and thus the G-values of H2O2. The average dose rate thresholds were in the order of 10 and 100 Gy/s for pulse dose rates of 2*10^5 and 10^6 Gy/s, respectively. At ultra-high dose rates, the brief intervals between pulses led to a reactive species build-up, which enhanced radical-radical reactions. This build-up is more likely to promote radical-radical reactions than the inter-track mechanism. The advancements in gMicroMC provide a sophisticated tool to study chemical dose rate dependencies.