A step-by-step Geant4-DNA framework for ultra-high dose rate radio-induced chemistry using fluence-based dose modeling

Experiments in pure water have reported significant changes in primary radio-induced chemical yields under ultra-high dose rate (UHDR) conditions, suggesting that spatial track proximity alters early radiolytic stages. However, standard simulation approaches often fail to fully reproduce these yield variations. Here, we present a computationally efficient Geant4-DNA-based framework that couples fluence-based dose deposition with Brownian transport of chemical species, preserving the spatial resolution of early aqueous radiochemistry up to 2 m s . A dedicated injection scheme enables systematic variations of pulse duration, track density, and radiation quality. Using the metric 𝜙 to quantify inter-track contributions, our results reveal a distinct dose- and LET-dependent transition in chemical reactivity. At high dose rates, deviations in chemical yields are driven predominantly by inter-track reactions, whereas the intra-track component remains unchanged within statistical uncertainty. Solvated electrons and hydroxyl radicals demonstrate the highest sensitivity to track overlap. For high-LET radiation ( 1 M e V protons), a dose-dependent reduction in 𝐺 ⁡ ( H 2 ⁡ O 2 ) is observed, consistent with experimental trends. In contrast, at low LET ( 1 0 0 M e V protons), the LET-dependent spatial distribution of solvated electrons enhances their self-reaction, altering solvated electron availability for competing pathways and leading to an increasing H 2 O 2 trend. While the high-LET model qualitatively reproduces experimental H 2 O 2 depletion, the absolute dose thresholds remain higher than reported experimentally, pointing toward uncertainties in the formation and spatial distribution of solvated electrons during the prechemical stage or the absence of explicit oxygen chemistry. Overall, this framework provides a baseline physicochemical reference model for isolating pulse structure and inter-track effects, serving as a flexible basis for future chemical extensions.