Investigation of hydrogen peroxide yields and oxygen consumption in high dose rate irradiation: a TOPAS-nBio Monte Carlo study

<i>Objective.</i>TOPAS-nBio enables users to simulate dose rate-dependent radiation chemical yields in water radiolysis accounting for inter-track and long-term chemistry for pulsed irradiation. This study aims to extend the TOPAS-nBio chemistry for the special case of continuous high-dose rate scenario, where both intertrack and longer time reactions need to be considered, and to quantitatively validate the extended framework by comparing the results with experimental data.<i>Approach.</i>The inter-track chemistry and escape<i>G</i>-values were first evaluated by the independent reaction time method. The escaping molecules were assumed to have a temporally continuous distribution based on the<i>G</i>-values using the Gillespie algorithm. The simulation results were comprehensively validated by comparing with the experimental data at different dose rates, temporal pulse shapes, and solutions. In addition, the influence of various factors, such as the chemistry model, simulation volume, temperature, pH concentration, and organic carbon contamination, was evaluated.<i>Main results.</i>The validation results showed that the H₂O₂concentration and O₂consumption increased with dose rate, and agreed within 3% with experimental data. Computational factors related to the chemistry model and volume size were negligible. pH and temperature had an impact of less than 10% in the experimental range. The presence of organic carbon and resulting reactions doubled H₂O₂yields and significantly increased O₂consumption by about an order of magnitude at lower dose rates, while the results are almost unchanged at higher dose rates. Consequently, the dose rate dependence of H₂O₂yields and O₂consumption were reversed at a certain organic carbon concentration compared to the pure water results.<i>Significance.</i>The extended TOPAS-nBio chemistry framework enables the reproduction of the dose-rate dependent radiation chemical yields of several experimental studies at different dose rates, temporal pulse shapes, and solutions. This new functionality is necessary to investigate recent high dose rate (FLASH) experimental results.