A Monte Carlo simulation framework for investigating the effect of inter-track coupling on H$_2$O$_2$ productions at ultra-high dose rates
Background: Lower production of H$_2$O$_2$ in water is a hallmark of ultra-high dose rate (UHDR) compared to the conventional dose rate (CDR). However, the current computational models based on the predicted yield of H$_2$O$_2$ are in opposite of the experimental data. Methods: We construct an analytical model for the rate equation in the production of H$_2$O$_2$ from \ce{^{.}OH}-radicals and use it as a guide to propose a hypothetical geometrical inhomogeneity in the configuration of particles in the FLASH-UHDR beams. We perform a series of Monte Carlo (MC) simulations of the track structures for a system of charged particles impinging the medium in the form of clusters and/or bunches. Results: We demonstrate the interplay of diffusion, reaction rates, and overlaps in track-spacing attribute to a lower yield of H$_2$O$_2$ at FLASH-UHDR vs. CDR. This trend is reversed if spacing among the tracks becomes larger than a critical value, with a length scale that is proportional to the diffusion length of \ce{^{.}OH}-radicals modulated by a rate of decay due to recombination with other species, available within a track, and the space among the tracks. The latter is substantial on the suppressing of the H$_2$O$_2$ population at FLASH-UHDR relative to CDR. Conclusions: Based on our analysis of the present work, at FLASH-UHDR, the lower yield in H$_2$O$_2$ can be interpreted as a signature of bunching the particles in beams of ionizing radiation. The beams enter the medium in closely packed clusters and form inhomogeneities in the track-structure distribution. Thus the MC simulations based on the assumption of uniformly distributed tracks are unable to explain the experimental data.