Medical physics · 2025
A Monte Carlo simulation framework for investigating the effect of inter-track coupling on H 2 O 2 ${\rm H}_{2}{\rm O}_{2}$ productions at ultra-high dose rates.
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Abolfath Ramin, Fardirad Sedigheh, Kacem Houda, Vozenin Marie-Catherine, Ghasemizad Abbas
Аннотация
BACKGROUND: Lower production of H 2 O 2 ${\rm H}_{2}{\rm O}_{2}$ 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 ${\rm H}_{2}{\rm O}_{2}$ are the opposite of the experimental data.
PURPOSE: To present a multi-scale formalism to reconcile the theoretical modeling and the experimental observations of H 2 O 2 ${\rm H}_{2}{\rm O}_{2}$ production and provide a mechanism for the suppression of H 2 O 2 ${\rm H}_{2}{\rm O}_{2}$ at FLASH-UHDR.
METHODS: An analytical model was constructed for the rate equation in the production of H 2 O 2 ${\rm H}_{2}{\rm O}_{2}$ from . OH ${^{.}{\rm OH}}$ -radicals and used as a guide to propose a hypothetical geometrical inhomogeneity in the configuration of particles in the UHDR beams. A series of Monte Carlo (MC) simulations of the track structures was performed for a system of charged particles impinging the medium in the form of clusters and/or bunches.
RESULTS: The interplay of diffusion, reaction rates, and overlaps in track-spacing contributes to a lower yield of H 2 O 2 ${\rm H}_{2}{\rm O}_{2}$ at UHDR versus 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 . OH ${^{.}{\rm OH}}$ -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 in the suppression of the H 2 O 2 ${\rm H}_{2}{\rm O}_{2}$ population at UHDR relative to CDR. Under these conditions in the present MC setup, the reduction of H 2 O 2 ${\rm H}_{2}{\rm O}_{2}$ dose rate effect within 1 ms time-scale is attributed mainly to several orders of magnitude earlier times, for example, 1 ns of the e aq - ${\rm e}^-_{\rm aq}$ reaction with . OH ${^{.}{\rm OH}}$ -radicals and negligible reaction with . H ${^{.}{\rm H}}$ .
CONCLUSIONS: Based on our analysis of the present work, at UHDR, the lower yield in H 2 O 2 ${\rm H}_{2}{\rm O}_{2}$ can be interpreted as a signature of bunching the particles in beams of ionizing radiation, and temporal correlations and time-dependent chain of reactions. 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.
PURPOSE: To present a multi-scale formalism to reconcile the theoretical modeling and the experimental observations of H 2 O 2 ${\rm H}_{2}{\rm O}_{2}$ production and provide a mechanism for the suppression of H 2 O 2 ${\rm H}_{2}{\rm O}_{2}$ at FLASH-UHDR.
METHODS: An analytical model was constructed for the rate equation in the production of H 2 O 2 ${\rm H}_{2}{\rm O}_{2}$ from . OH ${^{.}{\rm OH}}$ -radicals and used as a guide to propose a hypothetical geometrical inhomogeneity in the configuration of particles in the UHDR beams. A series of Monte Carlo (MC) simulations of the track structures was performed for a system of charged particles impinging the medium in the form of clusters and/or bunches.
RESULTS: The interplay of diffusion, reaction rates, and overlaps in track-spacing contributes to a lower yield of H 2 O 2 ${\rm H}_{2}{\rm O}_{2}$ at UHDR versus 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 . OH ${^{.}{\rm OH}}$ -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 in the suppression of the H 2 O 2 ${\rm H}_{2}{\rm O}_{2}$ population at UHDR relative to CDR. Under these conditions in the present MC setup, the reduction of H 2 O 2 ${\rm H}_{2}{\rm O}_{2}$ dose rate effect within 1 ms time-scale is attributed mainly to several orders of magnitude earlier times, for example, 1 ns of the e aq - ${\rm e}^-_{\rm aq}$ reaction with . OH ${^{.}{\rm OH}}$ -radicals and negligible reaction with . H ${^{.}{\rm H}}$ .
CONCLUSIONS: Based on our analysis of the present work, at UHDR, the lower yield in H 2 O 2 ${\rm H}_{2}{\rm O}_{2}$ can be interpreted as a signature of bunching the particles in beams of ionizing radiation, and temporal correlations and time-dependent chain of reactions. 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.