Author: Ramin Abolfath, Sedigheh Fardirad, Abbas Ghasemizad, Houda Kacem, Marie Catherine Vozenin 👨🔬
Affiliation: University of Guilan, Universite de Lausanne, University Hospital, Geneva, Howard University 🌍
Purpose: Lower production of H2O2 in water is a hallmark of ultra-high dose rate (UHDR) compared to the conventional dose rate (CDR). The current computational models are in the opposite of the experimental data. We present a multi-scale formalism to reconcile this disagreement.
Methods: We construct analytical rate equations and Monte Carlo (MC) models for the H2O2 production and use it as a guide to propose a hypothetical geometrical and chemical inhomogeneities in the configuration of particles in form of clusters and/or bunches in the UHDR beams.
Results: We demonstrate the interplay of diffusion, reaction rates, and overlaps in track-spacing attribute to a lower yield of H2O2 at 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 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 H2O2 population at UHDR relative to CDR. Under these conditions in our MC setup, the reduction of H2O2 dose rate effect within 1ms time-scale is attributed mainly to several orders of magnitude earlier times, e.g., 1 ns of the e_aq reaction with OH-radicals and negligible reaction with H-atom.
Conclusion: Based on our analysis of the present work, at UHDR, the lower yield in H2O2 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.