Author: Konstantinos Chatzipapas, George C. Kagadis, Panagiotis Konstas, Dimitris N. Mihailidis, Dimitris Visvikis 👨🔬
Affiliation: LaTIM, INSERM UMR1101, University of Pennsylvania, LaTIM, INSERM, University of Brest, University of Patras 🌍
Purpose: This study investigates the temporal evolution of radiolytic species (H3O+, OH•, OH-) in water under electron and proton irradiation using the mesoscopic chemistry model that allows longer temporal tracking of reactive oxygen species(ROS).
Methods: Monte Carlo simulations with Geant4-DNA were executed to track the temporal dynamics of ROS within a defined time window (10-12 s - 10-3 s), which is extended due to the use of the mesoscopic model, instead of the previously implemented in Geant4-DNA, allowing intertrack interactions, which are an important parameter in ultra-high dose rate conditions. Electron energies ranged between 0.01 - 1 MeV, to simulate secondary electrons. The pH was maintained at 5.5 and oxygen concentration was initially set at the normal level of 21% and can be adapted to investigate the oxygen enhancement ratio (OER) under UHDR conditions. Proton energies ranged from 67 MeV to 197 MeV. G-values, representing the number of ROS produced per 100 eV of absorbed energy, were calculated for each ROS and plotted as a function of particles energy.
Results: The simulations demonstrated a consistent trend: H3O+ and OH• concentrations decreased over time, while OH- concentration increased. Initial concentrations were H3O+ = 4.25 ± 0.1, OH• = 5.1 ± 0.1, and OH- = 0.09 ± 0.01 for all cases. The observed trends are attributed to the complex interplay of radiolytic reactions, including radical-radical recombination, radical-molecule reactions, and ion-molecule reactions. G-values exhibited dependence on electron energy, which is directly linked to linear energy transfer (LET). As investigated particles’ energy increases, LET decreases, leading to changes in the interaction patterns and consequently the production of ROS.
Conclusion: Valuable insights into the temporal and energy-dependent behavior of ROS in irradiated water have been presented. Further investigation is needed to explore the impact of pH, oxygen concentration, dose rate, and the presence of scavengers.