Simulation of Radiation Chemistry by a One-Shot Hybrid Continuum / Monte Carlo Method

Abstract: Understanding the spatio-temporal evolution of radiolytic species created by high-energy electrons in water underpins key applications from radiotherapy and nuclear safety to environmental processing and electron microscopy. Here, using the Manchester Inhomogeneous Radiation Chemistry by Linear Expansions (MIRaCLE) toolkit, we introduce and benchmark a novel approach to simulating these processes. Although the initial conditions are determined stochastically, the subsequent time evolution is calculated deterministically using a continuum representation, derived from those initial conditions. This hybrid approach essentially averages over many chemistry ``trajectories'' simultaneously, often converging to the 1% level in one shot, not requiring multiple runs. We demonstrate this new approach through the calculation of time-dependent G-values for e_{aq}^-$, \dot{\mathrm{OH}} and other radiolytic products, including at unprecedented dose rates where calculations which would take years with a conventional Monte Carlo approach can be performed in mere hours on a commercial laptop. We demonstrate that the main artifact of continuum modelling can be mitigated by a correction term. These results establish MIRaCLE as a flexible and efficient platform for modelling long-timescale radiolysis, providing a bridge between Monte Carlo approaches and macroscopic reaction--diffusion schemes, with broad implications for radiation chemistry in medicine, energy, and materials science.