High-LET ion radiolysis of water: Visualization of the formation and evolution of ion tracks and relevance to the radiation-induced bystander effect

Ionizing radiation-induced bystander effects, commonly observed in cell populations exposed to high-linear energy transfer (LET) radiations, are initiated by damage to a cellular molecule which then gives rise to a toxic signal exported to neighboring cells not directly hit by radiation. A major goal in studies of this phenomenon is the identification of this initial radiation-induced lesion. Liquid water being the main constituent of biological matter, reactive species produced by water radiolysis in the cellular environment are likely to be major contributors to the induction of this lesion. In this context, the radiation track structure is of crucial importance in specifying the precise location and identity of all the radiolytic species and their subsequent signaling or damaging effects. We report here Monte Carlo track structure simulations of the radiolysis of liquid water by four different impacting ions ¹H⁺, ⁴He²⁺, ¹²C⁶⁺ and ²⁰Ne¹⁰⁺, with the same LET (∼70 keV/μm). The initial radial distribution profiles of the various water decomposition products (e_aq^-, ^.OH, H ^., H₂ and H₂O₂) for the different ions considered are presented and discussed briefly in the context of track structure theory. As an example, the formation and temporal evolution of simulated 24 MeV ⁴He²⁺ ion tracks (LET ∼26 keV/μm) are reported for each radiolytic species from 1 ps to 10 μs. The calculations reveal that the ion track structure is completely lost by ∼1 μs.