Ionization and excitation cross sections for the interaction of HZE particles in liquid water and application to Monte Carlo simulation of radiation tracks

Abstract

Relativistic heavy ions of high charge (Z) and energy (E) (HZE) in galactic cosmic rays (GCR) are important contributors to space radiation risk because they cannot be shielded completely and their relative biological effectiveness is very high. To understand these risks, Monte Carlo track structure simulations by radiation transport codes are widely used in radiation biology to provide information on energy deposition and production of radiolytic species that damage cellular structures. In this paper, we show that relativistic corrections can be applied to existing semi-empirical cross section models for the ionization and excitation of water molecules by ions to extend the validity of their energy range up to ∼104 MeV amu−1. Similarly, an effective charge value correction is applied for Z>2 ions. Simulations of HZE tracks have been performed by a new C++ Monte Carlo transport code, named RITRACKS, that uses these cross sections to calculate the stopping power, radial dose, XY-plane projections of track segments and radial distributions of primary radiolytic species (H•, •OH, H2, H2O2 and e−aq) at ∼10−12 s. These new data will be useful to understand results from experiments performed at ion accelerators by discriminating the role of the so-called core and penumbra of the tracks.