Simulating the nanometric track-structure of carbon ion beams in liquid water at energies relevant for hadrontherapy

Abstract

The nanometric track-structure of energetic ion beams in biological media determines the direct physical damage to living cells, which is one of the main responsibles of their killing or inactivation during radiotherapy treatments or under cosmic radiation bombardment. In the present work, detailed track-structure Monte Carlo simulations, performed with the code SEED (Secondary Electron Energy Deposition), are presented for carbon ions in a wide energy range in liquid water. Liquid water is the main constituent of biological tissues, and carbon ions are one of the most promising projectiles currently available for ion beam cancer therapy. The simulations are based on accurate cross sections for the different elastic and inelastic events determining the interaction of charged particles with condensed-phase materials. The latter are derived from the ab initio calculation of the electronic excitation spectrum of liquid water by means of time-dependent density functional theory (TDDFT), which is then used within the dielectric formalism to obtain inelastic electronic cross sections for both carbon ions and secondary electrons. Both the ionisation cross sections of water by carbon ions and the excitation and ionisation cross sections for electron impact are obtained in very good agreement with known experimental data. The elastic scattering cross sections for electrons in condensed-phase water are also obtained from ab initio calculations by solving the Dirac-Hartree-Fock equation. The detailed simulations fed with reliable cross sections allow to assess the contribution of different physical mechanisms (electronic excitation, ionisation and dissociative electron attachment –DEA–) to the carbon ion-induced direct biodamage.