  The effective use of swift ion beams in cancer treatment (known as
hadrontherapy) as well as an appropriate protection in manned space missions
rely on the accurate understanding of energy delivery to cells damaging their
genetic information. The key ingredient characterizing the response of a medium
to the perturbation induced by charged particles is its electronic excitation
spectrum. By using linear response time-dependent density functional theory, we
obtain the energy and momentum transfer excitation spectrum (the energy-loss
function, ELF) of liquid water (main constituent of biological tissues), in
excellent agreement with experimental data. The inelastic scattering cross
sections obtained from this ELF, together with the elastic scattering cross
sections derived considering the condensed phase nature of the medium, are used
to perform accurate Monte Carlo simulations of the energy deposited by swift
carbon ions in liquid water and carried away by the generated secondary
electrons producing inelastic events (ionization, excitation, and dissociation
electron attachment, DEA), strongly correlated with cellular death, which are
scored in sensitive volumes having the size of two DNA convolutions. The sizes
of clusters of damaging events for a wide range of carbon ion energies, from
those relevant to hadrontherapy up to cosmic radiation, predict with
unprecedented statistical accuracy the nature and relative magnitude of the
main inelastic processes contributing to radiation biodamage, confirming that
ionization accounts for the vast majority of complex damage. DEA, typically
regarded as a very relevant biodamage mechanism, surprisingly plays a minor
role in carbon-ion induced clusters of harmful events.
