Ion-beam cancer therapy has become increasingly favored worldwide in treatment of certain types of cancer over the last decade. Whereas the clinical effects of the therapy are well documented, the understanding of the underlying physical mechanisms is somewhat incomplete. The problem arises due to the multiscale nature of the effects involved in ion-beam cancer therapy, as the effects range from quantum-mechanical to macroscopic scales. The present study investigates the production of free electrons in the vicinity of the Bragg peak through quantum-mechanical simulations of the collision between a C 4+ ion with a cytosine-guanine nucleotide pair taken from a DNA double helix. Time-dependent density-functional theory was employed using the OCTOPUS 6.0 software. The results show that such a collision triggers the release of a large amount of electrons into the cellular environment, as only a fraction is captured by the C 4+ ion. Furthermore, it is demonstrated that the impact angle and projectile kinetic energy have much more influence on the number of ejected electrons than the impact parameter.

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The presented investigation aims to establish a foundation for the study of ion beam cancer therapy employing time-dependent density functional theory for calculating collision cross-sections and energies of secondary electrons produced by a charged ion impacting on a biological target of arbitrary size and shape. The obtained collision cross-sections compare well to values obtained using the popular PASS code, which relies on the modified Bohr theory. Furthermore, we demonstrate that the differential cross-sections obtained in this study seem to be affected by post-collision electron recapture processes occurring inside the target.