Simulation of the energy spectra of swift light ion beams after traversing cylindrical targets: a consistent interpretation of experimental data relevant for hadron therapy

Abstract

We have performed detailed simulations of the energy spectra, recorded at several angles, of proton and helium ion beams after traversing thin cylindrical targets of different nature (liquid water and ethanol jets, as well as a solid aluminium wire), in order to reproduce a series of measurements intended to assess the stopping power of 0.3–2 MeV ions. The authors of these experiments derived values of the stopping power of liquid water (a quantity essential for the evaluation of radiation effects in materials, particularly for radiotherapy purposes) that are ~10% lower than what is expected from other measurements and theories. In our simulations, instead of treating the stopping power as an unknown free parameter to be determined, we use as input the electronic stopping power accurately calculated within the dielectric formalism. We take into account in the simulations the different interactions that each projectile can experience when moving through the target, such as electronic stopping, nuclear scattering or electron charge-exchange processes. The detailed geometry of the target is also accounted for. We find that our simulated energy distributions are in excellent agreement with the published measurements when the diameter of the cylindrical targets is slightly reduced, what is compatible with the potential evaporation of the liquid jets. On the basis of such an excellent agreement, we validate the accuracy of the model we use to calculate electronic excitation cross sections for ions in condensed matter in its range of applicability (particularly the electronic stopping power) needed for charged particle transport models, and we offer a consistent, but alternative, interpretation for these experiments on ion irradiation of cylindrical targets.