We have calculated the electronic stopping power and the energy-loss straggling parameter of swift He, Li, B, and N ions moving through several oxides, namely SiO 2 , Al 2 O 3 , and ZrO 2 . The evaluation of these stopping magnitudes was done in the framework of the dielectric formalism. The target properties are described by means of a combination of Mermin-type energy-loss functions that characterize the response of valence-band electrons, together with generalized oscillator strengths to take into account the ionization of inner-shell electrons. We have considered the different charge states that the projectile can have, as a result of electron capture and loss processes, during its motion through the target. The electron density for each charge state was described using the Brandt-Kitagawa statistical model and, for He and Li ions, also hydrogenic orbitals. This procedure provides a realistic representation of both the excitation properties of the target electrons and the projectile charge density, yielding stopping powers that compare reasonably well with available experimental data above a few tens of keV/amu.
We have evaluated the spatial distribution of energy deposition by proton beams in liquid water using the simulation code SEICS (Simulation of Energetic Ions and Clusters through Solids), which combines molecular dynamics and Monte Carlo techniques and includes the main interaction phenomena between the projectile and the target constituents: (i) the electronic stopping force due to energy loss to target electronic excitations, including fluctuations due to the energy-loss straggling, (ii) the elastic scattering with the target nuclei, with their corresponding energy loss and (iii) the dynamical changes in projectile charge state due to electronic capture and loss processes. An important feature of SEICS is the accurate account of the excitation spectrum of liquid water, based on a consistent solid-state description of its energy-loss-function over the whole energy and momentum space. We analyse how the above-mentioned interactions affect the depth distribution of the energy delivered in liquid water by proton beams with incident energies of the order of several MeV. Our simulations show that the position of the Bragg peak is determined mainly by the stopping power, whereas its width can be attributed to the energy-loss straggling. Multiple elastic scattering processes contribute slightly only at the distal part of the Bragg peak. The charge state of the projectiles only changes when approaching the end of their trajectories, i.e. near the Bragg peak. We have also simulated the proton-beam energy distribution at several depths in the liquid water target, and found that it is determined mainly by the fluctuation in the energy loss of the projectile, evaluated through the energy-loss straggling. We conclude that a proper description of the target excitation spectrum as well as the inclusion of the energy-loss straggling is essential in the calculation of the proton beam depth-dose distribution.
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