Double-differential cross section for ionization of H2O molecules by 4-MeV/u C6+ and Si13+ ions

Double-differential cross section (DDCS) for electrons ejected in collisions of fast ${\mathrm{C}}^{6+}$ and ${\mathrm{Si}}^{13+}$ projectiles, with a ${\mathrm{H}}_{2}\mathrm{O}$ vapor target, were measured. The electrons were detected over an energy range of 1--600 eV and an angular range of ${20}^{\ensuremath{\circ}}$--${160}^{\ensuremath{\circ}}$. The obtained DDCS spectra, for both the ions, were compared with the CDW-EIS model. Occasional reference has been made to the DDCS data for the case of 3.75-MeV/u ${\mathrm{O}}^{8+}$ colliding on the same molecule for an overall comparison. A reasonable agreement with theoretical results was seen for the case of ${\mathrm{C}}^{6+}$ and ${\mathrm{O}}^{8+}$ projectiles. However, between ${\mathrm{C}}^{6+}$ and ${\mathrm{O}}^{8+}$ projectiles, the deviation from theory is larger for the case of the carbon projectile. Substantial deviation starts to show up for the case of the ${\mathrm{Si}}^{13+}$ projectile. By numerical integration of the DDCS data, the single-differential cross section (SDCS) and total cross section (TCS) were obtained and compared with theoretical models. The present TCS data along with the other available data for $p$, $\mathrm{He}$, and $\mathrm{C}$ ions were plotted together. A clear and gradual deviation from the Bethe-Born predicted ${q}^{2}$ scaling was observed, where $q$ is the projectile charge state. From all the data we find TCS varies as ${q}^{n}$ where $n$ = 1.7 $\ifmmode\pm\else\textpm\fi{}$ 0.1. The provided data set will be valuable in order to help model the radiation damage in hadron therapy, particularly in the Bragg peak region.