Inelastic collision processes of low-energy protons in liquid water

Production probabilities of ions and excited particle species along the proton beam track in liquid water are estimated around the Bragg peak region, taking into account charge-changing processes and energetic secondary electron (δ-ray) behavior. Ionization and excitation processes are divided into two categories in this study: primary processes associated with direct proton (or hydrogen) interaction and secondary processes arising from the electrons ejected by the primary process. We show that the number of events in the secondary processes producing ions and excited particles is larger than that of the primary processes around the Bragg peak while neutralized protons (i.e., hydrogen) with low energy have a large contribution to direct ionization. Effects of charge-changing processes on ionization and excitation are also discussed.