Transport of low- and medium-energy electron and ion beams in seawater and its vapors

A general theory of stopping power for electrons and ions in a target medium (gas, liquid, or thin solid foil) containing neutral as well as positive and negative ions is developed. The approach is quite accurate in the low- and medium-energy range (<1 MeV), where the Bethe stopping-power cross section is inaccurate. The energy transfer during individual collisions is treated by the binary-encounter approximation. The theory is applied to determine the stopping power of seawater, whose major components are ${\mathrm{H}}_2$O molecules and ${\mathrm{Na}}^{+}$ and ${\mathrm{Cl}}^{\mathrm{-}}$ ions. The stopping-power cross sections for low- and medium-energy protons in water show excellent agreement with existing measurements. The theory is extended to relativistic energies and to cases where the excitation of the incident and target particles is allowed. The approach is quite general and can be applied to other ionic, atomic, and molecular systems.