Experimental ionization cross sections for fast helium atoms and ions in a water gas as a function of impact speed. “AR” refers to the additive rule: when no experimental results for H₂O are available, results for H₂ and O₂ are combined to give an estimate (see text for details); experimental uncertainties for these estimates are at least 25%. As no experimental data are available for He⁰, the composite recommendation of Uehara & Nikjoo (2002) was chosen (see text). Polynomial fits in thick continuous and dashed lines are also shown, and the coefficients are listed in Table 3. Smooth extrapolations in power laws at low and high energies are indicated as thin dotted lines.