Photoionization and Compton Double Ionization of Helium from Threshold to 20 keV

We report new measurements of ${\mathrm{He}}^{++}/{\mathrm{He}}^{+}$ ratios in three energy ranges. Near threshold, where only photoionization contributes, we find a ratio systematically lower than previously published measurements. From 2 to 6 keV, where Compton ionization gradually supplants photoionization as the dominant ionization mechanism, theory and experiment are in substantial agreement. Far above threshold where Compton ionization dominates, theories diverge markedly. Comparison with present results indicates asymptotic values have been established for photoionization but not Compton ionization.