Double and Single Ionization of Helium by 58-keV X Rays

We have measured the ratio of cross sections for double to single ionization of helium by Compton scattering, $R_C={\sigma }_C^{++}/{\sigma }_C^{+}$, at a photon energy of 58 keV using cold target recoil ion momentum spectroscopy. We find a value $R_C=\left({0.84}_{-0.11\mathrm{}}^{+0.08\mathrm{}}\right)\%$ that is in agreement with the asymptotic limits predicted by Andersson and Burgdörfer [Phys. Rev. A 50, R2810 (1994)] and Surić et al. [Phys. Rev. Lett. 73, 790 (1994)].