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, {ital R}{sub {ital C}}={sigma}{sub {ital C}}{sup ++}/{sigma}{sup +}{sub {ital C}}, at a photon energy of 58 keV using cold target recoil ion momentum spectroscopy. We find a value {ital R}{sub {ital C}}=(0.84{sub {minus}0.11}{sup +0.08}){percent} that is in agreement with the asymptotic limits predicted by Andersson and Burgd{umlt o}rfer [Phys. Rev. A {bold 50}, R2810 (1994)] and Suri{acute c} {ital et} {ital al}.[Phys. Rev. Lett. {bold 73}, 790 (1994)]. {copyright} {ital 1996 The American Physical Society.}