4. Conclusion

We have reviewed recent progress in calculations of the ratio of double to single photoionization of helium-like atoms at high energies. These ratios, both for ionization through photoeffect and Compton scattering, are largely to be understood through the shake-off mechanism, which assumes that one electron is suddenly removed from a two-electron system allowing the other only to shake off, as the residual system relaxes. Retardation does not change the photoeffect shake-off ratio (retardation is already included in the Compton process). Relativistic effects do not significantly change the ratio for either process. Both retardation and relativity affect the separate cross sections for double ionization and single ionization processes significantly. In the case of photoeffect we have also called attention to the recently proposed additional double ionization mechanism, involving equal energy sharing, which modifies the shake-off prediction for R_P at high energy.