Double $K$-shell ionization of atoms by a single photon

We investigate the double $K$-shell ionization of heliumlike ions by a single photon. A fast convergency of QED perturbation theory with respect to parameter $1/Z$ is demonstrated in entire nonrelativistic domain for moderate nuclear charge numbers $Z \ge 2$. The ratio of double-to-single photoionization cross sections is calculated for light heliumlike ions, taking into account the leading orders of $1/Z$ and $\alpha Z$ expansions. A comparison of our results with available experimental data for a number of neutral atoms is presented.