DoubleK-Shell Ionization in the Decay ofCs131andEr165

Double ionization of the $K$ shell has been studied in the electron capture decays of ${}_{}{}^{131}{}_{}{}^{}\mathrm{Cs}$ and ${}_{}{}^{165}{}_{}{}^{}\mathrm{Er}$ by recording coincidences between $K$ x rays emitted when the double vacancies are filled. A Si(Li) x-ray detector was used in conjunction with a NaI(T1) detector for these measurements. The results obtained for $P_{\mathrm{KK}}$, the probability per $K$ capture that a double $K$-shell vacancy is formed, were (1.33 ± 0.33) × ${10}^{-5\mathrm{}}$ for ${}_{}{}^{131}{}_{}{}^{}\mathrm{Cs}$ and (6.7 ± 3.9) × for ${}_{}{}^{165}{}_{}{}^{}\mathrm{Er}$. Both of these values are a factor of 2 smaller than previous measurements and, within the experimental uncertainties, are in agreement with predictions based on the relativistic theory of Intemann.