Internal Ionization During Electron Capture

When an atom decays by $K$ capture, the $K$ shell will be left completely vacant if the second $K$ electron is ejected from the atom (internal ionization) or makes a transition to an unoccupied bound state (internal excitation). Relativistic and nonrelativistic calculations are presented which describe internal ionization for both allowed and forbidden decays; the results are compared with those of previous theories and with pertinent experimental results. Evidence is provided that the total probability for internal ionization can be calculated from the nonrelativistic theory, while the spectral shape is described by the relativistic theory. The total probability calculated from the relativistic theory is larger than values obtained from experiment, owing to an overly simplified representation for the Coulomb screening and spatial corrections of the two $K$ electrons in the parent atom. The shape of the ejected-electron spectrum depends on the degree of forbiddenness of the electron-capture transition, and therefore may be analyzed to provide new information for nuclear spectroscopists.