Conversion of an atomic electron into a positron and double β+ decay
Voloshin M. B. , Mitsel'makher G. V., Eramzhyan R. A.
The probability for the conversion of an atomic electron into a positron should exceed the probability for 1β + decay in all nuclei. The calculated ratio of probabilities does not depend on the nuclear model; for most nuclei it is ~10 for conversion without neutrinos and —103 for two-neutrino processes.