The probability of production of an atom with a completely vacant $K$ shell by excitation of the second $K$ electron during the $K$-electron capture process has been determined to be (2.5±0.2)×${10}^{-5\mathrm{}}$ per $K$-capture event in ${\mathrm{Cs}}^{131}$. A scintillation coincidence spectrometer with a coincidence resolution time of ten millimicroseconds for 30-kev x rays was used for the measurement. The half-life of ${\mathrm{Cs}}^{131}$ has been redetermined to be 9.69±0.05 days. Previous measurements of electron excitation phenomena accompanying $K$ capture are summarized and the results are compared with the theory of Primakoff and Porter.

• Received 10 June 1960

DOI:https://doi.org/10.1103/PhysRev.120.536