Observation of the MnKααx rays in the electron-capture decay ofFe55

$K\alpha \alpha$ x rays have been observed for the first time in the electron-capture decay of ${}_{}{}^{55}{}_{}{}^{}\mathrm{Fe}$. These x rays are due to the two-electron—one-photon transition to the $K$ shell doubly ionized through $K$-shell shakeoff and shakeup during the $K$-electron capture decay. The measured $K\alpha \alpha$ x-ray energy is 11 907 ± 20 eV and the intensity ratio of $K\alpha \alpha$ x rays to $K\alpha$ x rays is (5.5 ± 1.2)×${10}^{-8\mathrm{}}$. The results are discussed and compared with theoretical calculations and other measurements.