First observation of bound-state ${\mathrm{\beta }}^{\mathrm{-}}$ decay

Bound-state ${\mathrm{\beta }}^{\mathrm{-}}$ decay was observed for the fist time by storing bare ${}_{66}{}^{163}{}_{}{}^{}\mathrm{Dy}_{}^{66+}{}_{}{}^{}$ ions in a heavy-ion storage ring. From the number of ${}_{67}{}^{163}{}_{}{}^{}\mathrm{Ho}_{}^{66+}{}_{}{}^{}$ daughter ions, measured as a function of the storage time, a half-life of ${47}_{\mathrm{-}5}^{+5\mathrm{}}$ d was derived. By comparing this result with reported half-lives for electron capture (EC) from the ${\mathit{M}}_1$ and ${\mathit{M}}_2$ shells of neutral ${}_{67}{}^{163}{}_{}{}^{}\mathrm{Ho}$, bounds for both the ${\mathit{Q}}_{\mathrm{EC}}$ value of neutral ${}_{67}{}^{163}{}_{}{}^{}\mathrm{Ho}$ and for the electron-neutrino mass were set.