We report the first measurement of a ratio ${\lambda }_{{\beta }_b}/{\lambda }_{{\beta }_c}$ of bound-state (${\lambda }_{{\beta }_b}$) and continuum-state $({\lambda }_{{\beta }_c})$ ${\beta }^{-}$-decay rates for the case of bare ${}^{207}\mathrm{Tl}^{81+}$ ions. These ions were produced at the GSI fragment separator FRS by projectile fragmentation of a ${}^{208}\mathrm{Pb}$ beam. After in-flight separation with the $B\rho \mathrm{\text{−}}\Delta E\mathrm{\text{−}}B\rho$ method, they were injected into the experimental storage-ring ESR at an energy of $400.5A\mathrm{MeV}$, stored, and electron cooled. The number of both the ${}^{207}\mathrm{Tl}^{81+}$ ions and their bound-state ${\beta }^{-}$-decay daughters, hydrogenlike ${}^{207}\mathrm{Pb}^{81+}$ ions, were measured as a function of storage time by recording their Schottky-noise intensities. The experimental result, ${\lambda }_{{\beta }_b}/{\lambda }_{{\beta }_c}=0.188(18)$, is in very good agreement with the value of 0.171(1) obtained from theory employing spectra of allowed transitions.

• Received 22 December 2004
• Publisher error corrected 28 July 2005

DOI:https://doi.org/10.1103/PhysRevLett.95.052501