Lifetimes of the 22-keV First Excited States inEu151andSm149

Abstract

Lifetimes of the first excited states in ${\mathrm{Eu}}^{151}$ and ${\mathrm{Sm}}^{149}$ were measured using the delayed coincidence technique. The half-lives obtained are (9.3\ifmmode\pm\else\textpm\fi{}0.7)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}9}$ and (7.6\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}9}$ sec for ${\mathrm{Eu}}^{151}$ and ${\mathrm{Sm}}^{149}$, respectively, which are in agreement with the lower limits determined previously by M\"ossbauer experiments. The two transitions, which are predominantly $M1$, are retarded by factors of 127 and 150 relative to Weisskopf estimates. The $E2$ speed of the 22-keV transition in ${\mathrm{Sm}}^{149}$ is also inferred from the known $\frac{E2}{M1}$ ratio and is found to be 83 times the single-particle speed.