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±0.7)×${10}^{-9\mathrm{}}$ and (7.6±0.5)×${10}^{-9\mathrm{}}$ sec for ${\mathrm{Eu}}^{151}$ and ${\mathrm{Sm}}^{149}$, respectively, which are in agreement with the lower limits determined previously by Mössbauer experiments. The two transitions, which are predominantly $M1\mathrm{}$, are retarded by factors of 127 and 150 relative to Weisskopf estimates. The $E2\mathrm{}$ speed of the 22-keV transition in ${\mathrm{Sm}}^{149}$ is also inferred from the known $\frac{E2\mathrm{}}{M1\mathrm{}}$ ratio and is found to be 83 times the single-particle speed.