Lifetimes of the First Excited States inRu99andXe129

Lifetimes of the first excited states in ${\mathrm{Ru}}^{99}$ (90 keV) and ${\mathrm{Xe}}^{129}$ (40 keV) were measured using the delayed coincidence technique. The half-lives obtained are (20±1)×${10}^{-9\mathrm{}}$ sec for ${\mathrm{Ru}}^{99}$ and (0.96±0.05)×${10}^{-9\mathrm{}}$ sec for ${\mathrm{Xe}}^{129}$. These are in agreement with the lower limits previously determined by Mössbauer experiments. The $E2\mathrm{}$ part of the 90-keV transition of ${\mathrm{Ru}}^{99}$ is enhanced by a factor of ∼50 relative to the Weisskopf estimate, while the $M1\mathrm{}$ part is retarded by a factor between 2400 and 7400. The 40-keV $M1\mathrm{}$ transition in ${\mathrm{Xe}}^{129}$ is retarded by at least a factor of 31 relative to Weisskopf estimates.