E1transition probabilities fromKπ=0−andKπ=1−states ofPu238

Levels of ${}_{}{}^{238}{}_{}{}^{}\mathrm{Pu}$ were studied in the ${\beta }^{-}$ decay of ${}_{}{}^{238}{}_{}{}^{}\mathrm{Np}$ and the $\alpha$ decay of ${}_{}{}^{242}{}_{}{}^{}\mathrm{Cm}$. Thirteen $\gamma$-ray transitions were observed for the first time, and additional information about multipolarities and mixing ratios was obtained. An analysis of the $\gamma$-ray branching ratios gives a measure of the $E1\mathrm{}$ transition probabilities between octupolevibrational states and the ground-state band, corresponding to $F_W=4.3\mathrm{}\times {10}^4$ for $\Delta K=1\mathrm{}$ transitions and $F_W=1.5\mathrm{}\times {10}^4$ for $\Delta K=0\mathrm{}$. The latter transitions are three orders of magnitude faster than those that occur in the isotone ${}_{}{}^{236}{}_{}{}^{}\mathrm{U}$, for which $F_W(\Delta K=0)=2.2\mathrm{}\times {10}^7$, from a direct measurement of the half-life of the 0,$1^{-}$ state. Estimates of the hindrance factors for $E1\mathrm{}$ transitions from octupole states in other heavy nuclei are given, and the validity of the calculations on which they are based is discussed.