Hindered $\Delta K=1$ Dipole Strength in octupole bands in $N=90$ ${}^{154}$Gd from Lifetime Measurements with $\gamma -\gamma$ fast timing technique

Abstract

The lifetimes of the low-lying negative-parity $1^{-}$ state at 1414~keV and $2^{-}$ state at 1398~keV in ${}^{154}$Gd have been measured using the $\gamma$--$\gamma$ fast-timing technique with the VENTURE array at VECC, Kolkata. The states were populated through the $\beta$ decay of ${}^{154}$Tb, produced in proton-induced reactions at the K130 cyclotron. From the measured lifetimes, absolute $B(E1)$ transition strengths were deduced. The extracted $B(E1)$ values are compared with those of neighboring Gd isotopes and with Gogny-HFB-based $sdf$-IBM calculations. The results show that the measured $E1$ strengths from these states are strongly hindered compared with the corresponding $\Delta K=0$ transitions, providing evidence for weak $\Delta K=1$ dipole strength in ${}^{154}$Gd.