Lifetime of the yrast $I^{\pi }=5^{-}$ state and $E1$ hindrance in the transitional nucleus ${}_{58}{}^{136}\mathrm{Ce}$

The half-lives of the $I^{\pi }=5^{-}$ and $I^{\pi }=7^{-}$ yrast states with $E_x=1978$ keV and $E_x=2307$ keV in the $N=78$ isotone ${}^{136}\mathrm{Ce}$, have been measured to be 496(23) ps and 270(24) ps, respectively, using the coincident fast-timing spectroscopy technique following population by a ${}^{124}\mathrm{Sn}({}^{16}\mathrm{O},4\mathrm{n})$ fusion-evaporation reaction at an incident beam energy of 68 MeV. The extracted $B(E1;5^{-}\to 4^{+})=1.8\left(3\right)\times {10}^{-6}$ W.u. value is compared with the systematics of the region and is found to be in line with typical $E1$ strengths for single-particle-type decays, rather than showing any evidence for the additional $E1$ decay hindrance associated with $K$-quantum number violation in axially deformed systems observed in lighter Ce isotopes.

Figure 1

Partial level scheme of ${}^{136}\mathrm{Ce}$.

Figure 2

(a) Total projection for all HPGe and ${\mathrm{LaBr}}_3\left(\mathrm{Ce}\right)$ detectors. (b) Total projection for all ${\mathrm{LaBr}}_3\left(\mathrm{Ce}\right)$ with a HPGe gate on the 971-keV transition. (c) A $\gamma$-ray spectrum obtained by gating on the 329-keV transition in the symmetric ${\mathrm{LaBr}}_3\left(\mathrm{Ce}\right) \left(E_{\gamma }\text{−}{E}_{\gamma }\right)$ coincidence energy matrix with an additional HPGe gate on the 971-keV transition. (d) A $\gamma$-ray spectrum obtained by gating on the 329-keV transition in the symmetric ${\mathrm{LaBr}}_3\left(\mathrm{Ce}\right) \left(E_{\gamma }\text{−}{E}_{\gamma }\right)$ coincidence energy matrix with an additional gate on the 806-keV transition in an HPGe detector.

Figure 3

Time difference spectra obtained in ${}^{136}\mathrm{Ce}$ from the ${\mathrm{LaBr}}_3\left(\mathrm{Ce}\right) E_{\gamma 1}\text{−}{E}_{\gamma 2}\text{−}\Delta T$ cube showing the time difference between the pairs of transitions (a) 664 and 762 keV, (b) 971 and 329 keV, and (c) 329 and 664 keV, respectively. The continuous lines in (b) and (c) are fits to the time distributions with an exponential decay convoluted with a Gaussian, resulting in the indicated half-life values. The dashed curve is a Gaussian prompt response function with a fixed FWHM of 546(22) ps.

Figure 4

Systematics of the yrast $B(E1;5^{-}\to 4^{+})$ in Barium and Cerium isotopes [11, 19] including the value of $B(E1;5^{-}\to 4^{+})$ in ${}^{136}\mathrm{Ce}$ from the current work.