Decay and Fission Hindrance of Two- and Four-Quasiparticle $K$ Isomers in ${}^{254}\mathrm{Rf}$

Two isomers decaying by electromagnetic transitions with half-lives of 4.7(1.1) and $247(73)\mu \mathrm{s}$ have been discovered in the heavy ${}^{254}\mathrm{Rf}$ nucleus. The observation of the shorter-lived isomer was made possible by a novel application of a digital data acquisition system. The isomers were interpreted as the $K^{\pi }=8^{-}$, ${\nu }^2(7/2^{+}[624],9/2^{-}[734])$ two-quasineutron and the $K^{\pi }=16^{+}$, $8^{-}{\nu }^2(7/2^{+}[624],9/2^{-}[734])\otimes 8^{-}{\pi }^2(7/2^{-}[514],9/2^{+}[624])$ four-quasiparticle configurations, respectively. Surprisingly, the lifetime of the two-quasiparticle isomer is more than 4 orders of magnitude shorter than what has been observed for analogous isomers in the lighter $N=150$ isotones. The four-quasiparticle isomer is longer lived than the ${}^{254}\mathrm{Rf}$ ground state that decays exclusively by spontaneous fission with a half-life of $23.2(1.1)\mu \mathrm{s}$. The absence of sizable fission branches from either of the isomers implies unprecedented fission hindrance relative to the ground state.

Figure 1

The decay time distribution for the ${}^{254}\mathrm{Rf}$ ground-state fission events. The dashed line corresponds to the exponential decay fit. The solid line corresponds to the fit, which takes into account undetected feeding from a short-lived isomer (the half-life of the isomer was fixed at $4.7\mu \mathrm{s}$). The latter fit indicates that about $15\%\pm 5\%$ of the fission events were delayed by the undetected isomer decays. The gap in the fitted function represents the dead time at the end of each event.

Figure 2

Decay-time distributions for the electrons emitted from both isomers in ${}^{254}\mathrm{Rf}$. The solid line corresponds to the two-component exponential decay fit.

Figure 3

Energy spectra for electrons emitted from (a) the 2-qp and (b) the 4-qp isomer, and (c) spectrum of $\gamma$ rays coincident with electrons emitted following the decay of the 2-qp isomer. Insets in panel (a) and (b) contain waveforms for implant-electron-fission ($i\text{−}e\text{−}sf$) and electron-electron ($e1\text{−}e2$) decay sequences, respectively.

Figure 4

Two- and four-quasiparticle states in ${}^{254}\mathrm{Rf}$ predicted using the multiquasiparticle calculations.