Search for shape-coexisting $0^{+}$ states in ${}^{66}\mathrm{Ni}$ from lifetime measurements

The lifetime of the $0_3^{+}$ state in ${}^{66}\mathrm{Ni}$, two neutrons below the $N=40$ subshell gap, has been measured. The transition $B(E2;0_3^{+}\to 2_1^{+}$) is one of the most hindered $E2$ transitions in the Ni isotopic chain and it implies that, unlike ${}^{68}\mathrm{Ni}$, there is a spherical structure at low excitation energy. We have performed extensive shell-model calculations that correctly predict this result, obtaining a spherical $0^{+}$ state at the correct energy and with an extremely low $B(E2;0_3^{+}\to 2_1^{+}$) value.

Figure 1

Energy of the $0^{+}$ states below 3 MeV in the even Ni isotopes. The $2_1^{+}$ states have been included for reference.

Figure 2

(Left) ${}^{66}\mathrm{Ni}$ level scheme populated in the $\beta$ decay of the $\left(1^{+}\right) {}^{66}\mathrm{Co}$ ground state. The dashed levels were not observed in this $\beta$-decay experiment, but have been included to help the discussion. The labels on the transitions are the energy in keV and the $B\left(E2\right)$ in W.u., while the width is proportional to their absolute intensity. (Right) Results from the LNPS calculations from this work.

Figure 3

Singles HPGe energy spectrum. A time gate of 300–1200 ms after the proton impact on the target was selected to enhance the ${}^{66}\mathrm{Co}$ decay activity. The long-lived ${}^{66}\mathrm{Ga}$ contaminant was subtracted. This did not suppress the ${}^{66}\mathrm{Fe}$ decay activity, so the transitions in ${}^{66}\mathrm{Co}$ are present and labeled in the spectrum.

Figure 4

Coincidence HPGe-HPGe energy spectrum with a gate on the 1425-keV transition. An asterisk (${}^{*}$) denotes a new transition of 2231.9 keV observed for the first time; see text for details.

Figure 5

Fit to the 2671-keV level half-life in ${}^{66}\mathrm{Ni}$ using the time difference between a $\beta$ and the 1246-keV decay transition detected in one of the ${\mathrm{LaBr}}_3\left(\mathrm{Ce}\right)$ crystals. The fit was done to a prompt Gaussian including the Compton background under the peak and a decay exponential. The result of $T_{1/2}=170\left(7\right)$ ps stems from the weighted average of four measurements. See text for details.

Figure 6

Coincidence HPGe-HPGe energy spectrum with a gate on the 1246-keV transition, showing the 1425-keV line.