Multifaceted Quadruplet of Low-Lying Spin-Zero States in ${}^{66}\mathrm{Ni}$: Emergence of Shape Isomerism in Light Nuclei

A search for shape isomers in the ${}^{66}\mathrm{Ni}$ nucleus was performed, following old suggestions of various mean–field models and recent ones, based on state-of-the-art Monte Carlo shell model (MCSM), all considering ${}^{66}\mathrm{Ni}$ as the lightest nuclear system with shape isomerism. By employing the two-neutron transfer reaction induced by an ${}^{18}\mathrm{O}$ beam on a ${}^{64}\mathrm{Ni}$ target, at the sub-Coulomb barrier energy of 39 MeV, all three lowest-excited $0^{+}$ states in ${}^{66}\mathrm{Ni}$ were populated and their $\gamma$ decay was observed by $\gamma$-coincidence technique. The $0^{+}$ states lifetimes were assessed with the plunger method, yielding for the $0_2^{+}$, $0_3^{+}$, and $0_4^{+}$ decay to the $2_1^{+}$ state the $B(E2)$ values of 4.3, 0.1, and 0.2 Weisskopf units (W.u.), respectively. MCSM calculations correctly predict the existence of all three excited $0^{+}$ states, pointing to the oblate, spherical, and prolate nature of the consecutive excitations. In addition, they account for the hindrance of the $E2$ decay from the prolate $0_4^{+}$ to the spherical $2_1^{+}$ state, although overestimating its value. This result makes ${}^{66}\mathrm{Ni}$ a unique nuclear system, apart from ${}^{236,238}U$, in which a retarded $\gamma$ transition from a $0^{+}$ deformed state to a spherical configuration is observed, resembling a shape-isomerlike behavior.

Figure 1

Potential energy surface (PES) for the lowest $0^{+}$ states of ${}^{66}\mathrm{Ni}$, as a function of prolate and oblate quadrupole moments. Circles on the PES represent shapes in the MCSM basis vectors: white (orange) circles for $0_1^{+}$ ($0_3^{+}$) spherical states, green (red) for $0_2^{+}$ oblate ($0_4^{+}$ prolate) states, respectively.

Figure 2

(a)–(c) $\gamma$-ray spectrum of ${}^{66}\mathrm{Ni}$ gated by $2_1^{+}\to 0_1^{+}1425\text{−}\mathrm{keV}$ transition, observed in the thick-target measurement. (d) Decay scheme of ${}^{66}\mathrm{Ni}$ observed in the experiment (black-thin arrows for known transitions [34] not seen here). (e) Results from MCSM calculations.

Figure 3

(a)–(b) Portions of $\gamma$ spectra obtained with the plunger setup, in coincidence with the 1425-keV transition [(a) sum over the short, (b) sum over the long distances]. The zoom around the 1546-keV transition is shown in the insets. The dashed histograms display the corresponding thick target spectrum, for comparison. (c) Decay curves of the three excited $0^{+}$ states of ${}^{66}\mathrm{Ni}$, as a function of the target-to-stopper distance.