New shape minimum in ${}^{160}\mathrm{Yb}$: Evidence for a triaxial, strongly deformed band

A high-spin rotational band was observed in the $N=90$ nucleus ${}^{160}\mathrm{Yb}$ with moment of inertia and decay characteristics very similar to recently discovered sequences in ${}^{157,158}\mathrm{Er}$. These latter structures were discussed in terms of strongly deformed triaxial bands. Detailed cranked Nilsson-Strutinsky calculations were performed that predict that well-deformed triaxial structures are also expected at high spin in ${}^{160}\mathrm{Yb}$. Within this interpretation the observed discontinuity in the dynamic moment of inertia around $\hslash \omega =0.40\text{−}0.45$ MeV can be explained as a crossing between $i_{13/2}$ neutron levels.

Figure 1

Background-subtracted spectrum illustrating the new high-spin sequence observed in ${}^{160}\mathrm{Yb}$. The spectrum was generated by requiring a coincidence of any three transitions in the band. Both the normal deformed positive- (filled circles) and negative-parity (filled triangles) yrast bands are labeled into which the new high-spin sequence decays.

Figure 2

(Top) Experimental dynamic moments of inertia as a function of rotational frequency for the new sequence in ${}^{160}\mathrm{Yb}$ and the yrast TSD bands in ${}^{157,158}\mathrm{Er}$ [9] and ${}^{161}\mathrm{Lu}$ [20]. (Bottom) Theoretical dynamic moments of inertia for the above bands (excluding ${}^{161}\mathrm{Lu}$) assuming configurations as labeled; see text for details.

Figure 3

Calculated potential-energy surfaces versus quadrupole deformation ${\varepsilon }_2$ and triaxiality parameter γ for spins $I=20,30,40,50,60$, and $70\hslash$ for $(\pi ,\alpha )=(+,0)$ structures in ${}^{160}\mathrm{Yb}$. The other combinations of ($\pi ,\alpha$) show similar features. The left-hand bottom corner corresponds to spherical shape, whereas the prolate axis ($\gamma =0^{°}$) and oblate axis ($\gamma ={60}^{°}$) are indicated. Note the strongly deformed triaxial minima at (${\varepsilon }_2~0.37\text{−}0.43,\gamma ~20\text{−}{25}^{°}$).

Figure 4

Calculated excitation energy relative to that of a rotating liquid drop as a function of angular momentum for selected near-yrast configurations in ${}^{160}\mathrm{Yb}$. Solid lines indicate positive parity, whereas dashed lines indicate negative parity. See text for an explanation of the configuration labeling scheme. In the legend, sd indicates one proton in an orbital of $s_{1/2}{d}_{3/2}$ character.

Figure 5

The single-particle proton (top panel) and neutron (bottom panel) energies as a function of rotational frequency at a deformation of ${\varepsilon }_2~0.37,\gamma ~{20}^{°}$. The orbitals are labeled at $\hslash \omega =0$ by the $N$ shell (or more accurately ${\mathrm{N}}_{\mathrm{rot}-\mathrm{shell}}$ [23]) to which they belong with the ordering within the $N$ shell as a subscript. A few orbitals important for the present interpretation are also labeled in a similar way by their dominating $j$ shell(s). The line types distinguish between different ($\pi ,\alpha$) combinations: solid lines represent $(+,+1/2)$, dotted lines $(+,-1/2)$, dashed lines $(-,+1/2)$, and dash-dotted lines $(-,-1/2)$.