Collective rotation of an oblate nucleus at very high spin

A sequence of nine almost equidistant quadrupole transitions is observed in ${}^{137}\mathrm{Nd}$. The sequence represents an extremely regular rotational band that extends to a spin of about 75/2 and an excitation energy of $\approx 4.5\mathrm{MeV}$ above yrast. Cranked mean-field calculations of the Nilsson-Strutinsky type suggest an oblate shape for the band. They reproduce the observed $I(I+1)$ dependence of the rotational energy whereas predicting a pronounced decrease in the deformation, which is the hallmark of antimagnetic rotation.

Figure 1

Partial level scheme of ${}^{137}\mathrm{Nd}$ showing the newly identified band O and its decay out. Possible decay-out transitions drawn with dashed lines are marked by an asterisk.

Figure 2

Spectrum constructed by double-gating on the observed transitions of band O of ${}^{137}\mathrm{Nd}$. The in-band transitions are marked with red labels, whereas the low-lying transition are marked in black. Possible decay-out transitions are marked by an asterisk. The levels deexcited by the 328-, 407-, and 876-keV transitions are fed by pathways which can be found in Ref. [22].

Figure 3

(a) Experimental energies relative to a rigid body rotor for the bands O, HD, and T of ${}^{137}\mathrm{Nd}$. (b) Energies relative to a rotating liquid drop calculated by means of the cranked Nilsson-Strutinsky (CNS) approach for the bands O, HD, and T. Open and filled symbols used for band T correspond to the two signature partners of the dipole band.

Figure 4

Angular momentum as a function of the rotational frequency $\hslash \omega =\left[E\right(I)-E(I-2\left)\right]/2$ of bands O and HD.

Figure 5

Deformation trajectories as a function of spin and potential-energy surfaces for the bands O, HD, and T of ${}^{137}\mathrm{Nd}$. Open and filled symbols correspond to $\alpha =-1/2$ and $\alpha =+1/2$, respectively. The distance between the contour lines is 0.25 MeV.

Figure 6

Single-particle Routhians of the $h_{11/2}$ shell as a function of the triaxiality parameter $\gamma$ for the $j$-shell model of Ref. [24]. The shown case corresponds to the bottom of band O and the middle of band T. The rotational frequency is $\hslash \omega =0.45\mathrm{MeV}$, and the energy scale is $\hslash {\omega }_0=8\mathrm{MeV}$. Full line: favored signature; dashed line: unfavored signature ($-1/2$ and 1/2, respectively, for $h_{11/2}$), blue circles: protons; cyan circles: neutron holes.

Figure 7

Angular momentum composition of band O. The $h_{11/2}$ orbitals are represented by their individual $j$ vectors. The sum of the contributions of the normal-parity orbitals, which are not shown in the figure, is denoted by $R$. The total angular momentum $I$ is the sum of the projections on the rotational $\left(x\right)$ axis.