Persistence of collective behavior at high spin in the $N=88$ nucleus ${}^{153}\mathrm{Tb}$

Excited states in the $N=88$ nucleus ${}^{153}\mathrm{Tb}$ were observed up to spin $\sim 40$ in an experiment utilizing the Gammasphere array. The ${}^{153}\mathrm{Tb}$ states were populated in a weak $\alpha 4n$ evaporation channel of the ${}^{37}\mathrm{Cl}$ + ${}^{124}\mathrm{Sn}$ reaction. Two previously known sequences were extended to higher spins, and a new decoupled structure was identified. The $\pi h_{11/2}$ band was observed in the spin region where other $N=88$ isotopes exhibit effects of prolate to oblate shape changes leading to band termination along the yrast line, whereas ${}^{153}\mathrm{Tb}$ displays a persistent collective behavior. However, minor perturbations of the very highest state in both signatures of this $h_{11/2}$ band are observed, which perhaps signal the start of the transition towards band termination.

Figure 1

Partial level scheme for ${}^{153}\mathrm{Tb}$ from the present analysis. Tentative levels and $\gamma$ rays are shown as dashed lines.

Figure 2

(a) A spectrum of the $\alpha =-1/2$ signature for band 1 $\left(\pi h_{11/2}\right)$ resulting from the sum of many triple coincidence gates of in-band transitions in the hypercube. (b) A spectrum produced in a similar manner as that in panel (a) to display the new transitions of the $\alpha =+1/2$ signature of band 1. (c) The newly observed band 3, where coincidences between all combinations of two in-band transitions with the 348-keV line in band 1 were summed.

Figure 3

Alignment for bands 1 and 2 in ${}^{153}\mathrm{Tb}$, as well as for the high-$K\nu (h_{11/2},i_{13/2})$ sequence in ${}^{152}\mathrm{Gd}$ [13]. Harris parameters [21] of ${\mathcal{J}}_0=12{\hslash }^2$/MeV and ${\mathcal{J}}_1=74{\hslash }^4/{\mathrm{MeV}}^3$ were used.

Figure 4

Yrast states (minus a rigid-rotor reference) with positive parity and even spins in the denoted even-even $N=88$ nuclei are displayed in the upper panels. The lower panels provide energies of the yrast negative-parity, $\alpha =+1/2,-1/2$ states in the odd-$Z$ isotopes (filled and empty blue [gray] circles, respectively). The negative-parity, $\alpha =-1/2$ states leading toward terminating states for ${}^{155}\mathrm{Ho}$ and ${}^{157}\mathrm{Tm}$ are shown as black squares. In addition, the cranked Nilsson-Strutinsky (CNS) calculations for the yrast negative-parity bands in ${}^{153}\mathrm{Tb}$ are given in the lower-left panel. Configurations for these calculated bands are described in the text, as is the choice of reference parameters. A dashed line at $I=30$ is given only to guide the eye.