Triaxial strongly deformed bands in ${}^{160,161}\mathrm{Tm}$

High-spin states in ${}^{160,161}\mathrm{Tm}$ were populated using the ${}^{128}\mathrm{Te}$(${}^{37}\mathrm{Cl}$, $5n$ and $4n$) reactions at a beam energy of 170  MeV. Emitted γ rays were detected in the Gammasphere spectrometer. Two rotational bands with high moments of inertia were discovered, one assigned to ${}^{160}\mathrm{Tm}$, while the other tentatively assigned to ${}^{161}\mathrm{Tm}$. These sequences display features similar to bands observed in neighboring Er, Tm, Yb, and Lu nuclei which have been discussed in terms of triaxial strongly deformed structures. Cranked Nilsson Strutinsky calculations have been performed that predict well-deformed triaxial shapes at high spin in ${}^{160,161}\mathrm{Tm}$.

Figure 1

Section of the nuclear chart showing nuclei that exhibit superdeformed (SD) and triaxial strongly deformed (TSD) band structures.

Figure 2

Background-subtracted spectra of γ rays resulting from summing all triple coincidences of in-band transitions in the TSD bands assigned to ${}^{160}\mathrm{Tm}$ (a) and ${}^{161}\mathrm{Tm}$ (b). The insets show the low-energy part of the spectra where the solid diamonds indicate γ rays associated with ${}^{160}\mathrm{Tm}$ while the open diamonds indicate known $\gamma$-ray transitions in ${}^{161}\mathrm{Tm}$.

Figure 3

Experimental dynamic moments of inertia as a function of rotational frequency for the new sequences in ${}^{161,160}\mathrm{Tm}$ along with selected structures showing TSD characteristics in neighboring nuclei.

Figure 4

Calculated potential-energy surfaces versus quadrupole deformation ${\varepsilon }_2$ and triaxiality parameter γ for spins $I=20.5,30.5,40.5,50.5,60.5$, and $70.5\hslash$ for (parity, signature) $=\hspace{0.5em}(\pi ,\alpha )=(+,1/2)$ structures in ${}^{161}\mathrm{Tm}$. The other combinations of $(\pi ,\alpha )$ show similar features. The bottom left corner corresponds to a spherical shape, and the prolate axis $(\gamma =0^{°})$ and oblate axis $(\gamma ={60}^{°})$ are indicated. Contour lines are 250 keV apart. The red regions indicates high energy and the blue regions represent low energy. The absolute minimum is labeled with a dot.

Figure 5

Calculated excitation energy relative to that of a rotating liquid drop as a function of angular momentum for selected bands in ${}^{160}\mathrm{Tm}$, ${}^{161}\mathrm{Tm}$, and ${}^{163}\mathrm{Lu}$ [2]. See the text for an explanation of the labeling scheme.