Evidence of nontermination of collective rotation near the maximum angular momentum in ${}^{75}\mathrm{Rb}$

Two of the four known rotational bands in ${}^{75}\mathrm{Rb}$ were studied via the ${}^{40}\mathrm{Ca}({}^{40}\mathrm{Ca},\alpha \mathrm{p}){}^{75}\mathrm{Rb}$ reaction at a beam energy of 165 MeV. Transitions were observed up to the maximum spin $I_{\max }$ of the assigned configuration in one case and one-transition short of $I_{\max }$ in the other. Lifetimes were determined using the residual Doppler shift attenuation method. The deduced transition quadrupole moments show a small decrease with increasing spin, but remain large at the highest spins. The results obtained are in good agreement with cranked Nilsson-Strutinsky calculations, which indicate that these rotational bands do not terminate, but remain collective at $I_{\max }$.

Figure 1

(a) Experimental kinematic and (b) dynamic moments of inertia as a function of rotational frequency.

Figure 2

Panels (a) and (b) show partial $\gamma$-ray spectra for band 2 in ${}^{75}\mathrm{Rb}$ that reveal the observed residual Doppler shifts. The energies of the $\gamma$ rays are given at the top of the figure. The weighted angles ${\theta }_w$, used to create these spectra, are shown in the two panels (see text for details). Note to clearly show the shift in the highest-energy transitions the binning changes from 2 keV per channel to 5 keV per channel at the thick dashed line.

Figure 3

Measured and calculated best fit for values of the residual Doppler shift, $F(\tau )$ for bands (a) 2 and (b) 3 in ${}^{75}\mathrm{Rb}$.

Figure 4

Panel (a) shows a comparison of the experimentally obtained transition quadrupole moments and those calculated using the CNS and CRMF formalisms. The [4, 4]($\alpha =+\frac{1}{2}$) configuration corresponds to band 2 and the [3, 4] ($\alpha =+\frac{1}{2}$) configuration corresponds to band 3. Panel (b) shows the normalized transition quadrupole moments of the bands in ${}^{74}\mathrm{Kr}$ [4], ${}^{75}\mathrm{Rb}$, and ${}^{109}\mathrm{Sb}$ [26], as a function of $I/I_{\max }$. The average of transition quadrupole moments of the lowest two transitions in ${}^{109}\mathrm{Sb}$ band 1 is used as a normalized value of $Q_t(I)/Q_t(I/I_{\max }=0.5)$ at $I/I_{\max }=0.5$ for this band.

Figure 5

Excitation energies relative to a rigid-rotor reference $E_{\mathit{RLD}}$. Panels (a)–(c) show CNS calculations, experimental data, and CRMF calculations, respectively. Solid and dashed lines represent positive and negative parity configurations, respectively. Open and solid circles are used for signatures $\alpha =-\frac{1}{2}$ and $\alpha =+\frac{1}{2}$, respectively. Terminating (aligned) states are shown by large open circles and the states that possess some collectivity at the maximum spin $I_{\max }$ of the respective configuration by large open squares. The yrast lines for four combinations of parity and signature are shown by small symbols and thin lines in panel (a).