${{}_{37}{}^{97}\mathrm{Rb}}_{60}$: The Cornerstone of the Region of Deformation around $A\sim 100$

Excited states of the neutron-rich nuclei ${}^{97,99}\mathrm{Rb}$ were populated for the first time using the multistep Coulomb excitation of radioactive beams. Comparisons of the results with particle-rotor model calculations provide clear identification for the ground-state rotational band of ${}^{97}\mathrm{Rb}$ as being built on the $\pi g_{9/2}$ $[431]3/2^{+}$ Nilsson-model configuration. The ground-state excitation spectra of the Rb isotopes show a marked distinction between single-particle-like structures below $N=60$ and rotational bands above. The present study defines the limits of the deformed region around $A\sim 100$ and indicates that the deformation of ${}^{97}\mathrm{Rb}$ is essentially the same as that observed well inside the deformed region. It further highlights the power of the Coulomb-excitation technique for obtaining spectroscopic information far from stability. The ${}^{99}\mathrm{Rb}$ case demonstrates the challenges of studies with very short-lived postaccelerated radioactive beams.

Figure 1

Doppler corrected $\gamma$-ray energy spectra for ${}^{97}\mathrm{Rb}$ (upper panel) and ${}^{99}\mathrm{Rb}$ (lower panel). Transitions, identified as belonging to Rb isotopes, are marked with their energies. Contaminating transitions, from the Coulomb excitation of Sr isotopes, are marked with circles.

Figure 2

Level schemes for ${}^{97}\mathrm{Rb}$ and ${}^{99}\mathrm{Rb}$ as obtained in the present experiment. Pure $E2$ transition are marked in red while mixed ($M1/E2$) are in blue.

Figure 3

Transitional quadrupole moments, $Q_0$, for the even-even Kr, Sr, Zr [12, 25, 26, 27, 28, 29] and the odd-mass Rb isotopes (present measurement) as a function of spin. The values for the ground states of ${}^{95,97,99}\mathrm{Rb}$ (left side) were obtained either from the analysis of the present data (squares), under the assumption of the rotational model, or from spectroscopic quadrupole moments (triangles) [8]. The values for the excited states were calculated from measured transition probabilities. Some of the values for the even-even cases are slightly displaced on the $x$ axis for clarity.

Figure 4

Nilsson diagram for protons in the $A\sim 100$ region. The single-particle energies are calculated using the Woods-Saxon model with the “universal parametrization” and ${\beta }_4={\beta }_2^2/6$ and ${\beta }_6=0$, where ${\beta }_2$, ${\beta }_4$, and ${\beta }_6$ are deformation parameters [31].

Figure 5

Particle-rotor model calculations for ${}^{97}\mathrm{Rb}$ compared with experiment. Upper panel: $B(M1)/(E2)$ ratios. Lower panel: $B(E2)$ values. See text for more details.