Lifetime measurements in ${}^{96}\mathrm{Rb}$ via fast-timing techniques: Investigating shape coexistence at $A\simeq 100$

Lifetime measurements of the ($3^{-}$), ($4^{-}$), and ($6^{-}$) intraband states in the neutron-rich, odd-odd ${}^{96}\mathrm{Rb}$ nucleus were performed at the LOHENGRIN spectrometer of Institut Laue-Langevin, using thermal-neutron-induced fission of ${}^{235}\mathrm{U}$ and fast-timing techniques with ${\mathrm{LaBr}}_3:\mathrm{Ce}$ scintillator detectors. The nanosecond isomeric nature of the ($3^{-}$) bandhead was established as well as the ${\beta }_2=0.39\left(3\right)$ deformation parameter of the band, pointing to a robust deformation in ${}^{96}\mathrm{Rb}$. Moreover, a hindered $B\left(E2\right)$ value of $3.9_{-13}^{+19}\times {10}^{-2}$ W.u. was found for the $\gamma$ decay of the deformed ($4^{-}$) state to the spherical $2^{-}$ ground state. A retardation was also found for the ($3^{-}$) $\to 2^{-}$ transition, possibly due to the shape change and giving strong support to a shape coexistence scenario in this nucleus, at the borders of the island of deformation at $N=60$. Analogies with the structure of the ${}^{98}\mathrm{Y}$ isotone are discussed.

Figure 1

HPGe (red) and ${\mathrm{LaBr}}_3:\mathrm{Ce}$ (blue) $\gamma$-ray energy spectra gated on the 461.6-keV, ($3^{-}$) $\to 2^{-}$ (a), 92.8-keV ($4^{-}$) $\to$ ($3^{-}$) (b), and 240.3-keV ($6^{-}$) $\to$ ($4^{-}$) (c) $\gamma$ rays in ${}^{96}\mathrm{Rb}$. Coincident transitions depopulating the (${10}^{-}$) isomer are marked in black, with weak lines displayed in gray. The peaks marked by a star come from the Compton scattering of $\gamma$ rays in the HPGe crystals of the clover detector and enter the spectrum as random coincidences.

Figure 2

Delayed (black) and antidelayed (red) time distributions for the ($3^{-}$) (a), ($4^{-}$) (b), and ($6^{-}$) (c) states in ${}^{96}\mathrm{Rb}$ located at 461.6, 554.5, and 794.8 keV, respectively. The feeding and decaying transitions used to measure the time distributions are shown in the inset. In (a) and (b), the fit results are also shown as solid lines with the same color code. In (c), the centroid positions of the delayed and antidelayed peak and background components are shown in the inset (see text for details). The obtained half-lives are $T_{1/2}=2.13\left(50\right)\times {10}^3\mathrm{ps}, T_{1/2}=599\left(55\right)\mathrm{ps}$, and $T_{1/2}=223\left(32\right)\mathrm{ps}$ for the ($3^{-}$), ($4^{-}$), and ($6^{-}$) states, respectively.

Figure 3

Comparison between the partial level schemes of ${}^{96}\mathrm{Rb}$ as measured in this work (left) and ${}^{98}\mathrm{Y}$ (right) [10, 11]. The half-lives and the $B\left(E2\right)$ values in ${}^{96}\mathrm{Rb}$ here obtained are displayed in red.

Figure 4

Excitation energy as a function of $I(I+1)$ for rotational bands built on the ($3^{-}$) and ($4^{-}$) states in ${}^{96}\mathrm{Rb}$ (red) and ${}^{98}\mathrm{Y}$ (black), respectively.