First evidence of an octupole rotational band in Ge isotopes

The spectroscopy of ${}^{71}\mathrm{Ge}$ has been investigated via the fusion-evaporation reaction ${}^{74}\mathrm{Ge}(\alpha ,\alpha 3n){}^{71}\mathrm{Ge}$. Collective structures including a rotational band built on the $15/2^{-}$ octupole state in ${}^{71}\mathrm{Ge}$ have been established. The observation of strong $E1$ transitions and the well-behaved rotational sequence built on the $15/2^{-}$ octupole state provide the first experimental evidence of an octupole rotational band in Ge isotopes, suggesting an enhanced octupole correlation around $N=40$ in the $A\approx 70$ region. A newly developed semimicroscopic cluster model provides a good description of the octupole characteristics of ${}^{71}\mathrm{Ge}$.

Figure 1

Partial level scheme of ${}^{71}\mathrm{Ge}$. Energies are given in keV and widths of the arrows are proportional to their relative intensities.

Figure 2

Coincident spectra for ${}^{71}\mathrm{Ge}$, gated on the (a) 853.2 $+$ 926.9 keV transitions, (b) 974.1 $+$ 1121.6 keV transitions, (c) 391.3 keV transition, and (d) 1664.7 keV transition. Transitions of ${}^{71}\mathrm{Ge}$ are marked in red. The peaks marked with asterisks are known contaminants, and the peaks marked with triangles are not included in the partial level scheme of Fig. 1. The inset in (b): Coincident $\gamma$-ray spectrum gated on the 314.1 keV transition.

Figure 3

(a) Excitation energies of the $15/2^{-}$ states (relative to the yrast $9/2^{+}$ states) in odd-A Ge isotopes [28, 29], and the $3^{-}$ and $2^{+}$ states in even-A Ge isotopes [15, 16, 17, 18, 30]. (b) The experimental $B\left(E1\right)/B\left(E2\right)$ branching ratios of $19/2^{-}$ state in ${}^{71}\mathrm{Ge}$ in comparison with experimental values in the neighboring odd-A Ge isotopes deduced from [28, 29], and the octupole-deformed nucleus ${}^{220}\mathrm{Ra}$ deduced from [31]. The average $B\left(E1\right)/B\left(E2\right)$ branching ratio with experimental uncertainty for the octupole band in ${}^{220}\mathrm{Ra}$ is displayed as the shaded region. Inset in (a): The neutron single-particle levels originating from the $g_{9/2}$ and $p_{3/2}$ orbitals in odd-$A$ Ge isotopes.

Figure 4

Aligned spins as a function of the rotational frequency for bands 2 in ${}^{71}\mathrm{Ge}$ and the negative-parity band in ${}^{72}\mathrm{Ge}$ [14, 15]. Harris parameters of $J_0=3.5{\hslash }^2/\mathrm{MeV}$ and $J_1=17.0{\hslash }^4/{\mathrm{MeV}}^3$ have been assumed.

Figure 5

The calculated excitation energies (in MeV) and intraband $B\left(E2\right)$ values (in $e^2{\mathrm{fm}}^4$) are compared with the experimental data of the octupole bands in ${}^{71}\mathrm{Ge}$ and ${}^{74}\mathrm{Se}$. The ratios of the intrinsic quadrupole moments $Q_0\left(s\right)$ (from spectroscopic properties) and $Q_0\left(t\right)$ (from $E2$ transitions) are shown on the right side of the calculated levels. The experimental data of ${}^{74}\mathrm{Se}$ are taken from Ref. [13].