First Evidence of Shape Coexistence in the ${}^{78}\mathrm{Ni}$ Region: Intruder $0_2^{+}$ State in ${}^{80}\mathrm{Ge}$

The $N=48$ ${}^{80}\mathrm{Ge}$ nucleus is studied by means of $\beta$-delayed electron-conversion spectroscopy at ALTO. The radioactive ${}^{80}\mathrm{Ga}$ beam is produced through the isotope separation on line photofission technique and collected on a movable tape for the measurement of $\gamma$ and $e^{-}$ emission following $\beta$ decay. An electric monopole $E0$ transition, which points to a 639(1) keV intruder $0_2^{+}$ state, is observed for the first time. This new state is lower than the $2_1^{+}$ level in ${}^{80}\mathrm{Ge}$, and provides evidence of shape coexistence close to one of the most neutron-rich doubly magic nuclei discovered so far, ${}^{78}\mathrm{Ni}$. This result is compared with theoretical estimates, helping to explain the role of monopole and quadrupole forces in the weakening of the $N=50$ gap at $Z=32$. The evolution of intruder $0_2^{+}$ states towards ${}^{78}\mathrm{Ni}$ is discussed.

Figure 1

Schematic view of the experimental setup. The tape moves the collected radioactivity in front of the Si(Li) detector.

Figure 2

Energy spectrum obtained from the Si(Li) detector from the decay of ${}^{80}\mathrm{Ga}$. The inset shows the decay curve of the 628 keV $e^{-}$, which is compatible with the ${}^{80}\mathrm{Ga}$ lifetime(s). The deduced partial level scheme of ${}^{80}\mathrm{Ge}$ is also drawn.

Figure 3

Upper panel: $\gamma$ spectrum gated on the 628 keV $e^{-}$. The peak at 1764 keV is clearly visible. The inset shows the inverse coincidence on the Si(Li) spectrum. Lower panel: $\gamma$ spectrum, $\beta$ gated, and enlarged in the region of interest. The inset shows the decay curve of the 1764 keV $\gamma$ ray, compatible with the ${}^{80}\mathrm{Ga}$ lifetime(s).

Figure 4

Evolution of the different contributions for the $N=48$ isotones $\nu (2p-2h)$ $0_2^{+}$ states. The minimum of the excitation energy is flattened out towards $Z=32$ (Ge), due to the monopole drift. The pairing and monopole energy gains are given with an error bar, which contributes to the final error bar on the $0^{+}$ state energy (see the text for details). The black dots are the known excited $0^{+}$ states, and the ones best fitting the $\nu (2p-2h)$ configuration are linked by a dashed line to show the trend.