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Shape coexistence—the occurrence of states with distinct nuclear shapes at similar excitation energies—has been observed in many regions of the nuclear chart [1-4] and is closely tied to the emergence of “islands of inversion”. Recent results indicate that even the neutron-rich doubly magic nucleus 78Ni exhibits signatures of deformation and shape coexistence [5], suggesting the onset of a new island of inversion at 𝑁=50 [6]. Just two neutrons below 78Ni50, 80Ge48 has attracted significant attention, with an earlier experiment suggesting shape coexistence through the observation of a low-lying 639-keV 0+ state below the first 2+ state at 659 keV [7].
We report on a comprehensive γ-ray spectroscopy study of 80Ge nuclear structure following the β-decay of 80Ga, populated by both its 6− ground state and 3− isomer with half-lives of 1.9 s and 1.3 s, respectively. The high-statistics experiment was performed with the high-efficiency GRIFFIN γ-ray spectrometer at TRIUMF [8-11]. More than 1100 γ-ray transitions and 400 excited states were observed, including several levels above the neutron separation energy at 8.08 MeV. Tentative spin and parity assignments are proposed, and β-feeding intensities deduced. Comparisons with large-scale shell-model calculations using NUSHELLX@MSU were carried out for both parent states. Our investigations [12,13] and subsequent β-decay [13,14] and Coulomb excitation studies [15] did not confirm the presence of the 639-keV 02+ state below the 21+ state or at 2 MeV, as suggested by theory. A recent paper reported two ~7 MeV high-energy γ rays populating low lying states that could, intriguingly, hint at pygmy dipole resonance in 80Ge [16,17]. Despite observing transitions with relative intensities as low as 10-3, our study did not observe these transitions.
This work provides one of the most extensive β-decay datasets analyzed to date, including the most comprehensive study of negative parity states in an even-even nucleus, and offers new insight into the structure of 80Ge [17] in the vicinity of the proposed 𝑁=50 island of inversion.
References
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[13] F. H. Garcia et al., Phys. Rev. C, to be published.
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[15] D. Rhodes et al., Phys. Rev. C 105, 024325 (2022).
[16] R. Li et al., Phys. Rev. C 111, 034303 (2025); R. Li et al., arXiv:2510.27125 (2025).
[17] L. T. Phuc, N. D. Dang, R. Li, and N. Q. Hung, Phys. Rev. C 110, 064323 (2024).