Direct evidence for the 𝜈⁢𝑑_5/2 orbital in ⁶⁹Ni: implications for the 𝑁=40 island of inversion

Shape coexistence, a collective manifestation of nuclear structure, emerges from the underlying single-particle dynamics and is prominently observed in the region below ⁶⁸Ni. Theoretical studies have emphasized the key role of the νd_5/2 orbital, the quadrupole partner of νg_9/2, in driving deformation. However, experimental constraints on the location and properties of neutron orbitals in neutron-rich isotopes in this region remain scarce. In this Letter, the single-particle structure of ⁶⁹Ni was investigated via the ⁶⁸Ni(d , p) reaction in inverse kinematics, performed at the ISOLDE Solenoidal Spectrometer at CERN. Several new excited states were observed, and comparisons with adiabatic distorted wave approximation (ADWA) calculations enabled l-value assignments for the most strongly populated states. In particular, a state at 2.56 MeV is interpreted as the dominant fragment of the νd_5/2 strength. The experimental findings are well reproduced by Large-Scale Shell Model calculations using a modified LNPS interaction with an increased νg_9/2–νd_5/2 energy gap. These results provide new insight into the structure of ⁶⁹Ni and underscore the crucial role of the νd_5/2 orbital in the onset of collectivity at the N = 40 island of inversion.