Direct Evidence for the πœˆβ’π‘‘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 πœˆβ’𝑑5/2 orbital, the quadrupole partner of πœˆβ’𝑔9/2, 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 β„“-value assignments for the most strongly populated states. In particular, a state at 2.56 MeV is interpreted as the dominant fragment of the πœˆβ’𝑑5/2 strength. The experimental findings are well reproduced by Large-Scale Shell Model calculations using a modified LNPS interaction with an increased πœˆβ’𝑔9/2β€“πœˆβ’π‘‘5/2 energy gap. These results provide new insight into the structure of 69Ni and underscore the crucial role of the πœˆβ’𝑑5/2 orbital in the onset of collectivity at the π‘=40 island of inversion.

DOI
https://doi.org/10.1103/tdfs-5qzw
Publication date
Reference
Physical Review Letters 135, 252502 (2025)
Authors
A. Ceulemans, R. Raabe, F. Nowacki, A. Alharbi, H. Ayatollahzadeh, S. A. Bennett, F. Browne, P. A. Butler, A. Camaiani, D. Clarke, A. J. Dolan, Z. Eleme, C. T. A. Everett, F. Flavigny, S. Fracassetti, S. J. Freeman, L. P. Gaffney, G. Georgiev, S. Goula, A. Heinz, A. Kawecka, J. M. Keatings, M. Labiche, I. Lazarus, P. T. MacGregor, M. V. Managlia, J. Ojala, B. Olaizola, R. D. Page, N. Patronis, O. Poleshchuk, A. M. SΓ‘nchez-BenΓ­tez, D. K. Sharp, H. TΓΆrnqvist, A. Youssef