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Direct Evidence for the νd5/2 Orbital in Ni69: Implications for the N=40 Island of Inversion

A. Ceulemans1,*, R. Raabe1,†, F. Nowacki2, A. Alharbi3,4, H. Ayatollahzadeh5,6, S. A. Bennett7, F. Browne8, P. A. Butler3, A. Camaiani1,‡ et al. (ISS Collaboration, ISOLDE Collaboration)

P. A. Butler3, A. Camaiani1,‡, D. Clarke7, A. J. Dolan3, Z. Eleme9, C. T. A. Everett3, F. Flavigny10, S. Fracassetti1, S. J. Freeman7,8, L. P. Gaffney3, G. Georgiev8,11, S. Goula9, A. Heinz12, A. Kawȩcka12, J. M. Keatings5,6, M. Labiche13, I. Lazarus13, P. T. MacGregor7,8, M. V. Managlia12, J. Ojala3,§, B. Olaizola8,∥, R. D. Page3, N. Patronis9, O. Poleshchuk1, A. M. Sánchez-Benítez14, D. K. Sharp7, H. Törnqvist12, and A. Youssef1 (ISS Collaboration, ISOLDE Collaboration)

  • *Contact author: andreas.ceulemans@kuleuven.be
  • †Contact author: riccardo.raabe@kuleuven.be
  • ‡Present address: Dipartimento di Fisica, Università degli Studi di Firenze e INFN-Firenze, Firenze, 50019, Italy.
  • §Present address: Accelerator Laboratory, Department of Physics, University of Jyväskylä, FI-40014 Jyväskylä, Finland.
  • ∥Present address: Instituto de Estructura de la Materia, CSIC, 28006 Madrid, Spain.
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Phys. Rev. Lett. 135, 252502 – Published 19 December, 2025

DOI: https://doi.org/10.1103/tdfs-5qzw

Abstract

Shape coexistence, a collective manifestation of nuclear structure, emerges from the underlying single-particle dynamics and is prominently observed in the region below Ni68. Theoretical studies have emphasized the key role of the νd5/2 orbital, the quadrupole partner of νg9/2, 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 Ni69 was investigated via the Ni68(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 νd5/2 strength. The experimental findings are well reproduced by Large-Scale Shell Model calculations using a modified LNPS interaction with an increased νg9/2–νd5/2 energy gap. These results provide new insight into the structure of Ni69 and underscore the crucial role of the νd5/2 orbital in the onset of collectivity at the N=40 island of inversion.

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Phys. Rev. Lett. 135, 252502– Published 19 December, 2025
  • Received 17 June 2025
  • Revised 11 September 2025
  • Accepted 12 November 2025
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