Neutron single-particle strengths at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>N</mml:mi><mml:mo>=</mml:mo><mml:mn>40</mml:mn></mml:mrow></mml:math> , 42: Neutron knockout from <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi mathvariant="normal">Ni</mml:mi><mml:mprescripts /><mml:none /><mml:mrow><mml:mn>68</mml:mn><mml:mo>,</mml:mo><mml:mn>70</mml:mn></mml:mrow></mml:mmultiscripts></mml:math> ground and isomeric states

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The distribution of single-particle strength in Ni67,69 was characterized with one-neutron knockout reactions from intermediate-energy Ni68,70 secondary beams, selectively populating neutron-hole configurations at N=39 and 41, respectively. The spectroscopic strengths deduced from the measured partial cross sections to the individual final states, as tagged by their γ-ray decays, are used to identify and quantify neutron configurations in the wave functions. While Ni69 compares well with shell-model predictions, the results for Ni67 challenge the validity of current effective shell-model Hamiltonians by revealing discrepancies that cannot be explained so far. These results suggest that our understanding of the low-lying states in the neutron-rich, semimagic Ni isotopes may be incomplete and requires further investigation on both the experimental and theoretical sides.

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