Experimental evidence for56Ni-core breaking from the low-spin structure of theN=Znucleus

Low-spin states in the odd-odd N=Z nucleus 58Cu were investigated with the58Ni(p,n gamma)58Cu fusion evaporation reaction at the FN-tandem accelerator inCologne. Seventeen low spin states below 3.6 MeV and 17 new transitions wereobserved. Ten multipole mixing ratios and 17 gamma-branching ratios weredetermined for the first time. New detailed spectroscopic information on the2+,2 state, the Isobaric Analogue State (IAS) of the 2+,1,T=1 state of 58Ni,makes 58Cu the heaviest odd-odd N=Z nucleus with known B(E2;2+,T=1 --> 0+,T=1)value. The 4^+ state at 2.751 MeV, observed here for the first time, isidentified as the IAS of the 4+,1,T=1 state in 58Ni. The new data are comparedto full pf-shell model calculations with the novel GXPF1 residual interactionand to calculations within a pf5/2 configurational space with a residualsurface delta interaction. The role of the 56Ni core excitations for thelow-spin structure in 58Cu is discussed.Comment: 15 pages, 7 figures, submitted to Phys. Rev.