Experimental evidence for56Ni-core breaking from the low-spin structure of theN=Znucleus2958Cu29

Abstract

Low-spin states in the odd-odd $N=Z$ nucleus ${}^{58}\mathrm{Cu}$ were investigated with the ${}^{58}\mathrm{Ni}(p,n\ensuremath{\gamma}{)}^{58}\mathrm{Cu}$ fusion evaporation reaction at the FN-TANDEM accelerator in Cologne. $\ensuremath{\gamma}\ensuremath{\gamma}$-coincidences, $\ensuremath{\gamma}\ensuremath{\gamma}$-angular correlations, and signs of $\ensuremath{\gamma}$-ray polarizations were measured. Seventeen low-spin states below 3.6 MeV and 17 new transitions were observed. Ten multipole mixing ratios and 17 $\ensuremath{\gamma}$-branching ratios were determined for the first time. New detailed spectroscopic information on the ${2}_{2}^{+}$ state, the isobaric analogue state (IAS) of the ${2}_{1}^{+},T=1$ state of ${}^{58}\mathrm{Ni},$ makes ${}^{58}\mathrm{Cu}$ the heaviest odd-odd $N=Z$ nucleus with known ${B(E2;2}^{+},T=\stackrel{\ensuremath{\rightarrow}}{1}{0}^{+},T=1)$ value. The ${4}^{+}$ state at 2.751 MeV, observed here for the first time, is identified as the IAS of the ${4}_{1}^{+},T=1$ state in ${}^{58}\mathrm{Ni}.$ The new data are compared to full $\mathrm{pf}$-shell-model calculations with the GXPF1 residual interaction and to calculations within a ${\mathrm{pf}}_{5/2}$ configurational space with a residual surface $\ensuremath{\delta}$ interaction. The role of the ${}^{56}\mathrm{Ni}$ core excitations for the low-spin structure in ${}^{58}\mathrm{Cu}$ is discussed.