Effects of core-excitation in 56, 57Ni and 55Co

The results of shell-model calculations on 56, 57Ni and 55Co nuclei are reported. Effects of one- and two-particle excitation from the 0f7/2 orbit for 56Ni are considered and those of one-particle excitation for 57Ni and 55Co. Observed low-lying spectra are well reproduced by this model. Calculated spectroscopic factors of one-nucleon transfer reactions are also in satisfactory agreement with those extracted from experimental data. The ground state of 56Ni is calculated to be a state composed mainly of the closed 0f7/2 shell component (about 80%). The one-particle excitation is dominant in the 21+ and 41+ states of 56Ni. The single-particle pictures for 57Ni and 55Co are not altered by the introduction of the core excitation. The analyses of coupling schemes in the 2p-1h, 1p-2h and 2p-2h systems have revealed that p-p(h-h) correlations with J = 0+ and 2+ act as leading interactions, while p-h correlations act as perturbations. Spin-isospin dependence of these p-h correlations is also investigated.