Spectroscopy of odd-mass cobalt isotopes toward the N=40 subshell closure and shell-model description of spherical and deformed states

The neutron-rich cobalt isotopes up to A=67 have been studied through multinucleon transfer reactions by bombarding a 238U target with a 460-MeV 70Zn beam. Unambiguous identification of prompt γ rays belonging to each nucleus has been achieved using coincidence relationships with the ions detected in a high-acceptance magnetic spectrometer. The new data are discussed in terms of the systematics of the cobalt isotopes and interpreted with large-scale shell-model calculations in the fpgd model space. In particular, very different shapes can be described in 67Co, at the edge of the island of inversion at N=40, where a low-lying highly deformed band coexists with a spherical structure.