Shape transition observed in neutron-rich pf-shell isotopes studied via proton inelastic scattering

The structure of the neutron rich pf-shell nuclei 58Ti36, 60Cr36 and 62Cr38 was studied by proton inelastic scattering in inverse kinematics. Large quadrupole deformation lengths of 1.12(16) fm and 1.36(14) fm were obtained for 60Cr and 62Cr, respectively, which confirm the enhanced collectivity in these nuclei. An excited state was observed in 62Cr at 1180(10) keV with tentative assignment of Jπ=4+. The increase in the Ex(4+)/Ex(2+) ratio indicates that the nature of collectivity changes from vibrational to rotational in 62Cr and affords evidence for the development of static deformation. The excitation energy of the newly observed 2+ state in 58Ti (1046(17) keV) shows hindered collectivity. The contrast between the structures of Cr and Ti suggests that the contributions of both protons and neutrons are crucial for the large collectivity in Cr close to N=40. From the comparison to shell model calculations, the large collectivity is found to originate from the admixture of the pf- and gd-shells across the N=40 sub-shell gap.