Shell model study of single-particle and collective structure in neutron-rich Cr isotopes

The structure of neutron-rich Cr isotopes is systematically investigated by using the spherical shell model. The calculations reproduce well the known energy levels for the even-even ${}^{52-62}$Cr and odd-mass ${}^{53-59}$Cr nuclei, and predict a lowering of excitation energies around neutron number N=40. The calculated $B(E2;2_1^{+}\to 0_1^{+})$ systematics shows a pronounced collectivity around N=40; a similar characteristic behavior has been suggested for Zn and Ge isotopes. Causes for the sudden drop of the $9\mathrm{/}{2}_1^{+}$ energy in ${}^{59}$Cr and the appearance of very low $0_2^{+}$ states around N=40 are discussed. We also predict a new band with strong collectivity built on the $0_2^{+}$ state in the N=40 isotope ${}^{64}$Cr.