Detailed γ-ray spectroscopy of 55Cr and 56Cr: Confirmation of the subshell closure at N=32

The nuclei 55,56Cr were populated to high spin following the reaction 48Ca(11B,pxn)58-xCr at a beam energy of 35–40 MeV. The level scheme of 55Cr has been confirmed and extended to Jπ=252+. The level scheme of 56Cr has been extended to J=13. The spins and parity of the majority of levels have been determined unambiguously for the first time. The first firm evidence for an intruder band based on the νg9/2 orbital has been observed in 55Cr. In 56Cr, the position of the first 4+ state has been determined unambiguously and the presence of a subshell closure at N=32 has been confirmed on the basis of the E41+/E21+ systematics. The properties of both nuclei are compared with total Routhian surface calculations and with previous predictions of subshell closures at N=32 and 34. The systematics in the region are discussed in terms of the strong νf5/2-πf7/2 interaction and its effect on the underlying single-particle structure.