Relativistic Coulomb excitation of neutron-rich 54,56,58Cr: On the pathway of magicity from N=40 to N=32

The first excited 2+ states in 54,56,58Cr were populated by Coulomb excitation at relativistic energies and γ rays were measured using the RISING setup at GSI. For 56Cr and 58Cr the B(E2,21+→0+) values relative to the previously known B(E2) value for 54Cr are determined as 8.7(3.0) and 14.8(4.2) W.u., respectively. The results are consistent with a subshell closure at neutron number N=32 which was already indicated by the higher energy of the 21+ state in 56Cr. Recent large-scale shell model calculations using effective interactions reproduce the trend in the excitation energies, but fail to account for the minimum in the B(E2) values at N=32.