Nuclear shell evolution in the "Island of Inversion"

Recent publications suggest a re-interpretation for the "Island of Inversion" (IOI) around Mg-32 at N=20. In contrast to earlier beliefs, significant amounts of intruder configurations might be present in the ground states of Mg-28 and Mg-30, already [N. Tsunoda et al. PRC 95, 021304(R)]. Also, the excited 0+ state in Mg-32 seems to be dominated by 2p2h and 4p4h contributions [A. O. Macchiavelli et al., PRC 94, 051303(R)]. We here confirm this picture for the region of the IOI around Ni-68, based on large-scale shell model calculations in the fpg shell using the new JJ44C effective interaction. We show that the nature of shell evolution in the IOI is not unique to neutron-rich, exotic nuclei. Instead, the tensor force is essential understanding the transition into the IOI, also affecting the interpretation of R4/2 values. We show that the mixing of np-nh configurations results in characteristic low-lying strong M1 transitions, which we confirm based on new lifetime measurements in 70Zn at N=40. The M1 strength systematics around N=20 and N=40 is found to be correlated with the behaviour of the presumed shape-coexisting 0+ states, offering a re-interpreation of low-lying M1 transitions in medium-light nuclei.