New isomeric transition in Mg36 : Bridging the N=20 and N=28 islands of inversion

Here, we observed a new isomeric γ transition at 168 keV in 36Mg, with a half-life of T1/2=90⁢($^{+410}_{-50}$) ns. We propose that the observed transition deexcites a new 0+ isomeric state at 833 keV and populates the previously known first 2+ state. The existence of this isomer is consistent with the predictions of the large-scale shell-model calculations of 36Mg using the sdpf-u-mix interaction. The observed excitation energy of the second 0+ state is caused by the small energy separation between two prolate-deformed configurations where the intruder configuration corresponds to two-neutron excitations from the sd to the pf shell. Within this interpretation, 36Mg becomes the crossing point between nuclei in which ground state deformed/superdeformed configurations are caused by the dominance of N=20 intruders (32,34Mg) and nuclei where deformed configurations are associated with the breaking of the N=28 closure and a large occupancy of the 1⁢p3/2 neutron orbit (38Mg and beyond). We found the lack of three-body monopole corrections in other effective interactions results in a predominance of N=20 intruder configurations past 38Mg incompatible with our observation. We conclude that 36Mg bridges the N=20 and N=28 islands of inversion, forming the so-called big island of deformation.