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

We observed a new isomeric γ transition at 168 keV in Mg36, with a half-life of T1/2=90(-50+410) 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 Mg36 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, Mg36 becomes the crossing point between nuclei in which ground state deformed/superdeformed configurations are caused by the dominance of N=20 intruders (Mg32,34) and nuclei where deformed configurations are associated with the breaking of the N=28 closure and a large occupancy of the 1p3/2 neutron orbit (Mg38 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 Mg38 incompatible with our observation. We conclude that Mg36 bridges the N=20 and N=28 islands of inversion, forming the so-called big island of deformation.