New isomeric transition in ${}^{36}$Mg: Bridging the N=20 and N=28 islands of inversion

Abstract

We observed a new isomeric gamma transition at 168 keV in ${}^{36}$Mg, with a half-life of T${}_{1/2}$=[130-500]$(\pm 40){(}_{-20}^{+800}{)}_{sys}$ ns. We propose that the observed transition de-excites a new 0${}^{+}$ isomeric state 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 ${}^{36}$Mg 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 {\it sd} to the {\it pf} shell. Within this interpretation, ${}^{36}$Mg becomes the crossing point between nuclei in which ground state deformed/superdeformed configurations are caused by the dominance of N=20 intruders (${}^{32,34}$Mg) and nuclei where deformed configurations are associated with N=28 intruders (${}^{38}$Mg 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 ${}^{38}$Mg incompatible with our observation. We conclude that ${}^{36}$Mg bridges the N=20 and N=28 islands of inversion, forming the so-called Big Island of Deformation.

Comments: 7 pages, 4 figures