We observed a new isomeric $\gamma$ transition at 168 keV in ${}^{36}\mathrm{Mg}$, with a half-life of $T_{1/2}=90{(}_{-50}^{+410})$ 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 ${}^{36}\mathrm{Mg}$ 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 sd to the pf shell. Within this interpretation, ${}^{36}\mathrm{Mg}$ becomes the crossing point between nuclei in which ground state deformed/superdeformed configurations are caused by the dominance of $N=20$ intruders $\left({}^{32,34}\mathrm{Mg}\right)$ and nuclei where deformed configurations are associated with the breaking of the $N=28$ closure and a large occupancy of the $1p_{3/2}$ neutron orbit (${}^{38}\mathrm{Mg}$ 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 ${}^{38}\mathrm{Mg}$ incompatible with our observation. We conclude that ${}^{36}\mathrm{Mg}$ bridges the $N=20$ and $N=28$ islands of inversion, forming the so-called big island of deformation.

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• Isomer decays
• Lifetimes & widths
• Nuclear structure & decays
• 20 ≤ A ≤ 38
• Radioactive beams
• Shell model