New shape isomer in the self-conjugate nucleus $^{72}$Kr

A new isomeric ${0}^{+}$ state was identified as the first excited state in the self-conjugate ($N=Z$) nucleus $^{\mathrm{72}}\mathrm{K}\mathrm{r}$. By combining for the first time conversion-electron and gamma-ray spectroscopy with the production of metastable states in high-energy fragmentation, the electric-monopole decay of the new isomer to the ground state was established. The new ${0}^{+}$ state is understood as the band head of the known prolate rotational structure, which strongly supports the interpretation that $^{\mathrm{72}}\mathrm{K}\mathrm{r}$ is one of the rare nuclei having an oblate-deformed ground state. This observation gives in fact the first evidence for a shape isomer…