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 in a $N=Z$ nucleus.

• Received 21 October 2002

DOI:https://doi.org/10.1103/PhysRevLett.90.082502