Nuclear isomers are crucial probes for studying the structure of nuclei. Unlike chemical isomers—which have the same chemical formula but different arrangements of atoms—nuclear isomers are nuclei that exist in a long-lived and relatively stable excited state.

Normally, an atomic nucleus resides in its lowest-energy state, known as the ground state. Under external perturbations, such as nucleus-nucleus collisions, however, a nucleus can be excited to a higher-energy state.

While most excited nuclear states are extremely short-lived and rapidly decay back to the ground state, some nuclei remain “trapped” in an excited state for a remarkably long time. Such isomeric states help reveal the structure of the nucleus due to its high sensitivity to the underlying shell structure as well as to changes in single-particle levels.