Investigation of the missing isomer in the rare-earth nucleus ¹⁵⁸Pm

Neutron-rich nuclei in the rare-earth region are important for understanding the effects of nuclear structure evolution and play a role in nuclear astrophysics. An enhanced rare-earth peak in the solar abundance pattern is also not fully understood, yet its formation may reveal information about possible astrophysical conditions. The odd-odd nucleus ¹⁵⁸Pm is particularly interesting as a predicted isomeric state has yet to be clearly established. A β-decay experiment was performed at ANL using the HPGe clover detectors of the X-Array combined with the SATURN decay station. Radioactive ions produced by the CARIBU facility were isobarically separated and delivered to the experimental setup. The tape system was optimized to focus on the decay of ¹⁵⁸Pm. It was discovered that the origin of the previously assigned 121-keV isomeric γ-ray transition in ¹⁵⁸Pm originates instead from the decay of ¹⁵⁸Nd. These observations provide the first high-resolution spectroscopic decay data in ¹⁵⁸Pm, extending our knowledge further into more neutron-rich systems.