Reassessment of the ¹⁰⁰Nb rotational band and its newfound connection with the (8⁻) isomer

In this work, a new spectroscopy is reported for the the odd-odd 100 Nb nucleus which lies one neutron below the well known N=60 island of deformation in the A~100 mass region where a sudden shape transition from semi-spherical to well deformed ground state occurs. The measurement involved prompt and prompt-delayed γ-ray spectroscopy of fission fragments isotopically identified using the combination of the Variable Mode Spectrometer (VAMOS++), the Advanced Gamma Tracking Array (AGATA) and the EXOtic GAMma-ray spectrometer (EXOGAM). In addition, a dataset of the spontaneous fission of 252 Cf measured using Gammasphere was used. A new link between the rotational band previously believed to be the ground state band and the isomer of 12.4(3) μ s isomer was found, resulting in a revision of the level scheme. The new level scheme suggests that the rotational band is built on the long-lived 5 + isomer instead. Moreover, a new rotational band was identified above the predicted 10 − isomer. These results are in agreement with IBFFM calculations predicting a non deformed ground state in this nucleus.