The gyromagnetic factor of the low-lying $E=251.96(9)\mathrm{keV}$ isomeric state of the nucleus ${}^{99}\mathrm{Zr}$ was measured using the time-dependent perturbed angular distribution technique. This level is assigned a spin and parity of $J^{\pi }=7/2^{+}$, with a half-life of $T_{1/2}=336(5)\mathrm{ns}$. The isomer was produced and spin aligned via the abrasion-fission of a ${}^{238}\mathrm{U}$ primary beam at RIKEN RIBF. A magnetic moment $|\mu |=2.31(14){\mu }_N$ was deduced showing that this isomer is not single particle in nature. A comparison of the experimental values with interacting boson-fermion model IBFM-1 results shows that this state is strongly mixed with a main $\nu d_{5/2}$ composition. Furthermore, it was found that monopole single-particle evolution changes significantly with the appearance of collective modes, likely due to type-II shell evolution.

• Received 22 August 2019
• Revised 28 November 2019
• Accepted 17 December 2019

DOI:https://doi.org/10.1103/PhysRevLett.124.112501