The gyromagnetic factor of the low-lying E=251.96(9) keV isomeric state of the nucleus Zr99 was measured using the time-dependent perturbed angular distribution technique. This level is assigned a spin and parity of Jπ=7/2+, with a half-life of T1/2=336(5) ns. The isomer was produced and spin aligned via the abrasion-fission of a U238 primary beam at RIKEN RIBF. A magnetic moment |μ|=2.31(14)μN 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 νd5/2 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.
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