Configuration assignments to isomers in the neutron-rich ${}^{186}\mathrm{Ta}$ $\left(Z=73\right)$ nucleus

Though the neutron-rich odd-odd nucleus ${}_{73}{}^{186}\mathrm{Ta}{}_{113}$ was first produced in 1955, even after 60 years its ground state (g.s.) and both of its two other isomers remain undefined. We use the well-tested two-quasiparticle rotor model, which explicitly includes residual neutron-proton $n\text{−}p$ interaction and other contributing factors, to evaluate the bandhead energies of the physically admissible low-lying two-particle structures in ${}^{186}\mathrm{Ta}$ with inputs from experimentally observed structures in neighboring isotopes and isotones to characterize these levels. Our analysis assigns $K^{\pi }$ = $5^{-}\{p$:$7/2\left[404\right]\otimes n$:$3/2\left[512\right]\}$ configuration to the 10.5 min ${}^{186}\mathrm{Ta}$(g.s.) and the antiparallel-spin $K^{\pi }=2^{-}$ of the same configuration to the 1.54 min isomer with $E_x$ = 90(10) keV. We further assign $K^{\pi }=8^{-}\{p$:$7/2\left[404\right]\otimes n$:$9/2\left[505\right]\}$ configuration to the recently identified 3.0 min isomer with $E_x=336$ (20) keV. These assignments are shown to be consistent with all the available experimental data. Further, they are seen to fit nicely as another instance of highly hindered $\Delta I=3$ isomeric transitions, and also of low-lying long-lived isomer triplets, frequently observed in numerous odd-odd $Z=61\left(2\right)75$ nuclides.

Figure 1

Plot of experimental low-lying (up to $\approx 500$ keV) [2] bandhead energies in isotopic ${}^{185}\mathrm{Ta}$ (on the left) and in isotonic ${}^{187}\mathrm{W}$ (on the right), and model-calculated 2qp level energies in ${}^{186}\mathrm{Ta}$ (in the middle). The thick lines represent the isomer triplets which are also labeled by their respective half-lives.