100 Years of Nuclear Isomers

Neutron-deficient nuclei around mid-shell at $N\sim 104$ in the lead region provide many examples of shape coexistence and shape isomers. In order to study shape coexistence in this region, prompt and delayed $\gamma$-ray spectroscopy of the ${}^{187}$Pb, ${}^{183}$Hg and ${}^{188}$Bi isotopes produced in the reaction ${}^{50}$Cr+${}^{142}$Nd$\to$${}^{192}$Po${}^{\ast }$ has been performed at the Argonne Gas-Filled Analyzer.
In ${}^{187}$Pb, a new 5.15(15)-$\mu$s isomeric state at 308 keV above the spherical 3/2${}^{-}$ ground state was identified. A strongly-coupled band is observed on top of this isomer, which is nearly identical to the one built on the prolate 7/2${}^{-}$[514] Nilsson state in the isotone ${}^{185}$Hg. Based on this similarity and on the result of the potential-energy surface calculations, the new isomer in ${}^{187}$Pb was proposed to be prolate with $J^{\pi }$ = 7/2${}^{-}$ and classified as a shape isomer. The retarded character of the 308-keV (7/2${}^{-}$)$\to$3/2${}_{gs}^{-}$ transition with a deduced $B(E2)$ = 5.6(2) $\times$ ${10}^{-4}$ W.u. can be well explained by the significant difference between the prolate parent and spherical daughter configurations, leading to the shape isomerism.
In ${}^{183}$Hg, the decay of the nearly spherical 13/2${}^{+}$ isomeric state was first observed following the $\alpha$ decay of the 13/2${}^{+}$ isomer in ${}^{187}$Pb. By the α − γ correlation measurement, the half-life of this isomer was measured to be T1/2 = 290(30) μs. This isomer was proposed to deexcite by retarded $M2$ transition, which can be explained by the notable shape change between the initial and the final states.
Recently, a strong shape staggering was found in the charge radii of ${}^{187,188,189}$Bi. To further characterize this phenomenon in ${}^{188}$Bi, its in-beam and decay spectroscopy was studied in the same experiment. A new 0.25(5)-$\mu$s isomeric state decaying via a 243-keV transition to the (10${}^{-}$) ${}^{188m}$Bi was identified.

[1] P. Möller et al., Phys. Rev. Lett. 103, 212501 (2009)
[2] W. Q. Zhang et al., submitted to Physics Letters B.
[3] A. Barzakh et al., Phys. Rev. Lett. 127, 192501 (2021).
.