100 Years of Nuclear Isomers

Isotopes of Hg and Tl in the $\mathit{\text{A}}\approx$ 200 region exhibit competition between collective and intrinsic modes of angular momentum generation. The neutron number $\mathit{\text{N}}=120$ appears to constitute a boundary, with lighter isotopes exhibiting collective behavior, and heavier ones displaying primarily single-particle excitations. Most of these isotopes lie close to the line of stability and are difficult to access through fusion-evaporation reactions involving heavy-ion beams. Therefore, multi-nucleon transfer reactions using $\approx$ 1.4 GeV ${}^{207}$Pb and ${}^{209}$Bi beams, with above-barrier energies, incident on a ${}^{197}$Au target, were used to populate highly-excited levels. The deexciting $\gamma$ rays were recorded by the Gammasphere detector array. The beams were pulsed in different intervals ranging from $<$ 1 $\mu$s to several seconds, to study isomers with a wide range of half-lives.

The evolution of collectivity in ${}^{198,200,202}$Hg has been studied through a measurement of the half-lives of the 7${}^{-}$, 9${}^{-}$ and 12${}^{+}$ states, and inferring the associated $B(E2)$ values. The half-lives of the $7^{-}$ and $9^{-}$ states in ${}^{202}$Hg are measured to be ${\mathit{\text{T}}}_{1/2}$ = 10.4(4) ns and $1.4(3)$ ns, respectively, while that of the ${12}^{+}$ state in ${}^{200}$Hg is ${\mathit{\text{T}}}_{1/2}$ = 1.0(3) ns. For even Hg isotopes, near the ground state, the extent of collective behavior is found to decrease from $\mathit{\text{N}}=112$ to $\mathit{\text{N}}=124$, while it increases for the 12${}^{+}$ and 9${}^{-}$ states up to $\mathit{\text{N}}=118$, and then reduces for higher neutron numbers [1]. Several new isomers were identified in the isotopes ${}^{200,202,203}$Tl. These include a six-nucleon-hole isomer with ${\mathit{\text{T}}}_{1/2}=57(2)$ ns in ${}^{200}$Tl [2]. The level structure of ${}^{202}$Tl has been studied up to the new ${\mathit{\text{I}}}^{\pi }$ = 20${}^{+}$ state, with ${\mathit{\text{T}}}_{1/2}$ = 215(10) $\mu$s, arising from a four-nucleon-hole excitation [3]. In ${}^{203}$Tl, isomeric states with $I^{\pi }=15/2^{-},35/2^{-},39/2^{-}$ and $49/2^{+}$ have been identified, with ${\mathit{\text{T}}}_{1/2}=7.9(5)$ ns, $4.0(5)$ ns, $1.9(2)$ ns, and $3.4(4)$ ns, respectively [4]. For the previously identified long-lived decay, the spin is reassigned as 29/2${}^{+}$ from the earlier suggested value of 25/2${}^{+}$. These new isomers provide a host of nuclear structure insights, including the magnitude of residual interactions for different configurations. Shell-model calculations, using the OXBASH code and the KHH7B interaction, have been performed for these nuclei.

References
[1] Saket Suman $\mathit{\text{et al.}}$, Phys. Rev. C $\mathbf{103}$, 014319 (2021).
[2] Poulomi Roy $\mathit{\text{et al.}}$, Phys. Rev. C $\mathbf{100}$, 024320 (2019).
[3] S.G. Wahid $\mathit{\text{et al.}}$, Phys. Rev. C $\mathbf{102}$, 024329 (2020).
[4] V. Bothe $\mathit{\text{et al.}}$, to be published in Phys. Rev. C (2022).