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Description
Isotopes of Hg and Tl in the $\textit{A} \approx $ 200 region exhibit competition between collective and intrinsic modes of angular momentum generation. The neutron number $\textit{N} = 120$ 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 $\textit{T}_{1/2}$ = 10.4(4) ns and $1.4(3)$ ns, respectively, while that of the $12^+$ state in $^{200}$Hg is $\textit{T}_{1/2}$ = 1.0(3) ns. For even Hg isotopes, near the ground state, the extent of collective behavior is found to decrease from $\textit{N} = 112$ to $\textit{N} = 124$, while it increases for the 12$^{+}$ and 9$^{-}$ states up to $\textit{N} = 118$, 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 $\textit{T}_{1/2} = 57(2)$ ns in $^{200}$Tl [2]. The level structure of $^{202}$Tl has been studied up to the new $\textit {I}^{\pi }$ = 20$^{+}$ state, with $\textit{T}_{1/2}$ = 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 $\textit{T}_{1/2} = 7.9(5)$ 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 $\textit{et al.}$, Phys. Rev. C ${\bf 103}$, 014319 (2021).
[2] Poulomi Roy $\textit{et al.}$, Phys. Rev. C ${\bf 100}$, 024320 (2019).
[3] S.G. Wahid $\textit{et al.}$, Phys. Rev. C ${\bf 102}$, 024329 (2020).
[4] V. Bothe $\textit{et al.}$, to be published in Phys. Rev. C (2022).
