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Two-neutron and core-excited states in Pb 210 : Tracing E 3 collectivity and evidence for a new β -decaying isomer in <…

2018; American Physical Society; Volume: 98; Issue: 2 Linguagem: Inglês

10.1103/physrevc.98.024324

2470-0002

R. Broda, Ł. W. Iskra, R. V. F. Janssens, B. A. Brown, B. Fornal, J. Wrzesiński, N. Cieplicka-Oryńczak, M. P. Carpenter, C. J. Chiara, C. R. Hoffman, F. G. Kondev, G. J. Lane, T. Lauritsen, Zs. Podolyák, D. Seweryniak, W. B. Walters, S. Zhu,

Atomic and Molecular Physics

Yrast and near-yrast levels up to an $I=17\ensuremath{\hbar}$ spin value and a 6-MeV excitation energy have been delineated in the ``two-neutron'' $^{210}\mathrm{Pb}$ nucleus following deep-inelastic reactions involving $^{208}\mathrm{Pb}$ targets and a number of heavy-ion beams at energies $\ensuremath{\sim}25%$ above the Coulomb barrier. The level scheme was established on the basis of multifold prompt and delayed coincidence relationships measured with the Gammasphere array. In addition to the previously known states, many new levels were identified. For most of the strongly populated states, spin-parity assignments are proposed on the basis of angular distributions. The reinvestigation of the $\ensuremath{\nu}{({g}_{9/2})}^{2}$, ${8}^{+}$ isomeric decay results in the firm identification of the low-energy $E2$ transitions involved in the ${8}^{+}\ensuremath{\rightarrow}{6}^{+}\ensuremath{\rightarrow}{4}^{+}$ cascade, and in a revised ${6}^{+}$ level half-life of 92(10) ns, nearly a factor of 2 longer than previously measured. Among the newly identified states figure spin $I=4--10\ensuremath{\hbar}$ levels associated with the $\ensuremath{\nu}{g}_{9/2}{i}_{11/2}$ multiplet, as well as yrast states involving $\ensuremath{\nu}{g}_{9/2}{j}_{15/2}$, $\ensuremath{\nu}{i}_{11/2}{j}_{15/2}$, and $\ensuremath{\nu}{({j}_{15/2})}^{2}$ neutron couplings. The highest-spin excitations are understood as $1p\text{\ensuremath{-}}1h$ core excitations and the yrast population is found to be fragmented to the extent that levels of spin higher than $I=17\ensuremath{\hbar}$ could not be reached. Four $E3$ transitions are present in the $^{210}\mathrm{Pb}$ yrast decay; three of these involve the ${g}_{9/2}\ensuremath{\rightarrow}{j}_{15/2}$ octupole component, as reflected in the 21(2) and $>10$ Weisskopf unit enhancements of the $B(E3)$ rates of the first two. The fourth, ${16}^{+}\ensuremath{\rightarrow}{13}^{\ensuremath{-}}\phantom{\rule{0.16em}{0ex}}E3$ transition corresponds to the ${3}^{\ensuremath{-}}$ core octupole excitation built on the $\ensuremath{\nu}{i}_{11/2}{j}_{15/2}$ state, in analogy to a similar $E3$ coupling to the $\ensuremath{\nu}{j}_{15/2}$ level in $^{209}\mathrm{Pb}$. Shell-model calculations performed for two-neutron states and $1p\text{\ensuremath{-}}1h$ $^{208}\mathrm{Pb}$ core excitations are in good agreement with the data. Evidence was found for the existence of a hitherto unknown high-spin $\ensuremath{\beta}$-decaying isomer in $^{210}\mathrm{Tl}$. Shell-model calculations of the $^{210}\mathrm{Tl}$ levels suggest the possibility of a ${11}^{+}$ long-lived, $\ensuremath{\beta}$-decaying state, and the delayed yields observed in various reactions fit rather well with a $^{210}\mathrm{Tl}$ assignment.