Speaker
Description
The two prevailing models for nuclear structure are the shell model and the collective model, which characterise predominantly separate regions of the nuclide chart. Nuclei at the edges of, and/or between these regions are essential for broadening our understanding of both the models themselves and nuclear structure more generally. The proton-pair, neutron-hole-pair nucleus of $^{208}$Po exhibits both shell and octupole collective behaviour, and therefore its structure is of particular interest for experimental research, particularly as theoretical models are limited in this region. $^{208}$At decays from a $6^+$ (Q$_{EC}$ = 5000(9)~keV\cite{QBeta_208At}) state and thus the populated states of the daughter nucleus are limited in both energy and spin-parity by beta decay selection rules. As such, $\beta-\gamma$ decay spectroscopy is ideal for isolating and studying the low-lying, low-spin states of $^{208}$Po. The large HPGe clusters at the IDS are superior to the smaller detectors used in prior experiments\cite{1981Exp,1983Exp,1985Exp}. This facilitated the placement of a number of gamma rays which were previously attributed to $^{208}$Po but were not placed into the level scheme due to low statistics.
Present in the structure of $^{208}$Po are two notable features. The first is a relatively long-lived ($T_{1/2}$ = 350(20)~ns\cite{Po208NNDC}) isomeric state, the half life of which was measured in this analysis using $\gamma-\gamma-\delta t$ coincidence matrices to be 377(9)~ns~\cite{MyConfProc}. The second is a 1995~keV $3^-$ state identified by this analysis, which can provide insight into the octupole collectivity prevalent in this mass region.
The $^{208}$Po level scheme has been significantly extended. 46 newly observed transitions and 26 new levels have been placed in an expanded level scheme alongside pre-existing and (re)assigned transitions and levels. Spin-parity assignments are based on decay patterns, previously measured conversion electron coefficients and log$ft$ values. Comparison with shell model calculations showed a good agreement for non-core-excited states.
Investigation into beta decay branching also yielded interesting conclusions. First-forbidden decays populate predominantly states at high excitation energies, which can be qualitatively explained by shell model considerations. First-forbidden and allowed $\beta$ decay have similar yields, a feature which is consistent with other nuclei in the region. The observation of many of the first-forbidden $\beta$ decay branches relied on the high detection efficiency for high-energy $\gamma$ rays which had not been observed in prior experiments. Observations of the $\beta$-decay properties of nuclei in the N$<$126, Z$>$82 region suggests that $^{208}$Po and its neighbouring nuclei provide a good testing ground for first-forbidden $\beta$-decay calculations, the understanding of which is important for the $r$-process nucleosynthesis.
In the presentation the details of the IDS experimental setup will be outlined and a general overview of the analysis and the new level scheme will be given. The implications of the new level scheme will be discussed further with regard to collective behaviour, comparison to shell model calculations, and beta decay branching. The analysis for which was recently submitted to Phys. Rev. C.
\bibliographystyle{ieeetr}
\begin{thebibliography}{widestlabel}
\bibitem{QBeta_208At}
M. Wang \textit{et. al.}, Chinese Physics C, \textbf{41}, 3 (2017)
\bibitem{1981Exp}
V. M. Vakhtel \textit{et. al.}, Izv. Akad. Nauk, SSSR, Ser. Fiz., \textbf{45}, 1841 \textit{[Engl. transl.: Bull. Acad. Sci. USSR, Phys. Ser. \textbf{45} no. 10, 29 (1981)}] (1981)
\bibitem{1983Exp}
B. S. Dzhelepov \textit{et. al.}, Izv. Akad. Nauk, SSSR, Ser. Fiz., \textbf{47}, 2 \textit{[Engl. transl.: Bull. Acad. Sci. USSR, Phys. Ser. \textbf{47} no. 1, 1 (1983)}] (1983)
\bibitem{1985Exp}
V. Rahkonen and T. Lonnroth, Z.Phys., \textbf{A322}, p. 333 (1985)
\bibitem{Po208NNDC}
M. J. Martin, Nucl. Data Sheets, \textbf{108}, 1583 (2007)
\bibitem{MyConfProc}
M. Brunet \textit{et. al.}, Jour. of Phys. Conf. Series, \textit{in press} (2020)
\end{thebibliography}
