Doubly magic ${}^{208}\mathrm{Pb}$ : High-spin states, isomers, and $E3$ collectivity in the yrast decay

Journal Article · 12 June 2017 · Physical Review C

DOI: https://doi.org/10.1103/PhysRevC.95.064308 · OSTI ID:1374540

Broda, R. ^[1]; Janssens, R. V. F. ^[2]; Iskra, Ł. W. ^[1]; Wrzesinski, J. ^[1]; Fornal, B. ^[1]; Carpenter, M. P. ^[2]; Chiara, C. J. ^[3]; Cieplicka-Oryńczak, N. ^[1]; Hoffman, C. R. ^[2]; Kondev, F. G. ^[4]; Królas, W. ^[1]; Lauritsen, T. ^[2]; Podolyak, Zs. ^[5]; Seweryniak, D. ^[2]; Shand, C. M. ^[5]; Szpak, B. ^[1]; Walters, W. B. ^[6]; Zhu, S. ^[2]; Brown, B. A. ^[7]

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1. Polish Academy of Sciences (PAS), Krakow (Poland). Inst. of Nuclear Physics
2. Argonne National Lab. (ANL), Argonne, IL (United States). Physics Division
3. Argonne National Lab. (ANL), Argonne, IL (United States). Physics Division; Univ. of Maryland, College Park, MD (United States). Dept. of Chemistry and Biochemistry
4. Argonne National Lab. (ANL), Argonne, IL (United States). Nuclear Engineering Division
5. Univ. of Surrey, Guildford (United Kingdom). Dept. of Physics
6. Univ. of Maryland, College Park, MD (United States). Dept. of Chemistry and Biochemistry
7. Michigan State Univ., East Lansing, MI (United States). Dept. of Physics and Astronomy and National Superconducting Cyclotron Lab.

Yrast and near-yrast levels up to spin values in excess of I = 30h have been delineated in the doubly-magic ²⁰⁸Pb nucleus following deep-inelastic reactions involving ²⁰⁸Pb targets and, mostly, 430-MeV ⁴⁸Ca and 1440-MeV ²⁰⁸Pb beams. The level scheme was established up to an excitation energy of 16.4 MeV, based on multi-fold γ-ray coincidence relationships measured with the Gammasphere array. Below the well-known, 0.5-μs 10⁺ isomer, ten new transitions were added to earlier work. The delineation of the higher parts of the level sequence benefited from analyses involving a number of prompt- and delayed-coincidence conditions. Three new isomeric states were established along the yrast line with I ^π = 20^- (10342 keV), 23⁺ (11361 keV), and 28^- (13675 keV), and respective half-lives of 22(3), 12.7(2), and 60(6) ns. Gamma transitions were also identified preceding in time the 28^- isomer, however, only a few could be placed in the level scheme and no firm spin-parity quantum numbers could be proposed. In contrast, for most states below this 28^- isomer, firm spin-parity values were assigned, based on total electron-conversion coefficients, deduced for low-energy (<500 keV) transitions from γ-intensity balances, and on measured γ-ray angular distributions. The latter also enabled the quantitative determination of mixing ratios. The transition probabilities extracted for all isomeric transitions in ²⁰⁸Pb have been reviewed and discussed in terms of the intrinsic structure of the initial and final levels involved. Particular emphasis was placed on the many observed E3 transitions as they often exhibit significant enhancements in strength (of the order of tens of W.u.) comparable to the one seen for the neutron j_15/2→g_9/2 E3 transition in ²⁰⁹Pb. In this context, the enhancement of the 725-keV E3 transition (56 W.u.) associated with the decay of the highest-lying 28^- isomer observed in this work remains particularly challenging to explain. Large-scale shell-model calculations were performed with two approaches, a first one where the 1, 2, and 3 particle-hole excitations do not mix with one another, and another more complex one, in which such mixing takes place. We compared the calculated levels with the data and a general agreement is observed for most of the ²⁰⁸Pb level scheme. At the highest spins and energies, however, the 2 correspondence between theory and experiment is less satisfactory and the experimental yrast line appears to be more regular than the calculated one. This regularity is notable when the level energies are plotted versus the I(I+1) product and the observed, nearly linear, behavior was considered within a simple “rotational” interpretation. Furthermore, within this approximate picture, the extracted moment of inertia suggests that only the 76 valence nucleons participate in the “rotation” and that the ¹³²Sn spherical core remains inert.

• Research Organization: Argonne National Laboratory (ANL), Argonne, IL (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Nuclear Physics (NP); Polish National Science Centre (NCN); National Science Foundation (NSF)
• Grant/Contract Number: AC02-06CH11357; FG02-94ER40834; PHY-1404442
• OSTI ID: 1374540
• Alternate ID(s): OSTI ID: 1363716
• Journal Information: Physical Review C, Vol. 95, Issue 6; ISSN 2469-9985
• Publisher: American Physical Society (APS)Copyright Statement
• Country of Publication: United States
• Language: English