Doubly magic Pb 208: High-spin states, isomers, and E3 collectivity in the yrast decay
Creators: name: Walters, W.B.
name: Lauritsen, T.
name: Janssens, R.V.F.
name: Chiara, C.J.
name: Kondev, F.G.
name: Broda, R.
name: Fornal, B.
name: Królas, W.
name: Shand, C.M.
name: Brown, B.A.
name: Carpenter, M.P.
name: Seweryniak, D.
name: Hoffman, C.R.
name: Podolyak, Z.
name: Wrzesinski, J.
name: Cieplicka-Oryńczak, N.
name: Zhu, S.
name: Szpak, B.
name: Iskra, Å.W.
Publisher: name: American Physical Society
Year: 2017

description: Yrast and near-yrast levels up to spin values in excess of I=30 have been delineated in the doubly magic Pb208 nucleus following deep-inelastic reactions involving Pb208 targets and, mostly, 430-MeV Ca48 and 1440-MeV Pb208 beams. The level scheme was established up to an excitation energy of 16.4 MeV, based on multifold 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 Iπ=20- (10 342 keV), 23+ (11 361 keV), and 28- (13 675 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 (<500keV) 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 Pb208 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 Weisskopf units (W.u.)] comparable to the one seen for the neutron j15/2→g9/2 E3 transition in Pb209. 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. The calculated levels were compared with the data and a general agreement is observed for most of the Pb208 level scheme. At the highest spins and energies, however, the 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. Within this approximate picture, the extracted moment of inertia suggests that only the 76 valence nucleons participate in the "rotation" and that the Sn132 spherical core remains inert.