S. Bayer

Rotational band on the 31 yr 16+ isomer in 178Hf

Abstract High-spin states in 178Hf have been identified using particle-γ-γ-time techniques and the incomplete fusion reaction, 176Yb(9Be,α3n)178Hf. The rotational band associated with the four-quasiparticle T 1 2 = 31 year, Kπ = 16+ isomer in 178Hf has been established. The gK−gR values obtained from the in-band decay properties confirm the configuration of the isomer as does its alignment which matches the sum of the alignments of the two Kπ = 8− two-quasiparticle bands that contain the components of the 16+ configuration. New information has also been obtained on the four-quasiparticle 14− band and on the two-quasiparticle 6+ and 8− bands.

Multi-quasiparticle states in 179Ta and structural changes in the yrast line of the odd tantalum isotopes

High-spin states in 179Ta have been studied by a variety of γ-ray spectroscopic techniques following the 176Yb(7Li,4n) reaction. The new results independently confirm and extend the one-, three- and five- quasiparticle bands proposed previously. The more comprehensive results obtained allow firm spin assignments to be made and also the identification of rotational bands associated with the higher seniority intrinsic states. Configuration assignments follow from analysis of these rotational band properties. To explain changes in the yrast line across the chain of isotopes from 173Ta to 179Ta, multi-quasiparticle calculations have been performed and compared with experiment. The calculations, which treat pairing correlations using the Lipkin-Nogami approach and include blocking, incorporate single-particle energies adjusted to reproduce the observed one-quasiparticle bandheads. Properties specific to 179Ta include evidence for configuration mixing in the 212− isomer configuration. Its decay is one of several anomalously fast K-hindered transitions observed and discussed in terms of configuration changes and other mechanisms.

Rotational and multi-quasiparticle excitations in 183Re

Abstract The high-spin structure of the nucleus 183 Re has been studied following the 176 Yb( 11 B,4n) reaction. High- K , multi-quasiparticle excitations compete with collective rotation in the formation of the yrast and near-yrast structures. Broken-pair (i 13/2 ) 2 excitations ( t -bands) play a dominant role, generating considerable angular momentum both along and perpendicular to the prolate symmetry axis. The stability of axially symmetric, prolate shapes is explored through comparisons with potential-energy-surface calculations. The possible onset of oblate rotation at high angular momentum is discussed.

Competing phenomena: high-seniority excitations and γ-softness in 184Os

Abstract The nuclear structure of the stable nucleus 184 76 Os has been studied including five newly identified rotational bands built on 2-quasiparticle K π =0 + , 6 − , 8 − couplings. The excitation energies of the proposed multi-quasiparticle excitations are in good agreement with predictions of blocked BCS calculations. The interaction responsible for the first band crossing, caused by a high- K tilted 2-neutron configuration, is calculated and a new interpretation presented for the yrast states after the interaction. A second crossing with an aligned s– β band is also reported. The yrast behaviour at high spins is dominated by a wealth of short-lived (nanoseconds and shorter) high- K levels. The low reduced hindrances for intrinsic state decays are discussed, and attributed to the triaxial shapes and fluctuations predicted by configuration-constrained potential-energy-surface calculations. Comparisons with neighbouring nuclei are made.

High-spin states, yrast isomers and residual interactions in the odd-odd nucleus 212At

Abstract Excited states in the odd-odd nucleus 212At have been studied using γ-ray and electron spectroscopy following the 208Pb(7Li,3n) reaction. Levels were identified to spins of 25 h and to excitation energies of about 4.8 MeV. New isomeric states which decay by enhanced E3 transitions have been established at spins 18+, 22− and 25−. Shell model calculations reproduce well the main features of the level scheme and show the configuration dependence of certain empirical residual interaction matrix elements.

High-spin isomers in 211Po and related structures in 210Po and 212Po

Abstract High-spin states in 211 Po have been populated through the incomplete fusion reactions 208 Pb ( 9 Be ,α2n ) and 208 Pb( 7 Li ,p3n) . The yrast sequence decaying to the τ m = 36.4 s isomer in 211 Po has been established to a spin of ( 43 2 + ) at an excitation energy of 4873 keV. Two other isomeric levels have been identified, with mean lives of 350(30) ns and 4(1) μs at excitation energies of 2135.7 keV and 4873.3 keV, respectively. Relationships between the structure of high-spin states in 211 Po and those previously identified in 210 Po and 212 Po are discussed.

Lifetimes of the 212− and 292+ states in 211At and their implication for effective charges and quadrupole moments

Abstract The lifetimes of the 21 2 − state (1416 keV) and the 29 2 + state (2641 keV) in 211At were measured to be τm = 50.7(10) ns and τm = 73.3(10) ns, respectively, using the γ-time technique. Using these lifetime measurements a value for the quadrupole moment of the 29 2 + state was derived. The explicit inclusion of octupole coupling effects into a description of the configuration for the 29 2 + level allows the extraction of an orbital independent effective charge of eeff = 1.5e. From this a nominal value for the quadrupole moment of the πi13/2 orbital can be deduced. New levels identified have allowed a determination of the strength of the πi13/2−πf7/2 residual interaction.

Triaxiality and tilted-bands in heavy osmium nuclei

Nuclei in the A ≈ 180 region around Z = 72 exhibit well deformed prolate shapes, giving rise to isomeric states as a result of the partial conservation of K, the projection of the total angular momentum on the nuclear symmetry axis. As more protons are added, away from Z = 72, the prolate deformation decreases and the nuclear potential becomes susceptible to triaxial fluctuations, governed by the γ degree of freedom. In these so called γ-soft nuclei, K is no longer a conserved quantity, resulting in the reduction or even loss of K-isomerism. Based on the excitation energies of the 2+ γ-vibrational bandheads, osmium isotopes (Z = 76) have the most γ-soft potentials in the A ≈ 180 region. Here, new results are reported for the highspin structure of 184Os and comparisons with neighbouring even-even osmium nuclei are made.