T. R. McGoram

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.

Incomplete fusion as a spectroscopic tool

Particle- coincidence studies have been a promising but not well exploited means of using incomplete fusion reactions to gain access to states in heavy nuclei near stability. At beam energies near the Coulomb barrier, fusion of the heavy fragment from break-up of the projectile with emission of non-equilibrated particles or other charged particles can be used to study relatively neutron-rich nuclei which cannot be reached by fusion, evaporation reactions with stable beams. Qualitative features which make the reactions attractive as a spectroscopic tool include a spin input which is higher than that achievable if fusion reactions were carried out with beams equivalent to the massive fragment and a correlation between the angle of emission of the light fragment and the number of evaporated neutrons which assists channel identification.

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 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.

Incomplete-fusion reactions for γ-ray spectroscopy: Application to the study of high-spin states in 234U

Incomplete-fusion reactions occur when breakup of the projectile results in only part of the beam particle fusing with the target, the remnant being emitted with an energy equivalent to the beam velocity. Such reactions have been demonstrated to populate slightly neutron-rich nuclei compared to conventional fusion-evaporation reactions, opening possibilities for the study of nuclei along the neutron-rich side of the line of stability. Results from a study of 211Po are presented to illustrate the use of incomplete-fusion reactions for γ-ray spectroscopy. New results from a test-run which populated high-spin states in 234U via the 232Th(9Be,α3n) reaction are also presented. An interesting feature of the latter reaction is that the high fission probabilities for the compound nuclei which follow complete fusion, results in the residues from incomplete fusion forming the dominant residue channels.

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.