R. Wyss

Structure of superdeformed states in AuRa nuclei

Abstract Energies of superdeformed states in nuclei around 192 Hg are calculated using the Strulinsky shell correction method with an average Woods-Saxon potential and a monopole pairing force. The influence of various terms in the model hamiltonian on the excitation energy of the superdeformed minimum is analysed. The systematics of calculated excitation energies of shape-isomeric minima and barrier heights in even-even Hgue5f8Ra nuclei are given together with predictions for one-quasiparticle band-head energies in odd-A Au, Hg, Tl, Pb and Bi nuclei. Possible occurrence of hyperdeformed states in very neutron-deficient Poue5f8Ra isotopes is investigated. It is shown that the presence of these exotic configurations is strongly related to the magnitude of pairing correlations at strongly elongated shapes. Finally, the role played by reflection-asymmetric deformations at superdeformed shapes is discussed.

First observation of a collective dipole rotational band in the A∼200 mass region

Abstract The nucleus 198 Pb was populated via the 186 W( 17 O,5n) 198 Pb reaction at beam energies of 92 and 98 MeV. A highly regular rotational ΔI =1 band has been found. Angular correlation measurements confirm the transitions to be dipoles. Assuming M1 multipolarity for these transitions the experimental data are interpreted in terms of a π([505] 9 2 − ⊗[606] 13 2 + ) K π =11 − ⊗ν( i 13 2 ) 2 configuration of oblate shape. This is the first such collective oblate structure identified in the A ∼200 mass region.

Collective oblate dipole rotational bands in 198Pb

Abstract The nucleus 198 Pb was populated via the 186 W( 17 O, 5n) 198 Pb reaction at beam energies of 92 and 98 MeV. Five collective rotational cascades of ΔI = 1 transitions have been found. Four are highly regular, one much more irregular. The structures are incorporated into a level scheme which extends up to approximately spin 32ħ and an excitation energy of about 10 MeV. Angular correlation measurements confirm the dipole character of the interband transitions. Their M1 multipolarity is inferred, and from this supposition the experimental data are interpreted in terms of oblate high- K two quasiproton configurations coupled to aligned neutron excitations. This interpretation is extended to include other ΔI = 1 oblate structures observed in 194–201 Pb. It is shown that the pattern of observed moments of inertia can be understood in the simple unpaired picture involving neutron i 13 2 excitations. The identical bands observed are interpreted in terms of the normal-parity weakly-coupled singlet orbital.

Intruder bands in 108Sn

Abstract The nucleus 108Sn has been populated via the 54Fe(58Ni, 4p) reaction channel at a beam energy of 243 MeV. The high-spin structure is dominated by three ΔI = 2 rotational sequences. These bands can be interpreted in terms of particle-hole excitations involving the proton g 7 2 , g 9 2 and h 11 2 orbitals and also aligned neutrons from the bottom of the h 11 2 shell. Lifet measurements have also been performed using the Doppler-shift attenuation method. These data have enabled quadrupole moments to be deduced for the two strongest bands. The results yield Q0 = 2.6±0.4 e·b for the positive-parity band and 3.4 ± 0.6 fse·b for one of the proposed negative-parity bands. These values yield quadrupole deformations of β2 = 0.20 and 0.26, respectively, for the two bands. The results obtained are discussed in terms of Woods-Saxon and total routhian surface calculations.

Shapes and rotational structures of neutron h112 configurations in the XeBaCe region

Abstract Rotational bands built on neutron h 11 2 states are investigated within the Strutinsky mean-field concept. Calculated deformation trajectories of one-, three- and five-quasiparticle configurations are presented. The experimentally observed signature splitting and crossing frequencies in Xeue5f8Baue5f8Ce nuclei are compared to theoretical predictions. The systematics of the observed band structures can only be accounted for by allowing a smooth transition from prolate to oblate structure via triaxial shapes as the number of neutrons increases.

Signature inversion in 120Cs: Evidence for a residual pn interaction

Abstract High-spin states have been observed in the odd-odd isotope 120Cs in 32S-induced reactions. The previously known band is extended to higher spin and several new bands are identified. Band-head configurations have been inferred by comparing the band properties with known bands in the odd nuclei 121Cs and 121Ba. A remarkable signature inversion is observed in the π h 11 2 ⊗ν h 11 2

Superdeformed bands in 189,190Hg

Abstract Experiments using the 160 Gd ( 34 S , x n ) reaction at 159 and 162 MeV have revealed two γ-ray cascades which have properties similar to those of the previously reported superdeformed bands in the heavier Hg isotopes. These sequences have been identified as superdeformed structures in 189Hg and 190Hg. For the band in 190Hg, an average quadrupole moment Q0 = 18 ± 3 e · b was obtained from a DSAM measurement. The data are interpreted within the cranked Woods-Saxon formalism and the rise in the dynamical moment of inertia for both bands is linked to the presence of pair correlations and quasiparticle alignments. For the 189–194Hg superdeformed bands, the variations in the observed population intensity and in the rotational frequencies at which decay out of the bands occur are found to be consistent with the calculated change in the well depth of the superdeformed minimum with mass.

Alignment processes in 119Cs, 121Cs and 123Cs

Rotational band structures have been observed in the odd-A Cs isotopes 119Cs, 121Cs and 123Cs. The previously known bands have been extended to higher spin values and several new bands have been es ...

High-spin states in 163Lu☆

Abstract High-spin states in 163 Lu were investigated by in-beam γ-ray spectroscopy using the multidetector array OSIRIS. One decoupled and two strongly coupled bands were observed up to high spins and a fragment of a second decoupled band was established. The band structures are connected to previously known low-spin states. An anomalously low-frequency second band crossing is observed in one of the strongly coupled bands. Band crossing frequencies and aligned angular momenta are interpreted within the framework of the cranked shell model. B (M1)/ B (E2) ratios are determined and compared to calculated values. Evidence is presented that the decoupled band may have a much larger deformation than the coupled bands. We suggest that it is built on the proton i 13 2 intruder configuration.

On the question of spin fitting and quantized alignment in rotational bands

Several methods employed to parametrize the spins of the superdeformed bands in terms of their observed gamma-ray energies are closely examined. It is pointed out that, since the proposed fitting procedures are variants of the Harris expansion formula, it is questionable if they could be applied to determine the unknown spins of the excited bands due to the possible presence of non-zero initial alignments. A comprehensive study of the normally deformed excited bands indicates that no correlation exists between the fitted spins and the experimentally known values. By analogy, we may conclude that there is no firm evidence for the presence of anomalous relative alignments between the superdeformed identical bands in the Hg region. Additionally, a critical review of several of the models and scenarios that purport to explain the origin of the identical bands, or the patterns of the fitted alignments is presented. It is concluded that none of these models can satisfactorily explain the identical moments of inertia in neighboring nuclei and the systematics of the observed or fitted alignments. Thus, the important question of the microscopic origin of the identical bands remains to be investigated.