[en] In terms of the interacting boson model (IBM), shape invariants for the ground state, formed by quadrupole moments up to sixth order, are studied in the dynamical symmetry limits and over the whole structural range of the IBM-1. The results are related to the effective deformation parameters and their fluctuations in the geometrical model. New signatures that can distinguish vibrator and γ-soft rotor structures, and one that is related to shape coexistence, are identified. (c) 2000 The American Physical Society

[en] We propose a description of cluster states in nuclei in terms of representations of unitary algebras U(ν+1), where ν is the number of space degrees of freedom. Within this framework, a variety of situations including both vibrational and rotational spectra, soft and rigid configurations, identical and nonidentical constituents can be described. As an example, we show how the method can be used to study α clustering configurations in ¹²C with point group symmetry D_3h. (c) 2000 The American Physical Society

[en] Using the GRID method the lifetimes of 12 states belonging to four negative parity bands in ¹⁶⁸Er were measured at the high flux reactor of the Institut Laue-Langevin (ILL). For K^π=0₁^- and K^π=2₁^- bands the absolute E1 transitions are in agreement with those obtained within the framework of the sdf IBA-1 model and their octupole vibrational character is confirmed. (c) 2000 The American Physical Society

[en] Excited states in ²³²Th were populated by Coulomb excitation with 80 MeV ¹⁶O ions. Gamma-gamma coincidences were detected with the detector array YRAST Ball. The one-phonon quadrupole and octupole vibrations are most strongly excited. These vibrations are in ²³²Th, particularly favorable cases for a study of their couplings, due to the small energy spacings of the β and γ bands and the K^π=1^- and 2^- bands, respectively. The need for additional experimental information on the γ-ray intensities and the location of members of the octupole-vibrational bands is emphasized. The higher-lying K^π=0⁺ bands with bandheads at 1079 and 1352 keV proposed previously are observed and some conflicting properties of these bands are discussed. The interpretation of the 1352 keV 0⁺ band as two-octupole-phonon vibration is not supported by our data. A level at 1054 keV, previously assigned as 2⁺ or 2^- level, is tentatively proposed to be the bandhead of a second-excited K^π=2⁺ band. Our γγ coincidence data establish that the unusually enhanced E2 transitions from a 2⁺ level at 1554 keV to members of the β and γ bands proposed previously do not exist. (c) 2000 The American Physical Society

[en] Inelastic excitation of ¹⁷⁰Er by a ²³⁸U beam, studied at near-barrier energies, has led to the observation of unusual features in rotational bands built on low-lying vibrations. The population of the high-spin members of the K^π=0⁺, β-vibrational band is enhanced due to mixing with the K^π=2⁺, γ-vibrational band at spin 4⁺. Strong mixing of the K^π=0⁺ band with a rotationally aligned 2 qp band results in this band losing its β-vibrational character and in a rapid gain in spin alignment leading to a crossing with the ground-state band between spins 20⁺ and 22⁺. The low-lying K^π=3⁺ band also is populated. It subsequently decays to both the γ-vibrational and the ground-state bands. The occurrence of appreciable K-forbidden E2 transitions from the K^π=3⁺ to the ground-state band is attributed to mixing with the K^π=2⁺ band, caused by the interaction between the quadrupole γ-vibrational and the hexadecapole vibrational motions. (c) 2000 The American Physical Society

[en] Collective structures in ¹⁷⁸Pt have been investigated with γ-ray spectroscopic techniques following mass selection. The present data expand considerably the level scheme of this nucleus. A new collective structure has been identified and the known bands have been extended to higher spins. Firm spin and parity assignments have been made to many levels. Of particular interest are the negative parity bands which are interpreted in terms of structures associated with octupole vibrations crossed at moderate spin by two-quasiparticle excitations. The systematics of the α-decay reduced widths in Pt nuclei has also been investigated. The differences of a factor of about 2 between the widths for the odd- and the even-A isotopes is attributed to changes in neutron pairing associated with the blocking effect. The trend is reproduced in calculations based on the Lipkin-Nogami pairing model. (c) 2000 The American Physical Society

[en] We examine the time-reversal-violating nuclear ''Schiff moment'' that induces electric dipole moments in atoms. After presenting a self-contained derivation of the form of the Schiff operator, we show that the distribution of Schiff strength, an important ingredient in the ground-state Schiff moment, is very different from the electric-dipole-strength distribution, with the Schiff moment receiving no strength from the giant dipole resonance in the Goldhaber-Teller model. We then present shell-model calculations in light nuclei that confirm the negligible role of the dipole resonance and show the Schiff strength to be strongly correlated with low-lying octupole strength. Next, we turn to heavy nuclei, examining recent arguments for the strong enhancement of Schiff moments in octupole-deformed nuclei over that of ¹⁹⁹Hg, for example. We concur that there is a significant enhancement while pointing to effects neglected in previous work (both in the octupole-deformed nuclides and ¹⁹⁹Hg) that may reduce it somewhat, and emphasizing the need for microscopic calculations to resolve the issue. Finally, we show that static octupole deformation is not essential for the development of collective Schiff moments; nuclei with strong octupole vibrations have them as well, and some could be exploited by experiment. (c) 2000 The American Physical Society