P.O. Lipas

Electron-gas clusters: the ultimate jellium model

The local spin-density approximation is used to calculate ground- and isomeric-state geometries of jellium clusters with 2 to 22 electrons. The positive background charge of the model is completely deformable, both in shape and in density. The model has no input parameters. The resulting shapes of the clusters exhibit breaking of axial and inversion symmetries; in general the shapes are far from ellipsoidal. Those clusters which lack inversion symmetry are extremely soft against odd-multipole deformations. Some clusters can be interpreted as molecules built from magic clusters. The deformation produces a gap at the Fermi level. This results in a regular odd-even staggering of the total energy per electron and of the HOMO level. The strongly deformed 14-electron cluster is semimagic. Stable isomers are predicted. The splitting of the plasmon resonance due to deformation is estimated on a classical argument.

IBA consistent-Q formalism extended to the vibrational region

Abstract The IBA-1 consistent- Q formalism developed by Warner and Casten is here extended to encompass the U(5) vibrational region. The extended model closely parallels the neutron-proton IBA (IBA-2) in form and parameter values. The transitional nucleus 154 Gd is used as a test case.

High-resolution study of the beta decay of 41Ti

Abstract An improved high-resolution study of the beta decay of 41 Ti, produced in the 40 Ca( 3 He, 2n) reaction at 40 MeV, has been performed at the Ion Guide Isotope Separator On-Line (IGISOL) facility. The beta-delayed radiation was detected by a low-energy charged-particle detector and a large Ge detector for gamma rays. The experimental beta-decay strength and its distribution, extracted from delayed-proton data, are compared with results of shell-model calculations in the sdfp space. The lowest J π = 3 2 + , T = 3 2 state in 41 Sc, the isobaric analogue state of the 41 Ti ground state, is estimated to contain 10% isospin impurity.

Microscopic IBA calculations of (e, e') M1 form factors☆

Abstract General theory of transition charge and current densities is formulated for inelastic electron scattering. Single-particle densities are calculated. The many-fermion matrix element of a general one-body operator is derived for states of generalized seniority two. The Otsuka-Arima-Iachello mapping is performed to yield the M1 transition current of the interacting-boson approximation (IBA). Transition currents and DWBA form factors are computed for the strong 1 + excitations in 154 Sm, 154–158 Gd, 164 Dy and 168 Er. Particular emphasis is placed on 164 Dy because of its rich experimental data. Boson g -factors are computed for these nuclei. All parameters are taken from the literature and not fitted to the scattering data. Particle-hole symmetry is discussed.

IBM-2 description of M1 properties in deformed nuclei

Abstract Results of schematic calculations are presented in which various terms breaking F -spin symmetry are considered in the hamiltonian of the neutron-proton interacting boson model (IBM-2). Specific attention is paid to the effect of F -spin symmetry breaking on γ → ground and γ → γ M1 transitions in deformed nuclei. A comparison with available M1 data in the rare-earth nuclei is presented. The constraints implied by these data on the form of the IBM-2 hamiltonian in well-deformed nuclei are discussed.

IBA-1 calculations of E2/M1 mixing ratios

Abstract The interacting-boson approximation is applied in its basic form (IBA-1) to the calculation of γ-ray E2/M1 mixing ratios. The description is tested on a variety of even nuclei with adequate experimental data: 146–152 Sm, 152–156 Gd, 162 Dy, 162–168 Er, 172 Yb, 190 Os. The consistent- Q formalism of Warner and Casten and its extension to the vibrational region are used. The three M1 transition parameters of IBA-1 are chosen so as to display acceptably smooth systematics, in contrast to earlier calculations reviewed for comparison. The calculated M1 matrix elements are found to depend strongly both on the wave functions and on the M1 parameters. Comparison is made with the few available IBA-2 results as well as with the two-fluid Frankfurt model and Kumars microscopic model.

Unrestricted shapes of light nuclei in the local-density approximation: Comparison with jellium clusters

Abstract The shapes of light nuclei are studied within density-functional theory. The Kohn-Sham method and the local-density approximation are used. No symmetry restrictions are imposed. A parallel study is made of monovalent atomic clusters described on the jellium model. The shapes obtained for nuclei with Z = N = 2–22 show a striking similarity to those of atomic clusters of an equal number of valence electrons. Moments of inertia, when suitably normalized, are virtually identical. The calculated nuclear quadrupole moments are found insensitive to the effective interaction and in good agreement with experiment. Similar shape coexistence is established in both systems.

IBA-1 calculation of E2M1 mixing ratios in 154Gd

Abstract The general theoretical scheme, with various special cases, is presented for calculating γ- ray E 2 M mixing ratios for even nuclei on the IBA-1 model. The M1 transition operator needed for unconstrained algebraic signs contains three parameters. Numerical results for 154Gd are compared with Warner;s simplified IBA treatment and with Kumars model. Sign flexibility is demonstrated by 168Er.

Unrestricted Shapes of Jellium Clusters

A jellium model with a completely relaxable background charge density is used to study metal clusters containing 2 to 22 electrons. The resulting shapes of the clusters exhibit breaking of axial and inversion symmetries, as well as molecular formation. The clusters without inversion symmetry are soft against deformation. The strongly deformed 14-electron cluster is found to be semi-magic. Stable-shape isomers are predicted.

IBA-1 calculation of E 2 M 1 mixing ratios in 154 Gd

Abstract The general theoretical scheme, with various special cases, is presented for calculating γ- ray E 2 M mixing ratios for even nuclei on the IBA-1 model. The M1 transition operator needed for unconstrained algebraic signs contains three parameters. Numerical results for 154Gd are compared with Warner;s simplified IBA treatment and with Kumars model. Sign flexibility is demonstrated by 168Er.