P. O. Lipas’s research while affiliated with University of Jyväskylä and other places

The microscopic shell model of nuclear physics is used to calculate the electronic structure and excitations of spherical atomic clusters described within the jellium model. The ground-state energies and ionization potentials compare well with those of the local-spin-density approximation. The spherical clusters do not show odd-even staggering in the ionization potential. The calculated photoabsorption cross-section has two dominant peaks for clusters of 9 and 10 atoms but only peak of the 8-atom cluster, in agreement with experiment.

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.

A phenomenological model is developed for the collective quadrupole properties of all even-even nuclei. Rotational. vibrational and transitional nuclei are included in the model on an equal footing. A Bohr-type intrinsic Hamiltonian for harmonic quadrupole vibrations about an axially deformed shape is solved exactly. States of good angular momentum are projected out of the intrinsic states, and they are made orthogonal by a Schmidt scheme. The angular-momentum and phonon-number composition of the states is analyzed at various stages; states with K = 1 are found spurious. Excitation energies for the ground, β, and γ bands are calculated as expectation values of a radically simplified nuclear Hamiltonian in our projected and orthogonalized states. With increasing deformation the calculated energies evolve smoothly from the evenly spaced phonon spectrum to the Bohr-Mottelson rotational-vibrational spectrum according to the scheme of Sheline and Sakai. Our basic model contains only two parameters (deformation d and energy scale) to fix the entire quadrupole spectrum of a nucleus. Our results are given in the form of graphs suitable for immediate application; numerical results are readily produced by our computer code. The ground bands are fitted comparably to the VMI model, while the β and γ bands are reproduced qualitatively. The nuclei 152Sm, 152Gd, and 114Cd are used as test cases. Quadrupole moments and E2 transition rates are also calculated. Intraground-band transition ratios and branching ratios from the β and γ bands are given in terms of the single parameter d. The results are applied to 152Sm with fair success. Finally we extend the model to include two more parameters (anisotropy). The improvement over the basic model is modest in view of the added parameters and computational effort.

We have studied low-spin states of 150,152,154,156Gd by nuclear orientation of β-decaying Tb in a Gd host. Especially by means of multipole mixing ratios, including E0/E2, we have checked and revised spin-parities and assignments to (quasi) rotational ground, β and γ bands. For 150Gd we propose the new interpretation 1207.2 keV (0β+), 1518.5 keV (2β+), 1700.1 keV (4β+), 1430.5 keV (2γ+), 1988.0 keV (3γ+), 2080.0 keV (4γ+). For 152,154,156Gd we find agreement with recent literature. Our comparison with theory includes available data on 158,160Gd and on states up to 10g+, 10β+, 7γ+. We review our previously proposed "projection model", which is basically of the Bohr-Mottelson geometrical type; its essential feature is angular-momentum projection from an intrinsic coherent phonon state. The algebraic boson model IBA-1 is also discussed. Both models are applied to energies and to E2 and E0 transition probabilities. Roughly viewed, the fits by both models are good for the energies, fair for the E2 transitions and poor for the E0 transitions. The four-parameter version of IBA-1 applied is unable to reproduce the nonadiabatic details of the bands, while even the two-parameter projection model does better. For 150Gd the simple boson models seem over-extended because of the proximity of doubly magic 14664Gd82. Description of the N = 88-90 shape transition requires discontinuous parameter behaviour in both models.

Beta decay of 23Al to excited states in 23Mg has been studied using low-energy proton and high-energy gamma-ray detection combined with ion-guide-based on-line mass separation. For the first time, a T=3/2 isobaric analogue state, at 7801(2) keV, was observed to decay by both proton and gamma emission, with a proton branching of 0.17(8)%. The deduced resonance strength ωγ=2.2(10) meV is in agreement with upper limits reported from 22Na(p,γ) reaction studies. Shell-model calculations are incorporated.

Theoretical estimates for isospin-forbidden beta-delayed proton and two-proton emission are calculated in a shell-model framework. The parent nuclei studied are 23Al and 31Ar. The results are compared to new experimental data. The reliability of the shell-model estimates is discussed. .

Truncation methods for the nuclear shell model are tested and compared in terms of properties of 28Si, 28Al, and 40Mg. Included are the traditional methods based on energy denominators and the number of particle-hole excitations, as well as a newer procedure which takes into account the two-body interactions. For nuclei with T≫0 the newer method is found appreciably better than the traditional methods, which are about equal. For nuclei with N≈Z there is little difference among the three methods.

Resonance ionization laser ion source (RILIS) technique has been used in the β-decay studies of 59Mn and 58Zn. The importance of the RILIS for production of these elements is discussed. The properties of the low-lying levels of the studied nuclei are discussed.

Shell-model and deformed Hartree-Fock plus BCS calculations are reported for even-even nuclei 18-30O, 18-36Ne, and 20-42Mg; shell-model calculations additionally included 38,40Ne and 44,46,48Mg. Ground-state binding energies and 21+ quadrupole moments are calculated by both models. Shell-model calculations, aided by a new truncation method, include 21+ excitation energies and magnetic moments. Hartree-Fock calculations with SkI6, RATP, Zσ*, and SkX Skyrme forces include ground-state deformations and rms radii; SkI6 gives the best overall agreement with experiment. The two models are compared with each other and with experiment. Two-neutron separation energies, evidence for a neutron halo or skin in heavy O isotopes, and deformation of Ne and Mg isotopes are discussed. Both models indicate disappearance of the shell gap at N=28 (Mg), and the shell model does so additionally at N=20 (Ne and Mg).

The authors have measured the angular distributions of 43 gamma rays in 148Sm following the decay of 148Eu polarised at low temperatures in an iron host. Multipole mixing ratios are deduced for 39 transitions; three X(E0/E2) values are also decided. They calculate these quantities, as well as excitation energies and B(E2) values, using the IBA-1 model. The mixing ratios are seen to be a sensitive probe of the wavefunctions.