K. Pomorski

Nuclear shell structure at very high angular momentum

Abstract A cranked modified-oscillator model (with triaxial shape coordinates ϵ and γ) is used to study the nuclear potential-energy surface (based on a Strutinsky type of shell correction method) for very high angular momenta (30 ≦ I ≦ 100). For this region of spin, pair correlation is assumed to have collapsed. The influence of rapid rotation on the shell structure has been studied in the light and heavy rare-earth region as well as the Te-Ba region. Preliminary studies have also been made in the regions of superheavy and light nuclei. The possible occurrence of yrast traps is discussed.

Self-consistent calculations of fission barriers in the Fm region

The fission barriers of the nuclei 254 Fm, 256 Fm, 258 Fm, 258 No, and 260 Rf are investigated in a fully microscopic way up to the scission point. The analysis is based on the constrained Hartree-Fock-Bogoliubov theory and Gogny’s D1S force. The quadrupole, octupole, and hexadecapole moments as well as the number of nucleons in the neck region are used as constraints. Two fission paths, corresponding to the bimodal fission, are found. The decrease with isotope mass of the half-life times of heavy Fm isotopes is also explained.

Microscopic dynamic calculations of collective states in xenon and barium isotopes

Abstract Dynamic calculations of collective states for neutron-deficient doubly even Xe and Ba isotopes are performed. The collective Hamiltonian is constructed in terms of the cranking inertial functions and the macroscopic-microscopic potential, both generated by the single-particle motion in the modified harmonic oscillator potential. The pairing correlations are included. Reduction of the standard pairing interaction strengths leads to results compatible with experiment. The deformation dependence of the inertial functions is found to play an important role.

Collective quadrupole excitations in the 50 < Z, N < 82 nuclei with the general Bohr Hamiltonian

Abstract The general Bohr Hamiltonian is applied to a description of low-lying collective excitations in even-even isotopes of Te, Xe, Ba, Ce, Nd and Sm. The collective potential and inertial functions are determined by means of the Strutinsky method and the cranking model, respectively. A shell-dependent parametrization of the Nilsson potential is used. An approximate particle-number projection is performed in the treatment of pairing correlations. The effect of coupling with the pairing vibrations is taken into account approximately when determining the inertial functions. The calculation does not contain any free parameter.

The mass parameters for the average mean-field potential☆

Starting from a mean-field hamiltonian with pairing interaction, we use the generator coordi- nate method (GCM) and a generalized gaussian overlap approximation to derive a multidimensional collective hamiltonian for large-amplitude motion. Numerical calculations are performed for Nilsson and Woods-Saxon potentials with BCS pairing. The BCS wave function is taken as the generator function and the deformation parameters of the single-particle mean field are used as the generator coordinates. We find thaf the GCM mass parameters on the average are smaller than those of the cranking ( + BCS) model by a factor of - ~. In the present approach, the zero-point energy correction to the collective potential is shown to vanish identically.

A more detailed calculation of particle evaporation and fission of compound nuclei

Abstract We consider particle evaporation and fission of an ensemble of hot, rotating compound nuclei as a stochastic process. We derive a set of coupled differential equations formed by a Fokker-Planck equation describing fission, and master equations for calculating particle evaporation. From these equations, we are able to determine multiplicities of prefission neutrons, protons and α-particles, their energy spectra and their angular momentum distributions. A comparison of our results with experimental data provides us with information regarding the reduced friction coefficient β, the fission barrier height and the level density parameter. For different iridium isotopes, ( 181,185,187 Ir), 185 Os and 158 Er, we obtain as an upper limit β ⩽8.0×10 21 s −1 .

Evaporation of light particles from a hot, deformed and rotating nucleus☆

Abstract The dependence of the transimission coefficient on the deformation, the collective rotation and excitation energy of the compound nucleus emitting light particles is introduced in the framework of Weisskopfs evaporation theory. The competition between fission and particle evaporation is treated by a Langevin equation for the fission variable coupled to the emission process. Detailed calculations are presented on the decay of different Gd and Yb isotopes at an excitation energy of about 250 MeV. These calculations demonstrate the importance of the effects of nuclear deformation and of the initial spin distribution on the evaporation.

On the stable octupole deformation of nuclei

Abstract The potential energy of nuclei in the Ra region is studied by a modified macroscopic-microscopic method which accounts for the consistency condition between the macroscopic and microscopic parts of the energy. A stable octupole deformation is obtained for a number of nuclei. The lowest Kπ=0− states are interpreted as being associated with this deformation.

Simple formula for nuclear charge radius

A new formula for the nuclear charge radius is proposed, dependent on the mass numberA and neutron excessN-Z in the nucleus. It is simple and it reproduces all the experimentally available mean square radii and their isotopic shifts of even-even nuclei much better than other frequently used relations.

The low-lying quadrupole collective excitations of Ru and Pd isotopes☆

Abstract Quadrupole excitations of even-even Ru and Pd isotopes are described within a microscopic approach based on the general collective Bohr model which includes the effect of coupling with the pairing vibrations. The excitation energies and E2 transition probabilities observed in 104–114 Ru and 106–110 Pd are reproduced in the frame of the calculation containing no free parameters. Particularly interesting are 104 Ru and 106–110 Pd where good agreement to very rich information based on Coulomb excitation experiments is achieved.