arXiv subjects
Publications and source records attributed to J. Dobaczewski.
We discuss pairing correlations in weakly bound neutron rich nuclei, by using the coordinate-space Hartree-Fock-Bogolyubov approach which allows to take properly into account the coupling to particle continuum. We show that the additional pairing binding energy acts against a development of an infinite rms radius, even in situations when an l=0 single-particle orbital becomes unbound.
Odd-even staggering of binding energies is studied in finite fermion systems with pairing correlations. We discuss contributions of the pairing and mean-field to the staggering, and we construct the binding-energy filters which measure the magnitude of pairing correlations and the effective single-particle spacings in a given system The analysis is based on studying several exactly-solvable many-body Hamiltonians as well as on the analytical formulas that can be applied in the weak and strong pairing limits.
We describe the new version (v1.75r) of the code HFODD which solves the nuclear Skyrme-Hartree-Fock problem by using the Cartesian deformed harmonic-oscillator basis. Three minor errors that went undetected in the previous version have been corrected. The new version contains an interface to the LAPACK subroutine ZHPEV. Several methods of terminating the Hartree-Fock iteration procedure, and an algorithm that allows to follow the diabatic configurations, have been implemented.
It is shown how to construct a basis in which two arbitrary complex antisymmetric matrices C and C' acquire simultaneously canonical forms. The present construction is not restricted by any conditions on properties of the C^+C' matrix. Canonical bases pertaining to the generator-coordinate-method treatment of many-fermion systems are discussed.
Three mutually perpendicular symmetry axes of the second order, inversion, and time reversal can be used to construct a double point group denoted by D2h(TD). Properties of this group are analyzed in relation to the symmetry and symmetry-breaking effects within the mean-field (Hartree-Fock) theories, both in even and odd fermion systems. We enumerate space symmetries of local one-body densities, and symmetries of electromagnetic moments, that appear when some or all of the D2h(TD) elements represent self-consistent mean-field symmetries.
We analyze breaking of symmetries that belong to the double point group D2h(TD) (three mutually perpendicular symmetry axes of the second order, inversion, and time reversal). Subgroup structure of the D2h(TD) group indicates that there can be as much as 28 physically different, broken-symmetry mean-field schemes --- starting with solutions obeying all the symmetries of the D2h(TD) group, through 26 generic schemes in which only a non-trivial subgroup of D2h(TD) is conserved, down to solutions that break all of the D2h(TD) symmetries. Choices of single-particle bases and the corresponding structures of single-particle hermitian operators are discussed for several subgroups of D2h(TD).
Nuclei with large neutron-to-proton ratios have neutron skins, which manifest themselves in an excess of neutrons at distances greater than the radius of the proton distribution. In addition, some drip-line nuclei develop very extended halo structures. The neutron halo is a threshold effect; it appears when the valence neutrons occupy weakly bound orbits. In this study, nuclear skins and halos are analyzed within the self-consistent Skyrme-Hartree-Fock-Bogoliubov and relativistic Hartree-Bogoliubov theories for spherical shapes. It is demonstrated that skins, halos, and surface thickness can be analyzed in a model-independent way in terms of nucleonic density form factors. Such an analysis allows for defining a quantitative measure of the halo size. The systematic behavior of skins, halos, and surface thickness in even-even nuclei is discussed.
We introduce a local-scaling point transformation to allow for modifying the asymptotic properties of the deformed three-dimensional Cartesian harmonic oscillator wave functions. The resulting single-particle bases are very well suited for solving the Hartree-Fock-Bogoliubov equations for deformed drip-line nuclei. We then present results of self-consistent calculations performed for the Mg isotopes and for light nuclei located near the two-neutron drip line. The results suggest that for all even-even elements with $Z$=10--18 the most weakly-bound nucleus has an oblate ground-state shape.
Superdeformed configurations in 32S, and in neighboring nuclei 33S, 31S, 33Cl, and 31P, are determined within the Hartree-Fock approach with the Skyrme interaction. Energies, angular momenta, quadrupole moments, particle-emission Q-values, and relative alignments and quadrupole moments are calculated for a number of superdeformed rotational bands in these nuclei. A new mechanism implying an existence of signature-separated rotational bands, distinct from the well-known signature-split bands, is discussed and associated with the time-odd channels of effective interactions.
Continuum effects in the weakly bound nuclei close to the drip-line are investigated using the analytically soluble Poschl-Teller-Ginocchio potential. Pairing correlations are studied within the Hartree-Fock-Bogoliubov method. We show that both resonant and non-resonant continuum phase space is active in creating the pairing field. The influence of positive-energy phase space is quantified in terms of localizations of states within the nuclear volume.
The phenomenon of shape coexistence is discussed within the self-consistent Hartree-Fock method and the nuclear shell model. The occurrence of the coexisting configurations with different intrinsic shapes is traced back to the properties of the effective Hamiltonian.
Beta-decay rates for spherical neutron-rich r-process waiting-point nuclei are calculated within a fully self-consistent Quasiparticle Random-Phase Approximation, formulated in the Hartree-Fock-Bogolyubov canonical single-particle basis. The same Skyrme force is used everywhere in the calculation except in the proton-neutron particle-particle channel, where a finite-range force is consistently employed. In all but the heaviest nuclei, the resulting half-lives are usually shorter by factors of 2 to 5 than those of calculations that ignore the proton-neutron particle-particle interaction. The shorter half-lives alter predictions for the abundance distribution of r-process elements and for the time it takes to synthesize them.
Physics of nuclei at extreme values of the isospin is at the focus of present-day nuclear science. Experimentally, thanks to existing and emerging radioactive-ion-beam facilities, we are on the verge of invading the territory of extreme N/Z ratios in an unprecedented way. Theoretically, nuclear exotica represent a formidable challenge for the nuclear many-body theories and their power to predict nuclear properties far from stability. Going to the limits of the nuclear binding is also important for an improvement of our description of normal nuclei from the neighborhood of the beta stability valley. In the present talk, we review several aspects of the present-day mean-field theoretical studies of weakly bound nuclei.
We present a review of the mean-field approaches describing superdeformed states, which are currently used and/or being developed. As an example, we discuss in more details the properties of superdeformed A~60 nuclei, and present results of calculations for the rotational band in the doubly magic superdeformed nucleus 32S.
The odd-even staggering of nuclear masses was recognized in the early days of nuclear physics. Recently, a similar effect was discovered in other finite fermion systems, such as ultrasmall metallic grains and metal clusters. It is believed that the staggering in nuclei and grains is primarily due to pairing correlations (superconductivity), while in clusters it is caused by the Jahn-Teller effect. We find that, for light and medium-mass nuclei, the staggering has two components. The first one originates from pairing while the second, comparable in magnitude, has its roots in the deformed mean field.
We present a very brief description of the Hartree-Fock method in nuclear structure physics, discuss the numerical methods used to solve the self-consistent equations, and analyze the precision and convergence properties of solutions. As an application we present results pertaining to quadrupole moments and single-particle quadrupole polarizations in superdeformed nuclei with A~60.
The prediction of cross sections for nuclei far off stability is crucial in the field of nuclear astrophysics. We calculate direct neutron capture on the even-even isotopes $^{124-145}$Sn and $^{208-238}$Pb with energy levels, masses, and nuclear density distributions taken from different nuclear-structure models. The utilized structure models are a Hartree-Fock-Bogoliubov model, a relativistic mean field theory, and a macroscopic-microscopic model based on the finite-range droplet model and a folded-Yukawa single-particle potential. Due to the differences in the resulting neutron separation and level energies, the investigated models yield capture cross sections sometimes differing by orders of magnitude. This may also lead to differences in the predicted astrophysical r-process paths. Astrophysical implications are discussed.
Recent experimental observation of the direct links between superdeformed and normal-deformed structures in the A~190 mass region offers a unique information on the absolute nuclear binding energy in the 2:1 minima, and hence on the magnitude of shell effects in the superdeformed well. In the present paper, the self-consistent mean-field theory with density-dependent pairing interaction is used to explain at the same time the two-particle separation energies in the first and second wells, and the excitation energies of superdeformed states in the A~190 and A~240 mass regions.