Division of Nuclear Structure Theory in 1998-1999
Permanent staff (January 1, 2000):
Prof. Jacek Dobaczewski, head
Prof. Witold Nazarewicz
Prof. Stanisaw G. Rohozinski
Dr. Wojciech Satua
Dr. Tomasz Werner
Students (January 1, 2000):
Rainald Kirchner | (PhD) |
Przemysaw Olbratowski | (PhD) |
Elzbieta Perlinska | (PhD) |
During past several years we worked on numerous scientific projects, some of them within the grant of the Polish Committee for Scientific Research, No. 2 P03B 040 14. Our activity is focused on investigating the following main research directions:
Hartree-Fock calculations for the excited well-deformed rotational band in ^{58}Cu has been presented in Ref. [5]. The first excited state in this band decays via emission to the spherical states associated with the first minimum in the potential, thus providing for its unambiguous assignment to ^{58}Cu. In contrast, its bandhead decays via emission of a prompt 2.4(1) MeV proton to an excited state in the daughter nucleus ^{57}Ni. This has been the first observation of proton decay from states associated with a deformed secondary minimum in the potential. Self-consistent Hartree-Fock calculations reproduce well both the large collectivity of this band and the general trend of its moment of inertia. In Ref. [15], the nuclear structure of the doubly magic nucleus ^{56}Ni has been investigated at high spins within the Hartree-Fock method. Configurations of two well-deformed rotational bands have been identified and compared with experimental data. Similar theoretical methods have also been used in Ref. [19] to describe the yrast superdeformed band in ^{61}Zn. Comparison of the J^{(2)} dynamical moments of inertia of this band with those in ^{60}Zn shows a nearly complete blocking of the observed alignment in ^{60}Zn, indicating that T=0 proton-neutron pair correlations may be present in ^{60}Zn.
The superdeformed (SD) bands in Hg-Pb nuclei of mass A190 are unique in many respects. Characteristic rise of their dynamical moments of inertia (MoI) versus rotational frequency clearly indicates importance of pairing correlations in these bands, unlike in SD bands in lighter nuclei. Stability of these highly elongated shapes against rotational distortion offers a unique test ground to study very subtle aspects of nuclear superconductivity. Ref. [22] attempts at a systematic study of the MoI in these bands in a framework of mean-field model. Both Strutinsky-type as well as fully self-consistent Hartree-Fock-Bogolyubov calculations have been presented. It has been shown that the calculations reproduce general experimental properties very well. They do encounter problems, however, in reproducing specific alignments of the SD bands or the identical bands.
In Ref. [6] shell corrections in finite one-body, spherically symmetric potentials have been analyzed. A new method has been employed, which allows for a description of shell corrections in exotic nuclei where continuum effects have to be taken into account. The method is based on solving the Schrödinger equation on complex energy plane. The results have been compared with those of the Wigner-Kirkwood expansion, and the asymptotic properties of solutions have been investigated in detail.
Continuum effects in the weakly bound nuclei close to the drip-line have been in Ref. [16] investigated using the analytically soluble Pöschl-Teller-Ginocchio potential. Pairing correlations have been studied within the Hartree-Fock-Bogolyubov method. We have shown that both resonant and non-resonant continuum phase space is active in creating the pairing field. The influence of positive-energy phase space has been quantified in terms of localizations of states within the nuclear volume.
Beta-decay rates for spherical neutron-rich r-process waiting-point nuclei have been calculated in Ref. [17] within a fully self-consistent Quasiparticle Random-Phase Approximation, formulated in the Hartree-Fock-Bogolyubov canonical single-particle basis. The same Skyrme force has been used everywhere in the calculation, except in the proton-neutron particle-particle channel, where a finite-range force has been 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.
The odd-even staggering (OES) of binding energies is a universal property of finite fermion (mesoscopic) systems. The underlying mechanism beyond OES is, however, system dependent. For example, in metallic clusters, it is predominantly due to the underlying non-spherical mean field, while in ultrasmall metallic grains OES is mainly related to the blocking mechanism of superconducting correlations by an unpaired electron. In atomic nuclei, additional complications arise because of the symmetry energy caused by a simultanous presence of two types of fermions. In Ref. [7] we have investigated the nuclear OES, and concluded that, in light nuclei, it has two competing components, one related to pairing and the other one to deformed mean field.
The phenomenon of shape coexistence has been discussed in Ref. [18] within the self-consistent Hartree-Fock method and the nuclear shell model. The occurrence of the coexisting configurations with different intrinsic shapes has been traced back to the properties of the effective Hamiltonian. The nucleus ^{32}Mg has been found to be a classic example of shape coexistence; the spherical and deformed configurations are close in energy and shape mixing is expected. For most Skyrme parameterizations used, the N=28 gap is predicted to be rather small. This gives rise to strong deformation effects around ^{44}S. The strong coexistence effects are also predicted for ^{80}Zr and ^{98}Zr. Both families of models applied in this work, i.e., the self-consistent mean-field models and the shell model, should be viewed as effective theories. That is, their predictive power crucially depends on the effective interaction assumed.
Structure of the odd-N superheavy elements with Z<120 and N<175 has been investigated in Ref. [21] using the self-consistent Skyrme-Hartree-Fock-Bogolyubov method with pairing. This is the first self-consistent analysis of one-quasiparticle states in this mass region. Microscopic analysis of -decay energies and deformations has been performed. Good agreement was obtained with the recently reported -decay chains of ^{289}114 and ^{293}118. The ground states of the N=175 isotones were calculated to be the high- isomeric states based on the [707]15/2^{-} orbital. Because of structural arguments, this state is probably bypassed by the -decays. (The same is true for the high- ground states at N=171.) This result may explain the observed lower cross section for the production of ^{293}118 (2pb) as compared to calculations by Smolanczuk (670pb).
Proton radioactivity is an excellent example of the elementary three-dimensional quantum-mechanical tunneling. Lifetimes of proton emitters provide a very direct information on the wave functions of the narrow proton resonances, and the energies of emitted protons tell us about the topology of the nuclear binding energy surface in the vicinity of the proton drip-line. In Refs. [28,29] experimental data on proton-emitting states in ^{141}Ho have been analyzed using the coupled-channel Schrödinger equation with outgoing boundary conditions. The observed resonances have been interpreted in terms of the [411]1/2^{+}and [523]7/2^{-} single-proton orbitals. It has been concluded, that the decay process of a Nilsson orbital is governed by the lowest- partial wave allowed by the angular momentum and parity conservation
Papers published in refereed periodicals in 1998 and 1999