Division of Nuclear Structure, 1998-1999 Activity Report

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Division of Nuclear Structure Theory in 1998-1999

Permanent staff (January 1, 2000):

Prof. Jacek Dobaczewski, head
Prof. Witold Nazarewicz
Prof. Stanis $\l$ aw G. Rohozinski
Dr. Wojciech Satu $\l$ a
Dr. Tomasz Werner

Students (January 1, 2000):

Rainald Kirchner (PhD)

Przemys $\l$ aw Olbratowski (PhD)

Elzbieta Perlinska (PhD)

Scientific activities:

The nuclear structure theory group works in close collaboration with the group at the Warsaw Institute of Technology (Prof. Stefan Cwiok, Dr. Wanda Dudek, and Dr. Piotr Magierski) and with Dr. Weronika P $\l$ óciennik of the So $\l$ tan Institute of Nuclear Studies. In 1999 we lost our teacher, friend, and collaborator; on September 5, 1999 Professor Zdzis $\l$ aw Szymanski, the father of nuclear theory in Poland, passed away during the Nuclear Physics Summer School in Krzyze. He was actively doing research till his very last days. We shall always remember him.

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:

Effective nucleon-nucleon interactions.
Nuclei with large neutron or proton excess.
Superheavy nuclei.
Nuclei with large deformations, super- and hyperdeformation.
Fast nuclear rotation.
Quadrupole collective states.
Collective vs. single-particle motion, dissipative phenomena.

Scientific achievements:

In 1998 and 1999, five members of our nuclear structure theory group have published 30 papers in refereed periodicals (see the list below), and have presented 31 invited talks and 14 contributions at international conferences. During this period of time we have realized the following main research projects:

Superdeformed states
Hartree-Fock calculations for the excited well-deformed rotational band in ⁵⁸Cu has been presented in Ref. [5]. The first excited state in this band decays via $\gamma$ emission to the spherical states associated with the first minimum in the potential, thus providing for its unambiguous assignment to ⁵⁸Cu. In contrast, its bandhead decays via emission of a prompt 2.4(1) MeV proton to an excited state in the daughter nucleus ⁵⁷Ni. 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 ⁵⁶Ni 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 ⁶¹Zn. Comparison of the J⁽²⁾ dynamical moments of inertia of this band with those in ⁶⁰Zn shows a nearly complete blocking of the observed alignment in ⁶⁰Zn, indicating that T=0 proton-neutron pair correlations may be present in ⁶⁰Zn.
The superdeformed (SD) bands in Hg-Pb nuclei of mass A $\sim$ 190 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.
Properties of weakly bound systems
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.
Pairing correlations in nuclei
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.
Shape-coexistence phenomena
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 ³²Mg 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 ⁴⁴S. The strong coexistence effects are also predicted for ⁸⁰Zr and ⁹⁸Zr. 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.
Super-heavy nuclei
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 $\alpha$ -decay energies and deformations has been performed. Good agreement was obtained with the recently reported $\alpha$ -decay chains of ²⁸⁹114 and ²⁹³118. The ground states of the N=175 isotones were calculated to be the high- $\Omega$ isomeric states based on the [707]15/2^- orbital. Because of structural arguments, this state is probably bypassed by the $\alpha$ -decays. (The same is true for the high- $\Omega$ ground states at N=171.) This result may explain the observed lower cross section for the production of ²⁹³118 ( $\sim$ 2pb) as compared to calculations by Smolanczuk ( $\sim$ 670pb).
Proton emitters
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 ¹⁴¹Ho 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- $\ell$ partial wave allowed by the angular momentum and parity conservation
Collective quadrupole excitations
Polarizational correlations of the $\gamma$ radiation emitted from oriented nuclei

Papers published in refereed periodicals in 1998 and 1999

1.: Theoretical Aspects of Science with Radioactive Nuclear Beams, J. Dobaczewski, W. Nazarewicz, Phil. Trans. R. Soc. Lond. A 356 (1998) 2007
2.: D_2h-symmetric nuclear shapes of higher multipolarities, S.G. Rohozinski, Heavy Ion Phys. 7 (1998) 63
3.: Dependence of direct neutron capture on nuclear structure models, T. Rauscher, R. Bieber, H. Oberhummer, K.-L. Kratz, J. Dobaczewski, P. Möller, M. Sharma, Phys. Rev. C57 (1998) 2031
4.: Shell effects in superdeformed minima, P.-H. Heenen, J. Dobaczewski, W. Nazarewicz, P. Bonche, T.L. Khoo, Phys. Rev. C57 (1998) 1719
5.: Prompt proton decay of a well-deformed rotational band in ⁵⁸Cu, D. Rudolph, C. Baktash, J. Dobaczewski, W. Nazarewicz, W. Satu $\l$ a, M.J. Brinkman, M. Devlin, H.-Q. Jin, D.R. LaFosse, L.L. Riedinger, D.G. Sarantites, C.-H. Yu, Phys. Rev. Lett. 80 (1998) 3018
6.: Shell corrections for finite depth potentials: Particle continuum effects, T. Vertse, A.T. Kruppa, R.J. Liotta, W. Nazarewicz, N. Sandulescu, T.R. Werner, Phys. Rev. C57 (1998) 3089
7.: Odd-even staggering of nuclear masses: pairing or shape effect?, W. Satu $\l$ a, J. Dobaczewski, W. Nazarewicz, Phys. Rev. Lett. 81 (1998) 3599
8.: Rotating pseudo-oscillator scheme: pseudo-spin symmetry and identical bands, Z. Szymanski, W. Nazarewicz, Phys. Lett. B433 (1998) 229
9.: Band structure in ⁷⁹Y and the question of T=0 pairing, S.D. Paul, C. Baktash, W. Satu $\l$ a, C.J. Gross, I. Birrel, R.M. Clark, M. Devlin, P. Fallon, A. Galindo-Uribarri, T. Ginter, D.R. LaFosse, F. Lerma, I.Y. Lee, A.O. Macchiavelli, B. Macdonald, A. Piechaczek, D.C. Radford, W. Reviol, L.L. Riedinger, D. Rudolph, K. Rykaczewski, D.G. Sarantites, J.X. Saladin, D. Shapira, G.N. Sylvan, S.L. Tabor, K.S. Toth, W. Weintraub, D.F. Winchell, V.Q. Wood, R. Wyss, C.H. Yu, Phys. Rev. C58 (1998) R3037
10.: Enahanced deformation in light Pr nuclei: the role of the $\nu h_{9/2}$ orbital., B.H. Smith, L.L. Riedinger, W. Reviol, W. Satu $\l$ a, A. Galindo-Uribarri, D.G. Sarantites, J.N. Wilson, S.M. Mullins, H.Q. Jin, D. LaFosse, Phys. Lett. B443 (1998) 89
11.: High-spin $\gamma$ -ray spectroscopy in the vicinity of ⁵⁶Ni, D. Rudolph, C. Baktash, W. Satu $\l$ a, J. Dobaczewski, W. Nazarewicz, M.J. Brinkman, M. Devlin, H.-Q. Jin, D.R. LaFosse, L.L. Riedinger, D.G. Sarantites, C.-H. Yu, Nucl. Phys. A630 (1998) 417c
12.: Frontiers of nuclear structure, W. Nazarewicz, Nucl. Phys. A630 (1998) 239c
13.: New discrete basis for nuclear structure studies, M.V. Stoitsov, W. Nazarewicz, S. Pittel, Phys. Rev. C58 (1998) 2092
14.: Masses and radii of spherical nuclei calculated in various microscopic approaches, Z. Patyk, A. Baran, J.F. Berger, J. Dechargé, J. Dobaczewski, P. Ring, A. Sobiczewski, Phys. Rev. C58 (1999) 704
15.: Rotational bands in the doubly magic nucleus ⁵⁶Ni, D. Rudolph, C. Baktash, M.J. Brinkman, E. Caurier, D.J. Dean, M. Devlin, J. Dobaczewski, P.H. Heenen, H.-Q. Jin, D.R. LaFosse, W. Nazarewicz, F. Nowacki, A. Poves, L.L. Riedinger, D.G. Sarantites, W. Satu $\l$ a, C.-H. Yu, Phys. Rev. Lett. 82 (1999) 3763
16.: Continuum effects for the mean-field and pairing properties of weakly bound nuclei, K. Bennaceur, J. Dobaczewski, M. P $\l$ oszajczak, Phys. Rev. C60 (1999) 034308
17.: Beta decay of r-process waiting-point nuclei in a self-consistent approach, J. Engel, M. Bender, J. Dobaczewski, W. Nazarewicz, R. Surman, Phys. Rev. C60 (1999) 014302
18.: Shape coexistence and the effective nucleon-nucleon interaction, P.-G. Reinhard, D.J. Dean, W. Nazarewicz, J. Dobaczewski, J.A. Maruhn, M.R. Strayer, Phys. Rev. C60 (1999) 014316
19.: Comparison of superdeformed bands in ⁶¹Zn and ⁶⁰Zn: Possible evidence for T=0 pairing, C.-H. Yu, C. Baktash, J. Dobaczewski, J.A. Cameron, C. Chitu, M. Devlin, J. Eberth, A. Galindo-Uribarri, D.S. Haslip, D.R. LaFosse, T.J. Lampman, I.-Y. Lee, F. Lerma, A.O. Macchiavelli, S.D. Paul, D.C. Radford, D. Rudolph, D.G. Sarantites, C.E. Svensson, J.C. Waddington, J.N. Wilson, Phys. Rev. C60 (1999) 031305
20.: Structure of nuclei at extreme values of the isospin, J. Dobaczewski, Acta Phys. Pol. B30 (1999) 1647
21.: Structure of Odd-N Superheavy Elements, S. Cwiok, W. Nazarewicz, and P.H. Heenen, Phys. Rev. Lett. 83 (1999) 1108
22.: Origin of unit alignment in superdeformed bands in A=190 nuclei, P. Fallon, P-H. Heenen, W. Satu $\l$ a, R.M. Clark, F.S. Stephens, M.A. Delaplanque, R.M. Diamond, I.Y. Lee, A.O. Machiavelli, K. Vetter, Phys. Rev. C60 (1999) 044301
23.: Experimental Test of the Polarization Direction Correlation Method (PDCO), K.Starosta, T. Morek, Ch. Droste, S.G. Rohozinski, J. Srebrny, A. Wierzchucka, M. Bergström, B. Herskind, E. Melby, T. Czosnyka, P.J. Napiórkowski, Nucl. Instruments and Methods 423 (1999) 16
24.: PPCO: Polarization-Polarization Correlation from Oriented Nuclei, Ch. Droste, K. Starosta, A. Wierzchucka, T. Morek, S.G. Rohozinski, J. Srebrny, M. Bergström, B. Herskind, E. Weso $\l$ owski, Nucl. Instruments and Methods 430 (1999) 260
25.: Collective Quadrupole Excitations in the 50<Z,N<82 Nuclei with the general Bohr Hamiltonian, L. Próchniak, K. Zajac, K. Pomorski, S.G. Rohozinski, J. Srebrny, Nucl. Phys. A648 (1999) 181
26.: Collective Quadrupole Excitations in Even-Even Ru Isotopes, K. Zajac, L. Próchniak, K. Pomorski, S.G. Rohozinski, J. Srebrny, Acta Phys. Pol. B30 (1999) 765
27.: The Low-lying Quadrupole Collective Excitations of Ru and Pd Isotopes, K. Zajac, L. Próchniak, K. Pomorski, S.G. Rohozinski, J. Srebrny, Nucl. Phys. A653 (1999) 71
28.: Proton Emitters ¹⁴⁰Ho and ¹⁴¹Ho: Probing the Structure of Unbound Nilsson Orbitals, K. Rykaczewski, J.C. Batchelder, C.R. Bingham, T. Davinson, T.N. Ginter, C.J. Gross, R. Grzywacz, M. Karny, B.D. MacDonald, J.F. Mas, J.W. McConnell, A. Piechaczek, R.C. Slinger, K.S. Toth, W.B. Walters, P.J. Woods, E.F. Zganjar, B. Barmore, L.Gr. Ixaru, A.T. Kruppa, W. Nazarewicz, M. Rizea, and T. Vertse, Phys. Rev. C60 (1999) 011301
29.: Studies of Nuclei at and Beyond the Proton Drip-Line with Stable and Radioactive Beams at HRIBF, K. Rykaczewski, J.C. Batchelder, C.R. Bingham, T. Davinson, T.N. Ginter, C.J. Gross, R. Grzywacz, Z. Janas, M. Karny, B.D. MacDonald, J.F. Mas, J.W. McConnell, A. Piechaczek, R.C. Slinger, J. Szerypo, K.S. Toth, W.B. Walters, P.J. Woods, E.F. Zganjar, W. Nazarewicz, and P.B. Semmes, Acta Phys. Pol. B30 (1999) 565
30.: Nuclear Structure, W. Nazarewicz, Nucl. Phys. A654 (1999) 195c

Master of Science, PhD, and Habilitation Theses:

Nuclear superconductivity, Wojciech Satu $\l$ a, Habilitation Thesis, 1999.
Mass and charge radii of atomic nuclei, Andrzej Godlewski, MSc Thesis, 1999.
Coupling of pairs of particles with the core in odd-odd nuclei, Mariusz Debowski, MSc Thesis, 1999.
Rotational bands in superdeformed nuclei in the rare-earth region, Jolanta Karny, MSc Thesis, 1999.

International cooperation:

The nuclear structure theory group collaborates very intensely with the theory groups at the Oak Ridge National Laboratory (USA), University of Knoxville (USA), University of Strasbourg (France), Stockholm University (Sweden), GANIL Laboratory (France), and many others. We participate on a regular basis in the activities of the European Centre for Theoretical Nuclear Physics in Trento (Italy) and of the Institute of Nuclear Theory in Seattle (USA).

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Jacek Dobaczewski
2000-03-02

Rainald Kirchner	(PhD)
Przemys $\l$ aw Olbratowski	(PhD)
Elzbieta Perlinska	(PhD)