162 research outputs found
Fission of heavy hypernuclei with the Skyrme-Hartree-Fock approach
Fission-related phenomena of heavy hypernuclei are discussed with
the constraint Skyrme-Hartree-Fock+BCS (SHF+BCS) method, in which a similar
Skyrme-type interaction is employed also for the interaction between a
particle and a nucleon. Assuming that the particle
adiabatically follows the fission motion, we discuss the fission barrier height
of U. We find that the fission barrier height increases
slightly when the particle occupies the lowest level. In this case,
the particle is always attached to the heavier fission fragment. This
indicates that one may produce heavy neutron-rich hypernuclei through
fission, whose weak decay is helpful for the nuclear transmutation of
long-lived fission products. We also discuss cases where the particle
occupies a higher single-particle level.Comment: 20 pages, 18 figures, to be submitted to Nucl. Phys.
The Origin of the Wigner Energy
Surfaces of experimental masses of even-even and odd-odd nuclei exhibit a
sharp slope discontinuity at N=Z. This cusp (Wigner energy), reflecting an
additional binding in nuclei with neutrons and protons occupying the same shell
model orbitals, is usually attributed to neutron-proton pairing correlations. A
method is developed to extract the Wigner term from experimental data. Both
empirical arguments and shell-model calculations suggest that the Wigner term
can be traced back to the isospin T=0 part of nuclear interaction. Our
calculations reveal the rather complex mechanism responsible for the nuclear
binding around the N=Z line. In particular, we find that the Wigner term cannot
be solely explained in terms of correlations between the neutron-proton J=1,
T=0 (deuteron-like) pairs.Comment: 10 RevTeX pages, 3 Postscript figures include
Photonuclear reactions of actinides in the giant dipole resonance region
Photonuclear reactions at energies covering the giant dipole resonance (GDR)
region are analyzed with an approach based on nuclear photoabsorption followed
by the process of competition between light particle evaporation and fission
for the excited nucleus. The photoabsorption cross section at energies covering
the GDR region is contributed by both the Lorentz type GDR cross section and
the quasideuteron cross section. The evaporation-fission process of the
compound nucleus is simulated in a Monte-Carlo framework. Photofission reaction
cross sections are analyzed in a systematic manner in the energy range of
10-20 MeV for the actinides Th, U and Np.
Photonuclear cross sections for the medium-mass nuclei Cu and Zn,
for which there are no fission events, are also presented. The study reproduces
satisfactorily the available experimental data of photofission cross sections
at GDR energy region and the increasing trend of nuclear fissility with the
fissility parameter for the actinides.Comment: 4 pages including 2 tables and 1 figur
Deformed nuclear halos
Deformation properties of weakly bound nuclei are discussed in the deformed
single-particle model. It is demonstrated that in the limit of a very small
binding energy the valence particles in specific orbitals, characterized by a
very small projection of single-particle angular momentum onto the symmetry
axis of a nucleus, can give rise to the halo structure which is completely
decoupled from the rest of the system. The quadrupole deformation of the
resulting halo is completely determined by the intrinsic structure of a weakly
bound orbital, irrespective of the shape of the core.Comment: LaTeX source (21 pages) and postscript file with figures (15 pages).
Accepted to Nucl. Phys.
Relations between fusion cross sections and average angular momenta
We study the relations between moments of fusion cross sections and averages
of angular momentum. The role of the centrifugal barrier and the target
deformation in determining the effective barrier radius are clarified. A simple
method for extracting average angular momentum from fusion cross sections is
demonstrated using numerical examples as well as actual data.Comment: 16 REVTeX pages plus 8 included Postscript figures (uses the epsf
macro); submitted to Phys. Rev. C; also available at
http://nucth.physics.wisc.edu/preprint
Self-Assembly of Supramolecular Triblock Copolymer Complexes
Four different poly(tert-butoxystyrene)-b-polystyrene-b-poly(4-vinylpyridine) (PtBOS-b-PS-b-P4VP) linear triblock copolymers, with the P4VP weight fraction varying from 0.08 to 0.39, were synthesized via sequential anionic polymerization. The values of the unknown interaction parameters between styrene and tert-butoxystyrene and between tert-butoxystyrene and 4-vinylpyridine were determined from random copolymer blend miscibility studies and found to satisfy 0.031<χS,tBOS<0.034 and 0.39<χ4VP,tBOS<0.43, the latter being slightly larger than the known 0.30<χS,4VP≤0.35 value range. All triblock copolymers synthesized adopted a P4VP/PS core/shell cylindrical self-assembled morphology. From these four triblock copolymers supramolecular complexes were prepared by hydrogen bonding a stoichiometric amount of pentadecylphenol (PDP) to the P4VP blocks. Three of these complexes formed a triple lamellar ordered state with additional short length scale ordering inside the P4VP(PDP) layers. The self-assembled state of the supramolecular complex based on the triblock copolymer with the largest fraction of P4VP consisted of alternating layers of PtBOS and P4VP(PDP) layers with PS cylinders inside the latter layers. The difference in morphology between the triblock copolymers and the supramolecular complexes is due to two effects: (i) a change in effective composition and, (ii) a reduction in interfacial tension between the PS and P4VP containing domains. The small angle X-ray scattering patterns of the supramolecules systems are very temperature sensitive. A striking feature is the disappearance of the first order scattering peak of the triple lamellar state in certain temperature intervals, while the higher order peaks (including the third order) remain. This is argued to be due to the thermal sensitivity of the hydrogen bonding and thus directly related to the very nature of these systems.
The T=0 neutron-proton pairing correlations in the superdeformed rotational bands around 60Zn
The superdeformed bands in 58Cu, 59Cu, 60Zn, and 61Zn are analyzed within the
frameworks of the Skyrme-Hartree-Fock as well as Strutinsky-Woods-Saxon total
routhian surface methods with and without the T=1 pairing correlations. It is
shown that a consistent description within these standard approaches cannot be
achieved. A T=0 neutron-proton pairing configuration mixing of
signature-separated bands in 60Zn is suggested as a possible solution to the
problem.Comment: 9 ReVTex pages, 10 figures, submitted to Phys. Rev.
Staggering effects in nuclear and molecular spectra
It is shown that the recently observed Delta J = 2 staggering effect (i.e.
the relative displacement of the levels with angular momenta J, J+4, J+8, ...,
relatively to the levels with angular momenta J+2, J+6, J+10, ...) seen in
superdeformed nuclear bands is also occurring in certain electronically excited
rotational bands of diatomic molecules (YD, CrD, CrH, CoH), in which it is
attributed to interband interactions (bandcrossings). In addition, the Delta J
= 1 staggering effect (i.e. the relative displacement of the levels with even
angular momentum J with respect to the levels of the same band with odd J) is
studied in molecular bands free from Delta J = 2 staggering (i.e. free from
interband interactions/bandcrossings). Bands of YD offer evidence for the
absence of any Delta J = 1 staggering effect due to the disparity of nuclear
masses, while bands of sextet electronic states of CrD demonstrate that Delta J
= 1 staggering is a sensitive probe of deviations from rotational behaviour,
due in this particular case to the spin-rotation and spin-spin interactions.Comment: LaTeX, 16 pages plus 30 figures given in separate .ps files. To
appear in the proceedings of the 4th European Workshop on Quantum Systems in
Chemistry and Physics (Marly-le-Roi, France, 1999), ed. J. Maruani et al.
(Kluwer, Dordrecht
Extended Hauser-Feshbach Method for Statistical Binary-Decay of Light-Mass Systems
An Extended Hauser-Feshbach Method (EHFM) is developed for light heavy-ion
fusion reactions in order to provide a detailed analysis of all the possible
decay channels by including explicitly the fusion-fission phase-space in the
description of the cascade chain. The mass-asymmetric fission component is
considered as a complex-fragment binary-decay which can be treated in the same
way as the light-particle evaporation from the compound nucleus in
statistical-model calculations. The method of the phase-space integrations for
the binary-decay is an extension of the usual Hauser-Feshbach formalism to be
applied to the mass-symmetric fission part. The EHFM calculations include
ground-state binding energies and discrete levels in the low excitation-energy
regions which are essential for an accurate evaluation of the phase-space
integrations of the complex-fragment emission (fission). In the present
calculations, EHFM is applied to the first-chance binary-decay by assuming that
the second-chance fission decay is negligible. In a similar manner to the
description of the fusion-evaporation process, the usual cascade calculation of
light-particle emission from the highly excited complex fragments is applied.
This complete calculation is then defined as EHFM+CASCADE. Calculated
quantities such as charge-, mass- and kinetic-energy distributions are compared
with inclusive and/or exclusive data for the S+Mg and
Cl+C reactions which have been selected as typical examples.
Finally, the missing charge distributions extracted from exclusive measurements
are also successfully compared with the EHFM+CASCADE predictions.Comment: 34 pages, 6 Figures available upon request, Phys. Rev. C (to be
published
Quantum Tunneling in Nuclear Fusion
Recent theoretical advances in the study of heavy ion fusion reactions below
the Coulomb barrier are reviewed. Particular emphasis is given to new ways of
analyzing data, such as studying barrier distributions; new approaches to
channel coupling, such as the path integral and Green function formalisms; and
alternative methods to describe nuclear structure effects, such as those using
the Interacting Boson Model. The roles of nucleon transfer, asymmetry effects,
higher-order couplings, and shape-phase transitions are elucidated. The current
status of the fusion of unstable nuclei and very massive systems are briefly
discussed.Comment: To appear in the January 1998 issue of Reviews of Modern Physics. 13
Figures (postscript file for Figure 6 is not available; a hard copy can be
requested from the authors). Full text and figures are also available at
http://nucth.physics.wisc.edu/preprints
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