72 research outputs found
Landau-Migdal vs. Skyrme
The magnitude and density-dependence of the non-spin dependent Landau-Migdal
parameters are derived from Skyrme energy functionals and compared with the
phenomenological ones. We perform RPA calculations with various approximations
for the Landau-Migdal particle-hole interaction and compare them with the
results obtained with the full Skyrme interaction. For the first time the next
to leading order in the Landau-Migdal approach is considered in nuclear
structure calculations.Comment: Dedicated to the memory of G.E. Brow
Spontaneous fission modes and lifetimes of super-heavy elements in the nuclear density functional theory
Lifetimes of super-heavy (SH) nuclei are primarily governed by alpha decay
and spontaneous fission (SF). Here we study the competing decay modes of
even-even SH isotopes with 108 <= Z <= 126 and 148 <= N <= 188 using the
state-of-the-art self-consistent nuclear density functional theory framework
capable of describing the competition between nuclear attraction and
electrostatic repulsion. The collective mass tensor of the fissioning
superfluid nucleus is computed by means of the cranking approximation to the
adiabatic time-dependent Hartree-Fock-Bogoliubov approach. Along the path to
fission, our calculations allow for the simultaneous breaking of axial and
space inversion symmetries; this may result in lowering SF lifetimes by more
than seven orders of magnitude in some cases. We predict two competing SF
modes: reflection-symmetric and reflection-asymmetric.The shortest-lived SH
isotopes decay by SF; they are expected to lie in a narrow corridor formed by
Hs, Fl, and Uuo that separates the regions of SH
nuclei synthesized in "cold fusion" and "hot fusion" reactions. The region of
long-lived SH nuclei is expected to be centered on Ds with a total
half-life of ?1.5 days.Comment: 6 pages, 4 figure
Extended Theory of Finite Fermi Systems: Application to the collective and non-collective E1 strength in Pb
The Extended Theory of Finite Fermi Systems is based on the conventional
Landau-Migdal theory and includes the coupling to the low-lying phonons in a
consistent way. The phonons give rise to a fragmentation of the single-particle
strength and to a compression of the single-particle spectrum. Both effects are
crucial for a quantitative understanding of nuclear structure properties. We
demonstrate the effects on the electric dipole states in Pb (which
possesses 50% more neutrons then protons) where we calculated the low-lying
non-collective spectrum as well as the high-lying collective resonances. Below
8 MeV, where one expects the so called isovector pygmy resonances, we also find
a strong admixture of isoscalar strength that comes from the coupling to the
high-lying isoscalar electric dipole resonance, which we obtain at about 22
MeV. The transition density of this resonance is very similar to the breathing
mode, which we also calculated. We shall show that the extended theory is the
correct approach for self-consistent calculations, where one starts with
effective Lagrangians and effective Hamiltonians, respectively, if one wishes
to describe simultaneously collective and non-collective properties of the
nuclear spectrum. In all cases for which experimental data exist the agreement
with the present theory results is good.Comment: 21 figures corrected typos in author fiel
Self-consistent calculations of the electric giant dipole resonances in light and heavy mass nuclei
While bulk properties of stable nuclei are successfully reproduced by
mean-field theories employing effective interactions, the dependence of the
centroid energy of the electric giant dipole resonance on the nucleon number A
is not. This problem is cured by considering many-particle correlations beyond
mean-field theory, which we do within the "Quasiparticle Time Blocking
Approximation". The electric giant dipole resonances in O, Ca,
and Pb are calculated using two new Skyrme interactions.Comment: 4 pages, 4 figure
A particle-hole model approach for hypernuclei
A particle-hole model is developed to describe the excitation spectrum of
single lambda hypernuclei and the possible presence of collective effects is
explored by making a comparison with the mean-field calculations. Results for
the spectra of 12C, 16O, 40Ca, 90Zr and 208Pb single lambda hypernuclei are
shown. The comparison with the available experimental data is satisfactory. We
find that collective phenomena are much less important in hypernuclei than in
ordinary nuclei.Comment: 24 pages, 5 eps figures, accepted for publication in Nucl. Phys.
Description of the Giant Monopole Resonance in the Even-A Sn Isotopes within the Microscopic Model Including Quasiparticle-Phonon Coupling
We have calculated the strength distributions of the giant monopole resonance
in the even-A tin isotopes (A = 112-124) which were recently measured in
inelastic -scattering. The calculations were performed within two
microscopic models: the quasiparticle random phase approximation (QRPA) and the
quasiparticle time blocking approximation which is an extension of the QRPA
including quasiparticle-phonon coupling. We used a self-consistent
calculational scheme based on the HF+BCS approximation. The single-particle
continuum was exactly included on the RPA level. The self-consistent mean field
and the effective interaction were derived from the Skyrme energy functional.
In the calculations, two Skyrme force parametrizations were used. The T5
parametrization with comparatively low value of the incompressibility of
infinite nuclear matter ( = 202 MeV) gives theoretical results in
good agreement with the experimental data including the resonance widths.Comment: 21 pages, 2 figures; the figures have been modified: experimental
data are shown in Fig.
Shape Coexistence and the Effective Nucleon-Nucleon Interaction
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.Comment: 40 pages (16 text, 24 figures). The file may also be retrieved at
http://csep2.phy.ornl.gov/theory_group/people/dean/shape_coex/shapes.htm
Evidence for bystander signalling between human trophoblast cells and human embryonic stem cells
Maternal exposure during pregnancy to toxins can occasionally lead to miscarriage and
malformation. It is currently thought that toxins pass through the placental barrier, albeit bilayered
in the first trimester, and damage the fetus directly, albeit at low concentration. Here we
examined the responses of human embryonic stem (hES) cells in tissue culture to two metals at low
concentration. We compared direct exposures with indirect exposures across a bi-layered model
of the placenta cell barrier. Direct exposure caused increased DNA damage without apoptosis or
a loss of cell number but with some evidence of altered differentiation. Indirect exposure caused
increased DNA damage and apoptosis but without loss of pluripotency. This was not caused by
metal ions passing through the barrier. Instead the hES cells responded to signalling molecules
(including TNF-α) secreted by the barrier cells. This mechanism was dependent on connexin 43
mediated intercellular ‘bystander signalling’ both within and between the trophoblast barrier and
the hES colonies. These results highlight key differences between direct and indirect exposure of hES
cells across a trophoblast barrier to metal toxins. It offers a theoretical possibility that an indirectly
mediated toxicity of hES cells might have biological relevance to fetal development
FGF4 Independent Derivation of Trophoblast Stem Cells from the Common Vole
The derivation of stable multipotent trophoblast stem (TS) cell lines from preimplantation, and early postimplantation mouse embryos has been reported previously. FGF4, and its receptor FGFR2, have been identified as embryonic signaling factors responsible for the maintenance of the undifferentiated state of multipotent TS cells. Here we report the derivation of stable TS-like cell lines from the vole M. rossiaemeridionalis, in the absence of FGF4 and heparin. Vole TS-like cells are similar to murine TS cells with respect to their morphology, transcription factor gene expression and differentiation in vitro into derivatives of the trophectoderm lineage, and with respect to their ability to invade and erode host tissues, forming haemorrhagic tumours after subcutaneous injection into nude mice. Moreover, vole TS-like cells carry an inactive paternal X chromosome, indicating that they have undergone imprinted X inactivation, which is characteristic of the trophoblast lineage. Our results indicate that an alternative signaling pathway may be responsible for the establishment and stable proliferation of vole TS-like cells
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