1,198 research outputs found
Large-scale QRPA calculation of E1-strength and its impact on the neutron capture cross section
Large-scale QRPA calculations of the E1-strength are performed as a first
attempt to microscopically derive the radiative neutron capture cross sections
for the whole nuclear chart. A folding procedure is applied to the QRPA
strength distribution to take the damping of the collective motion into
account. It is shown that the resulting E1-strength function based on the SLy4
Skyrme force is in close agreement with photoabsorption data as well as the
available experimental E1-strength at low energies. The increase of the
E1-strength at low energies for neutron-rich nuclei is qualitatively analyzed
and shown to affect the corresponding radiative neutron capture cross section
significantly. A complete set of E1-strength function is made available for
practical applications in a table format for all 7<Z<111 nuclei lying between
the proton and the neutron drip lines.Comment: 17 pages, 8 figures, accepted for publication in Nucl. Phys.
Hope and Inquietudes in Nucleo-cosmochronology
Critical views are presented on some nucleo-cosmochronological questions.
Progress has been made recently in the development of the 187Re-187Os
cosmochronometry. From this, there is good hope for this clock to become of the
highest quality for the nuclear dating of the Universe. The simultaneous
observation of Th and U in ultra-metal-poor stars would also be a most
interesting prospect. In contrast, a serious inquietude is expressed about the
reliability of the chronometric attempts based on the classical 232Th-238U and
235U-238U pairs, as well as on the Th (without U) abundance determinations in
ultra-metal poor stars.Comment: 9 pages, no figures; ASP Conference Series: "Astrophysical Ages and
Time Scales
Large-scale Continuum Random Phase Approximation predictions of dipole strength for astrophysical applications
Large-scale calculations of the E1 strength are performed within the random
phase approximation (RPA) based on the relativistic point-coupling mean field
approach in order to derive the radiative neutron capture cross sections for
all nuclei of astrophysical interest. While the coupling to the single-particle
continuum is taken into account in an explicit and self-consistent way,
additional corrections like the coupling to complex configurations and the
temperature and deformation effects are included in a phenomenological way to
account for a complete description of the nuclear dynamical problem. It is
shown that the resulting E1-strength function based on the PCF1 force is in
close agreement with photoabsorption data as well as the available experimental
E1 strength data at low energies. For neutron-rich nuclei, as well as light
neutron-deficient nuclei, a low-lying so-called pygmy resonance is found
systematically in the 5-10 MeV region. The corresponding strength can reach 10%
of the giant dipole strength in the neutron-rich region and about 5% in the
neutron-deficient region, and is found to be reduced in the vicinity of the
shell closures. Finally, the neutron capture reaction rates of neutron-rich
nuclei is found to be about 2-5 times larger than those predicted on the basis
of the nonrelativistic RPA calculation and about a factor 50 larger than
obtained with traditional Lorentzian-type approaches.Comment: 11 pages, 12 figure
Recent breakthroughs in Skyrme-Hartree-Fock-Bogoliubov mass formulas
We review our recent achievements in the construction of microscopic mass
tables based on the Hartree-Fock-Bogoliubov method with Skyrme effective
interactions. In the latest of our series of HFB-mass models, we have obtained
our best fit ever to essentially all the available mass data, by treating the
pairing more realistically than in any of our earlier models. The rms deviation
on the 2149 measured masses of nuclei with N and Z>8 has been reduced for the
first time in a mean field approach to 0.581 MeV. With the additional
constraint on the neutron-matter equation of state, this new force is thus very
well-suited for the study of neutron-rich nuclei and for the description of
astrophysical environments like supernova cores and neutron-star crusts.Comment: Proceedings of the Fifth International Conference on Exotic Nuclei
and Atomic Masses, September 7-13 2008, Ryn (Poland). To appear in the
European Physical Journal
Impact of the phonon coupling on the dipole strength and radiative neutron capture
The E1 strength functions and radiative capture cross sections for several
compound Sn isotopes, including unstable 132S n and 150S n, have been
calculated using the self-consistent microscopic theory. In addition to the
standard RPA or QRPA approaches, the method includes the quasiparticle-phonon
coupling and the single-particle continuum. The results obtained show that the
phonon contribution is very noticeable for the pygmy-dipole resonance, which,
as it is known, is important for a description of the radiative neutron
capture. The phonon contribution to the pygmy-dipole resonance and to the
radiative neutron capture cross sections is increased with the (N-Z) difference
growth. For example, in the (0-10) MeV interval the full theory gives 17% of
EWSR for 150S n and 2.8% for 124S n, whereas within the continuum QRPA approach
we have 5.1% and 1.7%, respectively. These facts indicate an important role of
the self-consistent calculations that are of astrophysical interest for
neutron-rich nuclei. The comparison with the phenomenological Generalized
Lorentzian approach by Kopecky-Uhl has shown that the (Q)RPA approach gives a
significant increase in the cross section by a factor of 2 for 132S n and a
factor of 10 for 150S n and inclusion of the phonon coupling increases the
cross sections for these nuclei even more, by a factor of 2-3.Comment: 4pages,3figures,International Conference on Nuclear Data for Science
and Technology 201
The r-Process in Supernovae: Impact of New Microscopic Mass Formulas
The astrophysical origin of -process nuclei remains a long-standing
mystery. Although some astrophysical scenarios show some promise, many
uncertainties involved in both the astrophysical conditions and in the nuclear
properties far from the -stability have inhibited us from understanding
the nature of the -process. The purpose of the present paper is to examine
the effects of the newly-derived microscopic Hartree-Fock-Bogoliubov (HFB) mass
formulas on the -process nucleosynthesis and analyse to what extent a
solar-like -abundance distribution can be obtained. The -process
calculations with the HFB-2 mass formula are performed, adopting the
parametrized model of the prompt explosion from a collapsing O-Ne-Mg core for
the physical conditions and compared with the results obtained with the HFB-7
and droplet-type mass formulas. Due to its weak shell effect at the neutron
magic numbers in the neutron-rich region, the microscopic mass formulas (HFB-2
and HFB-7) give rise to a spread of the abundance distribution in the vicinity
of the -process peaks ( and 195). While this effect resolves the
large underproduction at and 140 obtained with droplet-type
mass formulas, large deviations compared to the solar pattern are found near
the third -process peak. It is shown that a solar-like -process pattern
can be obtained if the dynamical timescales of the outgoing mass trajectories
are increased by a factor of about 2-3, or if the -decay rates are
systematically increased by the same factor.Comment: 22 pages, 12 figures, accepted for publication in ApJ, some color
figures converted to B&W due to size constraint
Non-explosive hydrogen and helium burnings: Abundance predictions from the NACRE reaction rate compilation
The abundances of the isotopes of the elements from C to Al produced by the
non-explosive CNO, NeNa and MgAl modes of hydrogen burning, as well as by
helium burning, are calculated with the thermonuclear rates recommended by the
European compilation of reaction rates for astrophysics (NACRE: details about
NACRE may be found at http://astro.ulb.ac.be. This electronic address provides
many data of nuclear astrophysics interest and also offers the possibility of
generating interactively tables of reaction rates for networks and temperature
grids selected by the user). The impact of nuclear physics uncertainties on the
derived abundances is discussed in the framework of a simple parametric
astrophysical model. These calculations have the virtue of being a guide in the
selection of the nuclear uncertainties that have to be duly analyzed in
detailed model stars, particularly in order to perform meaningful
confrontations between abundance observations and predictions. They are also
hoped to help nuclear astrophysicists pinpointing the rate uncertainties that
have to be reduced most urgently.Comment: 13 pages, 13 figures, Latex, accepted for publication in Astronomy
and Astrophysics main journal. Also available at
http://astro.ulb.ac.be/Htm/iaa0.ht
Singular inextensible limit in the vibrations of post-buckled rods: analytical derivation and role of boundary conditions
In-plane vibrations of an elastic rod clamped at both extremities are studied. The rod is modeled as an extensible planar Kirchhoff elastic rod under large displacements and rotations. Equilibrium configurations and vibrations around these configurations are computed analytically in the incipient post-buckling regime. Of particular interest is the variation of the first mode frequency as the load is increased through the buckling threshold. The loading type is found to have a crucial importance as the first mode frequency is shown to behave singularly in the zero thickness limit in case of prescribed axial displacement, whereas a regular behavior is found in the case of prescribed axial load
Microscopic HFB+QRPA predictions of dipole strength for astrophysics applications
Large-scale QRPA calculations of the E1 strength are performed on top of HFB
calculations in order to derive the radiative neutron capture cross sections
for the whole nuclear chart. The spreading width of the GDR is taken into
account by analogy with the second-RPA (SRPA) method. The accuracy of HFB+QRPA
model based on various Skyrme forces with different pairing prescription and
parameterization is analyzed. It is shown that the present model allows to
constrain the effective nucleon-nucleon interaction with the GDR data and to
provide quantitative predictions of dipole strengths.Comment: 21 pages, 9 figure
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