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 $r$-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 $\beta$-stability have inhibited us from understanding the nature of the $r$-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 $r$-process nucleosynthesis and analyse to what extent a solar-like $r$-abundance distribution can be obtained. The $r$-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 $r$-process peaks ($A = 130$ and 195). While this effect resolves the large underproduction at $A \approx 115$ and 140 obtained with droplet-type mass formulas, large deviations compared to the solar pattern are found near the third $r$-process peak. It is shown that a solar-like $r$-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 $\beta$-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