17 research outputs found
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
Relativistic Continuum Quasiparticle Random Phase Approximation in Spherical Nuclei
We have calculated the strength distributions of the dipole response in
spherical nuclei, ranging all over the periodic table. The calculations were
performed within two microscopic models: the discretized quasiparticle random
phase approximation (QRPA) and the quasiparticle continuum RPA, which takes
into account the coupling of the single-particle continuum in an exact way.
Pairing correlations are treated with the BCS model. In the calculations, two
density functionals were used, namely the functional PC-F1 and the functional
DD-PC1. Both are based on relativistic point coupling Lagrangians. It is
explicitly shown that this model is capable of reproducing the giant as well as
the pygmy dipole resonance for open-shell nuclei in a high level of
quantitative agreement with the available experimental observations.Comment: 9 pages, 6 figures, accepted for publication in Phys. Pev.
CH in stellar atmospheres: an extensive linelist
The advent of high-resolution spectrographs and detailed stellar atmosphere
modelling has strengthened the need for accurate molecular data.
Carbon-enhanced metal-poor (CEMP) stars spectra are interesting objects with
which to study transitions from the CH molecule. We combine programs for
spectral analysis of molecules and stellar-radiative transfer codes to build an
extensive CH linelist, including predissociation broadening as well as newly
identified levels. We show examples of strong predissociation CH lines in CEMP
stars, and we stress the important role played by the CH features in the
Bond-Neff feature depressing the spectra of barium stars by as much as 0.2
magnitudes in the 3000 -- 5500 \AA\ range. Because of the extreme
thermodynamic conditions prevailing in stellar atmospheres (compared to the
laboratory), molecular transitions with high energy levels can be observed.
Stellar spectra can thus be used to constrain and improve molecular data.Comment: 33pages, 15 figures, accepted in A&A external data available at
http://www.astro.ulb.ac.be/~spectrotools
Gamma-ray strength at low energies using relativistic QRPA with exact coupling to the continuum
Continuum-quasiparticle random-phase Approximation (CQRPA) within the relativistic point-coupling model with density-dependent coupling constants is applied to investigate collective excitations in spherical nuclei. In particular we study the impact of the exact continuum on the giant-dipole and pygmy resonance of several Sn isotopes as well as the radiative neutron capture rates of importance for astrophysical calculations.SCOPUS: cp.jinfo:eu-repo/semantics/publishe
Total and partial photoneutron cross sections for Pb isotopes
Using quasimonochromatic laser-Compton scattering γ rays, total photoneutron cross sections were measured for 206 ,207 ,208Pb near neutron threshold with a high-efficiency 4π neutron detector. Partial E1 and M1 photoneutron cross sections along with total cross sections were determined for 207 ,208Pb at four energies near threshold by measuring anisotropies in photoneutron emission with linearly polarized γ rays. The E1 strength dominates over the M1 strength in the neutron channel where E1 photoneutron cross sections show extra strength of the pygmy dipole resonance in 207 ,208Pb near the neutron threshold corresponding to 0.32%-0.42% of the Thomas-Reiche-Kuhn sum rule. Several μN2 units of B(M1)→ strength were observed in 207 ,208Pb just above neutron threshold, which correspond to an M1 cross section less than 10% of the total photoneutron cross section. © 2012 American Physical Society.SCOPUS: ar.jinfo:eu-repo/semantics/publishe