2,466 research outputs found
Dynamical r-process studies within the neutrino-driven wind scenario and its sensitivity to the nuclear physics input
We use results from long-time core-collapse supernovae simulations to
investigate the impact of the late time evolution of the ejecta and of the
nuclear physics input on the calculated r-process abundances. Based on the
latest hydrodynamical simulations, heavy r-process elements cannot be
synthesized in the neutrino-driven winds that follow the supernova explosion.
However, by artificially increasing the wind entropy, elements up to A=195 can
be made. In this way one can reproduce the typical behavior of high-entropy
ejecta where the r-process is expected to occur. We identify which nuclear
physics input is more important depending on the dynamical evolution of the
ejecta. When the evolution proceeds at high temperatures (hot r-process), an
(n,g)-(g,n) equilibrium is reached. While at low temperature (cold r-process)
there is a competition between neutron captures and beta decays. In the first
phase of the r-process, while enough neutrons are available, the most relevant
nuclear physics input are the nuclear masses for the hot r-process and the
neutron capture and beta-decay rates for the cold r-process. At the end of this
phase, the abundances follow a steady beta flow for the hot r-process and a
steady flow of neutron captures and beta decays for the cold r-process. After
neutrons are almost exhausted, matter decays to stability and our results show
that in both cases neutron captures are key for determining the final
abundances, the position of the r-process peaks, and the formation of the
rare-earth peak. In all the cases studied, we find that the freeze out occurs
in a timescale of several seconds.Comment: 20 pages, 12 figures, submitted to Phys. Rev. C (improved version
Neutrino Nucleosynthesis of radioactive nuclei in supernovae
We study the neutrino-induced production of nuclides in explosive supernova
nucleosynthesis for progenitor stars with solar metallicity and initial main
sequence masses between 15 M and 40 M. We improve previous
investigations i) by using a global set of partial differential cross sections
for neutrino-induced charged- and neutral-current reactions on nuclei with
charge numbers and ii) by considering modern supernova neutrino
spectra which have substantially lower average energies compared to those
previously adopted in neutrino nucleosynthesis studies. We confirm the
production of Li, B, La, and Ta by neutrino
nucleosynthesis, albeit at slightly smaller abundances due to the changed
neutrino spectra. We find that for stars with a mass smaller than 20 M,
F is produced mainly by explosive nucleosynthesis while for higher mass
stars it is produced by the process. We also find that neutrino-induced
reactions, either directly or indirectly by providing an enhanced abundance of
light particles, noticeably contribute to the production of the radioactive
nuclides Na and Al. Both nuclei are prime candidates for
gamma-ray astronomy. Other prime targets, Ti and Fe, however, are
insignificantly produced by neutrino-induced reactions. We also find a large
increase in the production of the long-lived nuclei Nb and Tc due
to charged-current neutrino capture.Comment: 6 pages, 2 figures, 2 table
Thermal QRPA with Skyrme interactions and supernova neutral-current neutrino-nucleus reactions
The Thermal Quasiparticle Random-Phase Approximation is combined with the
Skyrme energy density functional method (Skyrme-TQRPA) to study the response of
a hot nucleus to an external perturbation. For the sample nuclei, Fe and
Ge, the Skyrme-TQRPA is applied to analyze thermal effects on the
strength function of charge-neutral Gamow-Teller transitions which dominate
neutrino-nucleus reactions at ~MeV. For the relevant
supernova temperatures we calculate the cross sections for inelastic neutrino
scattering. We also apply the method to examine the rate of
neutrino-antineutrino pair emission by hot nuclei. The cross sections and rates
are compared with those obtained earlier from the TQRPA calculations based on
the phenomenological Quasiparticle-Phonon Model Hamiltonian. For inelastic
neutrino scattering on Fe we also compare the Skyrme-TQRPA results to
those obtained earlier from a hybrid approach that combines shell-model and RPA
calculations.Comment: Minor revisions according to referee's recomendation
Imaging of fuel mixture fraction oscillations in a driven system using acetone PLIF
Measurements of fuel mixture fraction are made for a jet flame in an acoustic chamber. Acoustic forcing creates a
spatially-uniform, temporally-varying pressure field which results in oscillatory behavior in the flame . Forcing is at 22,27, 32, 37, and 55 Hz. To asses the oscillatory behavior, previous work included chemiluminescence, OH PUF, nitric oxide PUF imaging, and fuel mixture fraction measurements by infrared laser absorption. While these results illuminated what was happening to the flame chemistry, they did not provide a complete explanation as to why these things were happening. In this work, the fuel mixture fraction is measured through PUF of acetone, which is introduced into the fuel stream as a marker. This technique enables a high degree of spatial resolution of fuel/air mixture value. Both non-reacting and reacting cases were measured and comparisons are drawn with the results from the previous work. It is found that structure in the mixture fraction oscillations is a major contributor to the magnitude of the flame oscillations
Coulomb displacement energies, energy differenced and neutron skins
A Fock space representation of the monopole part of the Coulomb potential is
presented. Quantum effects show through a small orbital term in . Once
it is averaged out, the classical electrostatic energy emerges as an
essentially exact expression, which makes it possible to eliminate the
Nolen-Schiffer anomaly, and to estimate neutron skins and the evolution of
radii along yrast states of mirror nuclei. The energy differences of the latter
are quantitatively reproduced by the monopole term and a schematic multipole
one.Comment: 4 pages, 3 figures, Revte
Improved estimate of electron capture rates on nuclei during stellar core collapse
Electron captures on nuclei play an important role in the dynamics of the
collapsing core of a massive star that leads to a supernova explosion. Recent
calculations of these capture rates were based on microscopic models which
account for relevant degrees of freedom. Due to computational restrictions such
calculations were limited to a modest number of nuclei, mainly in the mass
range A=45-110. Recent supernova simulations show that this pool of nuclei,
however, omits the very neutron-rich and heavy nuclei which dominate the
nuclear composition during the last phase of the collapse before neutrino
trapping. Assuming that the composition is given by Nuclear Statistical
Equilibrium we present here electron capture rates for collapse conditions
derived from individual rates for roughly 2700 individual nuclei. For those
nuclei which dominate in the early stage of the collapse, the individual rates
are derived within the framework of microscopic models, while for the nuclei
which dominate at high densities we have derived the rates based on the Random
Phase Approximation with a global parametrization of the single particle
occupation numbers. In addition, we have improved previous rate evaluations by
properly including screening corrections to the reaction rates into account.Comment: 32 pages, 13 figures, 1 table; elsart; to appear in Nuclear Physics
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