305 research outputs found
Neutrino-nucleus reactions and their role for supernova dynamics and nucleosynthesis
The description of nuclear reactions induced by supernova neutrinos has
witnessed significant progress during the recent years. At the energies and
momentum transfers relevant for supernova neutrinos neutrino-nucleus cross
sections are dominated by allowed transitions, however, often with
non-negligible contributions from (first) forbidden transitions. For several
nuclei allowed Gamow-Teller strength distributions could be derived from
charge-exchange reactions and from inelastic electron scattering data.
Importantly the diagonalization shell model has been proven to accurately
describe these data and hence became the appropriate tool to calculate the
allowed contributions to neutrino-nucleus cross sections for supernova
neutrinos. Higher multipole contributions are usually calculated within the
framework of the Quasiparticle Random Phase Approximation, which describes the
total strength and the position of the giant resonances quite well.
This manuscript reviews the recent progress achieved in calculating
supernova-relevant neutrino-nucleus cross sections and discusses its
verification by data. Moreover, the review summarizes also the impact which
neutrino-nucleus reactions have on the dynamics of supernovae and on the
associated nucleosynthesis. These include the absorption of neutrinos by nuclei
(the inverse of nuclear electron capture which is the dominating
weak-interaction process during collapse), inelastic neutrino-nucleus
scattering and nuclear de-excitation by neutrino-pair emission. We also discuss
the role of neutrino-induced reactions for the recently discovered
process, for the r-process and for the neutrino process, for which
neutrino-nucleus reactions have the largest impact. Finally, we briefly review
neutrino-nucleus reactions important for the observation of supernova neutrinos
by earthbound detectors. (Abridged)Comment: 77 pages, 29 figures, 4 tables, submitted to Progress in Particle and
Nuclear Physic
Supernova neutrinos and nucleosynthesis
Observations of metal-poor stars indicate that at least two different
nucleosynthesis sites contribute to the production of r-process elements. One
site is responsible for the production of light r-process elements Z<~50 while
the other produces the heavy r-process elements. We have analyzed recent
observations of metal-poor stars selecting only stars that are enriched in
light r-process elements and poor in heavy r-process elements. We find a strong
correlation between the observed abundances of the N=50 elements (Sr, Y and Zr)
and Fe. It suggest that neutrino-driven winds from core-collapse supernova are
the main site for the production of these elements. We explore this possibility
by performing nucleosynthesis calculations based on long term Boltzmann
neutrino transport simulations. They are based on an Equation of State that
reproduces recent constrains on the nuclear symmetry energy. We predict that
the early ejecta is neutron-rich with Ye ~ 0.48, it becomes proton rich around
4 s and reaches Ye = 0.586 at 9 s when our simulation stops. The
nucleosynthesis in this model produces elements between Zn and Mo, including
92Mo. The elemental abundances are consistent with the observations of the
metal-poor star HD 12263. For the elements between Ge and Mo, we produce mainly
the neutron-deficient isotopes. This prediction can be confirmed by
observations of isotopic abundances in metal-poor stars. No elements heavier
than Mo (Z=42) and no heavy r-process elements are produced in our
calculations.Comment: 18 pages, 5 figures, submitted to J. Phys. G: Nucl. Part. Phys.
(Focus issue "Nucleosynthesis and the role of neutrinos", ed. Baha Balantekin
and Cristina Volpe
Neutrino–nucleus reactions and nuclear structure
The methods used in the evaluation of the neutrino–nucleus cross section are reviewed. Results are shown for a variety of targets of practical importance. Many of the described reactions are accessible in future experiments with neutrino sources from the pion and muon decays at rest, which might be available at the neutron spallation facilities. Detailed comparison between the experimental and theoretical results would establish benchmarks needed for verification and/or parameter adjustment of the nuclear models. Having a reliable tool for such calculation is of great importance in a variety of applications, e.g. the neutrino oscillation studies, detection of supernova neutrinos, description of the neutrino transport in supernovae and description of the r-process nucleosynthesis
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
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
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
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