21 research outputs found
The Importance of Parity-Dependence of the Nuclear Level Density in the Prediction of Astrophysical Reaction Rates
A simple description for obtaining the parity distribution of nuclear levels
in the pf + g9/2 shell as a function of excitation energy was recently derived.
We implement this in a global nuclear level density model. In the framework of
the statistical model, cross sections and astrophysical reaction rates are
calculated in the Fe region and compared to rates obtained with the common
assumption of an equal distribution of parities. We find considerable
differences, especially for reactions involving particles in the exit channel.Comment: 4 pages, to appear in the proceedings of CGS11 (Prague), World
Scientifi
Large-scale prediction of the parity distribution in the nuclear level density and application to astrophysical reaction rates
A generalized method to calculate the excitation-energy dependent parity
ratio in the nuclear level density is presented, using the assumption of
Poisson distributed independent quasi particles combined with BCS occupation
numbers. It is found that it is crucial to employ a sufficiently large model
space to allow excitations both from low-lying shells and to higher shells
beyond a single major shell. Parity ratios are only found to equilibrate above
at least 5-10 MeV of excitation energy. Furthermore, an overshooting effect
close to major shells is found where the parity opposite to the ground state
parity may dominate across a range of several MeV before the parity ratio
finally equilibrates. The method is suited for large-scale calculations as
needed, for example, in astrophysical applications. Parity distributions were
computed for all nuclei from the proton dripline to the neutron dripline and
from Ne up to Bi. These results were then used to recalculate astrophysical
reaction rates in a Hauser-Feshbach statistical model. Although certain
transitions can be considerably enhanced or suppressed, the impact on
astrophysically relevant reactions remains limited, mainly due to the thermal
population of target states in stellar reaction rates.Comment: 15 pages, 17 figures; corrected/updated references in v2; additional
material can be found at http://nucastro.org/adndt.html#parit
Parity-Dependence in the Nuclear Level Density
Astrophysical reaction rates are sensitive to the parity distribution at low
excitation energies. We combine a formula for the energy-dependent parity
distribution with a microscopic-macroscopic nuclear level density. This
approach describes well the transition from low excitation energies, where a
single parity dominates, to high excitations where the two densities are equal.Comment: 4 pages, 3 figures; contribution to Nuclei In The Cosmos VIII, to
appear in Nucl. Phys.
Nucleosynthesis in neutrino heated matter: The vp-process and the r-process
This manuscript reviews recent progress in our understanding of the
nucleosynthesis of medium and heavy elements in supernovae. Recent
hydrodynamical models of core-collapse supernovae show that a large amount of
proton rich matter is ejected under strong neutrino fluxes. This matter
constitutes the site of the vp-process where antineutrino absorption reactions
catalyze the nucleosynthesis of nuclei with A > 64. Supernovae are also
associated with the r-process responsible for the synthesis of the heaviest
elements in nature. Fission during the r-process can play a major role in
determining the final abundance patter and in explaining the almost universal
features seen in metal-poor r-process-rich stars.Comment: 10 pages, 3 figures, invited talk at NIC-IX, International Symposium
on Nuclear Astrophysics - Nuclei in the Cosmos - IX, CERN, Geneva,
Switzerland, 25-30 June, 200
The Nuclear Reaction Network WinNet
© 2023. The Author(s). Published by the American Astronomical Society. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0/We present the state-of-the-art single-zone nuclear reaction network WinNet, which is capable of calculating the nucleosynthetic yields of a large variety of astrophysical environments and conditions. This ranges from the calculation of the primordial nucleosynthesis, where only a few nuclei are considered, to the ejecta of neutron star mergers with several thousands of involved nuclei. Here we describe the underlying physics and implementation details of the reaction network. We additionally present the numerical implementation of two different integration methods, the implicit Euler method and Gears method, along with their advantages and disadvantages. We furthermore describe basic example cases of thermodynamic conditions that we provide together with the network and demonstrate the reliability of the code by using simple test cases. With this publication, WinNet will be publicly available and open source at GitHub and Zenodo.Peer reviewe
Correlated Strength in Nuclear Spectral Function
We have carried out an (e,e'p) experiment at high momentum transfer and in
parallel kinematics to measure the strength of the nuclear spectral function
S(k,E) at high nucleon momenta k and large removal energies E. This strength is
related to the presence of short-range and tensor correlations, and was known
hitherto only indirectly and with considerable uncertainty from the lack of
strength in the independent-particle region. This experiment confirms by direct
measurement the correlated strength predicted by theory.Comment: 4 pages, 2 figures, accepted by Phys. Rev. Let
Have Superheavy Elements been Produced in Nature?
We discuss the possibility whether superheavy elements can be produced in
Nature by the astrophysical rapid neutron capture process. To this end we have
performed fully dynamical network r-process calculations assuming an
environment with neutron-to-seed ratio large enough to produce superheavy
nuclei. Our calculations include two sets of nuclear masses and fission
barriers and include all possible fission channels and the associated fission
yield distributions. Our calculations produce superheavy nuclei with A ~ 300
that however decay on timescales of days.Comment: 12 pages, 11 figure
Towards a parity-dependent level density for astrophysics
Astrophysical reaction rates are sensitive to the parity distribution at low excitation energies. We combine a formula for the energy-dependent parity distribution with a microscopic-macroscopic nuclear level density. This approach describes well the transition from low excitation energies where a single parity dominates to high excitations where the two densities are equal.Peer reviewe
Nuclear physics : a key ingredient in astrophysical modeling
Nuclear physics is a basic ingredient in a large number of energetic astrophysical environments which are characterized by sufficient temperatures and densities to permit scattering events among particles, leading to nuclear reactions and possibly the production of unstable reaction poducts. Strong, electromagnetic and weak interactions (fusion, exchange reactions, photodisintegrations, beta-decays, electron [and positron] captures on nucleons and nuclei, neutrino scattering and captures [i.e. neutral and charged current reactions]) can produce nuclei far form stability and require extended knowledge of nuclear structure near and far from stability, including decay and fission properties. Last, but not least, the nucleon-nucleon interaction determines the nuclear equation of state at and beyond nuclear densities and is ultimately connected to the question under which conditions a phase transitions from hadrons to the quark-gluon plasma occurs. In this review we will survey how these aspects of nuclear physics enter the modeling of astrophysical objects
The r-process in supernovae
This introductory review aims at understanding r-process nucleosynthesis by addressing the issues involved, nuclear properties, necessary environment conditions, properties of different suggested r-process sites, observational constraints and Galactic evolution. We summarize the remaining challenges and uncertainties which need to be overcome for a full understanding of the nature and site(s) of the r-process