1,090 research outputs found
Neutron-induced astrophysical reaction rates for translead nuclei
Neutron-induced reaction rates, including fission, are calculated in the
temperature range 1.d8 <T (K) < 1.d10 within the framework of the statistical
model for targets with atomic number 83 < Z < 119 (from Po to Uuo) from the
neutron to the proton drip-line. Four sets of rates have been calculated,
utilizing - where possible - consistent nuclear data for neutron separation
energies and fission barriers from Thomas-Fermi (TF), Extended Thomas-Fermi
plus Strutinsky Integral (ETFSI), Finite-Range Droplet Model (FRDM) and
Hartree-Fock-Bogolyubov (HFB) predictions. Tables of calculated values as well
as analytic seven parameter fits in the standard REACLIB format are supplied.
We also discuss the sensitivity of the rates to the input, aiming at a better
understanding of the uncertainties introduced by the nuclear input.Comment: 14 pages, 10 figures, 2 tables in paper, 2 in Annex and online tables
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The Role of Fission in Neutron Star Mergers and Its Impact on the r-Process Peaks
Comparing observational abundance features with nucleosynthesis predictions of stellar evolution or explosion simulations, we can scrutinize two aspects: (a) the conditions in the astrophysical production site and (b) the quality of the nuclear physics input utilized. We test the abundance features of r-process nucleosynthesis calculations for the dynamical ejecta of neutron star merger simulations based on three different nuclear mass models: The Finite Range Droplet Model, the (quenched version of the) Extended Thomas Fermi Model with Strutinsky Integral, and the Hartree-Fock-Bogoliubov mass model. We make use of corresponding fission barrier heights and compare the impact of four different fission fragment distribution models on the final r-process abundance distribution. In particular, we explore the abundance distribution in the second r-process peak and the rare-earth sub-peak as a function of mass models and fission fragment distributions, as well as the origin of a shift in the third r-process peak position. The latter has been noticed in a number of merger nucleosynthesis predictions. We show that the shift occurs during the r-process freeze-out when neutron captures and β-decays compete and an (n,γ)-(γ,n) equilibrium is no longer maintained. During this phase neutrons originate mainly from fission of material above A = 240. We also investigate the role of β-decay half-lives from recent theoretical advances, which lead either to a smaller amount of fissioning nuclei during freeze-out or a faster (and thus earlier) release of fission neutrons, which can (partially) prevent this shift and has an impact on the second and rare-earth peak as well.Peer reviewe
Nematic twist-bend phase with nanoscale modulation of molecular orientation
A state of matter in which molecules show a long-range orientational order and no positional order is called a nematic liquid crystal. The best known and most widely used (for example, in modern displays) is the uniaxial nematic, with the rod-like molecules aligned along a single axis, called the director. When the molecules are chiral, the director twists in space, drawing a right-angle helicoid and remaining perpendicular to the helix axis; the structure is called a chiral nematic. Here using transmission electron and optical microscopy, we experimentally demonstrate a new nematic order, formed by achiral molecules, in which the director follows an oblique helicoid, maintaining a constant oblique angle with the helix axis and experiencing twist and bend. The oblique helicoids have a nanoscale pitch. The new twist-bend nematic represents a structural link between the uniaxial nematic (no tilt) and a chiral nematic (helicoids with right-angle tilt)
R-process nucleosynthesis calculations with complete nuclear physics input
The r-process constitutes one of the major challenges in nuclear
astrophysics. Its astrophysical site has not yet been identified but there is
observational evidence suggesting that at least two possible sites should
contribute to the solar system abundance of r-process elements and that the
r-process responsible for the production of elements heavier than Z=56 operates
quite robustly producing always the same relative abundances. From the
nuclear-physics point of view the r-process requires the knowledge of a large
number of reaction rates involving exotic nuclei. These include neutron capture
rates, beta-decays and fission rates, the latter for the heavier nuclei
produced in the r-process. We have developed for the first time a complete
database of reaction rates that in addition to neutron-capture rates and
beta-decay half-lives includes all possible reactions that can induce fission
(neutron-capture, beta-decay and spontaneous fission) and the corresponding
fission yields. In addition, we have implemented these reaction rates in a
fully implicit reaction network. We have performed r-process calculations for
the neutrino-driven wind scenario to explore whether or not fission can
contribute to provide a robust r-process pattern
Contribution of the magnetic resonance to the third harmonic generation from a fishnet metamaterial
We investigate experimentally and theoretically the third harmonic generated
by a double-layer fishnet metamaterial. To unambiguously disclose most notably
the influence of the magnetic resonance, the generated third harmonic was
measured as a function of the angle of incidence. It is shown experimentally
and numerically that when the magnetic resonance is excited by pump beam, the
angular dependence of the third harmonic signal has a local maximum at an
incidence angle of {\theta} \simeq 20{\deg}. This maximum is shown to be a
fingerprint of the antisymmetric distribution of currents in the gold layers.
An analytical model based on the nonlinear dynamics of the electrons inside the
gold shows excellent agreement with experimental and numerical results. This
clearly indicates the difference in the third harmonic angular pattern at
electric and magnetic resonances of the metamaterial.Comment: 7 pages, 5 figure
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
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