203 research outputs found
Experimental (n,) cross sections of the p-process nuclei Se and Sr
The nucleosynthesis of elements beyond iron is dominated by the s and r
processes. However, a small amount of stable isotopes on the proton-rich side
cannot be made by neutron capture and are thought to be produced by
photodisintegration reactions on existing seed nuclei in the so-called "p
process". So far most of the p-process reactions are not yet accessible by
experimental techniques and have to be inferred from statistical
Hauser-Feshbach model calculations. The parametrization of these models has to
be constrained by measurements on stable proton-rich nuclei. A series of
(n,) activation measurements, related by detailed balance to the
respective photodisintegrations, were carried out at the Karlsruhe Van de
Graaff accelerator using the Li(p,n)Be source for simulating a
Maxwellian neutron distribution of kT= 25 keV. First results for the
experimental (n,) cross sections of the light p nuclei Se and
Sr are reported. These experimental values were used for an
extrapolation to the Maxwellian averaged cross section at 30 keV,
, yielding 27115 mb for Se, and 30017 mb for
the total capture cross section of Sr. The partial cross section to the
isomer in Sr was found to be 19010 mb.Comment: 10 pages, 5 figure
Astrophysical S-factors for fusion reactions involving C, O, Ne and Mg isotopes
Using the Sao Paulo potential and the barrier penetration formalism we have
calculated the astrophysical factor S(E) for 946 fusion reactions involving
stable and neutron-rich isotopes of C, O, Ne, and Mg for center-of-mass
energies E varying from 2 MeV to 18-30 MeV (covering the range below and above
the Coulomb barrier). We have parameterized the energy dependence S(E) by an
accurate universal 9-parameter analytic expression and present tables of fit
parameters for all the reactions. We also discuss the reduced 3-parameter
version of our fit which is highly accurate at energies below the Coulomb
barrier, and outline the procedure for calculating the reaction rates. The
results can be easily converted to thermonuclear or pycnonuclear reaction rates
to simulate various nuclear burning phenomena, in particular, stellar burning
at high temperatures and nucleosynthesis in high density environments.Comment: 30 pages including 11 tables, 4 figures, ADNDT, accepte
Skyrme Interaction and Nuclear Matter Constraints
This paper presents a detailed assessment of the ability of the 240 Skyrme
interaction parameter sets in the literature to satisfy a series of criteria
derived from macroscopic properties of nuclear matter in the vicinity of
nuclear saturation density at zero temperature and their density dependence,
derived by the liquid drop model, experiments with giant resonances and
heavy-ion collisions. The objective is to identify those parameterizations
which best satisfy the current understanding of the physics of nuclear matter
over a wide range of applications. Out of the 240 models, only 16 are shown to
satisfy all these constraints. Additional, more microscopic, constraints on
density dependence of the neutron and proton effective mass beta-equilibrium
matter, Landau parameters of symmetric and pure neutron nuclear matter, and
observational data on high- and low-mass cold neutron stars further reduce this
number to 5, a very small group of recommended Skyrme parameterizations to be
used in future applications of the Skyrme interaction of nuclear matter related
observables. Full information on partial fulfillment of individual constraints
by all Skyrme models considered is given. The results are discussed in terms of
the physical interpretation of the Skyrme interaction and the validity of its
use in mean-field models. Future work on application of the Skyrme forces,
selected on the basis of variables of nuclear matter, in Hartree-Fock
calculation of properties of finite nuclei, is outlined.Comment: 86 pages, 14 figure
Instabilities of infinite matter with effective Skyrme-type interactions
The stability of the equation of state predicted by Skyrme-type interactions
is examined. We consider simultaneously symmetric nuclear matter and pure
neutron matter. The stability is defined by the inequalities that the Landau
parameters must satisfy simultaneously. A systematic study is carried out to
define interaction parameter domains where the inequalities are fulfilled. It
is found that there is always a critical density beyond which the
system becomes unstable. The results indicate in which parameter regions one
can find effective forces to describe correctly finite nuclei and give at the
same time a stable equation of state up to densities of 3-4 times the
saturation density of symmetric nuclear matter.Comment: 20 pages, 5 figures, submitted to Phys.Rev.
alpha-nucleus potentials for the neutron-deficient p nuclei
alpha-nucleus potentials are one important ingredient for the understanding
of the nucleosynthesis of heavy neutron-deficient p nuclei in the astrophysical
gamma-process where these p nuclei are produced by a series of (gamma,n),
(gamma,p), and (gamma,alpha) reactions. I present an improved alpha-nucleus
potential at the astrophysically relevant sub-Coulomb energies which is derived
from the analysis of alpha decay data and from a previously established
systematic behavior of double-folding potentials.Comment: 6 pages, 3 figures, accepted for publication in Phys. Rev.
Proton Drip-Line Calculations and the Rp-process
One-proton and two-proton separation energies are calculated for proton-rich
nuclei in the region . The method is based on Skyrme Hartree-Fock
calculations of Coulomb displacement energies of mirror nuclei in combination
with the experimental masses of the neutron-rich nuclei. The implications for
the proton drip line and the astrophysical rp-process are discussed. This is
done within the framework of a detailed analysis of the sensitivity of rp
process calculations in type I X-ray burst models on nuclear masses. We find
that the remaining mass uncertainties, in particular for some nuclei with
, still lead to large uncertainties in calculations of X-ray burst light
curves. Further experimental or theoretical improvements of nuclear mass data
are necessary before observed X-ray burst light curves can be used to obtain
quantitative constraints on ignition conditions and neutron star properties. We
identify a list of nuclei for which improved mass data would be most important.Comment: 20 pages, 9 figures, 2 table
Short-lived p-nuclides in the early solar system and implications on the nucleosynthetic role of X-ray binaries
The data available for short-lived -nuclides are used in an open nonlinear
model of the chemical evolution of the Galaxy in order to discuss the origin of
extinct radionuclides, the stellar sources of -nuclides, and the chronology
of solar system formation. It is concluded that the observed abundances of
Tc, Tc, Nb, and Sm in the early solar system are
consistent with nucleosynthesis in type II supernovae during continuous
chemical evolution of the Galaxy and a subsequent short isolation of the
presolar molecular cloud from fresh nucleosynthetic inputs. However, further
work on supernova models is needed before -radionuclides will comprise
reliable cosmochronometers. Despite these limitations, we argue that niobium-92
can be used to test whether the {\it rp}-process contributed to the synthesis
of light {\it p}-nuclides in the Mo-Ru region.Comment: 8 pages, 2 figures. Proceedings of the Nuclear Physics in
Astrophysics Conference, Debrecen, Hungary, 2002, to appear in Nucl. Phys.
Analysis of 26 Barium Stars II. Contributions of s-, r- and p-processes in the production of heavy elements
Barium stars show enhanced abundances of the slow neutron capture (s-process)
heavy elements, and for this reason they are suitable objects for the study of
s-process elements. The aim of this work is to quantify the contributions of
the s-, r- and p-processes for the total abundance of heavy elements from
abundances derived for a sample of 26 barium stars. The abundance ratios
between these processes and neutron exposures were studied. The abundances of
the sample stars were compared to those of normal stars thus identifying the
fraction relative to the s-process main component. The fittings of the sigmaN
curves (neutron capture cross section times abundance, plotted against atomic
mass number) for the sample stars suggest that the material from the companion
asymptotic giant branch star had approximately the solar isotopic composition
as concerns fractions of abundances relative to the s-process main component.
The abundance ratios of heavy elements, hs, ls and s and the computed neutron
exposure are similar to those of post-AGB stars. For some sample stars, an
exponential neutron exposure fits well the observed data, whereas for others, a
single neutron exposure provides a better fit. The comparison between barium
and AGB stars supports the hypothesis of binarity for the barium star
formation. Abundances of r-elements that are part of the s-process path in
barium stars are usually higher than those in normal stars,and for this reason,
barium stars seemed to be also enriched in r-elements, although in a lower
degree than s-elements. No dependence on luminosity classes was found in the
abundance ratios behaviour among the dwarfs and giants of the sample barium
stars.Comment: 30 pages including 24 figures, accepted to A&
Nucleosynthesis in Electron Capture Supernovae of AGB Stars
We examine nucleosynthesis in the electron capture supernovae of progenitor
AGB stars with an O-Ne-Mg core (with the initial stellar mass of 8.8 M_\odot).
Thermodynamic trajectories for the first 810 ms after core bounce are taken
from a recent state-of-the-art hydrodynamic simulation. The presented
nucleosynthesis results are characterized by a number of distinct features that
are not shared with those of other supernovae from the collapse of stars with
iron core (with initial stellar masses of more than 10 M_\odot). First is the
small amount of 56Ni (= 0.002-0.004 M_\odot) in the ejecta, which can be an
explanation for observed properties of faint supernovae such as SNe 2008S and
1997D. In addition, the large Ni/Fe ratio is in reasonable agreement with the
spectroscopic result of the Crab nebula (the relic of SN 1054). Second is the
large production of 64Zn, 70Ge, light p-nuclei (74Se, 78Kr, 84Sr, and 92Mo),
and in particular, 90Zr, which originates from the low Y_e (= 0.46-0.49, the
number of electrons per nucleon) ejecta. We find, however, that only a 1-2%
increase of the minimum Y_e moderates the overproduction of 90Zr. In contrast,
the production of 64Zn is fairly robust against a small variation of Y_e. This
provides the upper limit of the occurrence of this type of events to be about
30% of all core-collapse supernovae.Comment: 13 pages, 14 figures, accepted for publication in Ap
- …