3,697 research outputs found
Nuclei embedded in an electron gas
The properties of nuclei embedded in an electron gas are studied within the
relativistic mean-field approach. These studies are relevant for nuclear
properties in astrophysical environments such as neutron-star crusts and
supernova explosions. The electron gas is treated as a constant background in
the Wigner-Seitz cell approximation. We investigate the stability of nuclei
with respect to alpha and beta decay. Furthermore, the influence of the
electronic background on spontaneous fission of heavy and superheavy nuclei is
analyzed. We find that the presence of the electrons leads to stabilizing
effects for both decay and spontaneous fission for high electron
densities. Furthermore, the screening effect shifts the proton dripline to more
proton-rich nuclei, and the stability line with respect to beta decay is
shifted to more neutron-rich nuclei. Implications for the creation and survival
of very heavy nuclear systems are discussed.Comment: 35 pages, latex+ep
Screened alpha decay in dense astrophysical plasmas and magnetars
This paper shows that ultrastrong magnetic fields (such as those of
magnetars) and dense astrophysical plasmas can reduce the half life of alpha
decaying nuclei by many orders of magnitude. In such environments the
conventional Geiger-Nuttall law is modifed so that all half lives are shifted
to dramatically lower values. Those effects, which have never been investigated
before, may have significant implications on the universal abundances of heavy
radioactive elements and the cosmochronological methods that rely on them.Comment: 15 RevTex pages, 3 ps figures (minor revision). This work was
presented during the conference ''Supernova, 10 years of SN1993J'', April
2003, Valencia, Spain. Accepted for publication in Phys.Rev.
Astrophysical reaction rate for Be by photodisintegration
We study the astrophysical reaction rate for the formation of Be
through the three body reaction . This reaction is one
of the key reactions which could bridge the mass gap at A = 8 nuclear systems
to produce intermediate-to-heavy mass elements in alpha- and neutron-rich
environments such as r-process nucleosynthesis in supernova explosions,
s-process nucleosynthesis in asymptotic giant branch (AGB) stars, and
primordial nucleosynthesis in baryon inhomogeneous cosmological models. To
calculate the thermonuclear reaction rate in a wide range of temperatures, we
numerically integrate the thermal average of cross sections assuming a
two-steps formation through a metastable Be. Off-resonant and on-resonant
contributions from the ground state in Be are taken into account. As
input cross section, we adopt the latest experimental data by
photodisintegration of Be with laser-electron photon beams, which covers
all relevant resonances in Be. We provide the reaction rate for
Be in the temperature range from T=10
to T=10 both in the tabular form and in the analytical form. The
calculated reaction rate is compared with the reaction rates of the CF88 and
the NACRE compilations. The CF88 rate is valid at due to lack
of the off-resonant contribution. The CF88 rate differs from the present rate
by a factor of two in a temperature range . The NACRE rate,
which adopted different sources of experimental information on resonance states
in Be, is 4--12 times larger than the present rate at ,
but is consistent with the present rate to within at .Comment: 32 pages (incl 6 figures), Nucl. Phys. in pres
Electron fraction constraints based on Nuclear Statistical Equilibrium with beta equilibrium
The electron-to-nucleon ratio or electron fraction is a key parameter in many
astrophysical studies. Its value is determined by weak-interaction rates that
are based on theoretical calculations subject to several nuclear physics
uncertainties. Consequently, it is important to have a model independent way of
constraining the electron fraction value in different astrophysical
environments. Here we show that nuclear statistical equilibrium combined with
beta equilibrium can provide such a constraint. We test the validity of this
approximation in presupernova models and give lower limits for the electron
fraction in type Ia supernova and accretion-induced collapse.Comment: 10 pages, 9 figures, Astronomy and Astrophysic
Nuclear Astrophysics in Storage Rings
Nuclear reaction cross sections are usually very small in typical
astrophysical environments. It has been one of the major challenges of
experimental nuclear astrophysics to assess the magnitude of these cross
sections in the laboratory. For a successful experiment high luminosity beams
are needed. Increasing the target width, one also increases the reaction
yields. But, this is of limited use due to multiple scattering in the target.
Storage rings are a very good way to overcome these difficulties. In principle,
they can be tuned to large luminosities, and have the advantage of crossing the
interaction region many times per second (typically one million/s),
compensating low density internal gas targets, or low reaction rates in
beam-beam collisions. Storage rings are also ideal tools for precise
measurements of masses and beta-decay lifetimes of nuclei of relevance for
astrophysics.Comment: 14 pages, LaTeX, figures available upon reques
The s-process nucleosynthesis in massive stars: current status and uncertainties due to convective overshooting
Context: It is well known that the so-called s-process is responsible for the
production of neutron-rich trans-iron elements, that form the bulk of the
"heavy nuclides" (i.e. nuclides more massive than the iron-group nuclei) in the
solar-system composition, considered as "standard of reference" dataset for
cosmic abundances. In particular, the s-process produces about half of all the
trans-iron isotopes by moving along the "valley of stability" through a series
of neutron capture reactions and beta decays. More than one s-process
"component" (i.e. a nucleosynthesis event with a single set of physical
conditions like neutron exposure, initial abundances and neutron density) is
required in order to explain the observed solar distribution of s-nuclei
abundances. Current views on the subject suggest the existence of several
components that, in terms of stellar environments, correspond to distinct
categories of stars in different evolutionary phases. Aims: The purpose of the
chapter is to review the s-process nucleosynthesis occurring in massive stars
(so-called weak component of s-process), pointing particular attention on the
recent studies devoted to analyze how the uncertainties due to stellar
evolution modeling and, specifically, due to convective overshooting affect the
efficiency of this nucleosynthesis process.Comment: 20 pages, 7 figures, invited chapter accepted for publication in the
book "Astrophysics" (ISBN 979-953-307-389-6) - Book editor: Ibrahim Kucuk -
InTech (some text added in the acknowledgements, typos corrected
Nucleosynthesis Basics and Applications to Supernovae
This review concentrates on nucleosynthesis processes in general and their
applications to massive stars and supernovae. A brief initial introduction is
given to the physics in astrophysical plasmas which governs composition
changes. We present the basic equations for thermonuclear reaction rates and
nuclear reaction networks. The required nuclear physics input for reaction
rates is discussed, i.e. cross sections for nuclear reactions,
photodisintegrations, electron and positron captures, neutrino captures,
inelastic neutrino scattering, and beta-decay half-lives. We examine especially
the present state of uncertainties in predicting thermonuclear reaction rates,
while the status of experiments is discussed by others in this volume (see M.
Wiescher). It follows a brief review of hydrostatic burning stages in stellar
evolution before discussing the fate of massive stars, i.e. the nucleosynthesis
in type II supernova explosions (SNe II). Except for SNe Ia, which are
explained by exploding white dwarfs in binary stellar systems (which will not
be discussed here), all other supernova types seem to be linked to the
gravitational collapse of massive stars (M8M) at the end of their
hydrostatic evolution. SN1987A, the first type II supernova for which the
progenitor star was known, is used as an example for nucleosynthesis
calculations. Finally, we discuss the production of heavy elements in the
r-process up to Th and U and its possible connection to supernovae.Comment: 52 pages, 20 figures, uses cupconf.sty (included); to appear in
"Nuclear and Particle Astrophysics", eds. J. Hirsch., D. Page, Cambridge
University Pres
r-Java 2.0: the nuclear physics
[Aims:] We present r-Java 2.0, a nucleosynthesis code for open use that
performs r-process calculations as well as a suite of other analysis tools.
[Methods:] Equipped with a straightforward graphical user interface, r-Java 2.0
is capable of; simulating nuclear statistical equilibrium (NSE), calculating
r-process abundances for a wide range of input parameters and astrophysical
environments, computing the mass fragmentation from neutron-induced fission as
well as the study of individual nucleosynthesis processes. [Results:] In this
paper we discuss enhancements made to this version of r-Java, paramount of
which is the ability to solve the full reaction network. The sophisticated
fission methodology incorporated into r-Java 2.0 which includes three fission
channels (beta-delayed, neutron-induced and spontaneous fission) as well as
computation of the mass fragmentation is compared to the upper limit on mass
fission approximation. The effects of including beta-delayed neutron emission
on r-process yield is studied. The role of coulomb interactions in NSE
abundances is shown to be significant, supporting previous findings. A
comparative analysis was undertaken during the development of r-Java 2.0
whereby we reproduced the results found in literature from three other
r-process codes. This code is capable of simulating the physical environment
of; the high-entropy wind around a proto-neutron star, the ejecta from a
neutron star merger or the relativistic ejecta from a quark nova. As well the
users of r-Java 2.0 are given the freedom to define a custom environment. This
software provides an even platform for comparison of different proposed
r-process sites and is available for download from the website of the
Quark-Nova Project: http://quarknova.ucalgary.ca/Comment: 26 pages, 18 figures, 1 tabl
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