210 research outputs found
Screening enhancement factors for laboratory CNO and rp astrophysical reactions
Cross sections of laboratory CNO and rp astrophysical reactions are enhanced
due to the presence of the multi-electron cloud that surrounds the target
nuclei. As a result the relevant astrophysical factors are overestimated unless
corrected appropriately. This study gives both an estimate of the error
committed if screening effects are not taken into account and a rough profile
of the laboratory energy thresholds at which the screening effect appears. The
results indicate that, for most practical purposes, screening corrections to
past relevant experiments can be disregarded. Regarding future experiments,
however, screening corrections to the CNO reactions will certainly be of
importance as they are closely related to the solar neutrino fluxes and the rp
process. Moreover, according to the present results, screening effects will
have to be taken into account particularly by the current and future LUNA
experiments, where screened astrophysical factors will be enhanced to a
significant degree.Comment: 6 RevTex pages + 2 ps figures. (Revised version). Accepted for
publication in Journal of Physics
Standard Solar models in the Light of New Helioseismic Constraints II. Mixing Below the Convective Zone
In previous work, we have shown that recent updated standard solar models
cannot reproduce the radial profile of the sound speed at the base of the
convective zone (CZ) and fail to predict the Li7 depletion. In parallel,
helioseismology has shown that the transition from differential rotation in the
CZ to almost uniform rotation in the radiative solar interior occurs in a
shallow layer called the tachocline. This layer is presumably the seat of large
scale circulation and of turbulent motions. Here, we introduce a macroscopic
transport term in the structure equations, which is based on a hydrodynamical
description of the tachocline proposed by Spiegel and Zahn, and we calculate
the mixing induced within this layer. We discuss the influence of different
parameters that represent the tachocline thickness, the Brunt-Vaissala
frequency at the base of the CZ, and the time dependence of this mixing process
along the Sun's evolution. We show that the introduction of such a process
inhibits the microscopic diffusion by about 25%. Starting from models including
a pre-main sequence evolution, we obtain: a) a good agreement with the observed
photospheric chemical abundance of light elements such as He3, He4, Li7 and
Be9, b) a smooth composition gradient at the base of the CZ, and c) a
significant improvement of the sound speed square difference between the
seismic sun and the models in this transition region, when we allow the
phostospheric heavy element abundance to adjust, within the observational
incertitude, due to the action of this mixing process. The impact on neutrino
predictions is also discussed.Comment: 15 pages, 7 figures, to be published in ApJ (used emulateapj style
for latex2e). New email for A. S. Brun: [email protected]
Astrophysical factors:Zero energy vs. Most effective energy
Effective astrophysical factors for non-resonant astrophysical nuclear
reaction are invariably calculated with respect to a zero energy limit. In the
present work that limit is shown to be very disadvantageous compared to the
more natural effective energy limit. The latter is used in order to modify the
thermonuclear reaction rate formula so that it takes into account both plasma
and laboratory screening effects.Comment: 7 RevTex pages. Accepted for publication in Phys.Rev.
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Macroscopic Self-Evolution of Dynamic Hydrogels to Create Hollow Interiors
A solid-to-hollow evolution in macroscopic structures is challenging in synthetic materials. A fundamentally new strategy is reported for guiding macroscopic, unidirectional shape evolution of materials without compromising the material's integrity. This strategy is based on the creation of a field with a âswelling poleâ and a âshrinking poleâ to drive polymers to disassemble, migrate, and resettle in the targeted region. This concept is demonstrated using dynamic hydrogels containing anchored acrylic ligands and hydrophobic long alkyl chains. Adding water molecules and ferric ions (Fe3+) to induce a swellingâshrinking field transforms the hydrogels from solid to hollow. The strategy is versatile in the generation of various closed hollow objects (for example, spheres, helix tubes, and cubes with different diameters) for different applications
Fusion rate enhancement due to energy spread of colliding nuclei
Experimental results for sub-barrier nuclear fusion reactions show cross
section enhancements with respect to bare nuclei which are generally larger
than those expected according to electron screening calculations. We point out
that energy spread of target or projectile nuclei is a mechanism which
generally provides fusion enhancement. We present a general formula for
calculating the enhancement factor and we provide quantitative estimate for
effects due to thermal motion, vibrations inside atomic, molecular or crystal
system, and due to finite beam energy width. All these effects are marginal at
the energies which are presently measurable, however they have to be considered
in future experiments at still lower energies. This study allows to exclude
several effects as possible explanation of the observed anomalous fusion
enhancements, which remain a mistery.Comment: 17 pages with 3 ps figure included. Revtex styl
Radiation correction to astrophysical fusion reactions and the electron screening problem
We discuss the effect of electromagnetic environment on laboratory
measurements of the nuclear fusion reactions of astrophysical interest. The
radiation field is eliminated using the path integral formalism in order to
obtain the influence functional, which we evaluate in the semi-classical
approximation. We show that enhancement of the tunneling probability due to the
radiation correction is extremely small and does not resolve the longstanding
problem that the observed electron screening effect is significantly larger
than theoretical predictions.Comment: 9 pages, 1 eps figure
Atomic effects in astrophysical nuclear reactions
Two models are presented for the description of the electron screening
effects that appear in laboratory nuclear reactions at astrophysical energies.
The two-electron screening energy of the first model agrees very well with the
recent LUNA experimental result for the break-up reaction , which so far defies all available theoretical models.
Moreover, multi-electron effects that enhance laboratory reactions of the CNO
cycle and other advanced nuclear burning stages, are also studied by means of
the Thomas-Fermi model, deriving analytical formulae that establish a lower and
upper limit for the associated screening energy. The results of the second
model, which show a very satisfactory compatibility with the adiabatic
approximation ones, are expected to be particularly useful in future
experiments for a more accurate determination of the CNO astrophysical factors.Comment: 14 RevTex pages + 2 ps (revised) figures. Phys.Rev.C (in production
Screening of Nuclear Reactions in the Sun and Solar Neutrinos
We quantitatively determine the effect and the uncertainty on solar neutrino
production arising from the screening process. We present predictions for the
solar neutrino fluxes and signals obtained with different screening models
available in the literature and by using our stellar evolution code. We explain
these numerical results in terms of simple laws relating the screening factors
with the neutrino fluxes. Futhermore we explore a wider range of models for
screening, obtained from the Mitler model by introducing and varying two
phenomenological parameters, taking into account effects not included in the
Mitler prescription. Screening implies, with respect to a no-screening case, a
central temperat reduction of 0.5%, a 2% (8%) increase of Beryllium
(Boron)-neutrino flux and a 2% (12%) increase of the Gallium (Chlorine) signal.
We also find that uncertainties due to the screening effect ar at the level of
1% for the predicted Beryllium-neutrino flux and Gallium signal, not exceeding
3% for the Boron-neutrino flux and the Chlorine signal.Comment: postscript file 11 pages + 4 figures compressed and uuencoded we have
replaced the previous paper with a uuencoded file (the text is the same) for
any problem please write to [email protected]
Nuclear Reaction Network for Primordial Nucleosynthesis: a detailed analysis of rates, uncertainties and light nuclei yields
We analyze in details the standard Primordial Nucleosynthesis scenario. In
particular we discuss the key theoretical issues which are involved in a
detailed prediction of light nuclide abundances, as the weak reaction rates,
neutrino decoupling and nuclear rate modeling. We also perform a new analysis
of available data on the main nuclear processes entering the nucleosynthesis
reaction network, with particular stress on their uncertainties as well as on
their role in determining the corresponding uncertainties on light nuclide
theoretical estimates. The current status of theoretical versus experimental
results for 2H, 3He, 4He and 7Li is then discussed using the determination of
the baryon density as obtained from Cosmic Microwave Background anisotropies.Comment: LaTeX, 83 pages, 30 .pdf figures. Some typos in the units of
R-functions in appendix D and relative plots fixe
Disciplining Skepticism Through Kantâs Critique, Fichteâs Idealism, and Hegelâs Negations
This chapter considers the encounter of skepticism with the Kantian and post-Kantian philosophical enterprise and focuses on the intriguing feature whereby it is assimilated into this enterprise. In this period, skepticism becomes interchangeable with its other, which helps understand the proliferation of many kinds of views under its name and which forms the background for transforming skepticism into an anonymous, routine practice of raising objections and counter-objections to oneâs own view. German philosophers of this era counterpose skepticism to dogmatism and criticism, ancient to modern skepticism, and, importantly, conceptualize the transitions from one form to another, which forms the conceptual matrix in which new disciplinary forms, such as psychology, anthropology, and historicism contend for cultural-intellectual standing beside philosophy. I present this assimilationist trajectory by reviewing three well-known moments of this encounter of skepticism and idealism: (1) Kantâs idealization of skepticism as a floating position amidst various philosophical positions through the dialectic, polemics, systematics, and history of pure reason; (2) Fichteâs schematic conception of skepticism as a dispute of systems in the early Wissenschaftslehre following his review of the skeptic G. E. Schulzeâs attacks on Critical philosophy; (3) Hegelâs historicizing conception of skepticism in the context of differences between subjective idealism and speculative thought and his early Jena review of another work by the same skeptic Schulze
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