5,478 research outputs found
Light radioactive nuclei capture reactions with phenomenological potential models
Light radioactive nuclei play an important role in many astrophysical
environments. Due to very low cross sections of some neutron and proton capture
reactions by these radioactive nuclei at energies of astrophysical interest,
direct laboratory measurements are very difficult. For radioactive nuclei such
as 8Li and 8B, the direct measurement of neutron capture reactions is
impossible. Indirect methods have been applied to overcome these difficulties.
In this work we will report on the results and discussion of phenomenological
potential models used to determine some proton and neutron capture reactions.
As a test we show the results for the 16O(p,g)17F_gs(5/2+) and
16O(p,g)17F_ex(1/2+) capture reactions.We also computed the nucleosynthesis
cross sections for the 7Li(n,g)8Li, 8Li(n,g)9Li and 8B(p,g)9C capture
reactions.Comment: Invited talk at Nuclear Physics Meeting in Brazil (2009). To be
published at AIP conference proceeding
Improved decision for a resource-efficient fusion scheme in cooperative spectrum sensing
Paper presented at at 2015 International Workshop on Telecommunications (IWT), 14th to 17th of June, Santa Rita do Sapucai, Brazil. Abstract Recently, a novel decision fusion scheme for cooperative spectrum sensing was proposed, aiming at saving resources in the reporting channel transmissions. Secondary users are allowed to report their local decisions through the symbols of binary modulations, at the same time and with the same carrier frequencies. As a consequence, the transmitted symbols add incoherently at the fusion center, forming a larger set of symbols in which a subset is associated to the presence of the primary signal, and another subset is associated to the absence of such a signal. A Bayesian decision criterion with uniform prior was applied for discriminating these subsets. In this paper we propose a modified decision rule in which the target probabilities of detection and false alarm are taken into account to produce a large performance improvement over the original decision criterion. This improvement comes with practically no cost in complexity and does not demand the knowledge of any additional information when compared to the original rule
Remarks on some vacuum solutions of scalar-tensor cosmological models
We present a class of exact vacuum solutions corresponding to de Sitter and
warm inflation models in the framework of scalar-tensor cosmologies. We show
that in both cases the field equations reduce to planar dynamical systems with
constraints. Then, we carry out a qualitative analysis of the models by
examining the phase diagrams of the solutions near the equilibrium points.Comment: 12 pages, 4 figures. To be published in the Brazilian Journal of
Physic
Scalar and Spinor Particles in the Spacetime of a Domain Wall in String Theory
We consider scalar and spinor particles in the spacetime of a domain wall in
the context of low energy effective string theories, such as the generalized
scalar-tensor gravity theories. This class of theories allows for an arbitrary
coupling of the wall and the (gravitational) scalar field. First, we derive the
metric of a wall in the weak-field approximation and we show that it depends on
the wall's surface energy density and on two post-Newtonian parameters. Then,
we solve the Klein-Gordon and the Dirac equations in this spacetime. We obtain
the spectrum of energy eigenvalues and the current density in the scalar and
spinor cases, respectively. We show that these quantities, except in the case
of the energy spectrum for a massless spinor particle, depend on the parameters
that characterize the scalar-tensor domain wall.Comment: LATEX file, 21 pages, revised version to appear in Phys. Rev.
Semiclassical gravitational effects near a singular magnetic flux
We consider the backreaction of the vacuum polarization effect for a massive
charged scalar field in the presence of a singular magnetic massless string on
the background metric. Using semiclassical approach, we find the first-order
(in units) metric modifications and the corresponding gravitational
potential and deficit angle. It is shown that, in certain region of values of
coupling constant and magnetic flux, the gravitational potential and deficit
angle can be positive as well as negative over all distances from the string
and can even change its sign. Unlike the case of massless scalar field, the
gravitational corrections were found to have short-range behavior.Comment: 14 pages, 4 figures, journal versio
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