480 research outputs found
Effects of nuclear molecular configurations on the astrophysical S-factor for O + O
The impact of nuclear molecular configurations on the astrophysical S-factor
for O + O is investigated within the realistic two-center shell
model based on Woods-Saxon potentials. These molecular effects refer to the
formation of a neck between the interacting nuclei and the radial dependent
collective mass parameter. It is demonstrated that the former is crucial to
explain the current experimental data with high accuracy and without any free
parameter, whilst in addition the latter predicts a pronounced maximum in the
S-factor. In contrast to very recent results by Jiang et al., the S-factor does
not decline towards extremely low values as energy decreases.Comment: In press in Physics Letters
Astrophysical S-factor for O+O within the adiabatic molecular picture
The astrophysical S-factor for O + O is investigated within the
adiabatic molecular picture. It very well explains the available experimental
data. The collective radial mass causes a pronounced resonant structure in the
S-factor excitation function, providing a motivation for measuring the O
+ O fusion cross section at deep sub-barrier energies.Comment: 5 pages, 2 figures, SOTANCP2008 Conference, Strasbourg, France, May
13-16, 2008, To appear in IJMP
A simple analytic model for astrophysical S-factors
We propose a physically transparent analytic model of astrophysical S-factors
as a function of a center-of-mass energy E of colliding nuclei (below and above
the Coulomb barrier) for non-resonant fusion reactions. For any given reaction,
the S(E)-model contains four parameters [two of which approximate the barrier
potential, U(r)]. They are easily interpolated along many reactions involving
isotopes of the same elements; they give accurate practical expressions for
S(E) with only several input parameters for many reactions. The model
reproduces the suppression of S(E) at low energies (of astrophysical
importance) due to the shape of the low-r wing of U(r). The model can be used
to reconstruct U(r) from computed or measured S(E). For illustration, we
parameterize our recent calculations of S(E) (using the Sao Paulo potential and
the barrier penetration formalism) for 946 reactions involving stable and
unstable isotopes of C, O, Ne, and Mg (with 9 parameters for all reactions
involving many isotopes of the same elements, e.g., C+O). In addition, we
analyze astrophysically important 12C+12C reaction, compare theoretical models
with experimental data, and discuss the problem of interpolating reliably known
S(E) values to low energies (E <= 2-3 MeV).Comment: 13 pages, 5 figures, Phys. Rev. C, accepte
Study of the effects of Pauli blocking and Pauli non-locality on the optical potential
Elastic scattering angular distributions for systems with reduced mass
between 3 and 34 and energies varying between 25 and 120 MeV/nucleon were
analyzed. The stable He, its exotic partner He, and the weakly bound
Li nuclei were included as projectiles in the systematics. Optical
model data analyzes were performed with an adjustable factor of normalization
included in the imaginary part of the potential. These analyzes indicated a
reduction of absorption for systems with small reduced masses that was detected
due to the refractive nature of the scattering by light systems.Comment: 16 pages, 7 figure
Suppression of complete fusion due to breakup in the reactions B + Bi
Above-barrier cross sections of -active heavy reaction products, as
well as fission, were measured for the reactions of B with
Bi. Detailed analysis showed that the heavy products include components
from incomplete fusion as well as complete fusion (CF), but fission originates
almost exclusively from CF. Compared with fusion calculations without breakup,
the CF cross sections are suppressed by 15% for B and 7% for B. A
consistent and systematic variation of the suppression of CF for reactions of
the weakly bound nuclei Li, Be, B on targets of
Pb and Bi is found as a function of the breakup threshold
energy
Relating breakup and incomplete fusion of weakly-bound nuclei through a classical trajectory model with stochastic breakup
A classical dynamical model that treats break-up stochastically is presented
for low energy reactions of weakly-bound nuclei. The three-dimensional model
allows a consistent calculation of breakup, incomplete and complete fusion
cross sections. The model is assessed by comparing the breakup observables with
CDCC quantum mechanical predictions, which are found to be in reasonable
agreement. Through the model, it is demonstrated that the breakup probability
of the projectile as a function of its distance from the target is of primary
importance for understanding complete and incomplete fusion at energies near
the Coulomb barrier.Comment: Accepted in Physical Review Letter
PREÇOS DA TERRA NO BRASIL
Este trabalho analisa o preço da terra no perÃodo 1975 a 2006, mas ocupa-se principalmente com o perÃodo mais recente, de 2000 a 2006. Para que o leitor tenha uma melhor dimensão dos preços de terras no Brasil, comparações serão realizadas com os preços de terras nos Estados Unidos em anos recentes. As principais fontes de informação são a Fundação Getúlio Vargas – FGV que fornece as informações sobre preço de venda de terras de lavouras, pastagens e preços de arrendamentos por estados, Regiões e Brasil e o Departamento de Agricultura dos Estados Unidos (USDA) que publica os preços de terras por regiões e estados. Complementarmente, são utilizadas outras fontes como a Secretaria da Agricultura e do Abastecimento do Paraná - Seab, através de seu Departamento de Economia Rural - Deral.-------------------------------------------------------------------- ----------------The paper analyses price land in Brazil during the 1975 – 2006 period. To best information, we compare prices in Brazil and in United States. We used the following information sources: Fundação Getúlio Vargas – FGV, that publishes price land and cash rents by state, region and Brazil. United States information are from United States Department of Agriculture - USDA. Another information source is Departamento de Economia Rural do estado do Paraná – Deral.Preço de Terra, Lavouras, Pastagens, Brasil, Price, Land, Crops, Pasture, Brazil, Land Economics/Use,
Disentangling effects of nuclear structure in heavy element formation
Forming the same heavy compound nucleus with different isotopes of the projectile and target elements
allows nuclear structure effects in the entrance channel (resulting in static deformation) and in the
dinuclear system to be disentangled. Using three isotopes of Ti and W, forming 232Cm, with measurement
spanning the capture barrier energies, alignment of the heavy prolate deformed nucleus is shown to be the
main reason for the broadening of the mass distribution of the quasifission fragments as the beam energy is
reduced. The complex, consistently evolving mass-angle correlations that are observed carry more
information than the integrated mass or angular distributions, and should severely test models of
quasifission
PLATYPUS: a code for fusion and breakup in reactions induced by weakly-bound nuclei within a classical trajectory model with stochastic breakup
A self-contained Fortran-90 program based on a classical trajectory model
with stochastic breakup is presented, which should be a powerful tool for
quantifying complete and incomplete fusion, and breakup in reactions induced by
weakly-bound two-body projectiles near the Coulomb barrier. The code calculates
complete and incomplete fusion cross sections and their angular momentum
distribution, as well as breakup observables (angle, kinetic energy and
relative energy distributions).Comment: Accepted in Computer Physics Communications (2011
Produtividade da agricultura: resultados para o Brasil e estados selecionados.
Este trabalho analisa a produtividade da agricultura nos anos recentes, em especial no perÃodo de 2000 a 2012. O artigo estima indicadores de produtividade não apenas para o Brasil, mas também para uma seleção de estados. Estes foram escolhidos pela sua relevância na produção nacional de grãos e carnes. Para isso, foram escolhidos os estados da Bahia, Minas Gerais, São Paulo, Paraná, Rio Grande do Sul, Mato Grosso e Goiás. Utilizou-se o conceito de produtividade total dos fatores (PTF) porque ele oferece uma definição mais abrangente de produtividade do que medidas que comparam o produto a apenas um insumo, como trabalho ou terra. Esta última forma de mensuração da produtividade é chamada produtividade parcial. A PTF considera, em sua definição, a reunião de todos os produtos das lavouras e da pecuária e os relaciona com todos os insumos usados na produção
- …