70 research outputs found
Fission widths of hot nuclei from Langevin dynamics
Fission dynamics of excited nuclei is studied in the framework of Langevin
equation. The one body wall-and-window friction is used as the dissipative
force in the Langevin equation. In addition to the usual wall formula friction,
the chaos weighted wall formula developed earlier to account for
nonintegrability of single-particle motion within the nuclear volume is also
considered here. The fission rate calculated with the chaos weighted wall
formula is found to be faster by about a factor of two than that obtained with
the usual wall friction. The systematic dependence of fission width on
temperature and spin of the fissioning nucleus is investigated and a simple
parametric form of fission width is obtained.Comment: RevTex, 12 pages including 9 Postscript figure
Prescission neutron multiplicity and fission probability from Langevin dynamics of nuclear fission
A theoretical model of one-body nuclear friction which was developed earlier,
namely the chaos-weighted wall formula, is applied to a dynamical description
of compound nuclear decay in the framework of the Langevin equation coupled
with statistical evaporation of light particles and photons. We have used both
the usual wall formula friction and its chaos-weighted version in the Langevin
equation to calculate the fission probability and prescission neutron
multiplicity for the compound nuclei W, Pt, Pb,
Fr, Th, and Es. We have also obtained the contributions
of the presaddle and postsaddle neutrons to the total prescission multiplicity.
A detailed analysis of our results leads us to conclude that the chaos-weighted
wall formula friction can adequately describe the fission dynamics in the
presaddle region. This friction, however, turns out to be too weak to describe
the postsaddle dynamics properly. This points to the need for a suitable
explanation for the enhanced neutron emission in the postsaddle stage of
nuclear fission.Comment: RevTex, 14 pages including 5 Postscript figures, results improved by
using a different potential, conclusions remain unchanged, to appear in Phys.
Rev.
CEM2k and LAQGSM as Event Generators for Space-Radiation-Shielding and Cosmic-Ray-Propagation Applications
The CEM2k and LAQGSM codes have been recently developed at Los Alamos
National Laboratory to simulate nuclear reactions for a number of applications.
We have benchmarked our codes against most available measured data at incident
particle energies from 10 MeV to 800 GeV and have compared our results with
predictions of other current models used by the nuclear community. Here, we
present a brief description of our codes and show illustrative results to show
that CEM2k and LAQGSM can be used as reliable event generators for
space-radiation-shielding, cosmic-ray-propagation, and other astrophysical
applications. Finally, we show the use of our calculated cross sections
together with experimental data from our LANL T-16 compilation to produce
evaluated files which we use in the GALPROP model of galactic particle
propagation to better constrain the size of the CR halo.Comment: 10 pages, 9 figures, LaTeX, talk given at the World Space Congress
2002, 34th COSPAR Scientific Assembly, Houston, Texas, USA, 10-19 October
2002, to appear in Advances in Space Researc
Small damping approach in Fermi-liquid theory
The validity of small damping approximation (SDA) for the quasi-classical
description of the averaged properties of nuclei at high temperatures is
studied within the framework of collisional kinetic theory. The isoscalar
collective quadrupole vibrations in hot nuclei are considered. We show that the
extension of the SDA, by accounting for the damping of the distribution
function in the collision integral reduces the rate of variation
with temperature of the Fermi surface distortion effects. The damping of the
in the collision integral increases significantly the collisional
width of the giant quadrupole resonance (GQR) for small enough values of the
relaxation time. The temperature dependence of the eigenenergy of the GQR
becomes much more weaker than in the corresponding SDA case.Comment: 11 pages, 3 figure
Highly deformed Ca configurations in Si + C
The possible occurrence of highly deformed configurations in the Ca
di-nuclear system formed in the Si + C reaction is investigated
by analyzing the spectra of emitted light charged particles. Both inclusive and
exclusive measurements of the heavy fragments (A 10) and their
associated light charged particles (protons and particles) have been
made at the IReS Strasbourg {\sc VIVITRON} Tandem facility at bombarding
energies of Si) = 112 MeV and 180 MeV by using the {\sc ICARE}
charged particle multidetector array. The energy spectra, velocity
distributions, and both in-plane and out-of-plane angular correlations of light
charged particles are compared to statistical-model calculations using a
consistent set of parameters with spin-dependent level densities. The analysis
suggests the onset of large nuclear deformation in Ca at high spin.Comment: 33 pages, 11 figure
Collinear cluster tripartition as sequential binary fission in the U(n,f) reaction
The mechanism leading to the formation of the observed products of the
collinear cluster tripartition is carried out within the framework of the model
based on the dinuclear system concept. The yield of fission products is
calculated using the statistical model based on the driving potentials for the
fissionable system. The minima of potential energy of the decaying system
correspond to the charge numbers of the products which are produced with large
probabilities in the sequential fission (partial case of the collinear cluster
tripartition) of the compound nucleus. The realization of this mechanism
supposes the asymmetric fission channel as the first stage of sequential
mechanism. It is shown that only the use of the driving potential calculated by
the binding energies with the shell correction allows us to explain the yield
of the true ternary fission products. The theoretical model is applied to
research collinear cluster tripartition in the reaction U(n,f). Calculations showed that in the first stage of this fission reaction,
the isotopes Ge and Nd are formed with relatively large
probabilities and in the second stage of sequential fission of the isotope Nd
mainly Ni and Ge are formed. This is in agreement with the yield of the isotope
Ni which is observed as the product of the collinear cluster
tripartition in the experiment.Comment: 20 pages, 9 figure
Spallation reactions. A successful interplay between modeling and applications
The spallation reactions are a type of nuclear reaction which occur in space
by interaction of the cosmic rays with interstellar bodies. The first
spallation reactions induced with an accelerator took place in 1947 at the
Berkeley cyclotron (University of California) with 200 MeV deuterons and 400
MeV alpha beams. They highlighted the multiple emission of neutrons and charged
particles and the production of a large number of residual nuclei far different
from the target nuclei. The same year R. Serber describes the reaction in two
steps: a first and fast one with high-energy particle emission leading to an
excited remnant nucleus, and a second one, much slower, the de-excitation of
the remnant. In 2010 IAEA organized a worskhop to present the results of the
most widely used spallation codes within a benchmark of spallation models. If
one of the goals was to understand the deficiencies, if any, in each code, one
remarkable outcome points out the overall high-quality level of some models and
so the great improvements achieved since Serber. Particle transport codes can
then rely on such spallation models to treat the reactions between a light
particle and an atomic nucleus with energies spanning from few tens of MeV up
to some GeV. An overview of the spallation reactions modeling is presented in
order to point out the incomparable contribution of models based on basic
physics to numerous applications where such reactions occur. Validations or
benchmarks, which are necessary steps in the improvement process, are also
addressed, as well as the potential future domains of development. Spallation
reactions modeling is a representative case of continuous studies aiming at
understanding a reaction mechanism and which end up in a powerful tool.Comment: 59 pages, 54 figures, Revie
Fusion and Binary-Decay Mechanisms in the Cl+Mg System at E/A 8 MeV/Nucleon
Compound-nucleus fusion and binary-reaction mechanisms have been investigated
for the Cl+Mg system at an incident beam energy of E= 282
MeV. Charge distributions, inclusive energy spectra, and angular distributions
have been obtained for the evaporation residues and the binary fragments.
Angle-integrated cross sections have been determined for evaporation residues
from both the complete and incomplete fusion mechanisms. Energy spectra for
binary fragment channels near to the entrance-channel mass partition are
characterized by an inelastic contribution that is in addition to a fully
energy damped component. The fully damped component which is observed in all
the binary mass channels can be associated with decay times that are comparable
to, or longer than the rotation period. The observed mass-dependent cross
sections for the fully damped component are well reproduced by the fission
transition-state model, suggesting a fusion followed by fission origin. The
present data cannot, however, rule out the possibility that a long-lived
orbiting mechanism accounts for part or all of this yield.Comment: 41 pages standard REVTeX file, 14 Figures available upon request -
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Effect of dissipation on dynamical fusion thresholds
The existence of dynamical thresholds to fusion in heavy nuclei (A greater than or equal to 200) due to the nature of the potential-energy surface is shown. These thresholds exist even in the absence of dissipative forces, due to the coupling between the various collective deformation degrees of freedom. Using a macroscopic model of nuclear shape dynamics, It is shown how three different suggested dissipation mechanisms increase by varying amounts the excitation energy over the one-dimensional barrier required to cause compound-nucleus formation. The recently introduced surface-plus-window dissipation may give a reasonable representation of experimental data on fusion thresholds, in addition to properly describing fission-fragment kinetic energies and isoscalar giant multipole widths. Scaling of threshold results to asymmetric systems is discussed. 48 refs., 10 figs
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