91 research outputs found
Statistical fluctuations for the fission process on its decent from saddle to scission
We reconsider the importance of statistical fluctuations for fission dynamics
beyond the saddle in the light of recent evaluations of transport coefficients
for average motion. The size of these fluctuations are estimated by means of
the Kramers-Ingold solution for the inverted oscillator, which allows for an
inclusion of quantum effects.Comment: 12 pages, Latex, 5 Postscript figures; submitted to PRC e-mail:
[email protected] www home page:
http://www.physik.tu-muenchen.de/tumphy/e/T36/hofmann.htm
Antisymmetrized molecular dynamics with quantum branching processes for collisions of heavy nuclei
Antisymmetrized molecular dynamics (AMD) with quantum branching processes is
reformulated so that it can be applicable to the collisions of heavy nuclei
such as Au + Au multifragmentation reactions. The quantum branching process due
to the wave packet diffusion effect is treated as a random term in a
Langevin-type equation of motion, whose numerical treatment is much easier than
the method of the previous papers. Furthermore a new approximation formula,
called the triple-loop approximation, is introduced in order to evaluate the
Hamiltonian in the equation of motion with much less computation time than the
exact calculation. A calculation is performed for the Au + Au central
collisions at 150 MeV/nucleon. The result shows that AMD almost reproduces the
copious fragment formation in this reaction.Comment: 24 pages, 5 figures embedde
Spinodal decomposition of expanding nuclear matter and multifragmentation
Density fluctuations of expanding nuclear matter are studied within a
mean-field model in which fluctuations are generated by an external stochastic
field. Fluctuations develop about a mean one-body phase-space density
corresponding to a hydrodinamic motion that describes a slow expansion of the
system. A fluctuation-dissipation relation suitable for a uniformly expanding
medium is obtained and used to constrain the strength of the stochastic field.
The distribution of the liquid domains in the spinodal decomposition is
derived. Comparison of the related distribution of the fragment size with
experimental data on the nuclear multifragmentation is quite satisfactory.Comment: 19 RevTex4 pages, 6 eps figures, to appear in Phys. Rev.
Nuclear fission: The "onset of dissipation" from a microscopic point of view
Semi-analytical expressions are suggested for the temperature dependence of
those combinations of transport coefficients which govern the fission process.
This is based on experience with numerical calculations within the linear
response approach and the locally harmonic approximation. A reduced version of
the latter is seen to comply with Kramers' simplified picture of fission. It is
argued that for variable inertia his formula has to be generalized, as already
required by the need that for overdamped motion the inertia must not appear at
all. This situation may already occur above T=2 MeV, where the rate is
determined by the Smoluchowski equation. Consequently, comparison with
experimental results do not give information on the effective damping rate, as
often claimed, but on a special combination of local stiffnesses and the
friction coefficient calculated at the barrier.Comment: 31 pages, LaTex, 9 postscript figures; final, more concise version,
accepted for publication in PRC, with new arguments about the T-dependence of
the inertia; e-mail: [email protected]
Nuclear Multifragmentation in the Non-extensive Statistics - Canonical Formulation
We apply the canonical quantum statistical model of nuclear
multifragmentation generalized in the framework of recently proposed Tsallis
non-extensive thermostatistics for the description of nuclear
multifragmentation process. The test calculation in the system with A=197
nucleons show strong modification of the 'critical' behaviour associated with
the nuclear liquid-gas phase transition for small deviations from the
conventional Boltzmann-Gibbs statistical mechanics.Comment: 4 pages, 4 figure
Compatibility of localized wave packets and unrestricted single particle dynamics for cluster formation in nuclear collisions
Antisymmetrized molecular dynamics with quantum branching is generalized so
as to allow finite time duration of the unrestricted coherent mean field
propagation which is followed by the decoherence into wave packets. In this new
model, the wave packet shrinking by the mean field propagation is respected as
well as the diffusion, so that it predicts a one-body dynamics similar to that
in mean field models. The shrinking effect is expected to change the diffusion
property of nucleons in nuclear matter and the global one-body dynamics. The
central \xenon+\tin collisions at 50 MeV/nucleon are calculated by the models
with and without shrinking, and it is shown that the inclusion of the wave
packet shrinking has a large effect on the multifragmentation in a big
expanding system with a moderate expansion velocity.Comment: 16 pages, 7 figure
Low density instability in a nuclear Fermi liquid drop
The instability of a Fermi-liquid drop with respect to bulk density
distortions is considered. It is shown that the presence of the surface
strongly reduces the growth rate of the bulk instability of the finite
Fermi-liquid drop because of the anomalous dispersion term in the dispersion
relation. The instability growth rate is reduced due to the Fermi surface
distortions and the relaxation processes. The dependence of the bulk
instability on the multipolarity of the particle density fluctuations is
demonstrated for two nuclei and .Comment: 12 pages, latex, 3 ps-figures, submitted to Phys. Rev.
Sound modes in hot nuclear matter
The propagation of the isoscalar and isovector sound modes in a hot nuclear
matter is considered. The approach is based on the collisional kinetic theory
and takes into account the temperature and memory effects. It is shown that the
sound velocity and the attenuation coefficient are significantly influenced by
the Fermi surface distortion (FSD). The corresponding influence is much
stronger for the isoscalar mode than for the isovector one. The memory effects
cause a non-monotonous behavior of the attenuation coefficient as a function of
the relaxation time leading to a zero-to-first sound transition with increasing
temperature. The mixing of both the isoscalar and the isovector sound modes in
an asymmetric nuclear matter is evaluated. The condition for the bulk
instability and the instability growth rate in the presence of the memory
effects is studied. It is shown that both the FSD and the relaxation processes
lead to a shift of the maximum of the instability growth rate to the longer
wave length region.Comment: 15 pages, 4 figures, submitted to Phys. Rev.
Memory effects on descent from nuclear fission barrier
Non-Markovian transport equations for nuclear large amplitude motion are
derived from the collisional kinetic equation. The memory effects are caused by
the Fermi surface distortions and depend on the relaxation time. It is shown
that the nuclear collective motion and the nuclear fission are influenced
strongly by the memory effects at the relaxation time . In particular, the descent of the nucleus from the fission
barrier is accompanied by characteristic shape oscillations. The eigenfrequency
and the damping of the shape oscillations depend on the contribution of the
memory integral in the equations of motion. The shape oscillations disappear at
the short relaxation time regime at , which corresponds to the
usual Markovian motion in the presence of friction forces. We show that the
elastic forces produced by the memory integral lead to a significant delay for
the descent of the nucleus from the barrier. Numerical calculations for the
nucleus U shows that due to the memory effect the saddle-to-scission
time grows by a factor of about 3 with respect to the corresponding
saddle-to-scission time obtained in liquid drop model calculations with
friction forces.Comment: 22 pages, 8 figures, submitted to Phys. Rev.
Molecular Dynamics for Fermions
The time-dependent variational principle for many-body trial states is used
to discuss the relation between the approaches of different molecular dynamics
models to describe indistinguishable fermions. Early attempts to include
effects of the Pauli principle by means of nonlocal potentials as well as more
recent models which work with antisymmetrized many-body states are reviewed
under these premises.
Keywords: Many-body theory; Fermion system; Molecular dynamics; Wave-packet
dynamics; Time-dependent variational principle; Statistical properties;
Canonical ensemble; Ergodicity; Time averagingComment: 97 pages, 13 postscript figures. To be published in July 2000 issue
of Reviews of Modern Physics. More information at http://www-aix.gsi.de/~fmd
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