411 research outputs found
Nuclear fragmentation by tunneling
Fragmentation of nuclear system by tunneling is discussed in a molecular
dynamics simulation coupled with imaginary time method. In this way we obtain
informations on the fragmenting systems at low densities and temperatures.
These conditions cannot be reached normally (i.e. above the barrier) in
nucleus-nucleus or nucleon-nucleus collisions. The price to pay is the small
probability of fragmentation by tunneling but we obtain observables which can
be a clear signature of such phenomena.Comment: Phys.Rev.C (submitted
Kinetic description of hadron-hadron collisions
A transport model based on the mean free path approach to describe pp
collisions is proposed. We assume that hadrons can be treated as bags of
partons similarly to the MIT bag model. When the energy density in the
collision is higher than a critical value, the bags break and partons are
liberated. The partons expand and can make coalescence to form new hadrons. The
results obtained compare very well with available data and some prediction for
higher energies collisions are discussed. Based on the model we suggest that a
QGP could already be formed in the pp collisions at high energies
Ambiguities in statistical calculations of nuclear fragmentation
The concept of freeze out volume used in many statistical approaches for
disassembly of hot nuclei leads to ambiguities. The fragmentation pattern and
the momentum distribution (temperature) of the emanated fragments are
determined by the phase space at the freeze-out volume where the interaction
among the fragments is supposedly frozen out. However, to get coherence with
the experimental momentum distribution of the charged particles, one introduces
Coulomb acceleration beyond this freeze-out. To be consistent, we investigate
the effect of the attractive nuclear force beyond this volume and find that the
possible recombination of the fragments alters the physical observables
significantly casting doubt on the consistency of the statistical model.Comment: 11 pages+3 figure
Experimental determination of the quasi-projectile mass with measured neutrons
The investigation of the isospin dependence of multifragmentation reactions
relies on precise reconstruction of the fragmenting source. The criteria used
to assign free emitted neutrons, detected with the TAMU Neutron Ball, to the
quasi-projectile source are investigated in the framework of two different
simulation codes. Overall and source-specific detection efficiencies for
multifragmentation events are found to be model independent. The equivalence of
the two different methods used to assign experimentally detected charged
particles and neutrons to the emitting source is shown. The method used
experimentally to determine quasi-projectile emitted free neutron multiplicity
is found to be reasonably accurate and sufficiently precise as to allow for the
study of well-defined quasi-projectile sources.Comment: 10 pages, 8 figures. To be submitted to Nucl. Instr. and Meth.
Constraint Molecular Dynamics approach to Fermionic systems
We propose a Constraint Molecular Dynamics model for Fermionic system. In
this approach the equations of motion of wave packets for the nuclear many-body
problem are solved by imposing that the one-body occupation probability
can assume only values less or equal to 1. This condition
reflects the Fermionic nature of the studied systems and it is implemented with
a fast algorithm which allows also the study of the heaviest colliding system.
The parameters of the model have been chosen to reproduce the average binding
energy and radii of nuclei in the mass region . Some comparison
to data is given.Comment: 11 pages and 6 figure
Constrained Molecular Dynamics Simulations of Atomic Ground-States
Constrained molecular dynamics(CoMD) model, previously introduced for nuclear
dynamics, has been extended to the atomic structure and collision calculations.
Quantum effects corresponding to the Pauli and Heisenberg principle are
enforced by constraints, in a parameter-free way. Our calculations for small
atomic system, H, He, Li, Be, F reproduce the ground-state binding energies
within 3%, compared with the results of quantum mechanical Hartree-Fock
calculations.Comment: 3 pages, 2 figure
A novel approach to Isoscaling: the role of the order parameter m = (N-Z)/A
Isoscaling is derived within a recently proposed modified Fisher model where
the free energy near the critical point is described by the Landau O(m^6)
theory. In this model m = (N-Z)/A is the order parameter, a consequence of (one
of) the symmetries of the nuclear Hamiltonian. Within this framework we show
that isoscaling depends mainly on this order parameter through the 'external
(conjugate) field' H. The external field is just given by the difference in
chemical potentials of the neutrons and protons of the two sources. To
distinguish from previously employed isoscaling relationships, this approach is
dubbed: m - scaling. We discuss the relationship between this framework and the
standard isoscaling formalism and point out some substantial differences in
interpretation of experimental results which might result. These should be
investigated further both theoretically and experimentally.Comment: 14 pages, 5 figure
The Quantum Nature of a Nuclear Phase Transition
In their ground states, atomic nuclei are quantum Fermi liquids. At finite
temperatures and low densities, these nuclei may undergo a phase change similar
to, but substantially different from, a classical liquid gas phase transition.
As in the classical case, temperature is the control parameter while density
and pressure are the conjugate variables. At variance with the classical case,
in the nucleus the difference between the proton and neutron concentrations
acts as an additional order parameter, for which the symmetry potential is the
conjugate variable. Different ratios of the neutron to proton concentrations
lead to different critical points for the phase transition. This is analogous
to the phase transitions occurring in He-He liquid mixtures. We
present experimental results which reveal the N/Z dependence of the phase
transition and discuss possible implications of these observations in terms of
the Landau Free Energy description of critical phenomena.Comment: 5 pages, 4 figure
Critical behavior of the isotope yield distributions in the Multifragmentation Regime of Heavy Ion Reactions
Isotope yields have been analyzed within the framework of a Modified Fisher
Model to study the power law yield distribution of isotopes in the
multifragmentation regime. Using the ratio of the mass dependent symmetry
energy coefficient relative to the temperature, , extracted in
previous work and that of the pairing term, , extracted from this
work, and assuming that both reflect secondary decay processes, the
experimentally observed isotope yields have been corrected for these effects.
For a given I = N - Z value, the corrected yields of isotopes relative to the
yield of show a power law distribution, , in the mass range of and the distributions are
almost identical for the different reactions studied. The observed power law
distributions change systematically when I of the isotopes changes and the
extracted value decreases from 3.9 to 1.0 as I increases from -1 to 3.
These observations are well reproduced by a simple de-excitation model, which
the power law distribution of the primary isotopes is determined to
, suggesting that the disassembling system at the
time of the fragment formation is indeed at or very near the critical point.Comment: 5 pages, 5 figure
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