341 research outputs found
Towards the critical behavior for the light nuclei by NIMROD detector
The critical behavior for the light nuclei with A has been
investigated experimentally by the NIMROD multi-detectors. The wide variety of
observables indicate the critical point has been reached in the disassembly of
hot nuclei at an excitation energy of 5.60.5 MeV/u.Comment: 4 pages, 2 figures; Proceeding of 18th Nuclear Physics Division
Conference of the Euro. Phys. Society (NPDC18) "Phase transitions in strongly
interacting matter", Prague, 23.8.-29.8. 2004. To be published in Nuclear
Physics
Proton-nucleus total reaction cross sections in the optical limit Glauber theory: Subtle dependence on the equation of state of nuclear matter
We calculate the proton-nucleus total reaction cross sections at different
energies of incident protons within the optical limit approximation of the
Glauber theory. The isospin effect has been taken into account. The nucleon
distribution is obtained in the framework of macroscopic nuclear models in a
way depending on the equation of state of uniform nuclear matter near the
saturation density. We find that at an energy of order 40 MeV, the reaction
cross section calculated for neutron- rich isotopes significantly increases as
the parameter L characterizing the density dependence of the symmetry energy
increases, while at energies of order 300 and 800 MeV, it is almost independent
of L. This is a feature of the optical limit Glauber theory in which an
exponential dependence of the reaction cross section on the neutron skin
thickness remains when the total proton-neutron cross section is small enough.Comment: 5 pages, 4 figure
Isocaling and the Symmetry Energy in the Multifragmentation Regime of Heavy Ion Collisions
The ratio of the symmetry energy coefficient to temperature, , in
Fermi energy heavy ion collisions, has been experimentally extracted as a
function of the fragment atomic number using isoscaling parameters and the
variance of the isotope distributions. The extracted values have been compared
to the results of calculations made with an Antisymmetrized Molecular Dynamics
(AMD) model employing a statistical decay code to account for deexcitation of
excited primary fragments. The experimental values are in good agreement with
the values calculated but are significantly different from those characterizing
the yields of the primary AMD fragments.Comment: 12 pages, 6 figure
Symmetry energy of dilute warm nuclear matter
The symmetry energy of nuclear matter is a fundamental ingredient in the
investigation of exotic nuclei, heavy-ion collisions and astrophysical
phenomena. New data from heavy-ion collisions can be used to extract the free
symmetry energy and the internal symmetry energy at subsaturation densities and
temperatures below 10 MeV. Conventional theoretical calculations of the
symmetry energy based on mean-field approaches fail to give the correct
low-temperature, low-density limit that is governed by correlations, in
particular by the appearance of bound states. A recently developed quantum
statistical (QS) approach that takes the formation of clusters into account
predicts symmetry energies that are in very good agreement with the
experimental data. A consistent description of the symmetry energy is given
that joins the correct low-density limit with quasiparticle approaches valid
near the saturation density.Comment: 4 pages, 2 figures, 1 tabl
Dissipation of angular momentum in light heavy ion collision
The inclusive energy distributions of fragments (4Z7) emitted in
the reactions O (116 MeV) + Al, Si, Ne (145 MeV) +
Al, Co have been measured in the angular range =
10 - 65. Fusion-fission and deep inelastic components of the
fragment emission have been extracted from the experimental data. The angular
mometum dissipations in fully damped deep inelastic collisions have been
estimated assming exit channel configuration similar to those for
fusion-fission process. It has been found that, the angular momentum
dissipations are more than those predicted by the empirical sticking limit in
all cases. The deviation is found to increase with increasing charge transfer
(lighter fragments). Qualitatively, this may be due to stronger friction in the
exit channel. Moreover, for the heavier system Ne + Co, the
overall magnitude of deviation is less as compared to those for the lighter
systems, {\it i.e.}, O + Al, Si, Ne + Al.
This may be due to lesser overlap in time scales of fusion and deep inelastic
time scales for heavier systems.Comment: 15 pages, 9 figures, accepted for publication in Phys. Rev.
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
Cluster emission and phase transition behaviours in nuclear disassembly
The features of the emissions of light particles (LP), charged particles
(CP), intermediate mass fragments (IMF) and the largest fragment (MAX) are
investigated for as functions of temperature and 'freeze-out'
density in the frameworks of the isospin-dependent lattice gas model and the
classical molecular dynamics model. Definite turning points for the slopes of
average multiplicity of LP, CP and IMF, and of the mean mass of the largest
fragment () are shown around a liquid-gas phase transition temperature
and while the largest variances of the distributions of LP, CP, IMF and MAX
appear there. It indicates that the cluster emission rate can be taken as a
probe of nuclear liquid--gas phase transition. Furthermore, the largest
fluctuation is simultaneously accompanied at the point of the phase transition
as can be noted by investigating both the variances of their cluster
multiplicity or mass distributions and the Campi scatter plots within the
lattice gas model and the molecular dynamics model, which is consistent with
the result of the traditional thermodynamical theory when a phase transition
occurs.Comment: replace nucl-th/0103009 due to the technique problem to access old
versio
Density determinations in heavy ion collisions
The experimental determination of freeze-out temperatures and densities from
the yields of light elements emitted in heavy ion collisions is discussed.
Results from different experimental approaches are compared with those of model
calculations carried out with and without the inclusion of medium effects.
Medium effects become of relevance for baryon densities above fm. A quantum statistical (QS) model incorporating medium
effects is in good agreement with the experimentally derived results at higher
densities. A densitometer based on calculated chemical equilibrium constants is
proposed.Comment: 5 pages, 3 figure
Measuring the Temperature of Hot Nuclear Fragments
A new thermometer based on fragment momentum fluctuations is presented. This
thermometer exhibited residual contamination from the collective motion of the
fragments along the beam axis. For this reason, the transverse direction has
been explored. Additionally, a mass dependence was observed for this
thermometer. This mass dependence may be the result of the Fermi momentum of
nucleons or the different properties of the fragments (binding energy, spin
etc..) which might be more sensitive to different densities and temperatures of
the exploding fragments. We expect some of these aspects to be smaller for
protons (and/or neutrons); consequently, the proton transverse momentum
fluctuations were used to investigate the temperature dependence of the source
Primary Isotope Yields and Characteristic Properties of the Fragmenting Source in Heavy-ion Reactions near the Fermi Energies
For central collisions of Ca Ca at 35 MeV/nucleon, the
density and temperature of a fragmenting source have been evaluated in a
self-consistent manner using the ratio of the symmetry energy coefficient
relative to the temperature, , extracted from the yields of primary
isotopes produced in antisymmetrized molecular dynamics (AMD) simulations. The
values are extracted from all isotope yields using an improved
method based on the Modified Fisher Model (MFM). The values of
obtained, using different interactions with different density dependencies of
the symmetry energy term, are correlated with the values of the symmetry
energies at the density of fragment formation. Using this correlation, the
fragment formation density is found to be . Using
the input symmetry energy value for each interaction temperature values are
extracted as a function of isotope mass . The extracted temperature values
are compared with those evaluated from the fluctuation thermometer with a
radial flow correction.Comment: 10 pages, 8 figure
- …