126 research outputs found
Asymmetry Dependence of the Nuclear Caloric Curve
A basic feature of the nuclear equation of state is not yet understood: the
dependence of the nuclear caloric curve on the neutron-proton asymmetry.
Predictions of theoretical models differ on the magnitude and even the sign of
this dependence. In this work, the nuclear caloric curve is examined for fully
reconstructed quasi-projectiles around mass A=50. The caloric curve extracted
with the momentum quadrupole fluctuation thermometer shows that the temperature
varies linearly with quasi-projectile asymmetry (N-Z)/A. An increase in
asymmetry of 0.15 units corresponds to a decrease in temperature on the order
of 1 MeV. These results also highlight the importance of a full
quasi-projectile reconstruction in the study of thermodynamic properties of hot
nuclei
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
Nucleation and cluster formation in low-density nucleonic matter: A mechanism for ternary fission
Ternary fission yields in the reaction 241Pu(nth,f) are calculated using a
new model which assumes a nucleation-time moderated chemical equilibrium in the
low density matter which constitutes the neck region of the scissioning system.
The temperature, density, proton fraction and fission time required to fit the
experimental data are derived and discussed. A reasonably good fit to the
experimental data is obtained. This model provides a natural explanation for
the observed yields of heavier isotopes relative to those of the lighter
isotopes, the observation of low proton yields relative to 2H and 3H yields and
the non-observation of 3He, all features which are shared by similar thermal
neutron induced and spontaneous fissioning systems.Comment: 6 pages, 3 figure
Symmetry energy and the isoscaling properties of the fragments produced in Ar, Ca + Fe, Ni reactions at 25 53 MeV/nucleon
The symmetry energy and the isoscaling properties of the fragments produced
in the multifragmentation of Ar, Ca + Fe, Ni
reactions at 25 - 53 MeV/nucleon were investigated within the framework of
statistical multifragmentation model. The isoscaling parameters , from
the primary (hot) and secondary (cold) fragment yield distributions, were
studied as a function of excitation energy, isospin (neutron-to-proton
asymmetry) and fragment symmetry energy. It is observed that the isoscaling
parameter decreases with increasing excitation energy and decreasing
symmetry energy. The parameter is also observed to increase with
increasing difference in the isospin of the fragmenting system. The sequential
decay of the primary fragments into secondary fragments, when studied as a
function of excitation energy and isospin of the fragmenting system, show very
little influence on the isoscaling parameter. The symmetry energy however, has
a strong influence on the isospin properties of the hot fragments. The
experimentally observed scaling parameters can be explained by symmetry energy
that is significantly lower than that for the ground state nuclei near
saturation density. The results indicate that the properties of hot nuclei at
excitation energies, densities and isospin away from the normal ground state
nuclei could be significantly different.Comment: 14 pages, 15 figure
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
Nuclear expansion and symmetry energy of hot nuclei
The decrease in the symmetry energy of hot nuclei populated in Ni +
Ni, Fe + Ni and Fe + Fe reactions at beam
energies of 30, 40, and 47 MeV/nucleon, as a function of excitation energy is
studied. It is observed that this decrease is mainly a consequence of
increasing expansion or decreasing density rather than the increasing
temperature. The results are in good agreement with the recently reported
microscopic calculation based on the Thomas-Fermi approach. An empirical
relation to study the symmetry energy of finite nuclei in various mass region
is proposed.Comment: 10 pages, 2 figure
Analysis of fragment yield ratios in the nuclear phase transition
The critical phenomena of the liquid-gas phase transition has been
investigated in the reactions 78,86Kr+58,64Ni at beam energy of 35 MeV/nucleon
using the Landau free energy approach with isospin asymmetry as an order
parameter. Fits to the free energy of fragments showed three minima suggesting
the system to be in the regime of a first order phase transition. The relation
m =-{\partial}F/{\partial}H, which defines the order parameter and its
conjugate field H, has been experimentally verified from the linear dependence
of the mirror nuclei yield ratio data, on the isospin asymmetry of the source.
The slope parameter, which is a measure of the distance from a critical
temperature, showed a systematic decrease with increasing excitation energy of
the source. Within the framework of the Landau free energy approach, isoscaling
provided similar results as obtained from the analysis of mirror nuclei yield
ratio data. We show that the external field is primarily related to the minimum
of the free energy, which implies a modification of the source concentration
\Delta used in isospin studies
- …