277 research outputs found
Effective nucleon mass and the nuclear caloric curve
Assuming a schematic form of the nucleon effective mass as a function of
nuclear excitation energy and mass, we provide a simple explanation for
understanding the experimentally observed mass dependence of the nuclear
caloric curve. It is observed that the excitation energy at which the caloric
curve enters into a plateau region, could be sensitive to the nuclear mass
evolution of the effective nucleon mass.Comment: 5 pages, 5 figures, Accepted for publication in Phys. Rev. C. Minor
changes mad
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
Proton induced reaction cross section measurements on Se isotopes for the astrophysical p process
As a continuation of a systematic study of reactions relevant to the
astrophysical p process, the cross sections of the 74,76Se(p,gamma)75,77Br and
82Se(p,n)82Br reactions have been measured at energies from 1.3 to 3.6 MeV
using an activation technique. The results are compared to the predictions of
Hauser-Feshbach statistical model calculations using the NON-SMOKER and MOST
codes. The sensitivity of the calculations to variations in the optical proton
potential and the nuclear level density was studied. Good agreement between
theoretical and experimental reaction rates was found for the reactions
74Se(p,gamma)75Br and 82Se(p,n)82Br.Comment: 9 pages, 6 figures (in 12 eps files), accepted for publication in
Phys. Rev C, RevTeX styl
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
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
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
Sensitivity of intermediate mass fragment flows to the symmetry energy
The NIMROD-ISiS array was used to study the transverse flow of intermediate mass fragments in 35 MeV/nucleon , , and reactions. The intermediate mass fragment flow was previously shown to be sensitive to the density dependence of the symmetry energy. To explore the model dependence of the results, the antisymmetrized molecular dynamics, constrained molecular dynamics, and stochastic mean-field models were each compared to the experimental results to extract information on the form of the symmetry energy. The results demonstrate that sensitivity of the models to the nuclear equation of state can vary significantly based on the treatment of the nuclear dynamics. Despite the differences in the sensitivity, improved agreement with the experimental data is observed for each model with a stiff density dependence of the symmetry energy
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