37 research outputs found
Isobaric Yield Ratios and The Symmetry Energy In Fermi Energy Heavy Ion Reactions
The relative isobaric yields of fragments produced in a series of heavy ion
induced multifragmentation reactions have been analyzed in the framework of a
Modified Fisher Model, primarily to determine the ratio of the symmetry energy
coefficient to the temperature, , as a function of fragment mass A. The
extracted values increase from 5 to ~16 as A increases from 9 to 37. These
values have been compared to the results of calculations using the
Antisymmetrized Molecular Dynamics (AMD) model together with the statistical
decay code Gemini. The calculated ratios are in good agreement with those
extracted from the experiment. In contrast, the ratios determined from fitting
the primary fragment distributions from the AMD model calculation are ~ 4 and
show little variation with A. This observation indicates that the value of the
symmetry energy coefficient derived from final fragment observables may be
significantly different than the actual value at the time of fragment
formation. The experimentally observed pairing effect is also studied within
the same simulations. The Coulomb coefficient is also discussed.Comment: 10 pages, 12 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
The Isospin Dependence Of The Nuclear Equation Of State Near The Critical Point
We discuss experimental evidence for a nuclear phase transition driven by the
different concentration of neutrons to protons. 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 4He-3He
liquid mixtures. We present experimental results which reveal the N/A (or Z/A)
dependence of the phase transition and discuss possible implications of these
observations in terms of the Landau Free Energy description of critical
phenomena.Comment: 14 pages, 18 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
Experimental reconstruction of primary hot isotopes and characteristic properties of the fragmenting source in the heavy ion reactions near the Fermi energy
The characteristic properties of the hot nuclear matter existing at the time
of fragment formation in the multifragmentation events produced in the reaction
Zn + Sn at 40 MeV/nucleon are studied. A kinematical focusing
method is employed to determine the multiplicities of evaporated light
particles, associated with isotopically identified detected fragments. From
these data the primary isotopic yield distributions are reconstructed using a
Monte Carlo method. The reconstructed yield distributions are in good agreement
with the primary isotope distributions obtained from AMD transport model
simulations. Utilizing the reconstructed yields, power distribution, Landau
free energy, characteristic properties of the emitting source are examined. The
primary mass distributions exhibit a power law distribution with the critical
exponent, , for isotopes, but significantly deviates from
that for the lighter isotopes. Landau free energy plots show no strong
signature of the first order phase transition. Based on the Modified Fisher
Model, the ratios of the Coulomb and symmetry energy coefficients relative to
the temperature, and , are extracted as a function of A.
The extracted values are compared with results of the AMD
simulations using Gogny interactions with different density dependencies of the
symmetry energy term. The calculated values show a close relation
to the symmetry energy at the density at the time of the fragment formation.
From this relation the density of the fragmenting source is determined to be
. Using this density, the symmetry energy
coefficient and the temperature of fragmenting source are determined in a
self-consistent manner as and
MeV
Experimental Determination of In-Medium Cluster Binding Energies and Mott Points in Nuclear Matter
In medium binding energies and Mott points for , , He and
clusters in low density nuclear matter have been determined at specific
combinations of temperature and density in low density nuclear matter produced
in collisions of 47 MeV Ar and Zn projectiles with Sn
and Sn target nuclei. The experimentally derived values of the in
medium modified binding energies are in good agreement with recent theoretical
predictions based upon the implementation of Pauli blocking effects in a
quantum statistical approach.Comment: 5 pages, 3 figure
A novel determination of density, temperature and symmetry energy for nuclear multi-fragmentation through primary fragment yield reconstruction
For the first time primary hot isotope distributions are experimentally
reconstructed in intermediate heavy ion collisions and used with
antisymmetrized molecular dynamics (AMD) calculations to determine density,
temperature and symmetry energy coefficient in a self-consistent manner. A
kinematical focusing method is employed to reconstruct the primary hot fragment
yield distributions for multifragmentation events observed in the reaction
system Zn + Sn at 40 MeV/nucleon. The reconstructed yield
distributions are in good agreement with the primary isotope distributions of
AMD simulations. The experimentally extracted values of the symmetry energy
coefficient relative to the temperature, , are compared with those
of the AMD simulations with different density dependence of the symmetry energy
term. The calculated values changes according to the different
interactions. By comparison of the experimental values of with
those of calculations, the density of the source at fragment formation was
determined to be . Using this density, the
symmetry energy coefficient and the temperature are determined in a
self-consistent manner as and
Me
Chemical potential and symmetry energy for intermediate-mass fragment production in heavy ion reactions near the Fermi energy
Ratios of differential chemical potential values relative to the temperature, ({\ensuremath{\mu}}_{n}\ensuremath{-}{\ensuremath{\mu}}_{p})/T, extracted from isotope yields of 13 reaction systems at 40 MeV/nucleon are compared to those of a quantum statistical model to determine the temperature and symmetry energy values of the fragmenting system. The experimental ({\ensuremath{\mu}}_{n}\ensuremath{-}{\ensuremath{\mu}}_{p})/T values are extracted based on the modified Fisher model. Using the density value of \ensuremath{\rho}/{\ensuremath{\rho}}_{0}=0.56 from the previous analysis, the temperature and symmetry energy values of T=4.6\ifmmode\pm\else\textpm\fi{}0.4 MeV and {a}_{\mathrm{sym}}=23.6\ifmmode\pm\else\textpm\fi{}2.1 MeV are extracted in a framework of a quantum statistical model. These values agree well with those of the previous work, in which a self-consistent method was utilized with antisymmetrized molecular dynamics simulations. The extracted temperature and symmetry energies are discussed together with other experimental values published in literature
The Nuclear Matter Symmetry Energy at
Measurements of the density dependence of the free symmetry energy in low
density clustered matter have been extended using the NIMROD multi-detector at
Texas A&M University. Thermal coalescence models were employed to extract
densities, , and temperatures, , for evolving systems formed in
collisions of 47 MeV Ar + Sn,Sn and Zn +
Sn, Sn. Densities of and
temperatures in the range 5 to 10 MeV have been sampled. The free symmetry
energy coefficients are found to be in good agreement with values calculated
using a quantum statistical model. Values of the corresponding symmetry energy
coefficient are derived from the data using entropies derived from the model.Comment: 6 pages, 6 figure