238 research outputs found
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
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
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
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
Tracing the Evolution of Temperature in Near Fermi Energy Heavy Ion Collisions
The kinetic energy variation of emitted light clusters has been employed as a
clock to explore the time evolution of the temperature for thermalizing
composite systems produced in the reactions of 26A, 35A and 47A MeV Zn
with Ni, Mo and Au. For each system investigated, the
double isotope ratio temperature curve exhibits a high maximum apparent
temperature, in the range of 10-25 MeV, at high ejectile velocity. These
maximum values increase with increasing projectile energy and decrease with
increasing target mass. The time at which the maximum in the temperature curve
is reached ranges from 80 to 130 fm/c after contact. For each different target,
the subsequent cooling curves for all three projectile energies are quite
similar. Temperatures comparable to those of limiting temperature systematics
are reached 30 to 40 fm/c after the times corresponding to the maxima, at a
time when AMD-V transport model calculations predict entry into the final
evaporative or fragmentation stage of de-excitation of the hot composite
systems. Evidence for the establishment of thermal and chemical equilibrium is
discussed.Comment: 9 pages, 5 figure
Evidence of Critical Behavior in the Disassembly of Nuclei with A ~ 36
A wide variety of observables indicate that maximal fluctuations in the
disassembly of hot nuclei with A ~ 36 occur at an excitation energy of 5.6 +-
0.5 MeV/u and temperature of 8.3 +- 0.5 MeV. Associated with this point of
maximal fluctuations are a number of quantitative indicators of apparent
critical behavior. The associated caloric curve does not appear to show a
flattening such as that seen for heavier systems. This suggests that, in
contrast to similar signals seen for liquid-gas transitions in heavier nuclei,
the observed behavior in these very light nuclei is associated with a
transition much closer to the critical point.Comment: v2: Major changes, new model calculations, new 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
Critical Behavior in Light Nuclear Systems: Experimental Aspects
An extensive experimental survey of the features of the disassembly of a
small quasi-projectile system with 36, produced in the reactions of 47
MeV/nucleon Ar + Al, Ti and Ni, has been carried
out. Nuclei in the excitation energy range of 1-9 MeV/u have been investigated
employing a new method to reconstruct the quasi-projectile source. At an
excitation energy 5.6 MeV/nucleon many observables indicate the presence
of maximal fluctuations in the de-excitation processes. The fragment
topological structure shows that the rank sorted fragments obey Zipf's law at
the point of largest fluctuations providing another indication of a liquid gas
phase transition. The caloric curve for this system shows a monotonic increase
of temperature with excitation energy and no apparent plateau. The temperature
at the point of maximal fluctuations is MeV. Taking this
temperature as the critical temperature and employing the caloric curve
information we have extracted the critical exponents , and
from the data. Their values are also consistent with the values of the
universality class of the liquid gas phase transition. Taken together, this
body of evidence strongly suggests a phase change in an equilibrated mesoscopic
system at, or extremely close to, the critical point.Comment: Physical Review C, in press; some discussions about the validity of
excitation energy in peripheral collisions have been added; 24 pages and 32
figures; longer abstract in the preprin
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