318 research outputs found
Heavy Residue Isoscaling as a Probe of the Symmetry Energy of Hot Fragments
The isoscaling properties of isotopically resolved projectile residues from
peripheral collisions of 86Kr (25 MeV/nucleon), 64Ni (25 MeV/nucleon) and 136Xe
(20 MeV/nucleon) beams on various target pairs are employed to probe the
symmetry energy coefficient of the nuclear binding energy. The present study
focuses on heavy projectile fragments produced in peripheral and semiperipheral
collisions near the onset of multifragment emission E*/A = 2-3 MeV). For these
fragments, the measured average velocities are used to extract excitation
energies. The excitation energies, in turn, are used to estimate the
temperatures of the fragmenting quasiprojectiles in the framework the Fermi gas
model. The isoscaling analysis of the fragment yields provided the isoscaling
parameters "alpha" which, in combination with temperatures and isospin
asymmetries provided the symmetry energy coefficient of the nuclear binding
energy of the hot fragmenting quasiprojectiles. The extracted values of the
symmetry energy coefficient at this excitation energy range (2-3 MeV/nucleon)
are lower than the typical liquid-drop model value ~25 MeV corresponding to
ground-state nuclei and show a monotonic decrease with increasing excitation
energy. This result is of importance in the formation of hot nuclei in
heavy-ion reactions and in hot stellar environments such as supernova.Comment: 11 pages, 9 figures, submitted to Phys. Rev.
Symmetry Energy in the Equation of State of Asymmetric Nuclear Matte
The symmetry energy is an important quantity in the equation of state of
isospin asymmetric nuclear matter. This currently unknown quantity is key to
understanding the structure of systems as diverse as the neutron-rich nuclei
and neutron stars. At TAMU, we have carried out studies, aimed at understanding
the symmetry energy, in a variety of reactions such as, the multifragmentation
of Ar, Ca + Fe, Ni and Ni, Fe +
Ni, Fe reactions at 25 - 53 AMeV, and deep-inelastic reactions of
Kr + Sn, Ni (25 AMeV), Ni + Ni,
Sn, Th, Pb (25 AMeV) and Xe + Ni,
Sn, Th, Au (20 AMeV). Here we present an overview
of some of the results obtained from these studies. The results are analyzed
within the framework of statistical and dynamical models, and have important
implications for future experiments using beams of neutron-rich nuclei.Comment: 10 pages, 4 figures, talk presented at VI Latin American Symposium on
Nuclear Physics and Application
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
Tracing the evolution of the symmetry energy of hot nuclear fragments from the compound nucleus towards multifragmentation
The evolution of the symmetry energy coefficient of the binding energy of hot
fragments with increasing excitation is explored in multifragmentation
processes following heavy-ion collisions below the Fermi energy. In this work,
high-resolution mass spectrometric data on isotopic distributions of
projectile-like fragments from collisions of 25 MeV/nucleon 86Kr and 64Ni beams
on heavy neutron-rich targets are systematically compared to calculations
involving the Statistical Multifragmentation Model. The study reveals a gradual
decrease of the symmetry energy coefficient from 25 MeV at the compound nucleus
regime (E*/A < 2 MeV) towards 15 MeV in the bulk multifragmentation regime
(E*/A > 4 MeV). The ensuing isotopic distributions of the hot fragments are
found to be very wide and extend towards the neutron drip-line. These findings
may have important implications to the composition and evolution of hot
astrophysical environments, such as core-collapse supernova.Comment: 5 pages, 4 figures, submitted to Phys. Rev.
Isotopic Scaling of Heavy Projectile Residues from the collisions of 25 MeV/nucleon 86Kr with 124Sn, 112Sn and 64Ni, 58Ni
The scaling of the yields of heavy projectile residues from the reactions of
25 MeV/nucleon 86Kr projectiles with 124Sn,112Sn and 64Ni, 58Nitargets is
studied. Isotopically resolved yield distributions of projectile fragments in
the range Z=10-36 from these reaction pairs were measured with the MARS recoil
separator in the angular range 2.7-5.3 degrees. The velocities of the residues,
monotonically decreasing with Z down to Z~26-28, are employed to characterize
the excitation energy. The yield ratios R21(N,Z) for each pair of systems are
found to exhibit isotopic scaling (isoscaling), namely, an exponential
dependence on the fragment atomic number Z and neutron number N. The isoscaling
is found to occur in the residue Z range corresponding to the maximum observed
excitation energies. The corresponding isoscaling parameters are alpha=0.43 and
beta=-0.50 for the Kr+Sn system and alpha=0.27 and beta=-0.34 for the Kr+Ni
system. For the Kr+Sn system, for which the experimental angular acceptance
range lies inside the grazing angle, isoscaling was found to occur for Z<26 and
N<34. For heavier fragments from Kr+Sn, the parameters vary monotonically,
alpha decreasing with Z and beta increasing with N. This variation is found to
be related to the evolution towards isospin equilibration and, as such, it can
serve as a tracer of the N/Z equilibration process. The present heavy-residue
data extend the observation of isotopic scaling from the intermediate mass
fragment region to the heavy-residue region. Such high-resolution mass
spectrometric data can provide important information on the role of isospin in
peripheral and mid-peripheral collisions, complementary to that accessible from
modern large-acceptance multidetector devices.Comment: 8 pages, 6 figures, submitted to Phys. Rev.
The decay time scale for highly excited nuclei as seen from asymmetrical emission of particles
A novel method was developed for the extraction of short emission times of
light particles from the projectile-like fragments in peripheral deep-inelastic
collisions in the Fermi energy domain. We have taken an advantage of the fact
that in the external Coulomb field particles are evaporated asymmetrically. It
was possible to determine the emission times in the interval 50-500 fm/c using
the backward emission anisotropy of alpha-particles relative to the largest
residue, in the reaction 28Si + 112Sn at 50 MeV/nucleon. The extracted times
are consistent with predictions based on the evaporation decay widths
calculated with the statistical evaporation model generalized for the case of
the Coulomb interaction with the target.Comment: 13 pages, 5 figures, submitted to Phys. Lett.
Timescale for equilibration of N/Z gradients in dinuclear systems
Equilibration of N/Z in binary breakup of an excited and transiently deformed
projectile-like fragment (PLF*), produced in peripheral collisions of 64Zn +
27Al, 64Zn, 209Bi at E/A = 45 MeV, is examined. The composition of emitted
light fragments (3<=Z<=6) changes with the decay angle of the PLF*. The most
neutron-rich fragments observed are associated with a small rotation angle. A
clear target dependence is observed with the largest initial N/Z correlated
with the heavy, neutron-rich target. Using the rotation angle as a clock, we
deduce that N/Z equilibration persists for times as long as 3-4 zs (1zs = 1 x
10^-21 s = 300 fm/c). The rate of N/Z equilibration is found to depend on the
initial neutron gradient within the PLF*.Comment: 6 pages, 4 figure
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