18 research outputs found
Thermal and Chemical Freeze-out in Spectator Fragmentation
Isotope temperatures from double ratios of hydrogen, helium, lithium,
beryllium, and carbon isotopic yields, and excited-state temperatures from
yield ratios of particle-unstable resonances in 4He, 5Li, and 8Be, were
determined for spectator fragmentation, following collisions of 197Au with
targets ranging from C to Au at incident energies of 600 and 1000 MeV per
nucleon. A deviation of the isotopic from the excited-state temperatures is
observed which coincides with the transition from residue formation to
multi-fragment production, suggesting a chemical freeze-out prior to thermal
freeze-out in bulk disintegrations.Comment: 14 pages, 10 figures, submitted to Phys. Rev. C, small changes as
suggested by the editors and referee
Fragmentation in Peripheral Heavy-Ion Collisions: from Neck Emission to Spectator Decays
Invariant cross sections of intermediate mass fragments in peripheral
collisions of Au on Au at incident energies between 40 and 150 AMeV have been
measured with the 4-pi multi-detector INDRA. The maximum of the fragment
production is located near mid-rapidity at the lower energies and moves
gradually towards the projectile and target rapidities as the energy is
increased. Schematic calculations within an extended Goldhaber model suggest
that the observed cross-section distributions and their evolution with energy
are predominantly the result of the clustering requirement for the emerging
fragments and of their Coulomb repulsion from the projectile and target
residues. The quantitative comparison with transverse energy spectra and
fragment charge distributions emphasizes the role of hard scattered nucleons in
the fragmentation process.Comment: 5 pages, 5 eps figures, RevTeX4, submitted to Phys. Lett.
Transition from participant to spectator fragmentation in Au+Au reaction between 60 AMeV and 150 AMeV
Using the quantum molecular dynamics approach, we analyze the results of the
recent INDRA Au+Au experiments at GSI in the energy range between 60 AMeV and
150 AMeV. It turns out that in this energy region the transition toward a
participant-spectator scenario takes place. The large Au+Au system displays in
the simulations as in the experiment simultaneously dynamical and statistical
behavior which we analyze in detail: The composition of fragments close to
midrapidity follows statistical laws and the system shows bi-modality, i.e. a
sudden transition between different fragmentation pattern as a function of the
centrality as expected for a phase transition. The fragment spectra at small
and large rapidities, on the other hand, are determined by dynamics and the
system as a whole does not come to equilibrium, an observation which is
confirmed by FOPI experiments for the same system.Comment: published versio
Neutron recognition in the LAND detector for large neutron multiplicity
The performance of the LAND neutron detector is studied. Using an
event-mixing technique based on one-neutron data obtained in the S107
experiment at the GSI laboratory, we test the efficiency of various analytic
tools used to determine the multiplicity and kinematic properties of detected
neutrons. A new algorithm developed recently for recognizing neutron showers
from spectator decays in the ALADIN experiment S254 is described in detail. Its
performance is assessed in comparison with other methods. The properties of the
observed neutron events are used to estimate the detection efficiency of LAND
in this experiment.Comment: 16 pages, 8 figure
Statistical Multifragmentation of Non-Spherical Expanding Sources in Central Heavy-Ion Collisions
We study the anisotropy effects measured with INDRA at GSI in central
collisions of Xe+Sn at 50 A.MeV and Au+Au at 60, 80, 100 A.MeV incident energy.
The microcanonical multifragmentation model with non-spherical sources is used
to simulate an incomplete shape relaxation of the multifragmenting system. This
model is employed to interpret observed anisotropic distributions in the
fragment size and mean kinetic energy. The data can be well reproduced if an
expanding prolate source aligned along the beam direction is assumed. An either
non-Hubblean or non-isotropic radial expansion is required to describe the
fragment kinetic energies and their anisotropy. The qualitative similarity of
the results for the studied reactions suggests that the concept of a
longitudinally elongated freeze-out configuration is generally applicable for
central collisions of heavy systems. The deformation decreases slightly with
increasing beam energy.Comment: 35 pages, 19 figures, submitted to Nuclear Physics
Discriminant Analysis and Secondary-Beam Charge Recognition
The discriminant-analysis method has been applied to optimize the exotic-beam
charge recognition in a projectile fragmentation experiment. The experiment was
carried out at the GSI using the fragment separator (FRS) to produce and select
the relativistic secondary beams, and the ALADIN setup to measure their
fragmentation products following collisions with Sn target nuclei. The beams of
neutron poor isotopes around 124La and 107Sn were selected to study the isospin
dependence of the limiting temperature of heavy nuclei by comparing with
results for stable 124Sn projectiles. A dedicated detector to measure the
projectile charge upstream of the reaction target was not used, and alternative
methods had to be developed. The presented method, based on the multivariate
discriminant analysis, allowed to increase the efficacy of charge recognition
up to about 90%, which was about 20% more than achieved with the simple scalar
methods.Comment: 6 pages, 7 eps figures, elsart, submitted to Nucl. Instr. and Meth.
Isospin dependent multifragmentation of relativistic projectiles
The N/Z dependence of projectile fragmentation at relativistic energies has
been studied with the ALADIN forward spectrometer at the GSI Schwerionen
Synchrotron (SIS). Stable and radioactive Sn and La beams with an incident
energy of 600 MeV per nucleon have been used in order to explore a wide range
of isotopic compositions. For the interpretation of the data, calculations with
the statistical multifragmentation model for a properly chosen ensemble of
excited sources were performed. The parameters of the ensemble, representing
the variety of excited spectator nuclei expected in a participant-spectator
scenario, are determined empirically by searching for an optimum reproduction
of the measured fragment-charge distributions and correlations. An overall very
good agreement is obtained. The possible modification of the liquid-drop
parameters of the fragment description in the hot freeze-out environment is
studied, and a significant reduction of the symmetry-term coefficient is found
necessary to reproduce the mean neutron-to-proton ratios /Z and the
isoscaling parameters of Z<=10 fragments. The calculations are, furthermore,
used to address open questions regarding the modification of the surface-term
coefficient at freeze-out, the N/Z dependence of the nuclear caloric curve, and
the isotopic evolution of the spectator system between its formation during the
initial cascade stage of the reaction and its subsequent breakup.Comment: 23 pages, 29 figures, published in Physical Review
Neutrons from projectile fragmentation at 600 MeV/nucleon
The neutron emission in projectile fragmentation at relativistic energies was
studied with the Large-Area-Neutron-Detector LAND coupled to the ALADIN forward
spectrometer at the GSI Schwerionen-Synchrotron (SIS). Stable 124Sn and
radioactive 107Sn and 124La beams with an incident energy of 600 MeV/nucleon
were used to explore the N/Z dependence of the identified neutron source. A
cluster-recognition algorithm is applied for identifying individual particles
within the hit distributions registered with LAND. The obtained momentum
distributions are extrapolated over the full phase space occupied by the
neutrons from the projectile-spectator source. The mean multiplicities of
spectator neutrons reach values of up to about 11 and depend strongly on the
isotopic composition of the projectile. An effective source temperature of T
\approx 2-5 MeV, monotonically increasing with decreasing impact parameter, is
deduced from the transverse momentum distributions. For the interpretation of
the data, calculations with the statistical multifragmentation model were
performed. The variety of excited projectile spectators assumed to decay
statistically is represented by an ensemble of excited sources with parameters
determined previously from the fragment production observed in the same
experiments. The obtained agreement is very satisfactory for more peripheral
collisions where, according to the model, neutrons are mainly emitted during
the secondary decays of excited fragments. The neutron multiplicity in more
central collisions is underestimated, indicating that other sources besides the
modeled statistical breakup contribute to the observed neutron yield. The
choice made for the symmetry-term coefficient of the liquid-drop description of
produced fragments has a weak effect on the predicted neutron multiplicities.Comment: 22 pages, 24 figures, to appear in Physical Review
Source shape determination with directional fragment-fragment velocity correlations
Correlation functions, constructed from directional projections of the
relative velocities of fragments, are used to determine the shape of the
breakup volume in coordinate space. For central collisions of 129Xe + natSn at
50 MeV per nucleon incident energy, measured with the 4pi multi-detector INDRA
at GSI, a prolate shape aligned along the beam direction with an axis ratio of
1:0.7 is deduced. The sensitivity of the method is discussed in comparison with
conventional fragment-fragment velocity correlations.Comment: 12 pages, 5 figures, subm. to Phys. Lett.