74 research outputs found
Competition of fusion and quasi-fission in the reactions leading to production of the superheavy elements
The mechanism of fusion hindrance, an effect observed in the reactions of
cold, warm and hot fusion leading to production of the superheavy elements, is
investigated. A systematics of transfermium production cross sections is used
to determine fusion probabilities. Mechanism of fusion hindrance is described
as a competition of fusion and quasi-fission. Available evaporation residue
cross sections in the superheavy region are reproduced satisfactorily. Analysis
of the measured capture cross sections is performed and a sudden disappearance
of the capture cross sections is observed at low fusion probabilities. A
dependence of the fusion hindrance on the asymmetry of the projectile-target
system is investigated using the available data. The most promising pathways
for further experiments are suggested.Comment: 8 pages, 7 figures, talk presented at 7th International
School-Seminar on Heavy-Ion Physics, May 27 - June 1, 2002, Dubna, Russi
Enhanced Production of Neutron-Rich Rare Isotopes in Peripheral Collisions at Fermi Energies
A large enhancement in the production of neutron-rich projectile residues is
observed in the reactions of a 25 MeV/nucleon 86Kr beam with the neutron rich
124Sn and 64Ni targets relative to the predictions of the EPAX parametrization
of high-energy fragmentation, as well as relative to the reaction with the less
neutron-rich 112Sn target. The data demonstrate the significant effect of the
target neutron-to-proton ratio (N/Z) in peripheral collisions at Fermi
energies. A hybrid model based on a deep-inelastic transfer code (DIT) followed
by a statistical de-excitation code appears to account for part of the observed
large cross sections. The DIT simulation indicates that the production of the
neutron-rich nuclides in these reactions is associated with peripheral nucleon
exchange. In such peripheral encounters, the neutron skins of the neutron-rich
124Sn and 64Ni target nuclei may play an important role. From a practical
viewpoint, such reactions between massive neutron-rich nuclei offer a novel and
attractive synthetic avenue to access extremely neutron-rich rare isotopes
towards the neutron-drip line.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
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
Identification of medium mass (A=60-80) ejectiles from 15 MeV/nucleon peripheral heavy-ion collisions with the MAGNEX large-acceptance spectrometer
An approach to identify medium-mass ejectiles from peripheral heavy-ion
reactions in the energy region of 15 MeV/nucleon is developed for data obtained
with a large acceptance magnetic spectrometer. This spectrometer is equipped
with a focal plane multidetector, providing position, angle, energy loss and
residual energy of the ions along with measurement of the time-of-flight. Ion
trajectory reconstruction is performed at high order and ion mass is obtained
with a resolution of better than 1/150. For the unambiguous particle
identification however, the reconstruction of both the atomic number Z and the
ionic charge q of the ions is critical and it is suggested, within this work,
to be performed prior to mass identification. The new proposed method was
successfully applied to MAGNEX spectrometer data, for identifying neutron-rich
ejectiles related to multinucleon transfer generated in the 70Zn+ 64Ni
collision at 15 MeV/nucleon. This approach opens up the possibility of
employing heavy-ion reactions with medium-mass beams below the Fermi energy
(i.e., in the region 15-25 MeV/nucleon) in conjunction with large acceptance
ray tracing spectrometers, first, to study the mechanism(s) of nucleon transfer
in these reactions and, second, to produce and study very neutron-rich or even
new nuclides in previously unexplored regions of the nuclear landscape.Comment: 6 pages, 6figure
Fragment Isospin as a Probe of Heavy-Ion Collisions
Isotope ratios of fragments produced at mid-rapidity in peripheral and
central collisions of 114Cd ions with 92Mo and 98Mo target nuclei at E/A = 50
MeV are compared. Neutron-rich isotopes are preferentially produced in central
collisions as compared to peripheral collisions. The influence of the size (A),
density, N/Z, E*/A, and Eflow/A of the emitting source on the measured isotope
ratios was explored by comparison with a statistical model (SMM). The
mid-rapidity region associated with peripheral collisions does not appear to be
neutron-enriched relative to central collisions.Comment: 12 pages including figure
Isotopic composition of fragments in multifragmentation of very large nuclear systems: effects of the chemical equilibrium
Studies on the isospin of fragments resulting from the disassembly of highly
excited large thermal-like nuclear emitting sources, formed in the ^{197}Au +
^{197}Au reaction at 35 MeV/nucleon beam energy, are presented. Two different
decay systems (the quasiprojectile formed in midperipheral reactions and the
unique source coming from the incomplete fusion of projectile and target in the
most central collisions) were considered; these emitting sources have the same
initial N/Z ratio and excitation energy (E^* ~= 5--6 MeV/nucleon), but
different size. Their charge yields and isotopic content of the fragments show
different distributions. It is observed that the neutron content of
intermediate mass fragments increases with the size of the source. These
evidences are consistent with chemical equilibrium reached in the systems. This
fact is confirmed by the analysis with the statistical multifragmentation
model.Comment: 9 pages, 4 ps figure
Caloric curve of 8 GeV/c negative pion and antiproton + Au reactions
The relationship between nuclear temperature and excitation energy of hot
nuclei formed by 8 GeV/c negative pion and antiproton beams incident on 197Au
has been investigated with the ISiS 4-pidetector array at the BNL AGS
accelerator. The double-isotope-ratio technique was used to calculate the
temperature of the hot system. The two thermometers used (p/d-3He/4He) and
(d/t-3He/4He) are in agreement below E*/A ~ 7 MeV when corrected for secondary
decay. Comparison of these caloric curves to those from other experiments shows
some differences that may be attributable to instrumentation and analysis
procedures. The caloric curves from this experiment are also compared with the
predictions from the SMM multifragmentation model.Comment: 34 pages, 11 figure
The ASY-EOS experiment at GSI: investigating the symmetry energy at supra-saturation densities
The elliptic-flow ratio of neutrons with respect to protons in reactions of
neutron rich heavy-ions systems at intermediate energies has been proposed as
an observable sensitive to the strength of the symmetry term in the nuclear
Equation Of State (EOS) at supra-saturation densities. The recent results
obtained from the existing FOPI/LAND data for Au+Au collisions
at 400 MeV/nucleon in comparison with the UrQMD model allowed a first estimate
of the symmetry term of the EOS but suffer from a considerable statistical
uncertainty. In order to obtain an improved data set for Au+Au collisions and
to extend the study to other systems, a new experiment was carried out at the
GSI laboratory by the ASY-EOS collaboration in May 2011.Comment: Talk given by P. Russotto at the 11th International Conference on
Nucleus-Nucleus Collisions (NN2012), San Antonio, Texas, USA, May 27-June 1,
2012. To appear in the NN2012 Proceedings in Journal of Physics: Conference
Series (JPCS
Experimental studies of N / Z equilibration in peripheral collisions using fragment yield ratios
- âŠ