102 research outputs found
Isoscaling Studies of Fission - a Sensitive Probe into the Dynamics of Scission
The fragment yield ratios were investigated in the fission of 238,233U
targets induced by 14 MeV neutrons. The isoscaling behavior was typically
observed for the isotopic chains of fragments ranging from the proton-rich to
the most neutron-rich ones. The observed high sensitivity of neutron-rich heavy
fragments to the target neutron content suggests fission as a source of
neutron-rich heavy nuclei for present and future rare ion beam facilities,
allowing studies of nuclear properties towards the neutron drip-line and
investigations of the conditions for nucleosynthesis of heavy nuclei. The
breakdowns of the isoscaling behavior around N=62 and N=80 manifest the effect
of two shell closures on the dynamics of scission. The shell closure around
N=64 can be explained by the deformed shell. The investigation of isoscaling in
the spontaneous fission of 248,244Cm further supports such conclusion. The
Z-dependence of the isoscaling parameter exhibits a structure which can be
possibly related to details of scission dynamics. The fission isoscaling
studies can be a suitable tool for the investigation of possible new pathways
to synthesize still heavier nuclei.Comment: 7 pages, 3 figures, RevTex, final version, to appear in Phys. Rev. C
as a regular articl
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.
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
A new technique for elucidating -decay schemes which involve daughter nuclei with very low energy excited states
A new technique of elucidating -decay schemes of isotopes with large
density of states at low excitation energies has been developed, in which a
Broad Energy Germanium (BEGe) detector is used in conjunction with coaxial
hyper-pure germanium detectors. The power of this technique has been
demonstrated on the example of 183Hg decay. Mass-separated samples of 183Hg
were produced by a deposition of the low-energy radioactive-ion beam delivered
by the ISOLDE facility at CERN. The excellent energy resolution of the BEGe
detector allowed rays energies to be determined with a precision of a
few tens of electronvolts, which was sufficient for the analysis of the
Rydberg-Ritz combinations in the level scheme. The timestamped structure of the
data was used for unambiguous separation of rays arising from the
decay of 183Hg from those due to the daughter decays
Inhomogeneous isospin distribution in the reactions of 28Si + 112Sn and 124Sn at 30 and 50 MeV/nucleon
We have created quasiprojectiles of varying isospin via peripheral reactions
of 28Si + 112Sn and 124Sn at 30 and 50 MeV/nucleon. The quasiprojectiles have
been reconstructed from completely isotopically identified fragments. The
difference in N/Z of the reconstructed quasiprojectiles allows the
investigation of the disassembly as a function of the isospin of the
fragmenting system. The isobaric yield ratio 3H/3He depends strongly on N/Z
ratio of quasiprojectiles. The dependences of mean fragment multiplicity and
mean N/Z ratio of the fragments on N/Z ratio of the quasiprojectile are
different for light charged particles and intermediate mass fragments.
Observation of a different N/Z ratio of light charged particles and
intermediate mass fragments is consistent with an inhomogeneous distribution of
isospin in the fragmenting system.Comment: 5 pages, 4 Postscript figures, RevTe
Effect of nucleon exchange on projectile multifragmentation in the reactions of 28Si + 112Sn and 124Sn at 30 and 50 MeV/nucleon
Multifragmentation of quasiprojectiles was studied in reactions of 28Si beam
with 112Sn and 124Sn targets at projectile energies 30 and 50 MeV/nucleon. The
quasiprojectile observables were reconstructed using isotopically identified
charged particles with Z_f <= 5 detected at forward angles. The nucleon
exchange between projectile and target was investigated using isospin and
excitation energy of reconstructed quasiprojectile. For events with total
reconstructed charge equal to the charge of the beam (Z_tot = 14) the influence
of beam energy and target isospin on neutron transfer was studied in detail.
Simulations employing subsequently model of deep inelastic transfer,
statistical model of multifragmentation and software replica of FAUST detector
array were carried out. A concept of deep inelastic transfer provides good
description of production of highly excited quasiprojectiles. The isospin and
excitation energy of quasiprojectile were described with good overall
agreement. The fragment multiplicity, charge and isospin were reproduced
satisfactorily. The range of contributing impact parameters was determined
using backtracing procedure.Comment: 11 pages, 8 Postscript figures, LaTeX, to appear in Phys. Rev. C (
Dec 2000
Asymmetry Dependence of the Nuclear Caloric Curve
A basic feature of the nuclear equation of state is not yet understood: the
dependence of the nuclear caloric curve on the neutron-proton asymmetry.
Predictions of theoretical models differ on the magnitude and even the sign of
this dependence. In this work, the nuclear caloric curve is examined for fully
reconstructed quasi-projectiles around mass A=50. The caloric curve extracted
with the momentum quadrupole fluctuation thermometer shows that the temperature
varies linearly with quasi-projectile asymmetry (N-Z)/A. An increase in
asymmetry of 0.15 units corresponds to a decrease in temperature on the order
of 1 MeV. These results also highlight the importance of a full
quasi-projectile reconstruction in the study of thermodynamic properties of hot
nuclei
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