215 research outputs found
Statistical evolution of isotope composition of nuclear fragments
Calculations within the statistical multifragmentation model show that the
neutron content of intermediate mass fragments can increase in the region of
liquid-gas phase transition in finite nuclei. The model predicts also
inhomogeneous distributions of fragments and their isospin in the freeze-out
volume caused by an angular momentum and external long-range Coulomb field.
These effects can take place in peripheral nucleus-nucleus collisions at
intermediate energies and lead to neutron-rich isotopes produced in the
midrapidity kinematic region.Comment: 14 pages with 4 figures. GSI preprint, Darmstadt, 200
Determination of the freeze-out temperature by the isospin thermometer
The high-resolution spectrometer FRS at GSI Darmstadt provides the full
isotopic and kinematical identification of fragmentation residues in
relativistic heavy-ion collisions. Recent measurements of the isotopic
distribution of heavy projectile fragments led to a very surprising new
physical finding: the residue production does not lose the memory of the N/Z of
the projectile ending up in a universal de-excitation corridor; an ordering of
the residues in relation to the neutron excess of the projectile has been
observed. These unexpected features can be interpreted as a new manifestation
of multifragmentation. We have found that at the last stage of the reaction the
temperature of the big clusters subjected to evaporation is limited to a
universal value. The thermometer to measure this limiting temperature is the
neutron excess of the residues.Comment: 8 pages, 6 figures, corrected some misprints in the abstract, to be
published in "Yadernaya Fizika" as a proceeding of the "VII International
School Seminar on Heavy-Ion Phyics", Dubna (Russia), May 27 - June 1, 200
Symmetry energy and the isospin dependent equation of state
The isoscaling parameter , from the fragments produced in the
multifragmentation of Ni + Ni, Fe + Ni and
Fe + Fe reactions at 30, 40 and 47 MeV/nucleon, was compared with
that predicted by the antisymmetrized molecular dynamic (AMD) calculation based
on two different nucleon-nucleon effective forces, namely the Gogny and
Gogny-AS interaction. The results show that the data agrees better with the
choice of Gogny-AS effective interaction, resulting in a symmetry energy of
18-20 MeV. The observed value indicate that the fragments are formed at
a reduced density of 0.08 fm.Comment: 5 pages, 5 figures, Accepted for publication in Phys. Rev. C (Rapid
Communication
Direct Test of the Scalar-Vector Lorentz Structure of the Nucleon- and Antinucleon-Nucleus Potential
Quantum Hadrodynamics in mean field approximation describes the effective
nucleon-nucleus potential (about -50 MeV deep) as resulting from a strong
repulsive vector (about 400 MeV) and a strong attractive scalar (about -450
MeV) contribution. This scalar-vector Lorentz structure implies a significant
lowering of the threshold for photoproduction on a nucleus by about
850 MeV as compared to the free case since charge conjugation reverses the sign
of the vector potential contribution in the equation of motion for the
states. It also implies a certain size of the photon induced
pair creation cross section near threshold which is calculated for a
target nucleus Pb. We also indicate a measurable second signature of
the photoproduction process by estimating the increased cross
section for emission of charged pions as a consequence of
annihilation within the nucleus.Comment: 18 pages latex, 5 PS figure
Mass Parameterizations and Predictions of Isotopic Observables
We discuss the accuracy of mass models for extrapolating to very asymmetric
nuclei and the impact of such extrapolations on the predictions of isotopic
observables in multifragmentation. We obtain improved mass predictions by
incorporating measured masses and extrapolating to unmeasured masses with a
mass formula that includes surface symmetry and Coulomb terms. We find that
using accurate masses has a significant impact on the predicted isotopic
observables.Comment: 12 pages, 4 figure
Symmetry energy and the isoscaling properties of the fragments produced in Ar, Ca + Fe, Ni reactions at 25 53 MeV/nucleon
The symmetry energy and the isoscaling properties of the fragments produced
in the multifragmentation of Ar, Ca + Fe, Ni
reactions at 25 - 53 MeV/nucleon were investigated within the framework of
statistical multifragmentation model. The isoscaling parameters , from
the primary (hot) and secondary (cold) fragment yield distributions, were
studied as a function of excitation energy, isospin (neutron-to-proton
asymmetry) and fragment symmetry energy. It is observed that the isoscaling
parameter decreases with increasing excitation energy and decreasing
symmetry energy. The parameter is also observed to increase with
increasing difference in the isospin of the fragmenting system. The sequential
decay of the primary fragments into secondary fragments, when studied as a
function of excitation energy and isospin of the fragmenting system, show very
little influence on the isoscaling parameter. The symmetry energy however, has
a strong influence on the isospin properties of the hot fragments. The
experimentally observed scaling parameters can be explained by symmetry energy
that is significantly lower than that for the ground state nuclei near
saturation density. The results indicate that the properties of hot nuclei at
excitation energies, densities and isospin away from the normal ground state
nuclei could be significantly different.Comment: 14 pages, 15 figure
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
Cross-sections for nuclide production in 56Fe target irradiated by 300, 500,750, 1000, 1500, and 2600 MeV protons compared with data on hydrogen target irradiation by 300, 500, 750, 1000, and 1500 MeV/nucleon 56Fe ions
Cross-sections for radioactive nuclide production in 56Fe(p,x) reactions at
300, 500, 750, 1000, 1500, and 2600 MeV were measured using the ITEP U-10
proton accelerator. In total, 221 independent and cumulative yields of products
of half-lives from 6.6 min to 312 days have been obtained via the
direct-spectrometry method. The measured data have been compared with the
experimental data obtained elsewhere by the direct and inverse kinematics
methods and with calculations by 15 codes, namely: MCNPX (INCL, CEM2k, BERTINI,
ISABEL), LAHET (BERTINI, ISABEL), CEM03 (.01, .G1, .S1), LAQGSM03 (.01, .G1,
>.S1), CASCADE-2004, LAHETO, and BRIEFF. Most of our data are in a good
agreement with the inverse kinematics results and disprove the results of some
earlier activation measurements that were quite different from the inverse
kinematics measurements. The most significant calculation-to-experiment
differences are observed in the yields of the A<30 light nuclei, indicating
that further improvements in nuclear reaction models are needed, and pointing
out as well to a necessity of more complete measurements of such reactions.Comment: 53 pages, 9 figures, 6 tables, only pdf file, submitted to Phys. Rev.
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