Fragment Production Mechanisms Research Articles

Study of nucleus orientation effect in the 238U+238U multinucleon transfer reaction

The improved quantum molecular dynamics model was employed to study the 238U+238U multinucleon transfer reaction at E c.m. = 833 MeV in this work. The influence of the orientation effect on the nuclear deformation at the contact moment, the composite system lifetime, the transfer rate and the fragment production mechanism are investigated. Statistical analysis reveals that at the contact moment, the orientation of the projectile and the target have less influence on each other and retain their respective initial characteristics to some extent. For collision parameters less than 6 fm, significant differences are observed in composite system lifetime and transfer rate under different orientation configurations; however, the orientation effect gradually diminishes with increasing collision parameters. Additionally, it is found that transfer reactions dominate when the collision parameter b ≤ 6 fm, while elastic and inelastic scattering events increase rapidly as the collision parameter exceeds 6 fm. Within the range of 10 ≤ b ≤ 13 fm, the transfer probability for side–side collisions is significantly higher compared to other cases.

Superasymmetric fission of heavy nuclei induced by intermediate-energy protons

In this work we present the results for the investigation of intermediate-mass fragment (IMF) production with the proton-induced reaction at 660 MeV on 238U and 237Np target. The data were obtained with the LNR Phasotron U-400M Cyclotron at Joint Institute for Nuclear Research (JINR), Dubna, Russia. A total of 93 isotopes, in the mass range of 30 < A < 200, were unambiguously identified with high precision. The fragment production cross sections were obtained by means of the induced-activation method in an off-line analysis. Mass-yield distributions were derived from the data and compared with the results of the simulation code CRISP for multimodal fission. A discussion of the super-asymmetric fragment production mechanism is also given.

CONSTRAINING THE SYMMETRY ENERGY: A JOURNEY IN THE ISOSPIN PHYSICS FROM COULOMB BARRIER TO DECONFINEMENT

Heavy Ion Collisions (HIC) represent a unique tool to probe the in-medium nuclear interaction in regions away from saturation. In this work we present a selection of reaction observables in dissipative collisions particularly sensitive to the isovector part of the interaction, i.e.to the symmetry term of the nuclear Equation of State (EoS). At low energies the behavior of the symmetry energy around saturation influences dissipation and fragment production mechanisms. We will first discuss the recently observed Dynamical Dipole Radiation, due to a collective neutron-proton oscillation during the charge equilibration in fusion and deep-inelastic collisions. Important Iso - EOS are stressed. Reactions induced by unstable132Sn beams appear to be very promising tools to test the sub-saturation Isovector EoS. New Isospin sensitive observables are also presented for deep-inelastic, fragmentation collisions and Isospin equilibration measurements (Imbalance Ratios).The high density symmetry term can be derived from isospin effects on heavy ion reactions at relativistic energies (few AGeV range), that can even allow a "direct" study of the covariant structure of the isovector interaction in the hadron medium. Rather sensitive observables are proposed from collective flows and from pion/kaon production. The possibility of the transition to a mixed hadron-quark phase, at high baryon and isospin density, is finally suggested. Some signatures could come from an expected "neutron trapping" effect. The importance of studying violent collisions with radioactive beams from low to relativistic energies is finally stressed.

THE DYNAMICAL DIPOLE RADIATION IN DISSIPATIVE COLLISIONS WITH EXOTIC BEAMS

Heavy Ion Collisions (HIC) represent a unique tool to probe the in-medium nuclear interaction in regions away from saturation. In this work we present a selection of reaction observables in dissipative collisions particularly sensitive to the isovector part of the interaction, i.e. to the symmetry term of the nuclear Equation of State (EoS). At low energies the behavior of the symmetry energy around saturation influences dissipation and fragment production mechanisms. We will first discuss the recently observed Dynamical Dipole Radiation, due to a collective neutron-proton oscillation during the charge equilibration in fusion and deep-inelastic collisions. We will review in detail all the main properties, yield, spectrum, damping and angular distributions, revealing important isospin effects. Reactions induced by unstable132Sn beams appear to be very promising tools to test the sub-saturation Isovector EoS. Predictions are also presented for deep-inelastic and fragmentation collisions induced by neutron rich projectiles. The importance of studying violent collisions with radioactive beams at low and Fermi energies is finally stressed.

Isospin Dynamics in Heavy Ion Collisions: EoS-sensitive Observables

Heavy Ion Collisions (HIC) represent a unique tool to probe the in-medium nuclear interaction in regions away from saturation and at high nucleon momenta. In this report we present a selection of reaction observables particularly sensitive to the isovector part of the interaction, i.e. to the symmetry term of the nuclear Equation of State (EoS) At low energies the behavior of the symmetry energy around saturation influences dissipation and fragment production mechanisms. Predictions are shown for deep-inelastic and fragmentation collisions induced by neutron rich projectiles. Differential flow measurements will also shed lights on the controversial neutron/proton effective mass splitting in asymmetric matter. The high density symmetry term can be derived from isospin effects on heavy ion reactions at relativistic energies (few AGeV range), that can even allow a “direct” study of the covariant structure of the isovector interaction in the hadron medium. Rather sensitive observables are proposed from collective flows and from pion/kaon production. The possibility of the transition to a mixed hadron-quark phase, at high baryon and isospin density, is finally suggested. Some signatures could come from an expected “neutron trapping” effect.

Fragmentation path of excited nuclear systems

We perform a study of the fragmentation path of excited nuclear sources, within the framework of a stochastic mean-field approach. We consider the reaction 129Xe + 119Sn at two beam energies: 32 and 50 MeV/ A, for central collisions. It is observed that, after the compression phase the system expands towards a dilute configuration from which it may recontract or evolve into a bubble-like structure. Then fragments are formed through the development of volume and/or surface instabilities. The two possibilities co-exist at 32 MeV/ A, leading to quite different fragment partitions, while at 50 MeV/ A the hollow configuration is observed in all events. Large variances are recovered in a way fully consistent with the presence of spinodal decomposition remnants. Kinematical properties of fragments are discussed and suggested as observables very sensitive to the dominant fragment production mechanism. A larger radial collective flow is observed at 50 MeV/ A, in agreement with experiments.

Limits of complete equilibration of fragments produced in central Au on Au collisions at intermediate energies

Experimental data related to fragment production in central Au on Au collisions were analyzed in the framework of a modified statistical model which considers cluster production both prior and at the equilibrated stage. The analysis provides limits to the number of nucleons and to the temperature of the equilibrated source. The rather moderate temperatures obtained from experimental double-yield ratios of d,t,3He and 4He are in agreement with the model calculations. A phenomenological relation was established between the collective flow and the chemical temperature in these reactions. It was shown that dynamical mechanisms of fragment production, e.g. coalescence, dominate at high energies. It is demonstrated that coalescence may be consistent with chemical equilibrium between the produced fragments. The different meaning of chemical and kinetic temperatures is discussed.

Source size scaling of fragment production in projectile breakup.

Fragment production has been studied as a function of the source mass and excitation energy in peripheral collisions of $^{35}\mathrm{Cl}$+$^{197}\mathrm{Au}$ at 43 MeV/nucleon and $^{70}\mathrm{Ge}$+$^{\mathrm{nat}}\mathrm{Ti}$ at 35 MeV/nucleon. The results are compared to the Au+Au data at 600 MeV/nucleon obtained by the ALADIN Collaboration. A mass scaling, by ${A}_{\mathrm{source}}\ensuremath{\sim}35 \mathrm{and} 190$, strongly correlated to excitation energy per nucleon, is presented, suggesting a thermal fragment production mechanism. Comparisons to a standard sequential decay model and the lattice-gas model are made. Fragment emission from a hot, rotating source is unable to reproduce the experimental source size scaling.

Molecular dynamics model of heavy ion collisions at medium energies

Molecular dynamics methods for describing medium-energy heavy-ion collisions are discussed on the basis of some recent studies of fragmentation reactions by QMD and AMD. Importance of the proper treatment of initialization and the decay of primordial fragments is emphasized, and some advantageous characters of AMD are explained. Fragment production mechanisms are discussed, and exclusive study of fragment and nucleon flow is shown to give important information on EOS and in-medium two-nucleon cross section.

Mechanisms of fragment production in heavy-ion reactions at intermediate energies

Comparative analysis of three typical models of nuclear disintegration, the statistical multifragmentation model (SMM), the quantum statistical model (QSM) and a generalized evaporation model (GEM), is carried out. The thermodynamical properties of a decaying system as well as observable characteristics in heavy ion collisions predicted by the different models are discussed. It is shown that these models yield quite similar results for low charge yields at higher excitation energies (E/A>6 MeV per nucleon) and it is suggested that the coincidence measurements of the intermediate mass fragment multiplicity and the neutron and proton multiplicity (or alternatively, the total bound charge) may be very useful for deducing the decay mechanism. The GEM is shown to differ from the other models in predicting a high Z residue peak.

Energy dependence of massive-fragment multiplicity.

The production of several massive fragments in the decay of a very excited compound nucleus is studied using simultaneous and sequential mechanisms of fragment production. The calculated massive-fragment multiplicities exhibit remarkably similar behaviors as functions of the excitation energy, thus ruling out using this observable as an experimental signature of the dominant fragmentation mechanism.

Intermediate mass fragment production mechanism in relativistic 4He+Au interactions

The cascade-percolation model is proposed to interpret the experimentally observed incident energy dependence of mass yield slopes for intermediate mass fragments generated in inclusive relativistic 4He+Au reactions. A mass moment correlation analysis is performed for the more sophisticated exclusive data.

Fragment emission in proton-xenon interactions in the near-threshold regime

Abstract Energy spectra of fragments, 3≤Z≤14, from the Interaction of 1–20 GeV protons with xenon have been measured over a nearly continuous range of proton energies using an Internal gas Jet target In the AGS. Analysis of the fragment energy spectra shows the presence of two distinct fragment production mechanisms below 6 GeV, which we Identify as binary and multi-fragment break up.The relative contribution of the binary process decreases with increasing proton energy and becomes negligible above 6 GeV. The data for the multi-fragment component above 4 GeV are consistent with the formation of fragments from a supersaturated gas of nucleons.The coexistence curve and the critical temperature are approached monotonically with increasing proton energy.

Formation of large target residues in intermediate energy nuclear collisions

We have used radiochemical techniques to measure the yields, angular distributions and velocity spectra of the large ( A frag ⩾ 2 3 A tqt ) target residues from the fragmentation of 197 Au by intermediate energy 12 C, 20 Ne, 32 S, 40 Ar, 84 Kr and 139 La projectiles. The fragment moving frame angular distributions are asymmetric for the lighter projectiles (C-Ar). The fragment velocity spectra are Maxwellian for the Kr induced reactions and non-Maxwellian for the reactions induced by the lighter ions. We interpret these results in terms of a change in the dominant fragment production mechanism(s) from one(s) involving a fast non-equilibrium process for the lighter ions to a slow, equilibrium process for Kr. Comparison of the measured yields and angular distributions with calculations made using a Boltzmann transport equation with appropriate modifications for Pauli blocking, etc. show excellent agreement between data and theory.

Fragment Production Mechanism Induced by High Energy Protons

Fragment Production Mechanism Induced by High Energy Protons