Production of energetic particles in heavy ion collisions

We calculate the effects of Pauli-blocking in the quasideuteron model of hard photon absorption. Our approach is based upon phase-space considerations and uses Fermi-gas state densities that conserve linear momentum. The Pauli-blocking function that we obtain differs from Levinger's phenomenological exponential factor and yields nuclear photoabsorption cross sections that are in good agreement with experimental data for photon energies up to the pion threshold. The temperature dependence of the nuclear photoabsorption cross section in the quasideuteron regime is also investigated. The model that we use to describe nuclear photoabsorption is particularly suited for application to preequilibrium models of hard photon emission.

Pauli-blocking in the quasideuteron model of photoabsorption

Birth, life and death of hot nuclei

Excitation functions of flow and stopping observables for the Au+Au system at energies from 40 to 1500 MeV per nucleon are presented. The systematics were obtained by merging the results of the INDRA and FOPI experiments, both performed at the GSI facility. The connection to the nuclear equation of state is discussed.

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Systematics of stopping and flow in Au+Au collisions

Heavy ion collisions (HICs) represent a unique tool to probe the in-medium nuclear interaction in regions away from saturation. In this review we present a selection of new reaction observables in dissipative collisions particularly sensitive to the symmetry term of the nuclear equation of state (Iso-EoS). We will first discuss the isospin equilibration dynamics. At low energies this manifests via the recently observed dynamical dipole radiation, due to a collective neutron–proton oscillation with the symmetry term acting as a restoring force. At higher beam energies Iso-EoS effects will be seen in an isospin diffusion mechanism, via imbalance ratio measurements, in particular from correlations to the total kinetic energy loss. For fragmentation reactions in central events we suggest to look at the coupling between isospin distillation and radial flow. In neck fragmentation reactions important Iso-EoS information can be obtained from the fragment isospin content, velocity and alignment correlations. The high-density symmetry term can be probed from isospin effects on heavy-ion reactions at relativistic energies (few A GeV range), in particular for high transverse momentum selections of the reaction products. Rather isospin sensitive observables are proposed from nucleon/cluster emissions, collective flows and meson production. The possibility of shedding light on the controversial neutron/proton effective mass splitting in asymmetric matter is also suggested. A large symmetry repulsion at high baryon density will also lead to an 'earlier' hadron-deconfinement transition in n-rich matter. The binodal transition line of the (T, ρB) diagram is lowered to a region accessible through heavy-ion collisions in the energy range of the new planned facilities, e.g. the FAIR/NICA projects. Some observable effects of the formation of a mixed phase are suggested, in particular a neutron trapping mechanism. The dependence of the results on a suitable treatment of the isovector part of the interaction in effective QCD Lagrangian approaches is critically discussed. We stress the interest of this study in nuclear astrophysics, in particular for supernovae explosions and neutron star structure, where the knowledge of the Iso-EoS is important at low as well as at high baryon density.

https://hal.archives-ouvertes.fr/hal-00600825/document

Probing the nuclear symmetry energy with heavy-ion collisions

Events with 2, 3 and 4 heavy fragments (A≥20) detected in the reactions100Mo +100Mo at 18.7, 23.7 A·MeV and120Sn +120Sn at 18.4 A·MeV were analyzed by means of an improved version of the kinematic coincidence method. The phase-space distributions prove that 3- (and possibly 4-) body events predominantly originate from a two-step mechanism and are compatible with the hypothesis of a binary deep-inelastic interaction followed by the further fissionlike decay of one (or both) of the primary fragments. The characteristics of the fission step — mass asymmetry, relative velocity, in-plane and out-of-plane angles — have been reconstructed for the 3-body events and indications are found that nonequilibrium effects at the end of the deep-inelastic phase may influence the fissionlike decay.

Analysis of the sequential fission observed in collisions of 100 Mo + 100 Mo and 120 Sn + 120 Sn around 20 A·MeV

Hard photons in heavy-ion collisions: Direct or statistical\?

Events with 2, 3 and 4 heavy-fragments (A≧20) have been detected in the reactions100Mo+100Mo atE/A =18.7, 23.7 MeV and120Sn+120Sn atE/A=18.4 MeV. The experiments were performed with an array of 12 detectors which together covered a large fraction of the forward hemisphere and allowed a high detection efficiency for these events. Masses and energies of all fragments have been reconstructed by means of an improved version of the kinematic coincidence method. The probabilitiesP
3 andP
4 of producing 3- and 4-body events were found to depend mainly on the dissipated energy rather than on the bombarding energy, thus indicating that their origin lies more in the decay properties of the excited fragments than in the dynamics of the interaction. Emission of light particles from the composite system is shown to become more relevant with increasing bombarding energy and may explain the drop of theP
3 andP
4 curves at high energy losses. Small deviations of theP
3 andP
4 curves at 23.7A · MeV from those at lower bombarding energies were used to estimate the amount of a possible pre-equilibrium light particle emission as a function of impact parameter.

Results on two-, three-, and four-body events from the 100 Mo+ 100 Mo and 120 Sn+ 120 Sn collisions around E/A =20 MeV

The fragmentation of the system /sup 40/Ar+/sup 58/Ni into three heavy nuclei is studied at 15 MeV/nucleon bombarding energy The kinematic quantities Z,E,p of the fragments are completely measured by three large ionization chambers Employing a correlation analysis on the fragment momenta p, the fragmentation mechanism is shown to be a fast process The data are in agreement with the assumption of a prompt breakup process competing with sequential fission, the latter being strongly influenced by the Coulomb residual interaction

M. Gnirs

Papers

Analysis of the sequential fission observed in collisions of 100 Mo + 100 Mo and 120 Sn + 120 Sn around 20 A·MeV

Hard photons in heavy-ion collisions: Direct or statistical\?

Results on two-, three-, and four-body events from the 100 Mo+ 100 Mo and 120 Sn+ 120 Sn collisions around E/A =20 MeV

Fragmentation mechanism in the 40Ar+58Ni reaction.