Reaction dynamics with exotic nuclei

Abstract

Iste magistrorum locus est simul et puerorum, mittunt quando volunt hic res quas perdere nolunt [1]. AbstractWe review the new possibilities offered by the reaction dynamics of asymmetric heavy ion collisions, using stable and unstable beams. We show that it represents a rather unique tool to probe regions of highly Asymmetric Nuclear Matter (AN M ) in compressed as well as dilute phases, and to test the in-medium isovector interaction for high momentum nucleons. The focus is on a detailed study of the symmetry term of the nuclear Equation of State (EOS) in regions far away from saturation conditions but always under laboratory controlled conditions. Thermodynamic properties of AN M are surveyed starting from nonrelativistic and relativistic effective interactions. In the relativistic case the role of the isovector scalar δ-meson is stressed. The qualitative new features of the liquid-gas phase transition, "diffusive" instability and isospin distillation, are discussed. The results of ab-initio simulations of n-rich, n-poor, heavy ion collisions, using stochastic isospin dependent transport equations, are analysed as a function of beam energy and centrality. The isospin dynamics plays an important role in all steps of the reaction, from prompt nucleon emissions to the final fragments. The isospin diffusion is also of large interest, due to the interplay of asymmetry and density gradients. In relativistic collisions, the possibility of a direct study of the covariant structure of the effective nucleon interaction is shown. Results are discussed for particle production, collective flows and iso-transparency.Perspectives of further developments of the field, in theory as well as in experiment, are presented.

Isospin fractionation: Equilibrium versus spinodal decomposition

“…It can in principle vary from zero (in the case of a purely isoscalar phase separation), to infinity (in the case of a purely isovector instability, corresponding to a separation of protons from neutrons into two phases at the same baryon density). In practice δρ 3 /δρ is always a small number [5,20]. This is easy to understand considering that i) proton and neutron kinetic energies favour equal-size Fermi spheres and ii) the nuclear interaction is more attractive in the proton-neutron than the proton-proton and neutron-neutron channels in any realistic nuclear physics model (T = 0 contribution), which favours the existence of a mixed isospin bound phase.…”

“…Dynamical transport models [5,7] indicate that clusterization takes place at a relatively well-defined density inside the spinodal region determined by the collision dynamics and corresponding to the first development of instabilities, for which we take a typical value around ρ 0 /3. Spinodal decomposition is then a phenomenon fast enough for fragment formation dynamics to be dominated by the amplification of the most unstable modes at this density [10].…”

“…Two others are used as a comparison : SGII [12], which has been shown to reproduce isospin effects studied through giant dipole resonances [13], and SIII [14], which is one of the first Skyrme parametrizations and is not constrained for asymmetric matter. Contrary to Sly230a and SGII, SIII is certainly not a realistic parameterization; we have included it in our comparison since it is still used in some transport codes because of its simplicity [5].…”

Isospin-dependent clusterization of neutron-star matter

“…In laboratory experiments, this phase diagram can be explored only through heavy-ion collisions; in particular the multifragmentation phenomenon, observed for collisions around the Fermi energy and above, has been interpreted as the occurrence of the nuclear liquid-gas phase transition rounded by finite-size effects [4,5,6,7,8]. From the theoretical point of view, the isospin properties of the phase diagram have been studied extensively [9,10,11] and we know that the phase transition concerns isospin-symmetric (ρ n = ρ p ) as well as asymmetric (ρ n = ρ p ) matter.…”

Cluster formation in asymmetric nuclear matter: Semi-classical and quantal approaches

“…In particular, the spinodal instabilities could be at the origin of the multifragmentation process [4,5,6]. Several models have been built in order to describe the dynamics leading to the formation of clusters [7,8]. Clustering is also expected to play an important role in compact stars [9,10].…”