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The dynamics of nuclear collisions

The above description of multifragmentation points to effects related to different dynamical paths followed by the system during the collision. A detailed understanding of the collision dynamics is therefore a priority. This is particularly the case for studies close to the Fermi energy where the transition between the low-energy mean field behaviour and the high energy domain where two body collisions prevail is expected to influence strongly the evolution of the system. Based on early data, dynamical models making use of BUU-LV equations have allowed the community to reach a significant understanding of the dynamical process. The more complete data collected with the new $4 \pi$ detectors now becoming available impose more stringent constraints on the model and will provide an important testing ground for our present understanding of the dynamical evolution.
 

The early phase of the collisions

    A unique probe of the Fermi energy domain is nuclear Bremsstrahlung (  MeV) which originates from the most energetic collisions between the nucleons in the reacting system. These photons probe the initial phase of the reaction and are a direct manifestation of two-body dissipation. In contrast to hadrons, photons escape the collision zone without further interaction. Tracing the Bremsstrahlung spectra as function of impact parameter and comparing with transport calculations, for beam energies around 100 MeV per nucleon an initial compression corresponding to a maximum density of 1.5 $\rho_0$ is found. Despite the low yields two-photon correlations can be measured and the results reveal that, for the larger systems, Bremsstrahlung continues to be emitted beyond the first chance collisions. This means the photons can be used as a tracer of the dynamical evolution of the system. The relation between photon production and the expansion of the system leading eventually to multifragmentation, will be explored by correlating the Bremsstrahlung and multifragmentation signals in future experimental studies.
 

A unique path towards multifragmentation?

    The latest experimental results confirm that a generalised overall binary reaction mechanism persists throughout the energy range of interest to multifragmentation studies. With increasing centrality an equilibrated central reaction volume is growing. This allows to identify and reconstruct projectile and target remnants whose sizes depend on the impact parameter of the collision and thus it appears to be possible to determine the size and the excitation energy of the reaction volume with high accuracy. An adequate investigation, both from experimental and theoretical points of view, of the collision dynamics of nuclei is strongly needed in order to understand the evolution of the multifragmentation phenomenon with impact parameter. The path leading towards a possible equilibrium state is governed by the far-from equilibrium dynamical evolution of the nuclear system, necessary to build a single excited piece of nuclear matter out of two cold colliding objects. In order to achieve a quantitative description of the equilibrium conditions the contributions to the final observables from particles emitted prior to equilibrium need to be identified and separated. Experimentally they are accessible by systematically varying the dynamical conditions by exploiting various beam energies or projectile and target sizes. Especially the impact parameter dependence of these phenomena, which is of extreme importance for a correct determination of the size and excitation of the observed piece of (equilibrated) nuclear matter, deserves further studies. A good understanding of the macroscopic, as well as microscopic nature of the interaction (for example how excitation energy and angular momentum are shared between the collision partners, what the fluctuations of such processes are, what the time scales,...) may be crucial to assess whether the values of excitation, temperature and density, which are the input for thermodynamical descriptions of multifragmentation are indeed consistent with the preceding dynamical path of the collision. Only a proper description of the dynamics as a function of impact parameter will give confidence in understanding this path.
 

Fragment production in peripheral reactions

    Signals of a transition between mean field behaviour and two-body collisions dominance may have been seen, below 100 MeV per nucleon, in the observation of significant fragment production associated with peripheral collisions. An example is shown in Figure [*]. Detailed studies of these fragments reveal that they originate from a region of velocity space intermediate between projectile and target. It is conjectured that they are produced at the contact zone between the colliding nuclei. Some of these fragments are supposed to break away very early from the system and could be associated with the formation of a neck, others are produced at a later stage from the fission of one of the nuclei but with a strong memory (angular orientation) of the collision dynamics.

 
Figure: Velocity distributions, perpendicular and parallel to the beam, of 4He (left) and 16O (right) fragments observed in peripheral collisions of Xe + Sn at 50 MeV per nucleon at GANIL.
 

The physics behind the production of these fragments (light particles are also seen) is potentially very interesting since the neck region could have quite different properties from those of the bulk. This could be an inroad for the study of nuclear matter at variable density and isospin ratios. The respective influence of surface and volume mechanical instabilities and the role of fluctuation terms can potentially be investigated. Indeed first results obtained at GANIL have shown that in fragments originating from the neck region neutrons are strongly enriched. The use of dynamical models in studying these results has met with a certain degree of success in reproducing the average characteristics of this fragment production. But many aspects are still beyond the scope of these models: fragment production, multiplicities and size, free neutron and proton densities, etc. 


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