Bimodalité en collisions périphériques et centrales : systématiques et comparaisons

Abstract : During the last few years, bimodality in heavy ions collisions has been observed for different systems, on large energy scale (from 35 MeV/u up to 1 GeV/u). In this thesis, the bimodal behaviour of the largest fragment distribution (Zmax) is studied for different INDRA data sets. For peripheral collisions (Au+Au from 60 to 150 MeV/u, Xe+Sn 80-100 MeV/u), the influence of sorting and selections on bimodality is tested. Then, two different approaches based on models are considered. In the first one (ELIE), bimodality would reflect mainly the collision geometry and the Fermi motion of the nucleon. In the second one (SMM), bimodality would reflect a phase transition of nuclear matter. The data are in favour of the second model. Zmax can then be considered as an order parameter of the transition. A reweighting procedure producing a flat excitation energy distribution is used to achieve comparisons between various bombarding energies and theoretical predictions based on a canonical approach. A latent heat of the transition is extracted. For central collisions (Ni+Ni from 32 to 74 MeV/u and Xe+Sn from 25 to 50 MeV/u) single source events are isolated by a Discriminant Factor Analysis. Bimodality is then looked for, in cumulating the different incident energies and in applying the reweighting procedure of the corresponding excitation energy as done for peripheral collisions. The bimodality behaviour is less evidence for central collisions than for peripheral ones. The possible reasons of this difference are discussed.
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D. Mercier. Bimodalité en collisions périphériques et centrales : systématiques et comparaisons. Physique Nucléaire Théorique [nucl-th]. Université de Caen, 2008. Français. ⟨tel-00354154⟩

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