Competition between dynamical and statistical particle emissions in ³⁶₁₈Ar + ⁵⁸₂₈Ni reactions. Search for a critical phenomenon signal in central collisions of ¹²⁹₅₄Xe + ^nat₅₀Sn

Nuclear reactions at the Fermi energy range provide a mean of studying nuclei interaction mechanisms, energy dissipation process and deexcitation phenomena. Heavy ion collisions offer an additional window to on the 'nuclear liquid' phase diagram, in particular its 'critical zone'. We study all these points by analysing the ³⁶₁₈Ar + ⁵⁸₂₈Ni and the ¹²⁹₅₄Xe + ^nat₅₀Sn reactions, measured with the 4π INDRA detector. We study light particles production properties from 32 MeV/u to 95 MeV/u. Inclusive measurements show that increasing the incident energies leads to a linear increase of the maximum number of Z = 1 and Z = 2 particles, whereas that of Z ≥ 3 increases weakly. The transverse energy of the Z = 1 and Z = 2 corresponds to a constant fraction of the available energy, whereas this of Z ≥ 3 decreases between 30 MeV/u and 50 MeV/u, and then converges. The same trends appear in the ¹²⁹₅₄Xe + ^nat₅₀Sn reactions, between 25 MeV/u and 50 MeV/u. The incident energy is mostly goes into producing more and more light particles. In addition, a more precise study of the reactions, selecting them according to the charge of the detected quasi-projectile residue (QPR). Whatever the incident energy is, the charge of the QPR is associated to the same mean impact parameter, and the production cross sections of QPR of a given charge also superimpose. These observations suggest a geometrical production mechanism of the QP. The transverse mean energy at a fixed parallel velocity reveals another emission source (MR) at an intermediate velocity between the remnants of the projectile (QP) and of the target (QT). The fit of two thermal surface emission (QP and QT) and one thermal volume emission (MR) on dσ/dν_z and (E-perpendicular)(ν_z) establishes that the temperature of the QP depends essentially on the impact parameter, not on the incident energy, whereas the apparent temperature of MR increases strongly with the available energy. The MR source results from the overlapping zone between the two nuclei, either from direct N-N scattering, or 'participant'-like interaction. A model of scattering between a preformed ⁴He in nuclei and a nucleon reproduces the excess of ⁴He observed behind the QP and in front of the QT. We search for a scaling invariance of the fluctuations, typical of critical phenomena, in the charge distributions of the central collisions of ¹²⁹₅₄Xe + ^nat₅₀Sn. After removing the Z = 1 and Z = 2 particles, which mostly come from pre-equilibrium emission, an intermittency signal appears in the 32 MeV/u events. Although percolation shows the expected behaviour, some doubts remain about an auto-similarity in the fragment size distributions. (author)