Isospin influence on the reaction mechanisms in the 78Kr+40Ca and 86Kr+48Ca collisions at 10 AMeV

The study of Heavy Ions Collisions constitutes a forefront area in nuclear research, the large palette of the projectiles, ranging from light ions to heavy ions, and the different bombarding energies available, allow the exploration in a wide range of densities and temperatures. These collisions allow the observation of a wide variety of reaction mechanisms, in fact a large number of theoretical and experimental studies were performed in order to investigate the properties of the fragments production processes and their dependence on the characteristics of the entrance channel. In this work of thesis, the subject experimentally investigated is the isospin influence on the reaction mechanisms in central and semi-peripheral collisions at low energy. In particular, the results of the analyses of the Fusion- Evaporation, Fission-like processes and of the break-up of the Projectile-like Fragment competitive processes in 78Kr+40Ca and 86Kr+48Ca collisions at 10 AMeV, will be presented and discussed in chapter 3. The experiment was performed in Catania, at Laboratori Nazionali del Sud. The precise measurements of the key observables, cross sections, charge distribution, angular distributions, multiplicities and velocities require a 4π solid angle coverage, high granularity and low energy threshold and for these reasons, the 4π multidetector CHIMERA was used (for the first time in the low energy regime, thanks to the implementation of its identification capabilities by means of the pulse shape discrimination on silicon detectors). The analyses of the velocity spectra and of the angular distributions of the reaction products, in the centre of mass reference frame, show a high relaxation of the degrees of freedom of a long lived source, suggesting the identification of their production mechanisms with Fusion-Fission and Quasi-Fission. It will be demonstrated that the emitted heavier fragments are produced in an evaporation process following complete and incomplete fusion of the two reaction partners, with the formation of a Compound Nucleus. The overlap with the contribution of another binary process, not completely relaxed in mass asymmetry is observed. The evaluation of the total cross section of each reaction mechanism in both systems, allowed to look for differences due to the different N/Z ratio. In particular, different weights of the contribution of the various involved processes are found, the neutron enrichment seems to discourage the formation of the composite system and to inhibit fission-like decay channel. The experimental production cross sections are compared to the theoretical prediction of different models: HIPSE followed by the statistical code GEMINI++, GEMINI++ and DNS. These models fail to fully reproduce the experimental data, probably because the contamination of a processes not fully relaxed is not accounted by them. In the above mentioned reactions we have also investigated the Projectile-Like Fragment (PLF) binary splitting into two massive fragments in non-central collisions, i.e. collisions that do not lead to a compound nucleus formation. In this analysis, first step was the development of a method capable of discriminating binary like-reaction leading to formation of the PLF from to other reaction mechanisms which populate the same region of the phase, i.e. fission of the compound nucleus, fast-fission in di-nuclear like processes. We have reconstructed the source of each combination of pair of fragments in events with 3 IMFs well identified and detected. When the reconstructed source has the primary PLF properties, charge and velocity similar to the ones of the projectile and the third fragment populates the kinematical and mass region of the TLF residue, the event is taken into account as a PLF break-up. In order to study the nature of the PLF break-up, we have looked at the angular distributions of the fragments produced in the splitting of the PLF, in the reference frame of their reconstructed source. A preference for an aligned break-up with the lighter fragment backward emitted, toward the TLF fragment, was observed. This asymmetry is a clear signature of dynamical effects, in fact in a fast non equilibrated splitting of the PLF, the preferred splitting direction indicates a memory of preceding reaction step, the binary PLF + TLF mechanism. The angular distributions were evaluated for different mass asymmetry between the 2 fragments coming from the PLF* break-up. The anisotropy observed in the angular distributions increases with the asymmetry of the splitting. Thus effect is more pronounced for the neutron rich system compared to neutron poor one. This is in agreement with other experiments, performed at higher energy and with heavier systems, from the CHIMERA collaboration, where the dynamical effect of the PLF breakup resulted to be more pronounced for the neutron-rich system.