Emission of fragments in Ca+Ca reaction at 25 MeV/nucleon

We discuss experimental data concerning 40,48Ca+40,48Ca reactions at 25 MeV/nucleon; the 4π multi-detector Chimera has been used as detection device. Effects that can be attributed to the neutron to proton ratios (N/Z) degree of freedom have been investigated. From the analysis of experimental data it seems that the neutron richness of the interacting system plays an important role on the evolution of fusion-like sources formed in semi-central collisions. In particular, it is observed that the larger is the neutron content and the larger is the emission of heavy residues. Experimental data have been compared with CoMD-II model calculations; a moderately stiff symmetry energy should be used to reproduce satisfactorily the data. A thermodynamical analysis on fusion-like sources has been also performed. In semi-peripheral collisions, isospin diffusion signals have been found. They have been investigated by analyzing isobaric emission (7Li/7Be) of quasi-projectile sources. Experimental data indicate that an incomplete N/Z mixing is reached during the interaction phase

Strong enhancement of dynamical emission of heavy fragments in the neutron-rich 124Sn+64Ni reaction at 35A MeV

A quantitative comparison is made between the absolute cross sections associated with statistical and dynamical emission of heavy fragments in the 124Sn+64Ni and 112Sn+58Ni collisions experimentally investigated at 35A MeV beam energy using the multidetector CHIMERA. The result shows that the dynamical process is about twice as probable in the neutron-rich 124Sn+64Ni system as in the 112Sn+58Ni neutron-poor one. This unexpected and significant difference indicates that the reaction mechanism is strongly dependent on the entrance-channel isospin (N/Z) content

Time sequence and time scale of intermediate mass fragments emission

Semiperipheral collisions in the 124Sn+64Ni reaction at 35MeV/nucleon were studied using the forward part of the Charged Heavy IonMass and Energy ResolvingArray. Nearly completely determined ternary events involving projectilelike fragments (PLF), targetlike fragments (TLF), and intermediatemass fragments (IMF) were selected. A new method of studying the reaction mechanism, focusing on the analysis of the correlations between relative velocities in the IMF+PLF and IMF+TLF subsystems, is proposed. The relative velocity correlations provide information on the time sequence and time scale of the neck fragmentation processes leading to production of IMFs. It is shown that the majority of light IMFs are produced within 40–80 fm/c after the system starts to reseparate. Heavy IMFs are formed at times of about 120 fm/c or later and can be viewed as resulting from two-step (sequential) neck rupture processes