E. Geraci

Decay competition in IMF production in the collisions 78 Kr+ 40 Ca and 86 Kr+ 48 Ca at 10 AMeV

De-excitation modes of compound systems 118 Ba and 134 Ba, produced respectively in the 78 Kr+ 40 Ca and 86 Kr+ 48 Ca collisions at 10 A.MeV, are investigated. In particular, the competition between the various disintegration decay paths of medium mass compound nuclei, formed by fusion processes and the isospin (related to N/Z) of the entrance channel influence on the decay process, are studied. Data were taken at the INFN-Laboratori Nazionali del Sud (LNS) in ISODEC experiment, by using the CHIMERA array. The experiment complements and improves the previous experiment performed at GANIL where the same mechanisms were studied at lower excitation energies. The results show the presence of a relaxed component in the reaction mechanism, evident staggering effects in the Z distributions, as well as different isotopic composition and neutron enrichment for the reaction products in the two systems.

Entrance channel effects in medium mass nuclear systems at 25 MeV/nucleon

We discuss experimental data concerning 48Ca+48Ca and 42Ca+54Fe reactions at 25 MeV/nucleon obtained by using the 4π multi-detector Chimera. Effects due due to the different neutron to proton ratios (N/Z) of the two systems have been investigated. Charge distributions of forwardly emitted fragments show even-odd staggering whose amplitude is larger for the neutron poor system. Moreover, the neutron excess of the total system influence the evolution of fusion-like sources formed in semi-central collisions. In particular, for the neutron rich system, the relative emission yield of heavy residues is larger, while multi-fragmentation and binary-like events prevail in the neutron poor system.

Search for α-particle condensation in nuclei from the Hoyle state deexcitation

Heavy ion reactions induced by few tens MeV/nucleon beams constitute an ideal framework for producing and studying nuclear excited states close to the decay threshold. The fragmentation of quasi-projectiles from the nuclear reaction 40Ca+12C at 25 MeV/nucleon performed at LNS-Catania was employed in order to populate the Hoyle state of 12C and investigate the theoretically predicted molecular structures. Complete kinematic characterization of individual decay events, made possible by the CHIMERA high-granularity 4π charged particle multi-detector, reveals that 7.5±4.0% of the particle decays of the Hoyle state correspond to direct decays in three equal-energy α-particles and thus fulfill the decay criteria of an α-particle condensate. Moreover, events with increased kinetic energy dispersion in the 12C center of mass, which amount to 9.5±4.0%, point toward the occurrence of a second competing molecular configuration, a linear α-chain type. Both α-particle condensate and linear chain structures have been theoretically predicted but, to our knowledge, not experimentally confirmed so far.

Competition between fusion-evaporation and multifragmentation in central collisions in 58Ni + 48Ca reaction at 25 AMeV

The experimental data concerning the 58Ni+48Ca reaction at Elab(Ni)=25A MeV, collected by the CHIMERA 4π device, have been analyzed in order to investigate the competition among different reaction mechanisms for central collisions in the Fermi energy domain. The method adopted to select such central collisions, by means of construction of the kinetic energy tensor and, consequently, the evaluation of the flow angle event by event is presented. Some global variables, able to characterize the pattern of central collisions, have been constructed, and the main features of the reaction products are analyzed in terms of some observable, like mass and velocity distributions, as well as their correlations. Much emphasis was devoted to the competition between fusion-evaporation processes with consequent identification of a heavy residue and multifragmentation of a well defined (if any) transient nuclear system formed in central collisions.

N/Z effects on dynamics of Ca+Ca collisions at 25 MeV/nucleon

Semi-central events of collisions 40,48Ca +40,48 Ca at 25 MeV/nucleon have been investigated by means of the Chimera multi-detector array. We find that the competition between evaporation residue and two or more fragment emission is regulated in a strong way by the neutron to proton ratio of the total reaction system. Comparisons of experimental data and Constrained Molecular Dynamics model calculations allow to obtain information about the behavior of the symmetry energy of the nuclear Equation of State.

Experimental evidence for alpha-particle condensation from the Hoyle state deexcitation

Fragmentation of quasi-projectiles from the nuclear reaction 40Ca+12C was used to produce excited states candidates to alpha-particle condensation. The methodology relies on high granularity 4π detection coupled to multi-particle correlations. Complete kinematic characterization reveals that 7.5±4.0% of the Hoyle state particle decays correspond to direct decay into three equal-energy alpha-particles. This is the first direct evidence for alpha-particle condensation in nuclei.