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Issue Date: 16-Feb-2015
Authors: Francalanza, Laura
Title: Exploring Central Collisions in the Reaction 58Ni+48Ca at 25 AMeV with CHIMERA
Abstract: In this PhD thesis it is presented the study concerning the competition of different reaction mechanisms in central collisions at Fermi energies, with particular interest on the onset of multifragmentation phenomena as a possible evolution of evaporative processes toward more explosive mechanisms. This study has been carried out through the analysis of the 58Ni + 48Ca reaction at 25 AMeV, performed at the Laboratori Nazionali del Sud in Catania by the ISOSPIN and NUCL-EX collaborations in 2003 by using the CHIMERA multidetector. As a main criterion for centrality selection we have chosen the event shape method, based on the construction of the kinetic energy tensor for the analysis of the configuration of the matter in the momentum space at the decay stage. It has been noticed how a cut on the shape variable "flow angle" (at 60 degrees) obtained by the diagonalization of the tensor resulted in a strong reduction of peripheral and semiperipheral less dissipative events, and in a very stringent selection of single compact sources, on an event by event basis. This latter feature has been highlighted by the analysis of other event shape variables like the sphericity and the coplanarity and their correlations. Moreover, the main properties of the reaction products were explored by using different constraints on some of the relevant observables, like mass and velocity distributions and their correlations. Much emphasis was devoted, for central collisions, to the competition between fusion-evaporation processes with subsequent identification of a heavy residue and a possible multifragmentation mechanism of a well defined (if any) transient nuclear system. A dominance of evaporative (sequential) mechanisms together with evidences of more fragmented processes has been highlighted and the two different contributions to emission have been firstly disentangled by a cut on the mass value of the heaviest fragment detected in each event (at A(1) = 40 amu) and then characterized. Dynamical evolution of the system and pre-equilibrium emission were taken into account by simulating (with BNV calculations) the reactions in the framework of transport theories. Afterwards, the de-excitation of such sources was carried out by means of the SIMON code, that followed decay of the formed nuclei via binary sequential emissions of fragments and light particles. Preliminary results show reasonable agreement with the assumption of sequential multifragmentation emission in the mass region of IMFs close to the heavy residues. Deviations from sequential processes were found for those IMFs in the region of masses intermediate between the mass of heavy residues and the mass of light IMFs. Such a region has been finally the object of a termodinamical analysis in terms of excitation energy estimation (both from calorimetric method and theoretical computations ) and apparent temperature evaluations through the isotopic ratio thermometer applied on light emitted nuclei (Li, Be and C): results are in quite in agreement with the existent systematic of excitation the energy and temperature correlation. It could be very interesting to enlarge such analysis by studying the evolution of the reaction mechanisms with increasing of the incident energies and by investigating its isospin dependence. This will be possible by exploring those variables sensitive to the symmetry energy in the comparison with other systems as 58,62Ni+40,48Ca in the reactions at 25 AMeV and 35 AMeV carried out by the collaboration.
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