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Register a new userExperimental evidence for spinodal decomposition in multifragmentation of heavy systems
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Multifragmentation of fused systems was observed for central collisions between 32 AMeV 129Xe and Sn, and 36 AMeV 155Gd and U. Previous extensive comparisons between the two systems led to the hypothesis of spinodal decomposition of finite systems as the origin of multifragmentation for incident energies around 30 AMeV. New results on velocity and charge correlations of fragments bring strong arguments in favor of this interpretation.
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Multifragmentation of a ``fused system was observed for central collisions between 32 MeV/nucleon 129Xe and natSn. Most of the resulting charged products were well identified thanks to the high performances of the INDRA 4pi array. Experimental higher-order charge correlations for fragments show a weak but non ambiguous enhancement of events with nearly equal-sized fragments. Supported by dynamical calculations in which spinodal decomposition is simulated, this observed enhancement is interpreted as a ``fossil signal of spinodal instabilities in finite nuclear systems.
Thermal multifragmentation of hot nuclei is interpreted as the nuclear liquid-fog phase transition inside the spinodal region. The experimental data for p(8.1GeV) + Au collisions are analyzed within the framework of the statistical multifragmentation model (SMM) for the events with emission of at least two IMFs. It is found that the partition of hot nuclei is specified after expansion to a volume equal to Vt = (2.6+-0.3) Vo, with Vo as the volume at normal density. However, the freeze-out volume is found to be twice as large: Vf = (5+-1) Vo.
Nuclei undergo a phase transition in nuclear reactions according to a caloric curve determined by the amount of entropy. Here, the generation of entropy is studied in relation to the size of the nuclear system.
The first experimental results of a new quantum method for calculating nuclear temperature and density of fragmenting heavy ions is presented. This method is based on fluctuations in the event quadrupole momentum and fragment multiplicity distributions of light Fermions. The cal- culated temperatures are lower than those obtained with a similar classical method. Quenching of the normalized multiplicity distributions of light fermions due to Pauli blocking is also observed. These results indicate a need for a quantum treatment when dealing with statistical properties of fragmenting heavy-ions.
Some characteristics of midvelocity emissions in semiperipheral heavy-ion collisions at Fermi energies are discussed in the framework of a multifragmenting scenario. We report on binary dissipative collisions of 93Nb + 93Nb at 38AMeV in which we measured an abundant emission of particles and fragments not originated from the usual evaporative decay of hot primary fragments. We checked the compatibility of these emissions with the multifragmentation of a source which forms in the overlap region. One can fairly well reproduce the data assuming a hot and dilute source, possibly more n-rich than the initial nuclei; the results appear to be insensitive to the source size.
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