No abstract

We present a simple model to describe the evolution of hot and compressed nuclei. It is based on irrotational hydrodynamics and a 3-dimensional site-bond percolation model. Hydrodynamics is used if the evolution of the system is dominated by the mean field. This situation occurs when the fluctuations of the mean field are small. These latter are evaluated using the percolation model. In some cases it turns out that these fluctuations become very large and the system breaks up into several pieces (multifragmentation). The results of this process are described by the percolation model. We have obtained analytical or fitted expressions for all the results which compare well with those obtained in a previous and more involved model based on the same physical ideas. In particular we found that a noncompressed nucleus undergoes multifragmentation if the thermal excitation energy is larger than 70% of its binding energy. If the nucleus is compressed this value is notably decreased.

A hydrodynamical approach and the Thomas Fermi approximation have been used to study the evolution of hot and compressed nuclei. Spherical symmetry was assumed in the calculation. The dynamical equations have been transformed into "SchrSdinger like" equations (using the Madelung transformation) and were solved numerically. Dissipation was simulated in the same way as in the Navier-Stokes equation by introducing shear and bulk viscosities. Global as well as local thermal equilibrium have been studied. The model has been applied to small amplitude oscillations (the breathing mode) and to the stability of hot and compressed nuclei. It was found that compression is more efficient to break nuclei than thermal excitation. The relaxation time for global equilibrium was estimated to be of the order of 10 .22 s. It was found that the results obtained in the case of global and local thermal equilibrium are very similar.

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We have used a spherical time dependent Thomas Fermi model to study the expansion of hot and/or compressed nuclei. This approach was coupled to a site-bond percolation model to study the disassembly of the nucleus into many pieces (multifragmentation). We find that a non compressed nucleus undergoes multifragmentation if the thermal excitation energy is larger than 70% of its binding energy. If the nucleus is compressed this value is notably decreased. 25.70.Np; 25.70.Gh; 25.70.Jj; 05.90 + m