BUU DYNAMICS OF UNBOUND SPHERICAL NUCLEI

Within the nuclear Boltzmann-Uhling-Uhlenbeck model, we investigate th e dynamical evolution of spherical calcium and gold nuclei that have b een agitated into unbound configurations by either compression or heat ing. Using a modified pseudo-particle method that preserves the spheri cal symmetry, we find that the conversion of the compressional energy into radial motion is only weakly dissipative and, remarkably, for a r ange of initial compressions between density doubling and tripling the nucleus expands to a quasi-stationary unstable bubble-like configurat ion. The same processes are also studied with the standard method of s olution in which perfect symmetry is absent and it is shown that while the bubbles then cluserize into bound fragments, the qualitative char acter of the outcome is different and sensitive to the employed number of pseudo-particles per nucleon, a purely numerical parameter. Our st udies suggest that for suitable initial compressions there exists a sp ecific nuclear multifragmentation process in which the decompression l eads to an unstable hollow configuration that subsequently clusterizes into massive fragments.