BUU dynamics of unbound spherical nuclei

Within the nuclear Boltzmann-Uehling-Uhlenbeck model, we investigate the dynamical evolution of spherical calcium and gold nuclei that have been agitated into unbound configurations by either compression or heating. Using a modified pseudo-particle method that preserves the spherical symmetry, we find that the conversion of the compressional energy into radial motion is only weakly dissipative and, remarkably, for a range of initial compressions between density doubling and tripling the nucleus expands to a quasi-stationary unstable bubble-like configuration. The same processes are also studied with the standard method of solution in which perfect symmetry is absent and it is shown that while the bubbles then clusterize into bound fragments, the qualitative character of the outcome is different and sensitive to the employed number of pseudo-particles per nucleon, a purely numerical parameter. Our studies suggest that for suitable initial compressions there exists a specific nuclear multifragmentation process in which the decompression leads to an unstable hollow configuration that subsequently clusterizes into massive fragments. (orig.)