GSI-Nachrichten 1/99 en

Simulations in Fermionic Molecular Dynamics simulations show, however, that even small systems exhibit all features of a phase transition, provided the total energy of the system can be determined well enough for each member of the statistical ensemble. The results are compared to the caloric curve deduced for finite nuclei by the ALADIN collaboration. The investigation of the equation of state of nuclear matter, in particular the search for the long predicted liquid-gas phase transition is an experimentally tough problem. Besides the fact that one can manipulate only finite nuclei, the measured information on the system is rather indirect. The difference to macro–physics is not only the smallness of the system, but also that one cannot control the thermodynamic quantities volume or pressure. TIn an experiment one is colliding two nuclei in order to produce excitation energy and compression. But as there is no container, the system begins to expand into the vacuum right after the compression and heating phase. Therefore, one is all the time in a transient state where equilibrium in its original meaning, i.e. a time–independent stationary macro-state, is not reached. The challenge to study the nuclear equation of state has been accepted not only for astrophysical reasons, like a deeper understanding of supernova explosions or neutron stars, but also because the subject in itself is of interest as one is dealing with a small charged quantum liquid which is self–bound by the strong interaction. In the following we investigate the liquid–gas phase transition within Fermionic Molecular Dynamics. This model can simulate nucleus–nucleus collisions as well as equilibrium situations. We will, however, concentrate on an experimentally not accessible situation, namely an excited nucleus which is put into an external field. This field plays the role of a container so that evaporated nucleons cannot escape, but The forces between the individual nucleons in a nucleus vary according to distance in a manner similar to those between molecules in a liquid. Thus the interesting question arises, whether with increasing excitation energy a liquid-gas phase transition can be observed in finite nuclei. Generally it is believed that signatures of phase transitions are washed out in systems with few constituents due to finite size effects. Theoretical