This is an animated GIF image, generated from a BUU simulation of a heavy-ion collision. The red particles are protons and the green particles are neutrons. At this beam energy, the nuclear matter heats up and expands to very near the point where the expansion becomes an irreversible explosion. At the peak of the expansion, a doughnut-like geometry forms before the matter collapses back into a hot nucleus. Also, note the rapid evaporation of nucleons which is characteristic of a Fermi system.

The BUU code was written by Wolfgang Bauer, and the code used to generate the movie was written by Scott Pratt. This code, which can generate a movie from any simulation which writes down phase space points at various times can be copied from Scott Pratt's home page.