Multifragment decay of hot nuclei: Dynamics or statistics?

Multifragment events in intermediate energy heavy ion reactions need to be characterized in terms of the emitting source, its size, excitation energy and angular momentum. The center of mass velocity of the multifragment events seems to provide a substantial amount of the needed information. By using the incomplete fusion model, this velocity allows one to calculate mass/charge and excitation energy of the source. A series of experiments carried for the reactions 139La + 12C, 27Al, 40Ca, 51V, natCu and 139La at bombarding energies of 35, 40, 45 and 55 MeV/u has been analyzed in this manner. When the relative probability of binary, ternary, quaternary and quinary events is plotted versus the calculated excitation energy, the resulting steeply rising excitation functions are independent of both the bombarding energy and the system. In other words, these probabilities depend mostly on the excitation energy of the hot intermediate system, no matter how it has been prepared. Thus, the role of dynamics seems to be confined to the formation of the hot system, while statistics governs its decay