Limitation of energy deposition in classicalNbody dynamics

Abstract

Energy transfers in collisions between classical clusters are studied with Classical N Body Dynamics calculations for different entrance channels. It is shown that the energy per particle transferred to thermalised classical clusters does not exceed the energy of the least bound particle in the cluster in its "ground state". This limitation is observed during the whole time of the collision, except for the heaviest system. PACS numbers: 24.10.Cn, 25.70.-z Keywords: Classical N-body dynamics, nucleus-nucleus collisions, energy depositionThe question of energy deposition in nuclei during nucleus-nucleus collisions is of great importance for nuclear matter studies. The maximum amount of energy which can be stored in hot equilibrated nuclei has been studied both experimentally [1,2,3,4,5,6] and theoretically [7,8]. Such studies have mainly been motivated by the determination of a plateau in the so called caloric curve (the evolution of the temperature with the excitation energy) which could be a signature of a first order liquid-gas phase transition [9,10].Experimentally, a lot of work has been done to determine the amount of thermal energy stored in nuclei. For central collisions, large energy deposits up to 22.5 A.MeV have been determined [9,11]. Other studies have shown that the excitation energy in primary products in multifragmenting systems is around 3-4 A.MeV [3,4,12,13], far below the total available energy in the center of mass frame. This energy does not seem to evolve strongly with the incident energy. These two measurements seem to be in contradiction. Possible limitations of energy deposition could result from prompt emission of energetic light charged particles at early times in the reaction [14,15,16].Theoretically, the maximum energy that a equilibrated nucleus can withstand corresponds to the energy (or temperature) at which the surface tension vanishes. This is often characterised by a critical temperature T c whose value is around 16 MeV [7,8]. This temperature is linked to the equation of state of nuclear matter. The main drawback of such studies is that they assume that the system is fully equilibrated and hence do not take into account possible limitations coming from the reaction mechanism.The aim of the present article is to study energy deposition during collisions between finite size systems in a well controlled framework. Results from the Classical N Body Dynamics code [17] will be shown and the mechanism of energy deposition in classical clusters will be studied. We will be interested in this paper by the energy transfered to thermally equilibrated clusters which corresponds to the energy that long-lived clusters can withstand. The paper is organised as follows. In a first section, the Classical N Body Dynamics will be briefly described. The excitation energy in clusters will be shown for various systems and incident energies in the second section. The third section will be devoted to the mechanism of energy deposition in clusters. Conclusions will be drawn in the last section I. DESCRIPT...