System Size and Isospin Effects in Central Heavy-Ion Collisions at SIS Energies

Abstract

Much interest has been devoted, for the last two decades, to the investigation of the bulk properties of hot and dense nuclear matter produced in energetic heavy-ion collisions. Despite the tremendous progress accomplished so far¹, the dynamical collision process is not yet sufficiently understood to extract unambiguous information on the fundamental properties of nuclear matter and the underlying equation of state. Further studies both experimentally and theoretically are therefore still needed. In this context, a large systematic study of different symmetric reactions has been achieved recently with the FOPI detector² at the SIS accelerator of GSI-Darmstadt, where different entrance channel parameters (the size of the system, its isospin and the beam energy) were varied. The production of intermediate mass fragments (IMFs) and the degree of equilibration attained during the collision were among the main aspects of interest. By varying the size of the interacting nuclei, one can explore the influence of phenomena like stopping, compression and decompression on the formation of IMFs, which is expected to be particularly important in central collisions³. A deep understanding of the mechanism of IMF production, the so-called “multifragmentation”, is of special interest as it might be related to the possible occurence of a liquid-gas phase transition in finite nuclear systems. Using, on the other hand, different combinations of equal mass projectile and target nuclei which differ only by their isospin content, one can investigate the degree of mixing of projectile and target nucleons. This offers a very promising method to probe the question of whether or not a global equilibrium is reached during the collision, which is the basic assumption in applying statistical concepts to describe the dissociation of the nuclear system. This issue is also of high interest in a microscopic approach of the collision process as the degree of mixing of projectile and target nucleons is expected to be particularly sensitive to the elementary in-medium interactions⁴.