Beramtane Djerroud

Sequential and Pre-Equilibrium Nucleon Emission in Sn + Ca Reactions at 35A MeV

Over the past decade, an enormous amount of work has been carried out to gain an understanding of heavy-ion reaction mechanisms in the Fermi-energy regime (E/A > 20MeV). At low energies, the phenomenology of heavy-ion reactions is reasonably well understood in terms of binary dissipative reaction mechanisms[1]. For Fermi bombarding energies, an essentially dissipative mechanism is still observed[2, 3, 4, 5], although the dissipation and equilibration of energy is less effective than at lower bombarding energies. New reaction phenomena discovered include incomplete energy dissipation[2,4,6], dynamical emission of multiple intermediate-mass fragments[7], and the nearly complete disassembly of projectile- and target-like fragments[8].

Dissipative Collisions and Multifragmentation in the Fermi Energy Domain

Multiple emission of intermediate-mass fragments (IMFs) is one of the salient reaction modes in heavy-ion collisions in the Fermi-energy domain (E/A ≈ 10 to 100 MeV).[1] It has been established that for heavy systems, and at the lower boundary of the Fermi energy regime (E/A ≈ 30 MeV), sequential thermal IMF emission from either the projectile-like fragment (PLF) or the target-like fragment (TLF) is rather weak.[3,5] IMFs are rather emitted from the “neck zone” formed between PLF and TLF, both in peripheral[2, 3, 4] and in central[6] collisions. It has also been shown that in peripheral collisions the “neck zone” is transiently formed from the neutron-rich projectile and target surface matter.[3, 7] These findings are consistent with a dynamical IMF production scenario, such as that of microscopic molecular dynamics models.[8, 9, 10] Since protons and α-particles are emitted mostly sequentially from PLF and TLF,[3, 5] while in the same events, IMFs are emitted mostly from a midrapidity “neck-like” source, a single fused system can be ruled out as a possible source of all particles and fragments, for nearly the entire observed cross section.

Incomplete Energy Damping and Heavy-Residue Production in 197AU+86KR Collisions at E/A=35 MeV

The study of reactions between heavy nuclei at intermediate energies (E/A = 20–200 MeV) has attracted considerable experimental and theoretical interest. This intermediate-energy domain offers an opportunity to produce highly excited nuclear systems and to observe possibly the onset of new phenomena leading over to the high-energy regime, characterized by two-body nucleon-nucleon collisions. One of the phenomena that has attracted significant interest was the appearance of very slow (E/A ~ 0.1 — 0.5 MeV), heavy residues (HR) with masses comparable to that of the target. These HRs were observed in asymmetric systems, with several barns of cross section mostly concentrated at forward angles. The conditions for this process, the origin of the HRs, and the reason for their survival have not yet been well understood. A number of possible production mechanisms have been proposed,[1]–[7] such as fusion-like reactions (complete and incomplete fusion), fast fission, fragmentation, dissipative collisions, and even spallation. A dynamical retardation of the fission process has been considered[4] as an explanation of the survival of the HRs. Hence, the process of HR production constitutes an important nuclear reaction mode which could possibly replace the fission mode at sufficiently high excitation energies. In the present work, ait exclusive 4π measurement of neutrons, light charged particles, and intermediate-mass products was performed in coincidence with projectile-like and target-like fragments (PLF and TLF), in order to determine the production mechanism of the HRs. The 197 Au+86 Kr reaction was studied at E/A = 35 MeV, since for this system, information[1] on HRs was available from radiochemical measurements.

From Dissipative Collisions to Multiple Fragment Production — A Unified View

Over the last decade, much of the effort in intermediate-energy heavy-ion reaction studies has been focused on multifragmentation, [1, 2, 3, 4] a phenomenon leading to multiple intermediate-mass fragments (IMF) in the reaction exit channel. Since the standard statistical model appeared to be unable to account for the large observed IMF multiplicities, various models and scenarios have been proposed [5, 6, 7], which favor copious production of IMFs. On the other hand, in several recent studies [8, 9, 10], it was found that even in the Fermi energy domain, the binary dissipative collision scenario, well established at lower bombarding energies,[11] still accounts for most of the reaction cross section. The present paper shows that, like dissipative collisions, multiple-IMF emission is a dynamical process, driven by the kinetic energy of relative motion of projectile- and target-like fragments. A new, more complete, intermediate-energy heavy-ion reaction scenario is proposed that connects in a natural way the domains of dissipative collisions and multiple IMF production.

Heavy Residue Production in Dissipative 197Au + 86Kr Collisions at E/A = 35 MeV

Massive residues of projectile — and target-like fragments from the 197Au + 86Kr reaction at E/A = 35 MeV have been measured in coincidence with neutrons, as well as light — and intermediate-mass charged products, using a highly efficient detector setup including two 4π devices — the Rochester SuperBall neutron detector and the St. Louis Microball. The observed joint distribution of neutron and charged-particle multiplicities, the emission patterns of charged-particles and projectile-like fragments, and the yield of slow, massive residues, are all indicative. of binary dissipative collisions, followed by statistical decay of the primary massive fragments. To a large extent, the slow massive residues are found to be remnants of target-like fragments, produced even in the most dissipative collisions identified in the present experiment.