Peripheral Ar induced reactions at 44 MeV/u — similarities and deviations with respect to a high energy fragmentation process

Abstract

Peripheral Ar reactions induced at GANIL at 44MeV/u have been analyzed in the framework of the high energy fragmentation model. Several deviations from this model have been interpreted as due to the persistence of some collective effects at this intermediate energy.Heavy ion induced reactions in the energy region between 20 and 100MeV/u appear to be very interesting as one expects a strong evolution in the reaction mechanism. A transition from the low energy one body dissipation process to two body friction should show up. As far as peripheral reactions are concerned it has been shown that at 20MeV/u with "light" heavy projectiles (M<20) the momentum widths of the ejectile were too narrow [1, 3] to be described by the standard high energy fragmentation models [4][5][6]. On the other hand, in the energy range 60-120 MeV/u there is a better agreement, the remaining difference with high energy data [7-10] arising from Final State Interactions (FSI). The latter have been described in terms of the combined Coulomb and nuclear field between the ejectile and the remaining target like nucleus or/and the Coulomb field between the ejectile and the cloud of removed protons and other light charged particles [-10]. The FSI are revealed by a broadening of the transverse momentum distributions, as compared to the longitudinal ones [7][8]. In order to further investigate these questions we have initiated Ar induced reactions at 44MeV/u, i.e. in an energy domain not explored so far with this relatively massive type of projectile. The fragmentation (or fragmentation-like data) will be compared to what has been observed at high energy by Viyogiet al. [11]. The advantage of Ar over lighter projectiles lies in the fact that struc-* This experiment has been performed at the GANIL national facility at CAEN ture effects in the projectile are less important. Furthermore the large diversity of ejectile (twelve elements with six or seven isotopes each were recorded in the present experiment [12]) makes easier to follow the evolution from the most peripheral collisions to some more inner ones. The data related to the mass and charge distributions of the fragments have been presented elsewhere [-12]. It was concluded that at this intermediate energy and for this particular aspect the dominant process was fragmentation. The present paper is mainly devoted to the energy and angular distributions of the fragments. The deviations from a fragmentation picture will be emphasized. At 44MeV/u bombarding energy such a simple model assuming a clean cut between participants and spectators does not seem to be adequate to reproduce all the observed features. It will be shown that quasi elastic exchanges of nucleons take place on the one hand and that dissipative phenomena manifest themselves clearly on the other hand. The paper will be arranged in the following way. In the first chapter a description of the experimental set up will be given, then, in a second part a general survey of the data will be made. Finally, analyzing the data in terms of a fragme...

Total cross sections of reactions induced by neutron-rich light nuclei

“…Therefore, the projectilelike fragments leave the target at 0 ~ with a velocity close to the one of the primary beam. Their A/Z ratio is governed by that of the incident projectile [11]. Contrary to the fragmentation process at relativistic energies, some residual mean field is still present which may be used advantageously to enhance the production of neutron-rich (neutron-deficient) fragments using neutron-rich (neutron-deficient) targets [12].…”

Projectile like fragment production in Ar induced reactions around the Fermi energy

“…The first experiments performed in this intermediate energy domain have demonstrated the existence of a process which, in many respects, resembles very much a fragmentation (or break-up) mechanism [6][7][8][9]. For instance in the systems we previously studied [6] (44 MeV/u Ar+ 5SNi, Au), and this was confirmed in several other systems, the isotopic distributions of the PLF are very similar to those observed * Experiment performed at the GANIL national facility at Caen at much higher energy with the same Ar projectile.…”

Observation of new neutron rich nuclei29F,35,36Mg,38,39Al,40,41Si,43,44P,45?47S,46?49Cl, and49?51Ar from the interactions of 55 MeV/U48Ca+Ta

“…It has been shown [1] that the isotopic distributions of projectile-like fragments at intermediate energy are strongly dependent on the N/Z ratio of the projectile. 4~ and S6Kr have been the most neutron-rich projectiles used so far at GANIL in experiments aiming at the production and identification of new neutron-rich nuclei.…”