B. Djerroud

Time scale in 24Mg projectile breakup at 25A and 35A MeV

Abstract The time scale involved in the breakup of 24 Mg projectiles into the 6α and the 5αpH channels has been investigated by examining distortions and shifts in the fragment velocity distributions due to the Coulomb field of the target. Assuming a fixed angular momentum range of 4 to 8 ħ strok; for both channels, we deduce time scales of (5.1 – 6.8) × 10 −22 s for the six-alpha channel at 3.4 MeV of excitation per nucleon and (3.0–5.9) × 10 −22 s for the 5αpH channel at 4.5A MeV of excitation.

Quasi-Projectile Formation and Decay Comparisons Between 58Ni+C and 58Ni+Au Reactions at 34.5A MeV

It is now well known that collisions between heavy ions in the Fermi energy domain produce mainly binary type events[1]-[5]. It seems that this binary character dominates even for the most violent reactions[1, 3, 4]. However, what is still not well understood is the deexcitation stage of the two principal emitters and the effects produced by the entrance channel dynamics. An important factor in this energy range is that many processes are possibly in competition and it is experimentaly difficult to isolate each of them. Processes such as the progressively vanishing fusion, binary deep inelastic collisions and the appearance of nucleon-nucleon scattering are all present in the Fermi energy range. Furthermore, detected particles could have been emitted on a large time scale from very different stages of the decay, ranging from pre-equilibrium process to evaporation. Within the statistical break-up hypothesis, where the two principal emitters are considered as thermalized nuclei, we expect only the excitation energy of each emitter, and not the way it is reached, to be a determinant quantity for the disintegration exit channels. On the other hand, typical violence of these collisions can also lead to important deformations of the two main products of the reaction. Such deformations were observed recently by the rupture of neck-like structures linking the reaction partners[6]-[9]. In an asymmetric collision, we could expect the biggest nucleus to sustain the largest deformation. By its subsequent disintegration toward a more stable state, it could be possible to observe the effects of such a deformation on the deexcitation mode of the nucleus.

Exclusive multidetection and study of projectile breakup at 25 and 35A MeV in 24Mg + 197Au

Abstract The breakup of the projectile 24 Mg, excited in peripheral collisions on a gold target, has been investigated at 25 and 35 A MeV with a large scintillation-detector array allowing exclusive measurements. Absolute breakup cross sections were deduced and the projectile-like nucleus velocity and excitation energy have been reconstructed. The excitation energy partition between the projectile and the target is found to lay between the limits of equal excitation energy sharing and equal temperature with some evolution from one limit to the other. The statistical nature of the decay mechanism is inferred from global variables. Small-relative-angle analysis is applied to the six-alpha exit channel and the corresponding data were found to be consistent with a sequential evaporation decay mechanism, with some contribution from sequential fission at higher excitation energies. The time scale involved in the breakup of 24 Mg projectiles into the 6α and the 5αHH channels has been investigated by examining distortions in the fragment velocity distributions due to the Coulomb field of the target. A decrease in the quasi-projectile lifetime is observed as the mean excitation energy increases from 3.4 to 4.5 A MeV.

Reaction Mechanisms of the Most Violent 24Mg + 12C Collisions at 25A and 35A MeV

A study of the reaction mechanisms in central 24Mg + 12C collisions at 25A and 35A MeV has been carried out. Global variables, such as anisotropy ratios and source-velocity ratios, computed for those events in which the total charge of the system has been detected, are compared to simulations based on statistical fragmentation codes. For violent events, a binary mechanism appears to be competing successfully with compound nucleus formation.