M. Samri

Origins of intermediate velocity particle production in heavy ion reactions

In this paper, we report for the first time clear distinctions between these prompt processes and an alternative phenomenon of delayed aligned asymmetric breakup that populates the intermediate-velocity zone by a deformation rupture of mainly the heavier of the colliding partners in mass asymmetric collisions. These distinctions were observed experimentally with intermediate-velocity particle correlation analysis of Ni+C and Ni+Au reac

Dissipative binary mechanisms in 24Mg + 12C collisions at 25A and 35A MeV☆

Abstract A study of the most violent collisions in the 24Mg+12C reactions at 25A and 35A MeV has been carried out. Experimental data, 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.

Statistical and sequential breakup of 24Mg in peripheral reactions at intermediate energies

Abstract The fragmentation of the projectile 24 Mg, excited in peripheral collisions on 197 Au and 12 C targets, has been investigated at 25A MeV and 35A MeV with a large scintillator detector array. Projectile breakup events were selected in the off-line analysis, by requiring that the total detected charge in one event be equal to that of the projectile. The projectile-like nucleus (PLN) excitation energies have been reconstructed and compared to results at 50A MeV and 70A MeV. The branching ratios of 2-, 4-, 5-, and 6-fold decays show a linear dependence, characteristic of statistical decay. Global variable analysis in terms of relative angles was applied to the six-alpha exit channel and the corresponding data have been compared to theoretical calculations based on simultaneous and sequential binary decay. The latter have been divided into sequential fission and sequential evaporation decays. The data has been found to be consistent with a sequential evaporation decay mechanism for a wide range of the intermediate-energy domain.

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.

An exclusive analysis of dissipation for light heavy-ion collisions at intermediate energy within the hybrid model

Abstract The reverse kinematics light heavy-ion system 35 Cl+ 12 C at 43 MeV/nucleon is investigated in the framework of a dynamical-statistical hybrid model. The modified quantum molecular dynamics model which includes a Pauli potential is used to describe the initial non-equilibrium stage of the reaction and the formation of highly excited pre-fragments. The decay of pre-fragments is further described with a statistical sequential decay code. The distributions of global observables from filtered simulations, such as the center-of-mass velocity of products, fragment charge, anisotropy ratio and flow angle, reproduce the experimental results remarkably well. The evolutions between the number of sources and flow angle with impact parameter are investigated and compared with experimental results. For the light heavy-ion system, binary dissipative collision is a dominant reaction process. The process is still present even in central collisions. The formation of a neck-like structure can be observed at mid-central collision in the simulations. From a comparison to Boltzmann-Uehling-Uhlenbeck model calculations, a possible origin of the neck-like structure for the present system could be the mean-field instabilities which originate from dynamical fluctuations through the time evolution of the collision.

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.