D. Gardès

A possible mechanism in heavy ion induced reactions: "Fast fission process"

The influence of the orbital angular momentuml on the mass distribution of fission fragments is studied, both on previously available data on heavy ion induced fission and in new specifically planned experiments: systems40Ar+165Ho and24Mg+181Ta at bombarding energies ranging from 180 up to 391 MeV and leading to the same fissioning nucleus205At with differentl distributions. Whenl values corresponding to a vanished fission barrier are reached, the mass distribution broadens. This suggest the existence of a specific process, “fast fission”, atl-values intermediate betweenl-values leading to compound nucleus formation and deep inelastic collisions, respectively. This process and its conditions of occurence are discussed; of special interest are the correlated differences between the limitations to the fusion cross-section and the fission mass distributions broadenings, respectively, for the Ar+Ho and Mg+Ta systems.

Deeply inelastic collisions as a source of intermediate mass fragments at E/A=27 MeV☆

Abstract Intermediate-mass fragments detected in coincidence with heavy residues were measured in 40 Ar induced reactions on Ag at E / A =27 MeV. From the observed characteristics, it is inferred that intermediate-mass fragments associated with the so-called intermediate-velocity source come mainly from deeply inelastic collisions occuring after or at the same time as preequilibrium particle emission.

Recoil study of transfer reactions: Evidence for α and 8Be transfers in 12C reactions with heavy nuclei (Au, Bi)

Abstract Transfer reactions induced with 12 C ions in Au and Bi targets have been studied. The experimental technique involves the measurement of the cross sections, angular distributions, and recoil range at each recoil angle, of the heavy residual nuclei. The processes leading to the production of At isotopes in the case of a Bi target, and Tl and Bi isotopes in the case of an Au target, have been identified by means of a kinematic analysis of the experimental data. They appear to be 2p, α and 8 Be transfers from the projectile to the target, followed by the evaporation of neutrons. The cross section for 8 Be transfer was derived from experimental data and is significant, of the order of 100 mb. The cross section for α-transfer was estimated to 30 to 80 mb. All the observed characteristics of the residual nuclei produced through α and 8 Be transfers (cross sections, ranges and angular distributions) are in good agreement with the cross sections and angular distributions of the direct α-particles which are the projectile residues of these transfers, and which were observed by other authors.

Light fragments produced in40Ar+natAg reactions at 27MeV/u

Angular and energy distributions have been measured for products of 4≦Z≦12 from reaction of 1,095 MeV40Ar+natAg. In addition to sources located near the projectile and fusion system velocities, the data show the presence of a source located aroundVbeam/2 which cannot be explained by any existing model. A possible interpretation is proposed.

Further experimental evidence for fast fission

Abstract Fusion-fission products have been studied for three reactions: Ar + Au, Ar + Bi and Ar + U (5.25–7.5 MeV/u). By measuring symmetric fragmentation components (fission-like events), cross sections for fusion were deduced and compared with the predictions of static and dynamic models. With increasing projectile energy, the width of the mass distributions strongly increases for the two lighter systems. By contrast, for Ar + U it remains essentially constant at a very large value. These results clearly demonstrate that the large increase of the width of the mass distribution cannot be attributed simply to large values of the angular momentum. However, they can be explained by the occurence of a different dissipative process, fast fission, which can be expected if there is no barrier to fission. For the reaction Ar + Au, the total kinetic-energy distributions were also studied in detail. In this case fast fission occurs only at high incident energy. The average total kinetic energy (TKE) was found to be constant with increasing energy whereas the widths of the TKE distribution increase.

Dynamics and thermalization in violent collisions between40Ar and Ag at 27 MeV/nucleon

For the violent collisions of 27 MeV/nucleon40Ar with Ag, coincidence measurements have been made between heavy residues and intermediate mass fragments (3≦Z≦14) or light charged particles. From the analysis of the correlation between heavy residues (mass and velocity) and intermediate mass fragments, the main characteristics of the dominant mechanisms, fusion and partially damped collisions preceded or accompanied by a preequilibrium emission, are presented. Balances concerning mean values of parallel linear momentum, mass and atomic number, are established and confirm that a complete description of violent collisions was obtained. Then thermalization is discussed, first in terms of excitation energies derived from kinematics between heavy residues and intermediate mass fragments, and secondly in terms of initial temperature estimates derived from light charged particle spectra. Very hot nuclei (T⋍5.7–6.6 MeV) are produced over a large impact parameter range from very central collisions to medium peripheral ones. Various experimental results are compared to predictions obtained with semi-classical calculations (Landau-Vlasov equation). From their good agreement one may conclude that, depending on the impact parameter, thermal equilibrium is achieved within 4–10×10−22 s.

Fission following carbon interactions on gold and uranium between 30 and 84 MeVu

Abstract The binary fission process in Au and U induced by 12 C at 30, 60 and 84 Me u has been investigated. Excitation energies of the fused nuclei are inferred from the mass measurements as a function of the transferred linear momentum. Incomplete-fusion models appear to account for the bulk of the data. The variance of the mass distribution of the fragments increases roughly linearly with the temperature derived at saddle point as expected in a thermal process.

Evolution from deep inelastic transfer to quasi-fission

Abstract Recoil techniques have been used to study the evolution of the features of four-charge transfer reactions in 148 Sm targets when the mass and energy of the projectile are varied. Beams of 63 Cu and 56 Fe were used, and their energies were chosen in order to be equal to 1.2 B and 1.5 B for both projectiles, B being the interaction barrier. The residual nuclei 149g Tb, 150 Dy and 151 Dy were identified by their radioactive properties. This study has shown a continuous evolution of the characteristics of the c.m. angular distributions, especially the position of the maxima, from typical deep inelastic transfer to quasi-fission features, when the value of E B decreases. For equal values of this ratio, no difference was observed between Fe and Cu induced reactions. The c.m, energies corresponding to the maximum cross section increases when the incident energy is increased, for a given projectile. At low incident energies (igE = 1.2B), the relative motion appears to be completely damped, and the distance of the two nuclei after separation is equal to about 5 fm. At higher incident energies, the c.m. energy is significantly higher. This may mean that the relative motion is not completely damped. Part of the effect may be explained by the differences in the angular momenta involved in the reaction.

Production and deexcitation of hot nuclei in collisions of 27 MeV/nucleon 40Ar with 238U ☆

Abstract Hot nuclei, produced in the reaction 1080 MeV 40Ar+238U, have been studied by means of the light charged particles emitted in coincidence with fission fragments. A dominant fraction of the light charged particles emitted in the backward hemisphere (60%, 75%, 80% and 85% for 1,2,3H and 4He, respectively) comes from evaporation from a composite nucleus prior to scission. Careful analysis of the cross sections and energy spectra of these evaporated particles yields several properties of the hot nuclear emitters: (i) they are thermalized with an average temperature of 4.0–4.6 MeV, (ii) they are quite deformed (mean axis ratio ≈ 2 1 ) and are rapidly spinning ( J r.m.s. of 100–140 h MeV ). Predictions from a dynamical model, based on the Landau-Vlasov equation, are consistent with the experimental results, and give insights into the time evolution of the fused system

Excitation energy partition in deeply inelastic collisions between40Ar and Ag at 27 MeV per nucleon

The dynamics of the two partners produced in dissipative collisions has been experimentally studied for the system40Ar + Ag at 27 MeV per nucleon. Primary masses of the fragments can then be calculated; the excitation energy partition between the two fragments is derived from the number of particles evaporated by each fragment. We found that this division evolves from equipartition to a repartition close to thermal equilibrium in the excitation energy range 300–350 MeV or interaction times 5-10×l0−22 s.