L'influence de la dynamique dans le processus de fusion-évaporation Les limites du modèles statistique

École thématiqu

The cross sections for different channels in heavy ion nuclear reactions

Estimates have been made for the critical value lc of the orbital angular momentum above which a complete fusion nucleus cannot be formed. The results have been obtained by measurements of cross sections for noncompound inelastic processes (inelastic scattering, transfer. reactions, α particle emission) when Ag targets were bombarded by 86 MeV 12C ions and by 78 and 113 MeV 14N ions. The average value of lc was found between 38 and 45 ħ. A short discussion is given on the range of angular momenta at which transfer reactions occur. It is shown that quasi elastic single and multi-nucleon transfer reactions are possible only for large values of l called lT. When the bombarding energy is high enough, theses values lT are larger than lc and there is a region of l between l c and lT where a very inelastic mechanism takes place. A model called « fusion prompt scission » process is proposed. It is suggested that a deformed shape for the two fissioning nuclei is temporarly formed and breaks off very shortly after.La valeur critique lc du moment angulaire au-delà duquel un noyau composé de fusion ne peut plus être formé, a été estimée. Les résultats ont été obtenus selon une nouvelle méthode qui consiste à mesurer la section efficace de tous les processus ne faisant pas intervenir le noyau composé (diffusion inélastique, réactions de transfert, émission de particules alpha vers l'avant). L'étude a été effectuée au moyen d'ions 12C de 86 MeV et d'ions 14N de 78 et 113 MeV bombardant des cibles d'argent. On a trouvé lc compris entre 38 et 45 ħ quelque soit l'ion incident et l'énergie. On discute ensuite brièvement pour quelle région de moments angulaires les réactions de transfert ont lieu. On montre que les cas de transferts quasi élastiques sont possibles seulement pour des l élevés, de valeur moyenne lT dépendant de l'énergie incidente. lT devient très supérieur à lc lorsque l'énergie croit et un mécanisme d'échange de nucléons très inélastique intervient dans la région comprise entre lc et l T. Un modèle est proposé qui consiste en une fusion de durée très brève des deux noyaux sous la forme d'une configuration très déformée

Effect of the intermediate velocity emissions on the quasi-projectile properties for the Ar+Ni system at 95 A.MeV

The quasi-projectile (QP) properties are investigated in the Ar+Ni collisions at 95 A.MeV taking into account the intermediate velocity emission. Indeed, in this reaction, between 52 and 95 A.MeV bombarding energies, the number of particles emitted in the intermediate velocity region is related to the overlap volume between projectile and target. Mean transverse energies of these particles are found particularly high. In this context, the mass of the QP decreases linearly with the impact parameter from peripheral to central collisions whereas its excitation energy increases up to 8 A.MeV. These results are compared to previous analyses assuming a pure binary scenario

Multifragmentation of a very heavy nuclear system (I): Selection of single-source events

A sample of `single-source' events, compatible with the multifragmentation of very heavy fused systems, are isolated among well-measured 155Gd+natU 36AMeV reactions by examining the evolution of the kinematics of fragments with Z>=5 as a function of the dissipated energy and loss of memory of the entrance channel. Single-source events are found to be the result of very central collisions. Such central collisions may also lead to multiple fragment emission due to the decay of excited projectile- and target-like nuclei and so-called `neck' emission, and for this reason the isolation of single-source events is very difficult. Event-selection criteria based on centrality of collisions, or on the isotropy of the emitted fragments in each event, are found to be inefficient to separate the two mechanisms, unless they take into account the redistribution of fragments' kinetic energies into directions perpendicular to the beam axis. The selected events are good candidates to look for bulk effects in the multifragmentation process.Comment: 39 pages including 15 figures; submitted to Nucl. Phys.

The SPIRAL radioactive ion beam facility

This document describes the scientific goals as well as the technical choices of the SPIRAL project (Système de Production d'Ions Radioactifs et d'Accélération en Ligne)

PRODUCTION OF NUCLEI FAR FROM THE BETA STABILITY LINE USING INTERMEDIATE-ENERGY HEAVY IONS

La production de noyaux exotiques à l'aide des ions lourds aux énergies intermédiaires est passée en revue. Les différents mécanismes de production, essentiellement la fragmentation du projectile et les réactions de transfert, sont présentées ainsi que les premiers résultats.The production of far unstable nuclei using heavy ion accelerators in the intermediate energy domain is reviewed. The various mechanisms responsible for the production of exotic species, mainly the projectile fragmentation and transfer reactions, are discussed, and the first experimental results presented