D. Jacquet

The decay of primary products in binary highly damped 208Pb + 197Au collisions at 29 MeV/u☆

Abstract Events with fragment multiplicities up to eight have been detected with a large detector array in Pb + Au reactions at 29 MeV/u. All collisions show a binary character irrespective of a possible further disassembly of the two highly excited primary partners. For the most violent collisions, dissipative orbiting is observed as well as full energy damping corresponding to excitation energies as high as 6 MeV/u.

Hot nuclei in reactions induced by 475 MeV, 2 GeV 1H and 2 GeV 3He

Abstract Inclusive neutron multiplicity distributions have been measured for 475 MeV, 2 GeV proton- and 2 GeV 3 He-induced reactions on Ag, Au, Bi, U targets. There is general agreement between these multiplicity data and results of intranuclear cascade calculations. The results indicate a broad distribution of excitation energies with 10% of the events exceeding 500 MeV. For a thermalized nucleus this would translate into temperatures exceeding 5 MeV

Neck formation and decay in Pb + Au collisions at 29 MeV/u☆

Abstract Three fragment production has been studied in peripheral 208 Pb + 197 Au collisions at 29 MeV/u. The data suggests the formation and subsequent decay of necklike structures producing a small fragment emitted at mid-rapidity in between the two partners of a deep inelastic scattering. Data and trajectory calculations suggest also the existence of a competing process in which the neck is absorbed by one of the reactants, this latter decaying further by binary fission.

Complete energy damping in 29 MeV/nucleon Pb+Au two-body final-state reactions☆

Abstract Decay products emitted in highly dissipative Pb+Au reactions at 29 MeV/nucleon have been detected using a large area array. Multiplicities of fragments as large as 8 have been detected with a sizeable cross section. The m -fragment exit channels are fully compatible with the formation of a transient excited dinuclear system formed in damped collisions. The excitation energy function shows that fully damped collisions are achieved even for m = 2 indicating that heavy nuclei are able to sustain high excitation energy and end up as evaporation residues.

Nuclear disassembly time scales using space-time correlations

Abstract The lifetime, τ, with respect to multifragmentation of highly excited nuclei is deduced from the analysis of strongly damped Pb + Au collisions at 29 MeV/u. The method is based on the study of space-time correlations induced by “proximity” effects between fragments emitted by the two primary products of the reaction and gives the time between the reseparation of the two primary products and the subsequent multifragment decay of one partner. A lifetime significantly longer than the interaction time is found. It is compared with time scales corresponding to different types of dynamical instabilities which may be responsible for nuclear disassembly.

Damped reaction dynamics in 197Au + 208Pb collisions at 29 MeV/nucleon

Abstract Velocity and angular distributions of neutrons from the reaction 197Au + 208Pb at E A =29 MeV were measured in coincidence with charged reaction products. The neutron emission pattern is found to erolve smoothly with atomic number and energy of the coincident fragments, up to the highest degrees of energy dissipation observed. The velocity distribution of neutrons in coincidence with any type of fragment is essentially bimodal and can be understood in terms of sequential emission from fully accelerated primary projectile-like and target-like fragments. A relatively weak nonequilibrium neutron component is also observed. Neutron multiplicities and spectral slope parameters, as well as the deduced emitter velocities indicate consistently that, even for highly dissipative collisions associated with intermediate-mass fragments, the average damping of kinetic energy available in the entrance channel is incomplete.

Fission of spin-aligned projectile-like nuclei in the interactions of 29 MeV/nucleon 208Pb with 197Au

Abstract Binary fission of projectile-like nuclei has been investigated in the interaction of 29 MeV/nucleon Pb on Au, together with the associated neutron multiplicity. Fission is only observed in rather peripheral collisions and represents approximately 20% of the total reaction cross-section. The fission process occurs after collisions in which up to 550 MeV have been dissipated. The angular and energy distribution of the fragments can be accounted for by assuming a noticeable spin alignment of the fissioning nuclei.

Evidence for Coulomb fission of 238U in the interaction of 24.3 MeV/nucleon 238U with 197Au: a new experimental approach

Abstract Coulomb fission of 238U has been studied in the interaction of a 24.3 MeV/nucleon U beam with a Au target. A novel experimental approach is followed, aiming at distinguishing fission of Coulomb origin from nuclear fission. The Z distribution of the fragments is studied for both fission processes.

Reaction mechanisms in 24.3 MeV/nucleon 238U induced reactions through a comprehensive study of fission

Abstract Nuclear reaction mechanisms for systems characterized by very different asymmetries (U + C, Si, Ni, Au) have been investigated at 24.3 MeV / nucleon, using as observables both the fission products and the neutron multiplicity. It is clearly observed that the fusion process — whatever its completeness — can only occur with rather light target nuclei, indicating the persistence of potential energy effects much above the interaction barrier.