F. Cosmo

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.

Energy damping and intermediate-velocity fragment emission in peripheral Kr+Au collisions at 43 MeV/u☆

Abstract Triple and four-fold coincidences among fragments have been measured in the reaction 84 Kr+Au at 43 MeV/u. All events showing the projectile-like nucleus and fission fragments of the target-like nucleus, and all events with one additional intermediate-velocity fragment, were analysed in the frame of a dissipative collision and a participant-spectator model. The mechanism is basically that of a dissipative collision but the emission of the intermediate velocity fragment by the target differs from an equilibrated evaporation.

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.

Evidence for fast and simultaneous multi-fragment emission in central Kr + Au collisions at 60 MeV/u

Abstract Decay products emitted in Kr+Au collisions at 60 MeV/u have been detected with a large experimental array. Events with fragment multiplicities up to 5 have been recorded. After a careful selection of central collisions leading to equilibrited events, the data are compared to two fragmentation models in which fragments are emitted either sequentially or simultaneously. It is shown that the data can be explained very well by the second model, thus suggesting a prompt multifragmentation mechanism. Possible expansion effects in the disassembly process are also investigated.

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.

Fragmentation pattern in sequential emission spectra

Abstract Coincident light particles and fragments produced in Kr+Au reactions at 27, 44 and 60 MeV/u have been measured with a large solid-angle set-up. Energy spectra of Z=1, 2 and 3 particles sequentially emitted by the slowed-down projectile-like nucleus evolve between an evaporation and a fragmentation pattern, suggesting a saturation in the thermalisation process.