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 Gd-155 + U-na t 36 A MeV reactions by examining the evolution of the kinematics of fragme nts with Z greater than or equal to 5 as a function of the dissipated energ y and loss of memory of the entrance channel. Single-source events are foun d to be the result of very central collisions. Such central collisions may also lead to multiple fragment emission due to the decay of excited project ile- and target-like nuclei and so-called 'neck' emission, and for this rea son the isolation of single-source events is very difficult. Event-selectio n criteria based on centrality of collisions, or on the isotropy of the emi tted fragments in each event, are found to be inefficient to separate the t wo mechanisms, unless they take into account the redistribution of fragment s' kinetic energies into directions perpendicular to the beam axis. The sel ected events are good candidates to look for bulk effects in the multifragm entation process. (C) 2001 Elsevier Science B.V. All rights reserved.