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

39 pages including 15 figures; submitted to Nucl. Phys. A - Matière Nucléaire, GANIL E202, GANIL E209International audienceA sample of `single-source' events, compatible with the multifragmentation of very heavy fused systems, are isolated among well-measured $^{155}$Gd+$^{nat}$U 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