Independence of fragment charge distributions of the size of heavy multifragmenting sources

Charged product multiplicities and Z distributions were measured for single multifragmenting sources produced in collisions between 129 Xe+ nat Sn and 155 Gd+ 238 U at the same available energy per nucleon. Z distributions are found identical for both reactions while fragment multiplicities scale as the charge of the total systems. A complete dynamical simulation, in which multifragmentation originates in the spinodal decomposition of a finite piece of nuclear matter resulting from an incomplete fusion of projectile and target, well accounts for this experimental observation.