  In highly dissipative collisions between heavy ions, the optimal conditions
to investigate different de-excitation channels of hot nuclei such as
evaporation, fission or multifragmentation are well known. One crucial issue
remains the excitation energy region where fission gives way to
multifragmentation. In this paper, the onset of multi-fragment exit channels is
investigated in terms of sequential fission. For the first time, the dynamical
approach based on solving Langevin transport equations in multidimensional
collective coordinate space is used to follow the de-excitation of highly
excited (up to E* =223-656 MeV) 248Rf compound nuclei. The sequential fission
model we propose contains two steps: (1) time evolution of the compound nucleus
up to either scission or residue formation, followed by (2) dynamical
calculations of each primary fragment separately. This procedure allows to
obtain from one to four cold fragments correlated with the light particles
emitted during the de-excitation process. Experimental data measured with the
INDRA detector for the 129Xe+ natSn reaction at beam energies 8, 12 and 15
MeV/nucleon provide strong constraints for this sequential fission scenario.
