  The atomic numbers and the masses of fragments formed in quasi-fission
reactions have been simultaneously measured at scission in 48 Ti + 238 U
reactions at a laboratory energy of 286 MeV. The atomic numbers were determined
from measured characteristic fluorescence X-rays whereas the masses were
obtained from the emission angles and times of flight of the two emerging
fragments. For the first time, thanks to this full identification of the
quasi-fission fragments on a broad angular range, the important role of the
proton shell closure at Z = 82 is evidenced by the associated maximum
production yield, a maximum predicted by time dependent Hartree-Fock
calculations. This new experimental approach gives now access to precise
studies of the time dependence of the N/Z (neutron over proton ratios of the
fragments) evolution in quasi-fission reactions.
