Nuclear structure in cold rearrangement processes in fission and fusion

In fission and fusion of heavy nuclei large numbers of nucleons are rearran ged at a scale of excitation energy very small compared with the binding en ergy of the nuclei. The energies involved are less than 40 MeV at nuclear t emperatures below 1.5 MeV. The shapes of the configurations in the rearrang ement of a binary system into a monosystem in fusion, or vice versa in fiss ion, change their elongations by as much as 8 fm, the radius of the monosys tem. The dynamics of the reactions macroscopically described by a potential energy surface, inertia parameters, dissipation, and a collision energy is strongly modified by the nuclear structure of the participating nuclei. Ex periments showing nuclear structure effects in fusion and fission of the he aviest nuclei are reviewed. The reaction kinematics and the multitude of is otopes involved are investigated by detector techniques and by recoil spect rometers. The advancement of the latter allows one to find very small react ion branches in the range 10(-5) 10(-10). The experiments reveal nuclear st ructure effects in all stages of the rearrangement processes. These are dis cussed, pointing to analogies in fusion and fission on the microscopic scal e, notwithstanding that both processes macroscopically are irreversible. He avy clusters, as Sn-132, Pb-208, nuclei with closed-shell configurations N = 82, 126, Z = 50, 82 survive in large parts of the nuclear rearrangement. They determine the asymmetry in the mass distribution of low-energy fission , and they allow the synthesis of superheavy elements, currently up to elem ent 112. Experiments on the cold rearrangement in fission and fusion are pr esented. Here, in the range of excitation energies below 12 MeV, the phenom ena are observed most convincingly.