Recent results on the fusion reactions between very heavy ions and nuclei

Results are given for measurements of complete fusion cross sections in 40 Ar and 84 Kr induced reactions on medium and heavy targets. After complete fusion, composite nuclei decay by particle and gamma ray emission and also, in large proportions, by binary fission. Cross sections were measured on the residual nuclei and on binary fissions following full momentum transfer. In the case of Ar ions, the fusion cross section is still a large part of the total reaction cross section and the restriction due to high angular momentum is not severe. With krypton projectiles there is also a large proportion of complete fusion if the compound nucleus is in the medium masses. On the other hand with targets of bismuth and uranium a very small cross section was observed for fission events following a fusion process. An intermediate situation was found in the case of holmium and tungsten targets. Most of the reaction cross section goes into incomplete fusion channels, with a large loss of kinetic energy. A discussion is given on the dynamical aspects of the collision between two heavy nuclei.