The processes of fusion-fission and quasi-fission of superheavy nuclei M. G. Itkis a , A. A. Bogachev, I. M. Itkis, J. Kliman, G. N. Knyazheva, N. A. Kondratiev, E. M. Kozulin, L. Krupa, Yu. Ts. Oganessian, I. V. Pokrovsky, E. V. Prokhorova, A. Ya. Rusanov a Flerov Laboratory of Nuclear Reactions, JINR, 141980 Dubna, Russia 1. INTRODUCTION One of the main problems of modern nuclear physics is the finding of extremal conditions for the existence of atomic nuclei. The synthesis of new superheavy elements in heavy ion induced reactions is an important part of these investigations. The survivability of superheavy elements (SHE) with respect to the fission process is defined by the fission barrier height, which completely depends on nuclear shells in the superheavy region. At approaching the shell closure the fission barrier grows and the survivability of superheavy nuclei increases. The recent success in the synthesis of new elements with Z = 112 - 118, produced in the reactions with 48 Ca ions and actinide targets 238 U, 244 Pu, 248 Cm and 249 Cf [1,2], confirms the existence of “the island of stability” of SHE near the proton shell closure with Z = 114, 120, 122 and neutron shell N = 184. However, the fusion process of massive nuclei with heavy ions (A i ≥ 27) differs greatly from the reactions with more light ions, in which the fusion always occurs after the capture stage. An increase in the Coulomb repulsion forces between interacting massive nuclei can lead to the decay of the composite nuclear system before it reaches a compact shape near the ground state. In this case the quasi-fission (QF) process [3,4] takes place. Quasi-fission is a fast process and its experimental features differ from those of the true fusion-fission (FF) process: large width of mass distributions, asymmetries in mass-angle correlation of fragments and large angular anisotropies [5]. The neutron [6] and γ -multiplicities [7,8] are also different in both processes. It has been found recently [9] that the total kinetic energy (TKE) of fragments is higher in the QF process than that in the FF-process. Different factors influence the FF/QF competition, such as the entrance channel mass asymmetry, N/Z ratio, excitation energy and angular momentum, the deformation and orientation of colliding nuclei, shell structure of the entrance channel and the formed compound nucleus (CN). It is well known that in heavy ion induced reactions leading to the formation of SHE the deep-inelastic and quasi-fission processes are the dominant reaction channels, whereas the fusion probability is much lower. In the reactions with 48 Ca-ions and actinide targets the probability of fusion relative to QF is less than 10%, and the ratio decreases for more symmetrical target-projectile combinations used in cold fusion reactions. The last SHE which can be produced in the reaction with 48 Ca-ions is the element with Z = 118 since the heaviest possible target is Cf. Thus to produce more heavy elements more heavy Nuclear Physics A 787 (2007) 150c–159c 0375-9474/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2006.12.026