[en] Recent experimental and theoretical investigations have led to the characterization of a new reaction mechanism called quasi-fission. This process is characterized by the emergence of fission-like fragments, which do not originate from the fission decay of a compound nucleus formed by heavy-ion fusion, but rather from the break up of a short-lived intermediate complex. The occurrence of quasi-fission processes appear to be limited to heavy reaction systems and/or large angular momenta, although the present work demonstrates that such reactions occur for somewhat lighter projectiles than previously believed. It is thus shown that measurements of fragment angular distributions provide a signature for quasifission by being sensitive to whether or not a compound nucleus was formed during the reaction. From an analysis of such data it is concluded that the possibilities for synthesizing super-heavy elements in the range Z=112-116 are reduced considerably over previous estimates

[en] A large number of recent experimental studies have shown that a substantial fraction of the total reaction cross section in heavy-ion reactions is found in fission-like processes, which do not result from the fission decay of a completely fused system. Following the suggestion of Swiatecki such processes, which represents a complete relaxation of the relative kinetic energy and a substantial amount of net mass transfer between the two fragments, are denoted quasi-fission reactions. They are distinct from compound fission reactions by bypassing the stage of a completely fused-system. This typically means that they are associated with short reaction times, which results in several measurable characteristics such as broken forward-backward symmetries, large anisotropies of the angular distributions and increased widths of the fragment mass distributions. The distinction between quasi-fission and deep inelastic reactions is less stringent and has the character of a gradual evolution from one reaction type to the other, as found also as quasi-elastic reaction evolves into deeply inelastic processes as a function of the total kinetic energy loss. In the present paper some of the experimental data characterizing quasi-fission reactions are reviewed and discussed. 22 refs., 6 figs

[en] Quantitative analyses of angular distributions and angle-mass correlations have been applied to the U + Ca reaction to obtain upper limit estimates for the cross sections for complete fusion near or below the interaction barrier. Extrapolating to the systems Ca + Cm and Ca + Es using the well established scaling properties of the extra push model, an estimate of the cross sections relevant to the efforts of synthesizing super-heavy elements in the region Z = 116 and N = 184 via heavy-ion fusion reactions are obtained. A simple evaporation calculation using properties of the super heavy elements shows that the failure to observe super-heavy elements with the Ca + Cm reaction is consistent with estimates of the complete fusion process. 33 refs., 9 figs., 1 tab

[en] In spite of the many attempts to synthesize superheavy elements in recent years, these efforts have not yet been successful. Recent improved theoretical models of heavy-ion fusion reactions suggest that the formation of super-heavy elements is hindered by the dynamics of the process. Several recent experiments lend support to these theories. The necessity of an excess radial velocity (extra push) over the Coulomb barrier in order to induce fusion is observed experimentally as predicted by the theory. So is a new reaction mechanism, called quasi-fission which tend to exhaust the part of the reaction cross section, which would otherwise lead to fusion. The present study shows that the angular distribution of fragments from quasi-fission processes are very sensitive to the occurrence of this reaction mechanism. A slight modification of one parameter in the theory demanded by the observation of quasi-fission for lighter projectiles via the angular distributions, has the consequence of posing even more-stringent limitations on heavy-ion-fusion reactions. This reduces even further the possibility for synthesizing and identifying superheavy elements in heavy-ion-fusion reactions

[en] The spin excitations of products from two-body reactions have two sources: transfer of orbital motion into intrinsic spins via tangential friction and thermal excitations of di-nuclear spin modes. The relative importance of these two mechanisms is discussed for deep inelastic scattering, quasi-fission and spontaneous fission processes. The results of simple model calculations are compared to measured γ-multiplicities in ²³⁸U induced quasi-fission reactions and it is concluded that the spin-excitation are only partially equilibrated during the interaction. 11 refs., 5 figs

[en] In order to select the most cost-effective method for the production of secondary ion beams, yield calculations for a variety of primary beams were performed ranging in mass from protons to ¹⁸O with energies of 100-200 MeV/u. For comparison, production yields for 600-1000 MeV protons were also calculated. For light ion-(A < ⁴He) induced reactions at energies above 50 MeV/u the LAHET code was used while the low energy calculations were performed with LPACE. Heavy-ion-induced production rates were calculated with the ISAPACE program. The results of these codes were checked against each other and wherever possible a comparison with experimental data was performed. These comparisons extended to very exotic reaction channels, such as the production of ¹⁰⁰Sn from ¹¹²Sn and ¹²⁴Xe induced fragmentation reactions. These comparisons indicate that the codes are able to predict production rates to within one order of magnitude

[en] The role of dynamics in fission has attracted much interest since the discovery of this process over fifty years ago. However, the study of the dynamical aspects of fission was for many years hampered by the lack of suitable experimental observables against which theoretical calculations could be tested. For example, it was found that the total kinetic energy release in fission can be described equally well by very different dissipation mechanisms, namely the wall formula, that is based on the collisions of the nucleons with the moving wall of the system, as well as a bulk viscosity of the nuclear matter. Although early theoretical work suggested that the fission process may be described as a diffusion process over the fission barrier, this was largely forgotten because of the success of a purely statistical model which instead of enumerating the ultimate final states of the process argues that the fission rate is determined at the open-quote transition state close-quote as the system traverses the fission saddle point. It was therefore significant when Gavron showed that the transition state model was unable to describe the number of neutrons emitted prior to scission at high excitation energy in reactions of ¹⁶O+¹⁴²Nd. Subsequent experimental work using different methods to measure the fission dissipation/viscosity has confirmed these initial observations. It was therefore very surprising when Moretto in recent publications concluded that their analysis of fission excitation functions obtained with a and α and ³He induced projectiles was perfectly in accord with the transition state model and left no room for fission viscosity. In this paper we'll show that Moretto's analysis is flawed by assuming first chance fission only (in direct contradiction to the experimental observation of pre-scission neutron emission in heavy-ion induced fission), and reveal why the systematics presented by Moretto looked so convincing despite these flaws

No abstract available

[en] Collisions in which two heavy ions come into close contact (touching) may be separated into three categories, namely Deep Inelastic Scattering, Quasi-Fission and Complete Fusion. The three processes are associated with the relaxation of the energy, the mass asymmetry and the shape degrees of freedom, respectively. Quasi-fission processes are characterized by a net mass flow from the heavy to the light reaction partner which takes place on a relatively short time scale. In lighter systems, the intermediate complex rotates several times during the quasi-fission process and final fragments show forward-backward symmetric angular distributions and symmetric mass distributions. In these cases, the quasifission process can be discerned only by a quantitative analysis of the angular anisotropies, which are sensitive to the shape of the intermediate system. In heavier systems, the intermediate complex rotates less than half a revolution during the process, and the reaction time may be determined from the angle of rotation. Studying the time dependence of the mass drift toward symmetry we find that it has the characteristics of an overdamped motion with a characteristic time constant of tausub(ch) = 3 x 10^-21 (sec) which appears to be independent of the scattering system. (author)

[en] Short communication