Heavy-ion fusion at low energies

Through precision measurements of fusion cross sections at energies close t o the Coulomb barrier and through the application of the method of "experim ental barrier distributions" which these permit, many recent advances have been made in our understanding of the dynamical processes occuring during a heavy-ion collision. It is now clear that the target and projectile reach one another in superpositions of states which correspond to different orien tations for rotational nuclei or to different induced deformations for vibr ational nuclei. The creation of a neck of neutron matter has also long been postulated and by studying the isotopic dependence of the fusion reaction, some recent results in the Ca-40 + Zr-90,Zr-96 systems appear to confirm t his result. For large Z(1)Z(2) a type of extra-push effect can arise from t he same inelastic entrance-channel effects which enhance the fusion of ligh ter systems, though this will be absent in cases where the enhancement aris es from neutron transfers. The existence of different barriers will of course influence all other reac tion channels. Fusion simply allows one to visualise the barriers most easi ly, since for this process, the total cross section is an incoherent sum of the contributions from all relevant eigenchannels. Some effects in other c hannels have already been observed. Other possible effects will be discusse d. These include; the exploitation of the lowest-energy barrier to produce exotic evaporation residues and strongly deformed high-spin states at low e xcitation energy.