Complete fusion and quasifission in reactions between heavy ions.

Abstract

It is shown that fission anisotropies from reactions with projectiles lighter than A=20 are well described by the standard theory, in which it is assumed that the distribution of K values (projection of total spin I onto the symmetry axis) is determined at the fission saddle point. Angular anisotropies of symmetric fragments from heavy ion reactions with projectiles heavier than A=24 are, however, substantially larger than expected on the basis of this theory, an observation which recently led to the suggestion that the K values adjust adiabatically during the descent from saddle to scission such that the observed final K distributions reflect a thermal equilibrium at the scission point. The predictions of such scission point models are compared with the available experimental data and it is shown that they fail to reproduce the observed trends as a function of bombarding energy for most systems. It is concluded that the observed deviation from the saddle point theory can be explained as a dynamical inhibition of complete fusion with heavy projectiles. An analysis is carried out in which the fraction of the symmetric fragment cross section originating from the fission decay of completely fused compound nuclei can be estimated. The systematics of the inhibition to complete heavy ion fusion is discussed in the framework of the extra push model.