Threshold behaviour of the compound-nucleus formation in the reactions Kr+Sb, Sn+Zr, and Sn+Mo

At the velocity filter SHIP of the Society for Heavy Ion Research in Darmstadt cross sections for the evaporation-residual nucleus formation in the reactions 86Kr+sup(121,123)Sb and 124Sn+sup(90,92,94,96)Zr, 100Mo have been measured. The probability for compound-nucleus formation in these reactions was determined by comparison of the measured cross sections with evaporation-residual nucleus cross sections and fusion cross sections from 40Ar-induced reactions which yield the same compound nuclei. It is valid for nearly central collisions of the reaction partners. The fusion probability is compared with the prediction of a conventional fusion model, in which it is described by the transmission coefficient of a one-dimensional potential barrier, at energies near the fusion barrier of the conventional model suppressed by up to three orders of magnitude. It reaches its saturation value one at energies between 9 and 26 MeV higher than expected by the conventional model. The increase of the fusion probability below the saturation value with the energy is very much weaker than expected in the conventional model. The results can be parametrized by means of Gauss-shaped fluctuations of a mean fusion barrier. The measured mean fusion barriers are compared with the prediction of a semiempirical macroscopic model for nucleus-nucleus collisions which predicts a dynamic increase of the fusion barrier for massive systems. (orig./HSI)