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Integral fission cross sections in the system z3su + 238U were measured at beam energies below the interaction barrier Vc. Scattering angle dependent probabilities and integral cross sections for Coulomb fission were calculated. It is concluded that earlier observed discrepancies between measured and calculated angular distributions for the one-neutron transfer product z39U cannot be explained by sequential fission. Multi-nucleon transfer induced fission is observed down to energies (0.90 + 0.02) x V c.

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Cross sections for nuclear reactions at beam energies near and below the spherical Coulomb barrier Vc were measured in the very heavy collision systems 23aU +238U and 23sU+19VAu. The most probable reaction channel with mass transfer is the one-neutron transfer. Its excitation function is understood in terms of Rutherford trajectories together with the quantal process of neutron tunnelling over large distances. In addition, the exchange of up to 15 nucleons is observed down to 0.90 Vc. The excitation functions for the multi-nucleon transfer products have much steeper slopes than that for one-neutron transfer, and are steeper for 23sU - than for 238U+23sU, suggesting that nuclear contact is established in the associated collisions. The angular distribution for one selected multi-nucleon transfer product, 227Th, shows that its formation occurs in more central collisions within contact times shorter than about 10 -21 s. There is no evidence for very longlived di-nuclear systems in the these reactions.

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