Characteristic Time for Mass Asymmetry Relaxation in Quasi-fission Reactions

Quasi-fission reactions induced by a beam of 238U particles on targets of 32S, 40Ca, 48Ca and natZn have been studied extensively at several bombarding energies ranging from 4.6 MeV/u to 7.5 MeV/u. The time required for a given drift in mass asymmetry to occur during the reaction is determined from the angular distributions and from the l-values, which in turn are derived from the cross-sections. The results imply a "universal" time constant of (5.3 ± 1) 10-21 s for relaxation of the mass asymmetry degree of freedom, irrespective of target mass and irrespective of the excitation energy. The nature of the conservative driving forces, the shapes, and the dissipation mechanism that appears to be common for all these reactions are discussed and found to strongly support the one-body dissipation picture.