Fission and quasifission in U-induced reactions.

Abstract

Triple-differential cross sections have been measured as a function of product mass, total kinetic energy, and center-of-mass scattering angle in reactions induced by $^{238}\mathrm{U}$ on $^{16}\mathrm{O}$, $^{26}\mathrm{Mg}$, $^{27}\mathrm{Al}$, $^{32}\mathrm{S}$, $^{35}\mathrm{Cl}$, $^{40}\mathrm{Ca}$, $^{48}\mathrm{Ca}$, and $^{\mathrm{nat}}\mathrm{Zn}$ targets at several bombarding energies between 4.6 and 7.5 MeV/nucleon. The analysis focuses on binary processes in which the product masses are substantially different from the target-projectile masses. These include the complete fusion followed by fission as well as quasifission processes in which large mass transfers occur on a short time scale. The relative contributions of these two components are estimated from the mass-angle correlations and analyzed within the extra and extra-extra push concepts. The time scale for mass transfer in quasifission reactions is derived from turning angles of the intermediate complex, and it is found that the mass drift toward symmetry shows the characteristics of an overdamped motion with a universal time constant independent of scattering system and bombarding energy. This is consistent with the one-body nuclear dissipation mechanism being responsible for the damping in the mass asymmetry degree of freedom. Also the average total kinetic energy of reaction products in quasifission is independent of temperature, supporting the one-body dissipation hypothesis.