Fission and quasifission in U-induced reactions

TLDR

The study examines complete fusion followed by fission and quasifission, where large mass transfers occur on a short timescale. The authors measured triple‑differential cross sections for 238U‑induced reactions on several targets at 4.6–7.5 MeV/nucleon, focused on binary processes with large mass differences, and estimated the relative contributions of fusion‑fission and quasifission from mass–angle correlations using the extra‑push and extra‑extra‑push models. The quasifission mass transfer proceeds with a universal overdamped time constant independent of system and energy, and the reaction products’ kinetic energy is temperature‑independent, both supporting a one‑body nuclear dissipation mechanism.

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.