Fusion window problem in time-dependent Hartree-Fock theory revisited

Head-on collisions of ${}^{16}\mathrm{O}{+}^{16}\mathrm{O}$ are calculated using an extended version of the time-dependent Hartree-Fock theory known as the time-dependent density-matrix theory (TDDM). TDDM deals with both one-body and two-body dissipation in a quantum-mechanical way. A newly developed TDDM code, which includes spin-orbit force in a mean field, is used. It is found that the threshold energy above which the colliding nuclei do not fuse increases due to two-body dissipation. Head-on collisions of ${}^{22}\mathrm{O}{+}^{22}\mathrm{O}$ are also calculated to investigate the effects of two-body collisions in exotic nuclei. In contrast to the case of ${}^{16}\mathrm{O}{+}^{16}\mathrm{O},$ no additional dissipation due to two-body collisions is found in the head-on collisions of ${}^{22}\mathrm{O}{+}^{22}\mathrm{O}.$