Effects of neck and nuclear orientations on the mass drift in heavy ion collisions

Background: We clarified that the fusion hindrance in heavy ion collisions is caused by the expansion of the neck bridge at the early stage of collision [Phys. Rev. C 108, 014612 (2023)]; however, our discussion was limited to the trajectory analysis. To get a reliable fusion cross section, it is important to understand the fusion process connecting with multinucleon transfer and also the process depending on the target orientation in detail. Especially, the effects of target orientation on the multinucleon transfer process have not been discussed so far in our model. Purpose: First, we investigate precisely the start time of the neck expansion relevant to the mass transfer. The main aims of this paper are to discuss the mass drift in the collision with the different target orientations within the dynamical approach based on the fluctuation-dissipation theorem in the reaction 36S + 232Th. Method: The orientation effects are incorporated within the framework of the Langevin equation with three nuclear deformation parameters as the degree of freedom and the two-center shell model (TCSM) for the potential energy of the system. Results: The start time of the neck expansion was presumed to be 10 zs (zs = 10-21 s) by analysis in several entrance channels. By taking account of the target nuclear orientation, a strong mass-angle correlation was obtained which is compatible with the experimental data. There was a large difference in the mass transfer or the mass drift mode in the fusion process between the tip and side collisions. Conclusions: Not only "delayed relaxation" of the neck but also the nuclear orientation effects have an important role in the strong correlation between fragment mass and its emitting angle. The mass evolution toward mass symmetry is slower than the standard mass drift mode assuming an exponential-type function. Particularly, the mass drift of the tip collision follows the slow mass drift mode assuming a Fermi-type function rather than an exponential-type function, which is related to the different features of the maximum neck cross-sectional area in the sticking process.