Shell effects in quasifission in reactions forming the Th226 compound nucleus

Background: Quasifission reactions occur in fully damped heavy-ion collisions without the formation of an equilibrated compound nucleus, leading to the formation of fragments with properties similar to those in fission reactions. In particular, similar shell effects are expected to affect fragment formation in both fission and quasifission. Experimentally, the role of shell effects in quasifission is still debated, and further theoretical predictions are needed.Purpose: We aim to investigate quasifission dynamics in different reactions forming the same compound nucleus and to search for possible signatures of shell effects in fragment formation.Methods: ⁵⁰Ca+¹⁷⁶Yb and ⁹⁶Zr+¹³⁰Sn quasifission reactions are simulated with the time-dependent Hartree-Fock code sky3d near the Coulomb barrier. Evolutions of the quadrupole (Q₂₀) and octupole (Q₃₀) moments are interpreted in terms of features of the potential energy surface (PES) of the ²²⁶Th compound nucleus.Results: Both reactions encounter quasifission. In ⁵⁰Ca+¹⁷⁶Yb, it only occurs at finite angular momenta. In the more symmetric ⁹⁶Zr+¹³⁰Sn reaction with stronger Coulomb repulsion in the entrance channel, quasifission also occurs in central collisions. In agreement with earlier predictions, ⁵⁰Ca+¹⁷⁶Yb encounters partial mass equilibration that is stopped when the heavy fragment reaches Z≈54 protons, as in the asymmetric fission mode of ²²⁶Th. Interestingly, ⁹⁶Zr+¹³⁰Sn encounters an ``inverse quasifission'' (multinucleon transfer increasing the mass asymmetry between the fragments) also leading to fragments similar to those in asymmetric fission. In both systems, quasifission trajectories in the (Q₂₀-Q₃₀) plane are found close to the asymmetric fission valley of the ²²⁶Th PES.Conclusions: The observation of an inverse quasifission that goes against expectations from a simple liquid drop picture suggests that shell effects have an influence in quasifission. In addition, the similarity between fragments formed in asymmetric fission and quasifission supports the idea that the same shell effects are at play in both mechanisms. In particular, these were recently attributed to octupole deformed shell effects in $Z=52--56$ fragments. Interpreting quasifission dynamics with PESs used in fission is naturally limited by the fact that these PESs are usually computed with axial symmetry, no angular momentum, and no excitation energy, thus motivating future developments of PESs for quasifission.