Background: Quasifission is a nonequilibrium phenomenon that strongly suppresses the formation of heavy nuclei in heavy-ion fusion reactions. This process shows a strong entrance channel dependence. Understanding quasifission is crucial for exploring potential avenues to synthesize new superheavy elements and their isotopes.

Purpose: To study the interplay of fusion-fission and quasifission with increasing $Z_p{Z}_t$ for the reactions using ${}^{197}\mathrm{Au}$ as the target. To explore the variation of average sticking time in these reactions with a constant target deformation.

Methods: The mass-angle distributions of fission and fission-like fragments were measured for seven reactions using ${}^{197}\mathrm{Au}$ as the target at energies near the Coulomb barrier. Negligible sequential fission is expected in these reactions; thus, the measured binary products represent the total quasifission yield. Pulsed beams of ${}^{32,36}\mathrm{S}$, ${}^{48}\mathrm{Ca}$, ${}^{50}\mathrm{Ti}$, ${}^{54}\mathrm{Cr}$, ${}^{58}\mathrm{Fe}$, and ${}^{64}\mathrm{Ni}$ were used in the study. A systematic analysis of mass ratio distributions and mass widths has been carried out. The experimental mass-angle distribution is simulated following a phenomenological approach. The average sticking time for the fast quasifission is extracted from the simulations.

Results: Dominant mass-symmetric fission fragments with a slight mass-angle correlation are observed in the most mass-asymmetric (${}^{32,36}\mathrm{S}+{}^{197}\mathrm{Au}$) reactions. In contrast, a $\mathsf{U}$-shaped mass ratio distribution has been observed in the most symmetric (${}^{64}\mathrm{Ni}+{}^{197}\mathrm{Au}$) reaction. A sharp decrease in fusion probability is inferred with increasing $Z_p{Z}_t$. The width of the mass ratio distribution is found to increase exponentially with the increase in mean fissility for a fixed beam energy relative to the barrier. The average sticking time of fast quasifission is found to vary from approximately 14 to 15 zeptoseconds to approximately 4 to 5 zeptoseconds for the systems under study.

Conclusion: The observed transition from a dominant symmetric mass split to an asymmetric mass split with mass-angle correlations indicates the increasing probability of fast quasifission and fusion suppression with increasing $Z_p{Z}_t$. The observation is correlated with the sharp fall in the extracted average sticking time with increasing $Z_p{Z}_t$ from 1264 to 2212.

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• Fission
• Nuclear reactions
• 190 ≤ A ≤ 219