Background: Quasifission substantially hinders the formation of the compound nucleus in heavy ion fusion reactions. It poses a major obstacle to superheavy element formation. This process has a strong entrance channel dependence.

Purpose: To study the interplay between fission and quasifission in ${}^{30}\mathrm{Si}+{}^{197}\mathrm{Au}$ reaction at energies around the Coulomb barrier.

Methods: Binary fragments from ${}^{30}\mathrm{Si}+{}^{197}\mathrm{Au}$ reaction were measured in coincidence using a pair of large area multiwire proportional counters. The mass-ratio and mass angle distributions of the fragments were obtained using the kinematic re-construction method. Experimental results are interpreted using different theoretical methods. Further, the results of present reaction have been compared with that of ${}^{28}\mathrm{Si}+{}^{197}\mathrm{Au}$ reaction available in literature.

Results: Broadened mass ratio distributions have been observed in the range of energy studied compared to the theoretical calculations assuming compound nucleus formation. No mass angle correlation is observed. Fission fragment mass distribution of the present reaction is compared with that of other neptunium isotopes at low excitation energies. A fast transition from asymmetric fission to symmetric fission is observed with decrease of $N/Z$ along the isotopic chain of neptunium. The difference between the asymmetric and symmetric fission barrier is higher for the ${}^{238}\mathrm{Np}$ at all temperatures, compared to ${}^{225,227}\mathrm{Np}$ resulting in asymmetric mass division in the fission of ${}^{238}\mathrm{Np}$ and symmetric split at low excitation energies in ${}^{225,227}\mathrm{Np}$. Calculated quasifission yields in the mass range $80\le A\le 150$ show significant dependence on collision energy for both ${}^{28,30}\mathrm{Si}+{}^{197}\mathrm{Au}$ reactions.

Conclusions: Measured fragment mass distribution has significant contribution from slow quasifission in ${}^{30}\mathrm{Si}+{}^{197}\mathrm{Au}$ reaction. $40–60\%$ fusion probability is obtained for the ${}^{28,30}\mathrm{Si}+{}^{197}\mathrm{Au}$ reactions. Quasifission yields show strong dependence on energy and angular momentum.

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• Fission
• Low & intermediate energy heavy-ion reactions
• Nuclear fragmentation