Entrance channel dependence of quasifission in reactions formingTh220

Mass-angle correlations of binary fragments produced in $^{16}\mathrm{O}+^{204}\mathrm{Pb}$, $^{34}\mathrm{S}+^{186}\mathrm{W}$, and $^{48,50}\mathrm{Ti}+^{166,170}\mathrm{Er}$ reactions have been measured for a range of bombarding energies around their Coulomb barriers. At above-barrier energies, the width of the mass distributions for the fission-like fragments in the $^{50}\mathrm{Ti}+^{170}\mathrm{Er}$ reaction are found to be higher than those from the same compound system at similar excitation energies populated via the more mass asymmetric entrance channel reaction $^{34}\mathrm{S}+^{186}\mathrm{W}$, which in turn is higher than those for the $^{16}\mathrm{O}+^{204}\mathrm{Pb}$ system. The width of the mass distributions of the $\mathrm{Ti}+\mathrm{Er}$ systems is found to increase with decreasing bombarding energies, in contrast with those of the $^{16}\mathrm{O}+^{204}\mathrm{Pb}$ and $^{34}\mathrm{S}+^{186}\mathrm{W}$ systems, which show a monotonic reduction in mass widths. This may be associated with the elongated contact configuration at sub-barrier energies.