Background: The fusion of heavy nuclei can be conceptually divided into two steps: capture of the projectile and target, then evolution towards a compact fully equilibrated compound nucleus. Reseparation through quasifission after capture inhibits compound nucleus formation, leading to fusion suppression.

Purpose: We aim to determine quantitatively the correlation between the degree of fusion suppression resulting from quasifission and the observed characteristics of the quasifission events themselves.

Method: Evaporation residues were measured for ${}^{28}\mathrm{Si}+{}^{192}\mathrm{Os}$ and ${}^{34}\mathrm{S}+{}^{186}\mathrm{W}$ forming the compound nucleus ${}^{220}\mathrm{Th}$, and identified using the implantation-decay method. Fusion suppression factors for these reactions (and others from the literature also forming ${}^{220}\mathrm{Th}$) were determined relative to the ${}^{16}\mathrm{O}+{}^{204}\mathrm{Pb}$ reaction. Fission characteristics were also measured for these three reactions, as well as for ${}^{48}\mathrm{Ca}+{}^{172}\mathrm{Yb}$.

Results: Overall, fusion probabilities are found to fall rapidly as the charge product $Z_p{Z}_t$ increases above the value for ${}^{16}\mathrm{O}+{}^{204}\mathrm{Pb}$. Crucially, although significant suppression of the $xn$ evaporation residue yield is observed for the ${}^{28}\mathrm{Si}$ and ${}^{34}\mathrm{S}$ reactions, there is no significant fission mass-angle correlation that would signal the presence of fast quasifission. However, fission mass widths are larger than those expected based on the ${}^{16}\mathrm{O}+{}^{204}\mathrm{Pb}$ mass widths, taken as corresponding to fusion-fission.

Conclusions: Formation of an equilibrated compact compound nucleus can be suppressed by over 50% due to (slow) quasifission, even when no significant evidence of a mass-angle correlation is seen. Evidence of slow quasifission is shown by the increased mass widths observed.

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• Fission
• Fusion evaporation reactions
• Low & intermediate energy heavy-ion reactions
• Nuclear fusion
• 190 ≤ A ≤ 219
• A ≥ 220
• Spectrometers & spectroscopic techniques