Evaporation residue cross sections have been measured following the fusion of ${}^{\mathrm{16}}\mathrm{O}$ with ${}^{\mathrm{204}}\mathrm{P}\mathrm{b}$, forming the compound nucleus ${}^{\mathrm{220}}\mathrm{T}\mathrm{h}$. These are compared with existing data for the same compound nucleus formed following fusion reactions with ${}^{\mathrm{40}}\mathrm{A}\mathrm{r}$, ${}^{\mathrm{48}}\mathrm{C}\mathrm{a}$, ${}^{\mathrm{82}}\mathrm{S}\mathrm{e}$, and ${}^{\mathrm{124}}\mathrm{S}\mathrm{n}$ projectiles. At energies where the reduced cross sections of $xn$ evaporation residues would be expected to be the same for all reactions, those for the heavier projectiles are typically a factor of 10 smaller than those for ${}^{\mathrm{16}}\mathrm{O}$. This inhibition is attributed to strong competition of quasifission with fusion at the low angular momenta associated with evaporation residue formation.

• Received 1 August 2002

DOI:https://doi.org/10.1103/PhysRevLett.89.282701