In order to study the effects of heavy projectile neutron number on complete fusion with a well-deformed target, we have measured the excitation functions of the evaporation residue cross sections for $xn,$ $pxn,$ and $\alpha \mathrm{xn}$ channels $(x=2\mathrm{}–6)$ by using the JAERI recoil mass separator in the reactions of ${}^{60}\mathrm{Ni}{+}^{154}\mathrm{Sm}$ and ${}^{64}\mathrm{Ni}{+}^{154}\mathrm{Sm}$ at energies around the Coulomb barrier. In the ${}^{64}\mathrm{Ni}$-induced reaction, the evaporation residue cross section was larger by about two orders of magnitude than that in the ${}^{60}\mathrm{Ni}$-induced reaction. This is mainly due to the exit channel effect of the small neutron separation energy in the more neutron-rich compound nucleus. The fusion probability has been obtained from the calculated survival probability, which had agreed with the evaporation residue cross sections measured in the same compound nucleus system of ${}^{32}\mathrm{S}{+}^{182}\mathrm{W}.$ No obvious difference in the extracted fusion probability between the ${}^{64}\mathrm{Ni}$- and ${}^{60}\mathrm{Ni}$-induced fusion reactions was observed in the excitation functions. In both reaction systems, the fusion probabilities at the lowest energies, where collisions only at the tip of the deformed ${}^{154}\mathrm{Sm}$ nucleus are possible, were significantly smaller than the coupled channel calculation by three orders of magnitude. On the other hand, fusion hindrance was negligible at higher energies where side collisions with the deformed ${}^{154}\mathrm{Sm}$ become possible. This is consistent with our previous conclusion that tip collisions need some extra kinetic energy over the fusion barrier in order to fuse, while side collisions lead to complete fusion without such extra energy.

• Received 8 October 2001

DOI:https://doi.org/10.1103/PhysRevC.65.054608