Fusion cross sections forTi46,50+90Zr,93Nb and some systematics of heavy-ion fusion at barrier and subbarrier energies

Measurements of the fusion cross sections in the barrier and subbarrier regions are presented for the four heavy-ion systems ${}_{}{}^{46,50}{}_{}{}^{}\mathrm{Ti}$ ${+}^{90}$Zr${,}^{93}$Nb. The measured cross sections varied from 0.04 to 400 mb. The evaporation residues were detected by the use of a velocity filter. Procedures are described for extracting fusion cross sections from such measurements. The observed differences in the subbarrier fusion cross sections for the ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$ and ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$ nuclei, both of which have small collectivity, have led us to question the view that excursions of the strong nuclear force due to collective motions of the colliding nuclei are the primary enhancement mechanisms for the observed heavy-ion subbarrier fusion cross sections.