Evidence for hindrance in fusion between sulfur and lead nuclei

Abstract

The influence of the structure of projectile and target nuclei on the capture cross sections was investigated for the reactions ${}^{34}\mathrm{S}\phantom{\rule{0.16em}{0ex}}+{\phantom{\rule{0.16em}{0ex}}}^{204,206,208}$Pb and ${}^{36}\mathrm{S}\phantom{\rule{0.16em}{0ex}}+{\phantom{\rule{0.16em}{0ex}}}^{204,206,208}$Pb. Capture cross sections were deduced by measuring the fission fragments using multiwire proportional counters. An enhancement of the capture cross sections relative to a one-dimensional barrier penetration model was observed for all reactions at energies below the interaction barriers. The enhancement is larger in the case of reactions with ${}^{34}$S than with ${}^{36}$S. This observation is explained by a stronger coupling to the vibrational states in the reactions with ${}^{34}$S. Comparing the capture cross sections and the evaporation-residue cross sections for the reactions ${}^{36}\mathrm{S}\phantom{\rule{0.16em}{0ex}}+{\phantom{\rule{0.16em}{0ex}}}^{206}$Pb and ${}^{34}\mathrm{S}\phantom{\rule{0.16em}{0ex}}+{\phantom{\rule{0.16em}{0ex}}}^{208}$Pb, both yielding the same compound nucleus ${}^{242}$Cf, it is shown that the latter reaction has a lower fusion probability.