Effect of shell structure in the fusion reactions82Se+134Baand82Se+138Ba

Abstract

The dependence of fusion on the nuclear shell structure was investigated for the two reaction systems ${}^{82}\mathrm{Se}{+}^{138}\mathrm{Ba}$ and ${}^{82}\mathrm{Se}{+}^{134}\mathrm{Ba},$ where the nucleus ${}^{138}\mathrm{Ba}$ has a closed neutron shell $N=82$ while the nucleus ${}^{134}\mathrm{Ba}$ has a neutron number $N=78,$ four neutrons less than the closed shell. Evaporation residues for these reaction systems were measured in the vicinity of the Coulomb barrier. The measured evaporation residue cross sections of $\mathrm{xn}$ and $\mathrm{pxn}$ channels for the reaction system ${}^{82}\mathrm{Se}{+}^{138}\mathrm{Ba}$ were considerably larger than those for the reaction system ${}^{82}\mathrm{Se}{+}^{134}\mathrm{Ba},$ almost 100 times larger at the excitation energy ${(E}_{\mathrm{ex}})$ region of 20--30 MeV. The fusion probabilities for these reaction systems were obtained from the evaporation residue cross sections with the aid of calculated survival probability and compared with those of the other reaction systems that make the same compound nucleus as the present systems. It was found that the fusion reaction ${}^{82}\mathrm{Se}{+}^{138}\mathrm{Ba}$ occurs without hindrance, while that of ${}^{82}\mathrm{Se}{+}^{134}\mathrm{Ba}$ is considerably hindered as commonly seen in the massive reaction system with the charge product ${Z}_{\mathrm{p}}{Z}_{\mathrm{t}}>~1800$ of projectile and target. The present result suggests that the neutron shell closure $N=82$ promotes fusion.