Effects of entrance channels on the evaporation residue yields in reactions leading to theTh220compound nucleus

Abstract

The evaporation residue yields from the compound nuclei $^{220}\mathrm{Th}$ formed in the $^{16}\mathrm{O}+^{204}\mathrm{Pb}, ^{40}\mathrm{Ar}+^{180}\mathrm{Hf}, ^{82}\mathrm{Se}+^{138}\mathrm{Ba}$, and $^{124}\mathrm{Sn}+^{96}\mathrm{Zr}$ reactions are analyzed to study the entrance channel effects by comparison of the capture, fusion, and evaporation residue cross sections calculated by the combined dinuclear system (DNS) and advanced statistical models. The difference between evaporation residue (ER) cross sections can be related to the stages of compound nucleus formation and/or its survival against fission. The sensitivity of both stages in the evolution of the DNS up to the evaporation residue formation to the angular momentum of DNS is studied. The difference between fusion excitation functions is explained by the hindrance to complete fusion due to the larger intrinsic fusion barrier ${B}_{\mathrm{fus}}^{*}$ for the transformation of the DNS into a compound nucleus and the increase of the quasifission contribution due to the decreasing of the quasifission barrier ${B}_{\mathrm{qf}}$ as a function of the angular momentum. The largest value of the ER residue yields in the very mass asymmetric $^{16}\mathrm{O}+^{204}\mathrm{Pb}$ reaction is related to the large fusion probability and to the relatively low threshold of the excitation energy of the compound nucleus. Due to the large threshold of the excitation energy (35 MeV) of the $^{40}\mathrm{Ar}+^{180}\mathrm{Hf}$ reaction, it produces ER yields less than the almost mass symmetric $^{82}\mathrm{Se}+^{138}\mathrm{Ba}$ reaction having the lowest threshold value (12 MeV).