Background: Presence of closed proton and/or neutron shells causes deviation from macroscopic properties of nuclei, which are understood in terms of the liquid-drop model. Efforts to synthesize artificial elements are driven by the prediction of the existence of closed shells beyond the heaviest doubly magic nucleus found in nature. It is important to investigate experimentally the stabilizing effects of shell closure, if any, against fission.

Purpose: This Rapid Communication aims to investigate probable effects of proton shell ($Z=82$) closure in the compound nucleus in enhancing survival probability of the evaporation residues formed in heavy ion-induced fusion-fission reactions.

Method: Evaporation residue cross sections have been measured for the reactions ${}^{19}\mathrm{F}+{}^{180}\mathrm{Hf},{}^{19}\mathrm{F}+{}^{181}\mathrm{Ta}$, and ${}^{19}\mathrm{F}+{}^{182}\mathrm{W}$ from $\simeq 9\%$ below to $\simeq 42\%$ above the Coulomb barrier; leading to the formation of compound nuclei with the same number of neutrons ($N=118$) but different numbers of protons across $Z=82$ employing the Heavy Ion Reaction Analyzer. Measured excitation functions have been compared with a statistical model calculation in which the reduced dissipation coefficient is the only adjustable parameter.

Results: Evaporation residue cross section, normalized by the capture cross section, is found to decrease gradually with increasing fissility of the compound nucleus. Measured evaporation residue cross sections require inclusion of nuclear viscosity in the model calculations. Reduced dissipation coefficient in the range of $1–3\times {10}^{21}{\mathrm{s}}^{-1}$ reproduces the data quite well.

Conclusions: Since entrance channel properties of the reactions and structural properties of the heavier reaction partners are very similar, the degree of presence of noncompound nuclear fission, if any, is not expected to be significantly different in the three cases. No abrupt enhancement of evaporation residue cross sections has been observed in the reaction forming a compound nucleus with $Z=82$. Thus, this Rapid Communication does not find enhanced stabilizing effects of the $Z=82$ shell closure against fission in the compound nucleus. One may attempt to measure cross sections of individual exit channels for further confirmation of our observation.

• Received 15 March 2019
• Revised 20 May 2019

DOI:https://doi.org/10.1103/PhysRevC.99.061601