Background: The role of hexadecapole deformation (${\beta }_4$) in sub-barrier fusion has been studied for the ${}^{16}\mathrm{O}+{}^{174,176}\mathrm{Yb}$ reactions; however, the extracted ${\beta }_4$ values are model dependent and carry systematic uncertainties that remain unresolved.

Purpose: We extend fusion cross-section measurements to the ${}^{16}\mathrm{O}+{}^{170,172}\mathrm{Yb}$ reactions to investigate isotopic dependence of fusion excitation functions and to clarify the role of higher-order static deformations across the complete ${}^{16}\mathrm{O}+{}^{170,172,174,176}\mathrm{Yb}$ isotopic chain.

Method: Evaporation residue (ER) cross sections for ${}^{16}\mathrm{O}+{}^{170,172}\mathrm{Yb}$ were measured from 12% below to 35% above the Coulomb barrier using the Heavy Ion Reaction Analyzer (HIRA) at IUAC, New Delhi. The data are analyzed within the coupled-channels and statistical model frameworks.

Results: A strong sub-barrier fusion enhancement is observed relative to one-dimensional barrier penetration model (1D-BPM) predictions. Coupled-channels calculations incorporating quadrupole and hexadecapole deformations of the target nuclei reproduce the measured fusion excitation functions for both reactions. Statistical model calculations with a single fission-barrier scaling factor $k_f=1.10$ reproduce the ER and fission cross sections simultaneously, over the entire measured energy range. The single-Gaussian barrier-distribution analysis shows that ${}^{16}\mathrm{O}+{}^{174}\mathrm{Yb}$ exhibits the largest sub-barrier enhancement among the ${}^{170,172,174,176}\mathrm{Yb}$ nuclei.

Conclusions: Coupled-channels calculations including both quadrupole ${\beta }_2$ and hexadecapole ${\beta }_4$ deformations of the target nuclei successfully reproduce the measured fusion excitation functions for ${}^{16}\mathrm{O}+{}^{170,172}\mathrm{Yb}$. Our analysis brings out the role of ${\beta }_4$ across the entire range of Yb nuclei. The reproduction of ER and fission excitation functions with $k_f=1.10$ rules out noncompound nuclear fission (NCNF) contributions for both systems. The pronounced sub-barrier enhancement in ${}^{16}\mathrm{O}+{}^{174}\mathrm{Yb}$ is attributed to the midshell character of ${}^{174}\mathrm{Yb}$ ($N$ = 104), where nuclear collectivity is maximal.

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• Low & intermediate energy heavy-ion reactions
• Nuclear fusion
• Nuclear shapes and moments
• 150 ≤ A ≤ 189