Background: Fusion hindrance refers to steeper descent of measured fusion excitation function compared with the predictions of standard coupled-channels models. Owing to experimental challenges, fusion measurements for asymmetric systems down to deep sub-barrier energies have been performed for only a handful of systems.

Purpose: We aim to investigate whether the entrance channel mass asymmetry affects the onset of fusion hindrance by measuring excitation functions for asymmetric systems and comparing the results with that of symmetric systems having similar values of the system parameter $\zeta$.

Method: Fusion cross sections were measured for the systems ${}^{16}\mathrm{O}+{}^{116}\mathrm{Cd}$ and ${}^{16}\mathrm{O}+{}^{142}\mathrm{Ce}$ down to energies $\simeq 13\%$ below the Bass barrier using a recoil mass spectrometer. Fusion barrier distributions were derived from the data employing the Gaussian analytic recipe, in addition to the usual double-derivative method. Experimental results were compared with predictions of coupled-channels calculations. Logarithmic slopes and astrophysical $S$-factors were extracted from the measured cross sections. Results of the present measurements were compared with the same from symmetric systems with nearly the same $\zeta$.

Results: Experimental excitation functions and barrier distributions were reproduced reasonably well by coupled-channels calculations in the range of energies considered in this study. Extrapolations of logarithmic slopes and $S$-factors to lower energies suggests onset of fusion hindrance for both systems. The energy, $4\sigma$ below the centroid of the lowest barrier, is found to be a good measure of the threshold for fusion hindrance.

Conclusions: Presence of fusion hindrance is inferred by extrapolation in both the systems. The Gaussian analytic recipe is found to be quite effective to obtain an unambiguous barrier distribution which, in turn, can be used for determining the threshold energy for fusion hindrance. Comparison of results between asymmetric and symmetric systems points to the fact that fusion hindrance is a generic feature of all the studied systems, independent of their entrance channel mass asymmetry.

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• Low & intermediate energy heavy-ion reactions
• Nuclear fusion
• Nuclear reactions
• Nuclear data analysis & compilation
• Optical, coupled-channel & distorted wave models
• Spectrometers & spectroscopic techniques