Fusion cross sections have been measured for the asymmetric system ${}^{11}\mathrm{B}+{}^{169}\mathrm{Tm}$ at near- and deep sub-barrier energies to investigate the occurrence of fusion hindrance. Cross sections down to $\approx 200$ nb have been measured. The fusion cross sections have been compared with coupled-channels calculations. The onset of fusion hindrance appears to occur at $E_{\mathrm{c}.\mathrm{m}.}=37.0\pm 1.5$ MeV, where a deviation in the slope of the experimental logarithmic derivative of $E\sigma$, compared to that of the coupled-channels calculations, has been observed. The adiabatic model with a Yukawa-plus-exponential potential and damping of coupling effects can reproduce the data at deep sub-barrier energies. The damping parameters for the present system are consistent with previous observations for far-asymmetric systems. Although the present system has a permanently deformed rotational rare-earth target nucleus, the extracted damping parameters are closer to those of stiff asymmetric systems such as ${}^{16}\mathrm{O}+{}^{208}\mathrm{Pb}$ and ${}^{48}\mathrm{Ca}+{}^{96}\mathrm{Zr}$, compared to the systems with vibrational target nuclei ${}^{11}\mathrm{B}+{}^{197}\mathrm{Au}$ and ${}^{12}\mathrm{C}+{}^{198}\mathrm{Pt}$. This may indicate a different trend of damping for rotational couplings, compared to the damping of vibrational couplings. More measurements of deep sub-barrier fusion with permanently deformed rare-earth target nuclei could help clarify the fundamental difference between the damping of rotational and vibrational couplings in the deep sub-barrier region.

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• Nuclear fusion
• 150 ≤ A ≤ 189
• Optical, coupled-channel & distorted wave models
• Radiation detectors