Observation of a Constant Average Angular Momentum for Fusion at Sub-Barrier Energies

The cross sections for the fusion reaction ${}_{}{}^{128}{}_{}{}^{}\mathrm{Te}({}_{}{}^{12}{}_{}{}^{}\mathrm{C}, 3n){}_{}{}^{137}{}_{}{}^{}\mathrm{Ce}$ were measured from $E_{\mathrm{c}.\mathrm{m}.\mathrm{}}=34.5\mathrm{} \mathrm{to} 50$ MeV by observing the decay of the ground ($J^{\pi }={\frac{3}{2}}^{+}$, $t_{\frac{1}{2}}=9.0\mathrm{}$ h) and isomeric (${\frac{11}{2}}^{-}$, 34.4 h) states in ${}_{}{}^{137}{}_{}{}^{}\mathrm{Ce}$. The isomer ratio decreases rapidly from a value of ≃34 at 47 MeV to an essentially constant value of ≃1.5 below 38 MeV. This confirms the prediction of an approximately constant value for the average angular momentum for fusion of heavy ions at energies well below the fusion barrier.