Beyond the Coherent Coupled Channels Description of Nuclear Fusion

New measurements of fusion cross sections at deep sub-barrier energies for the reactions ${}^{16}\mathrm{O}+{}^{204,208}\mathrm{Pb}$ show a steep but almost saturated logarithmic slope, unlike ${}^{64}\mathrm{Ni}$-induced reactions. Coupled channels calculations cannot simultaneously reproduce these new data and above-barrier cross-sections with the same Woods-Saxon nuclear potential. It is argued that this highlights an inadequacy of the coherent coupled channels approach. It is proposed that a new approach explicitly including gradual decoherence is needed to allow a consistent description of nuclear fusion.

Figure 1

Ratio of evaporation residue to fission cross sections as a function of excitation energy of the compound nucleus. The lines are fits to the data (see text).

Figure 2

Fusion cross sections as a function of the center-of-mass energy with respect to the barrier energies $B=74.5$ and 74.9 MeV for ${}^{208}\mathrm{Pb}$ and ${}^{204}\mathrm{Pb}$, respectively.

Figure 3

Logarithmic slope as a function of energy with respect to the barrier. Calculation with standard parameters fail to match the measurements at low energy.

Figure 4

Calculations which reproduce $\sigma$ above the barrier (a) fail to reproduce the low energy data (b).