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A large number of precision fusion excitation functions, at energies above the average fusion barriers, have been fitted using the Woods-Saxon form for the nuclear potential in a barrier passing model of fusion. They give values for the empirical diffuseness parameter a ranging between 0.75 and 1.5 fm, compared with values of about 0.65 fm which generally reproduce elastic scattering data. There is a clear tendency for the deduced a to increase strongly with the reaction charge product Z 1 Z 2 , and some evidence for the effect of nuclear structure on the value of a, particularly with regard to the degree of neutron richness of the fusing nuclei, and possibly with regard to deformation. The measured fusion-barrier energies are always lower than those of the bare potentials used, which is expected as a result of adiabatic coupling to high energy collective states. This difference increases with increasing Z 1 Z 2 and calculations show that about 1 / 3 of it may be attributed to coupling to the isoscalar giant-quadrupole resonances in the target and projectile. Coupling to all giant resonances may account for a significant part. Fluctuations about the trend line may be due to systematic errors in the data and/or structure effects such as coupling to collective octupole states. Previously suggested reasons for the large values of a have been related to departures from the Woods-Saxon potential and to dissipative effects. This work suggests that the apparently large values of a may be an artifact of trying to describe the dynamical fusion process by use of a static potential. Another partial explaination might reside in fusion inhibition, due for example to deep-inelastic scattering, again a process requiring dynamical calculations.

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We calculate the capture (fusion) cross sections for nine reactions involving spherical nuclei: 16 O + 16 O, 28 Si, 92 Zr, 144 Sm, 208 Pb; 28 Si + 28 Si, 92 Zr, 208 Pb; 32 S + 208 Pb. For six of them precision data are available in the literature. Analysis of these precision data within the framework of the single-barrier penetration model based on the Woods-Saxon profile for the strong nucleus-nucleus interaction potential (SnnP) gave rise to the problem of the apparently large diffuseness of the SnnP [Newton et al., Phys. Rev. C 70, 024605 (2004)]. Our fluctuation-dissipation trajectory model is based on the double-folding approach with the density-dependent M3Y NN forces including the finite-range exchange part. For the nuclear matter density the Skyrme-Hartree-Fock approach including the tensor interaction is applied. The resulting nucleus-nucleus potential possesses rather small (normal) diffuseness. The strength of the radial friction K R is used as the free parameter of the model. It turns out that for four of the five reactions induced by 16 O (except 16 O + 208 Pb) the calculated cross sections cannot be brought into agreement with the data within the experimental errors. This suggests that the calculated nuclear density is incorrect for 16 O. For the reactions not involving 16 O and, surprisingly, for the 16 O + 208 Pb reaction the agreement with the data within 2-5% is achieved at K R = 1.2 × 10 −2 to 3.0 × 10 −2 MeV −1 zs which is in accord with the previous works.

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