Isotopic differences in the fusion cross sections for 0+ 0, observed by Thomas et al. , are interpreted with a coupled-channel model, We find that most of the subbarrier enhancement of 0+ 0 over '60+ 0 is attributable to the excitation of the low 2+ state in 0. However, the model predicts the enhancement to persist above the barrier, contrary to experimental findings.The role of intrinsic degrees of freedom in subbarrier fusion reactions has become a subject of considerable interest. An overview of the present experimental and theoretical status of subbarrier fusion is given in the proceedings of a recent topical conference. ' Strong isotopic dependences are found for the cross sections, particlarly in heavy nuclei. The calculation of the cross sections is numerically difficult for heavy nuclei, since many channels must be included. This point was brought out in the semiclassical treatment of Ref. 2. Theory has not been entirely successful so far in reproducing the cross sections for heavy systems, so it is important to find simpler cases more amenable to complete theoretical treatment. A recent study comparing fusion cross sections of ' 0 and ' 0 on ' 0 targets provides a nice case that we will examine here ( Fig. 1). For the '60+'60 system, the internal degrees of freedom are relatively unimportant because of the high excitation energies of inelastic channels and particle transfer channels. The nucleus ' 0, on the other hand, has a 2+ state at 1.98 MeV excitation with a transition strength from the ground state of 16.6 %eisskopf units.The experimental finding of Ref. 4 is that the ' 0 fusion cross section is enhanced over ' 0 in the subbarrier region. %e show this in Fig. 2 with the cross section ratio plotted as a function of the center-of-mass energy. The enhancement is small compared to that found in heavier nuclei, but there is certainly an effect present. In Ref. 4 it is suggested that the difference is mainly due to changes in the potential barrier going from one system to another. So, as a first step in our analysis, we will examine the expected effect of potential changes adding two nucleons to ' O. %e also expect a significant effect from the low 2+ state, which may be roughly estimated from the frozen approximation of Esbensen. ' According to Eq. (24) of Ref. 2, the enhancement in the subbarrier fusion may be estimated as 10 10

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