Abstract
The phenomenon of hindrance in sub-barrier heavy-ion fusion has been confirmed by several experimentalevidences and it is now recognised as a general phenomenon of heavy-ion fusion process. In many cases the signature of fusion hindrance lies in the trend of the logarithmic slope of the excitation function and of the S factor at low energies. The comparison with stadard Coupled-Channels calculations is a more quantitative evidence for its existence. In many medium-heavy systems the hindrance effect has been recognised with different features depending on the various couplings to the inelastic and transfer channels. Different theoretical appro ches have been proposed to explain the hindrance but the underlying physics is still a matter of debate. Hindrance is observed in light systems, independent of the sign of the fusion Q-value, with different features. In the case of the 12C + 30Si system the effect is smallbut it is clearly observed. Near-by cases show evidence for systematic behaviours. A very recent experiment has concerned the lighter case 12C + 24Mg where hindrance shows up clearly, because a maximum of the S factor appears already at a relatively high cross section σ=1.6mb. The consequences for the dynamics of stellar evolution have to be clarified by further experimental and theoretical work.
Highlights
Heavy-ion fusion reactions near and below the barrier are typical phenomena where the interplay of reaction dynamics and nuclear structure leads to a large enhancement of the fusion probability with respect to the one-dimensional quantum tunnelling limit
An important step forward in the fusion studies has been the introduction of the concept of barrier distribution [1] generated by the channel couplings and the possibility to obtain it from the experimental excitation functions
The phenomenon of hindrance in sub-barrier heavy-ion fusion was discovered about 15 years ago and it is presently established to be a general phenomenon
Summary
Heavy-ion fusion reactions near and below the barrier are typical phenomena where the interplay of reaction dynamics and nuclear structure leads to a large enhancement of the fusion probability with respect to the one-dimensional quantum tunnelling limit. Extending down in energy it has been observed that the fusion excitation functions of many systems decrease more steeply than the predictions of standard Coupled-Channels (CC) calculations based on a Wood-Saxon potential This phenomenon known as fusion hindrance has been recognised as a general feature forcing us to rethink the shape of the inner part of the potential barrier as some theoretical models indicate. In this talk the cases of 12C + 30Si, 24Mg, recently studied at LNL, will be presented and discussed, showing the sub-barrier trend of their excitation function and of the S factor. The two medium-light systems 12C + 30Si, 24Mg may be helpful to establish a reliable systematic behaviour to guide the extrapolation to lighter cases of astrophysical interest
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