Abstract
This paper is devoted mainly to the fusion hindrance phenomenon, in its various aspects. Recent ex- perimental results on medium-light systems where the fusion Q-value is positive are discussed. The application of the coupled-channels model using a shallow ion-ion potential is illustrated for 32,36 S + 48 Ca. The detailed in- fluence of nuclear structure on the low-energy fusion cross sections is shown for the pair of systems 48 Ti + 58 Fe and 58 Ni + 54 Fe, where the low-lying quadrupole modes have a different degree of collectivity. The sub-barrier excitation function of 48 Ti + 58 Fe is much larger than for 58 Ni + 54 Fe. The lighter symmetric system 28 Si + 28 Si has been the object of recent experimental investigations. Its fusion cross sections have been measured in a wide energy range down to ≤1μb. Above the barrier, we have a clear indication of oscillations in the excitation function, probably due to the penetration of successive centrifugal barriers, that are in rather good agreement with previous calculations. The CC model using the shallow M3Y+ repulsion potential is able to reproduce also the sub-barrier part of the excitation function of 28 Si + 28 Si .
Highlights
The hindrance of heavy-ion fusion process at deep subbarrier energies has been observed for several systems [1,2,3] in the last decade
Measurements of fusion cross sections in the far subbarrier energy region are very challenging from the experimental point of view, but the results one can obtain have a far-reaching value for our understanding of fusion dynam
Two examples of the application of the CC model using shallow M3Y + repulsion potentials have been illustrated. They deal with the different behavior of the two systems 32,36S + 48Ca that were studied at Legnaro in recent years
Summary
The hindrance of heavy-ion fusion process at deep subbarrier energies has been observed for several systems [1,2,3] in the last decade. The onset of fusion hindrance has often been associated with the energy where L(E) reaches the value (named LCS ) expected for a constant astrophysical S factor. At that energy the S -factor develops a maximum as a function of the energy. Identifying its onset requires a comparison of the experimental data to standard coupled-channels (CC) calculations. Is really the fusion hindrance phenomenon a general one, and does it occur systematically in all systems with positive fusion Q-value? How (quantitatively) does the structure of the two colliding nuclei and, possibly, couplings to transfer channels, affect the energy threshold below which hindrance shows up? Is really the fusion hindrance phenomenon a general one, and does it occur systematically in all systems with positive fusion Q-value? How (quantitatively) does the structure of the two colliding nuclei and, possibly, couplings to transfer channels, affect the energy threshold below which hindrance shows up? And what is the basic mechanism underlying the experimental evidences?
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