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Fusion cross sections of 28 Si + 28 Si have been measured in a range above the barrier with a very small energy step ( Δ E lab = 0.5 MeV ). Regular oscillations have been observed, best evidenced in the first derivative of the energy-weighted excitation function. For the first time, quite different behaviors (the appearance of oscillations and the trend of sub-barrier cross sections) have been reproduced within the same theoretical frame, i.e., the coupled-channel model using the shallow M3Y + repulsion potential. The calculations suggest that channel couplings play an important role in the appearance of the oscillations, and that the simple relation between a peak in the derivative of the energy-weighted cross section and the height of a centrifugal barrier is lost, and so is the interpretation of the second derivative of the excitation function as a barrier distribution for this system, at energies above the Coulomb barrier. [ABSTRACT FROM AUTHOR]

Measurements of fusion excitation functions of 58Ni + 124Sn and 64Ni + 124Sn are extended towards lower energy to cross sections of 1 fib and are compared to detailed coupled-channels calculations. The calculations clearly show the importance of including transfer reactions in a coupled-channels treatment for such heavy systems. This result is different from the conclusion made in a previous article which claimed that the influence of transfer on fusion is not important for fusion reactions of Ni + Sn. In the energy region studied in this experiment no indication of fusion hindrance has been observed, which is consistent with a systematic study of this behavior. [ABSTRACT FROM AUTHOR]

Background: The fusion excitation function of the system 28Si + 28 at energies near and below the Coulomb barrier is known only down to ≃ 15 µb. This precludes any information on both coupling effects on sub-barrier cross sections and the possible appearance of hindrance. For 28Si + 30 even if the fusion cross section is measured down to ≃50 µb, the evidence of hindrance is marginal. Both systems have positive fusion Q values. While 28Si has a deformed oblate shape, 30Si is spherical. Purpose: We investigate 1. the possible influence of the different structure of the two Si isotopes on the fusion excitation functions in the deep sub-barrier region and 2. whether hindrance exists in the Si+Si systems and whether it is strong enough to generate an S-factor maximum, thus allowing a comparison with lighter heavy-ion systems of astrophysical interest. Methods: 28Si beams from the XTU Tandem accelerator of the INFN Laboratori Nazionali di Legnaro were used. The setup was based on an electrostatic beam separator, and fusion evaporation residues (ER) were detected at very forward angles. Angular distributions of ER were measured. Results: Fusion cross sections of 28Si + 28 have been obtained down to ≃600 nb. The slope of the excitation function has a clear irregularity below the barrier, but no indication of a S-factor maximum is found. For 28Si + 30 the previous data have been confirmed and two smaller cross sections have been measured down to ≃4 µb. The trend of the S-factor reinforces the previous weak evidence of hindrance. Conclusions: The sub-barrier cross sections for 28Si + 28 are overestimated by coupled-channels calculations based on a standard Woods-Saxon potential, except for the lowest energies. Calculations using the M3Y+repulsion potential are adjusted to fit the 28Si + 28 and the existing 30Si + 30 data. An additional weak imaginary potential (probably simulating the effect of the oblate 28Si deformation) is required to fit the low-energy trend of 28Si + 28. The parameters of these calculations are applied to predict the ion-ion potential for 28Si + 30. Its cross sections are well reproduced by also including one- and successive two-neutron transfer channels, besides the low-lying surface excitations. [ABSTRACT FROM AUTHOR]

The fusion excitation function of 32S + 48Ca has been experimentally studied in a wide energy range, from above the Coulomb barrier down to cross sections in the sub-µb region. The measurements were done at INFN-Laboratori Nazionali di Legnaro, using the 32S beam from the XTU Tandem accelerator. The excitation function has a smooth behavior below the barrier, and no evident hindrance character shows up in the measured energy region. The fusion barrier distribution has a peculiar shape with two distinct peaks of similar height, lower and higher than the Akyüz-Winther barrier. Coupled-channels calculations using the M3Y + repulsion potential are presented for this system and for 36S + 48Ca. The results of these calculations give a good account of the data, and indicate the influence of one- and two-nucleon transfer channels with positive Q values, which are only open for 32S + 48Ca. [ABSTRACT FROM AUTHOR]

The fusion excitation function of 40Ca + 40Ca has been measured from well above the Coulomb barrier, down to low energies where the cross section is as small as ≈ 20 μb, and the astrophysical S factor possibly reaches a maximum vs energy. The results of coupled-channels calculations using the M3Y + repulsion potential are presented. A detailed comparison is made with recently published data on the fusion of 40Ca + 48Ca and of 48Ca + 48Ca, including the excitation functions, their low-energy slopes and the barrier distributions. The presence of the fusion hindrance phenomenon in all cases is pointed out, as well as the influence of the strong octupole excitation in 40Ca and of nucleon transfer channels with positive Q values in 48Ca + 48Ca. [ABSTRACT FROM AUTHOR]