Abstract
Background: The fusion reaction is a link between heavier cases studied in recent years, and the light heavy-ion systems, e.g., that have a prominent role in the dynamics of stellar evolution. fusion itself is not a relevant process for astrophysics, but it is important to establish its behavior below the barrier, where couplings to low-lying collective modes and the hindrance phenomenon may determine the cross sections. The excitation function is presently completely unknown below the barrier for the reaction, thus no reliable extrapolation into the astrophysical regime for the C+C and O+O cases can be performed.
Purpose: Our aim was to carry out a complete measurement of the fusion excitation function of from well below to above the Coulomb barrier, so as to clear up the consequence of couplings to low-lying states of , and whether the hindrance effect appears in this relatively light system which has a positive value for fusion. This would have consequences for the extrapolated behavior to even lighter systems.
Methods: The inverse kinematics was used by sending beams delivered from the XTU Tandem accelerator of INFN-Laboratori Nazionali di Legnaro onto thin () targets enriched to in mass 12. The fusion evaporation residues (ER) were detected at very forward angles, following beam separation by means of an electrostatic deflector. Angular distributions of ER were measured at , 59, and 80 MeV, and they were angle integrated to derive total fusion cross sections.
Results: The fusion excitation function of was measured with high statistical accuracy, covering more than five orders of magnitude down to a lowest cross section . The logarithmic slope and the factor have been extracted and we have convincing phenomenological evidence of the hindrance effect. These results have been compared with the calculations performed within the model that considers a damping of the coupling strength well inside the Coulomb barrier.
Conclusions: The experimental data are consistent with the coupled-channels calculations. A better fit is obtained by using the Yukawa-plus-exponential potential and a damping of the coupling strengths inside the barrier. The degree of hindrance is much smaller than the one in heavier systems. Also a phenomenological estimate reproduces quite closely the hindrance threshold for , so that an extrapolation to the C+C and O+O cases can be reliably performed.
- Received 6 November 2017
DOI:https://doi.org/10.1103/PhysRevC.97.024610
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