Fusion hindrance for the positive Q -value system C 12 + Si 30

G. Montagnoli, A. M. Stefanini, C. L. Jiang, K. Hagino, F. Galtarossa, G. Colucci, S. Bottoni, C. Broggini, A. Caciolli, P. Čolović, L. Corradi, S. Courtin, R. Depalo, E. Fioretto, G. Fruet, A. Gal, A. Goasduff, M. Heine, S. P. Hu, M. KaurT. Mijatović, M. Mazzocco, D. Montanari, F. Scarlassara, E. Strano, S. Szilner, G. X. Zhang

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Abstract

Background: The fusion reaction C12+Si30 is a link between heavier cases studied in recent years, and the light heavy-ion systems, e.g., C12+C12, O16+O16 that have a prominent role in the dynamics of stellar evolution. C12+Si30 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 C12+Si30 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 C12+Si30 from well below to above the Coulomb barrier, so as to clear up the consequence of couplings to low-lying states of Si30, and whether the hindrance effect appears in this relatively light system which has a positive Q value for fusion. This would have consequences for the extrapolated behavior to even lighter systems. Methods: The inverse kinematics was used by sending Si30 beams delivered from the XTU Tandem accelerator of INFN-Laboratori Nazionali di Legnaro onto thin C12 (50μg/cm2) targets enriched to 99.9% 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 Ebeam=45, 59, and 80 MeV, and they were angle integrated to derive total fusion cross sections. Results: The fusion excitation function of C12+Si30 was measured with high statistical accuracy, covering more than five orders of magnitude down to a lowest cross section ≃3μb. The logarithmic slope and the S 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 C12+Si30, so that an extrapolation to the C+C and O+O cases can be reliably performed.

Original languageEnglish
Article number024610
JournalPhysical Review C
Volume97
Issue number2
DOIs
Publication statusPublished - 2018 Feb 12

ASJC Scopus subject areas

  • Nuclear and High Energy Physics

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    Montagnoli, G., Stefanini, A. M., Jiang, C. L., Hagino, K., Galtarossa, F., Colucci, G., Bottoni, S., Broggini, C., Caciolli, A., Čolović, P., Corradi, L., Courtin, S., Depalo, R., Fioretto, E., Fruet, G., Gal, A., Goasduff, A., Heine, M., Hu, S. P., ... Zhang, G. X. (2018). Fusion hindrance for the positive Q -value system C 12 + Si 30. Physical Review C, 97(2), [024610]. https://doi.org/10.1103/PhysRevC.97.024610