Measurement of elastic12C+αscattering: Above the proton separation energy

Abstract

Background: Reactions that probe the compound nucleus structure of ${}^{16}$O near the $\ensuremath{\alpha}$ and proton separation energies have important implications for nuclear astrophysics. New, more sensitive measurements of the reaction ${}^{15}$N${(p,\ensuremath{\gamma})}^{16}$O have motivated the need for improved compound-nucleus reaction data in other channels. A previous work [P. Tischhauser et al., Phys. Rev. C 79, 055803 (2009)] reported extensive $\ensuremath{\alpha}$-scattering data over the energy region from ${E}_{\ensuremath{\alpha}}=2.6$ to ${E}_{\ensuremath{\alpha}}=6.6$ MeV. During this experiment data for higher energies, from ${E}_{\ensuremath{\alpha}}=6.6$ to ${E}_{\ensuremath{\alpha}}=8.2$ MeV, were also recorded but were not analyzed. The unpublished data cover the excitation energy range from the proton separation energy at ${E}_{x}$ $=$ 12.13 MeV up to 13.31 MeV, the same energy range important for the reaction ${}^{15}$N${(p,\ensuremath{\gamma})}^{16}$O.Purpose: These previously unpublished data for ${}^{12}$C${(\ensuremath{\alpha},\ensuremath{\alpha})}^{12}$C as well as the reactions ${}^{12}$C${(\ensuremath{\alpha},{\ensuremath{\alpha}}_{1})}^{12}$C and ${}^{12}$C${(\ensuremath{\alpha},p)}^{15}$N are analyzed in order to provide additional data for a comprehensive $R$-matrix analysis of compound-nucleus reactions populating ${}^{16}$O.Methods: Expanding on the previous publication's analysis, the ratio of the reaction yields was analyzed using a multiple-channel $R$-matrix calculation.Results: The excitation curves resolved resonances at ${E}_{\ensuremath{\alpha}}$ (${J}^{\ensuremath{\pi}})=7.05\phantom{\rule{0.28em}{0ex}}({1}^{\ensuremath{-}}$), 7.75 (2${}^{+}$), 7.91 (1${}^{\ensuremath{-}}$), 7.97 (3${}^{\ensuremath{-}}$), and 8.13 (3${}^{\ensuremath{-}}$) MeV. The $R$-matrix analysis revealed excellent agreement between the measured yield ratios and cross-section data from the literature. The two 1${}^{\ensuremath{-}}$ levels at ${E}_{\ensuremath{\alpha}}=7.05$ and 7.91 MeV are those that dominate the ${}^{15}$N${(p,\ensuremath{\gamma})}^{16}$O cross section.Conclusions: In a future publication, the additional constraints provided by these data will be used in a comprehensive multiple-channel $R$-matrix analysis to investigate the low-energy cross section of the reaction ${}^{15}$N${(p,\ensuremath{\gamma})}^{16}$O.