Measurement ofγrays from15N(p,γ)16O cascade and15N(p,α1γ)12C reactions

Abstract

Background The main energy production mechanism for massive stars during hydrogen burning is the CNO cycle. The reactions ${}^{15}$N${(p,\phantom{\rule{-0.16em}{0ex}}\ensuremath{\gamma})}^{16}$O and ${}^{15}$N${(p,\phantom{\rule{-0.16em}{0ex}}{\ensuremath{\alpha}}_{0})}^{12}$C form a branch point in this cycle. The ratio of the corresponding reaction rates determines the CNO abundances evolving during this early stage of the star's life which affects the subsequent nucleosynthesis in later phases of stellar evolution. Determining the cross sections for these reactions at stellar energies is often very difficult. Measurements of other reactions that populate the same compound nucleus can often be used to indirectly determine the cross section of interest.Purpose The nuclear level properties of broad resonances in ${}^{16}$O which characterize the cross section of the reactions ${}^{15}$N${(p,\phantom{\rule{-0.16em}{0ex}}\ensuremath{\gamma})}^{16}$O and ${}^{15}$N${(p,\phantom{\rule{-0.16em}{0ex}}{\ensuremath{\alpha}}_{0})}^{12}$C must be well known in order to accurately extrapolate the measured cross sections to the stellar energy range. The $R$-matrix formalism is a powerful technique for interpreting these cross sections and is greatly enhanced by additional data in other reaction channels. In a previous publication, measurements were reported for the cross section of the reaction ${}^{15}$N${(p,\phantom{\rule{-0.16em}{0ex}}\ensuremath{\gamma})}^{16}$O for the ground state transition only. Concurrently, $\ensuremath{\gamma}$-ray measurements were recorded for the cascade transitions to the ${E}_{x}$ $=$ 6.050, 6.130, and 7.117 MeV bound states of ${}^{16}$O as well as from the reaction ${}^{15}$N${(p,\phantom{\rule{-0.16em}{0ex}}{\ensuremath{\alpha}}_{1}\ensuremath{\gamma})}^{12}$C. Excitation curves for the cascade transitions have never been measured and the excitation curve data for the ${}^{15}$N${(p,\phantom{\rule{-0.16em}{0ex}}{\ensuremath{\alpha}}_{1}\ensuremath{\gamma})}^{12}$C reaction found in the literature may suffer from substantial errors due to target contamination.Methods Angle integrated cross sections are measured over the proton energy range from ${E}_{p}$ $=$ 0.14 to 1.80 MeV for the $\ensuremath{\gamma}$-ray cascade transitions and for the reaction ${}^{15}$N${(p,\phantom{\rule{-0.16em}{0ex}}{\ensuremath{\alpha}}_{1}\ensuremath{\gamma})}^{12}$C.Results De-excitations associated with several compound nucleus states in ${}^{16}$O are observed in both the $\ensuremath{\gamma}$-ray and ${\ensuremath{\alpha}}_{1}$ channels. An $R$-matrix analysis is performed and partial decay widths are deduced for several previously unobserved decay branchings from these states.Conclusion For the first time, excitation curves for the cascade transitions to the ${}^{16}$O bound states at ${E}_{x}$ $=$ 6.050, 6.130, and 7.117 MeV are reported over the energy range from ${E}_{p}$ $=$ 0.14 to 1.80 MeV. In addition, an improved measurement of the ${}^{15}$N${(p,\phantom{\rule{-0.16em}{0ex}}{\ensuremath{\alpha}}_{1}\ensuremath{\gamma})}^{12}$C excitation curve has been made over a similar energy range.