Measurement of γ rays from 15N(p,γ)16O cascade and 15N(p,α1γ)12C reactions

Background The main energy production mechanism for massive stars during hydrogen burning is the CNO cycle. The reactions 15N(p, γ)16O and 15N(p, α0)12C 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 16O which characterize the cross section of the reactions 15N(p, γ)16O and 15N(p, α0)12C 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 15N(p, γ)16O for the ground state transition only. Concurrently, γ-ray measurements were recorded for the cascade transitions to the Ex = 6.050, 6.130, and 7.117 MeV bound states of 16O as well as from the reaction 15N(p, α1γ)12C. Excitation curves for the cascade transitions have never been measured and the excitation curve data for the 15N(p, α1γ)12C 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 Ep = 0.14 to 1.80 MeV for the γ-ray cascade transitions and for the reaction 15N(p, α1γ)12C. Results De-excitations associated with several compound nucleus states in 16O are observed in both the γ-ray and α1 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 16O bound states at Ex = 6.050, 6.130, and 7.117 MeV are reported over the energy range from Ep = 0.14 to 1.80 MeV. In addition, an improved measurement of the 15N(p, α1γ)12C excitation curve has been made over a similar energy range.