Charged-particle branching ratios above the neutron threshold in $^{19}$F: constraining $^{15}$N production in core-collapse supernovae
Résumé
Background: Spatially correlated overabundances of N15 and O18 observed in some low-density graphite meteoritic grains have been connected to nucleosynthesis taking place in the helium-burning shell during core-collapse supernovae. Two of the reactions which have been identified as important to the final abundances of N15 and O18 are F18(n,α)N15 and F18(n,p)O18. The relative strengths of the F18(n,α)N15 and F18(n,p)O18 reactions depend sensitively on the relative α0 and p0 decay branches from states above the neutron threshold in F19 in addition to other properties such as the spins, parities, and neutron widths. However, experimental data on the charged-particle decays from these highly excited states are lacking or inconsistent. Purpose: We measure the charged-particle decay branches from states around the neutron threshold in F19. Method: Two experiments were performed using proton inelastic scattering from LiF targets and magnetic spectrographs. The first experiment used the high-resolution Q3D spectrograph at Munich to constrain the properties of levels in F19. A second experiment using the Orsay split-pole spectrograph and an array of silicon detectors was performed in order To measure the charged-particle decay branches from states around the neutron threshold in F19. Results: A number of levels in F19 have been identified along with their corresponding charged-particle decays. The first state above the neutron threshold which has an observed proton-decay branch to the ground state of O18 lies 68 keV (Ex=10.5 MeV) above the neutron threshold. The α-particle decays from the neutron-unbound levels are generally observed to be much stronger than the proton decays. Conclusion:Neutron-unbound levels in F19 are observed to decay predominantly by α-particle emission, supporting the role of F18(n,α)N15 in the production of N15 in the helium-burning shell of supernovae. Improved resonant-scattering reaction data are required in order to be able to determine the reaction rates accurately.