Charged-particle branching ratios above the neutron threshold in F19 : Constraining N15 production in core-collapse supern

Abstract

Background: Spatially-correlated overabundances of 15 N and 18 O 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 15 N and 18 O are 18 F(n, α) 15 N and 18 F(n, p) 18 O.Purpose: The relative strengths of the 18 F(n, α) 15 N and 18 F(n, p) 18 O reactions depend sensitively on the relative α0 and p0 decay branches from states above the neutron threshold in 19 F in addition to other properties such as the spins and parities, and the neutron widths. However, experimental data on the charged-particle decays from these highly excited states are lacking or inconsistent.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 19 F. A second experiment using the Orsay Split-Pole spectrograph and an array of silicon detectors was performed in order to measure the charged-particle decays of neutron-unbound levels in 19 F.Results: A number of levels in 19 F 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 18 O 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 19 F are observed to decay predominantly by α-particle emission, supporting the role of 18 F(n, α) 15 N in the production of 15 N 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. I. ASTROPHYSICAL BACKGROUND *

Understanding globular cluster abundances through nuclear reactions

“…This means that the states populated in 23 Na are not being populated in resonance scattering but in some other, rather non-selective, reaction mechanism which appears to be compound in nature. This reaction has been used to study a number of different isotopes already, such as 31 P, 28 Si, 27 Al, 24,26 Mg, 23 Na, and 19 F [10][11][12][13][14][15][16].…”