Low-energy resonances in the O 18 (p,γ)19 F reaction

Background: Shell hydrogen burning during the asymptotic giant branch (AGB) phase through the oxygen isotopes has been indicated as a key process that is needed to understand the observed O18/O16 relative abundance in presolar grains and in stellar atmospheres. This ratio is strongly influenced by the relative strengths of the reactions O18(p,α)N15 and O18(p,γ)F19 in low-mass AGB stars. While the former channel has been the focus of a large number of measurements, the (p,γ) reaction path has only recently received some attention and its stellar reaction rate over a wide temperature range rests on only one measurement. Purpose: Our aim is the direct measurement of states in F19 as populated through the reaction O18(p,γ)F19 to better determine their influence on the astrophysical reaction rate, and more generally to improve the understanding of the nuclear structure of F19.Method: Branchings and resonance strengths were measured in the proton energy range Eplab=150-400keV, using a high-purity germanium detector inside a massive lead shield. The measurement took place in the ultra-low-background environment of the Laboratory for Underground Nuclear Astrophysics (LUNA) experiment at the Gran Sasso National Laboratory, leading to a highly increased sensitivity. Results: The uncertainty of the γ branchings and strengths was improved for all four resonances in the studied energy range; many new transitions were observed in the case of the 334keV resonance, and individual γ decays of the 215keV resonance were measured for the first time. In addition a number of transitions to intermediate states that decay through α emission were identified. The strengths of the observed resonances are generally in agreement with literature values. Conclusions: Our measurements substantially confirm previous determinations of the relevant resonance strengths. Therefore the O18(p,γ)F19 reaction rate does not change with respect to the reaction rate reported in the compilations commonly adopted in the extant computations of red-giant branch and AGB stellar models. Nevertheless, our measurements definitely exclude a nonstandard scenario for the fluorine nucleosynthesis and a nuclear physics solution for the O18 depletion observed in Group 2 oxygen-rich stardust grains.