Low-energy resonances in the O-18(p, gamma)F-19 reaction [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 O-18/O-16 relative abundance in presolar grains and in stellar atmospheres. This ratio is strongly influenced by the relative strengths of the reactions O-18(p, alpha)N-15 and O-18(p, gamma)F-19 in low-mass AGB stars. While the former channel has been the focus of a large number of measurements, the (p, gamma) 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 F-19 as populated through the reaction O-18(p, gamma)F-19 to better determine their influence on the astrophysical reaction rate, and more generally to improve the understanding of the nuclear structure of F-19. Method: Branchings and resonance strengths were measured in the proton energy range E-p(lab) = 150-400 keV, using a high-purity germanium detector inside a massive lead shield. The measurement took place in the ultralow-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 gamma branchings and strengths was improved for all four resonances in the studied energy range; many new transitions were observed in the case of the 334 keV resonance, and individual gamma decays of the 215 keV resonance were measured for the first time. In addition a number of transitions to intermediate states that decay through alpha 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 O-18(p, gamma)F-19 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 O-18 depletion observed in Group 2 oxygen-rich stardust grains.