Low-energy resonances in the 18O (p,Υ) 19F 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 18O/16O relative abundance in pre-solar grains and in stellar atmospheres. This ratio is strongly in uenced by the relative strengths of the reactions 18O (p;α)15N and 18O (p,Υ )19F in low-mass AGB stars. While the former channel has been the focus of a large number of measurements, the (p,ϒ) reaction path has only recently received some attention and its stellar
reaction rate over a wide temperature range rests on only one measurement.
Purpose: The direct measurement of states in 19F as populated through the reaction 18O(p,ϒ)19 F to better determine their influence on the astrophysical reaction rate, and more generally improve the understanding of the nuclear structure of 19F.
Method: Branchings and resonance strengths were measured in the proton energy range Elabp = 150 - 400 keV, 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 ϒ-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 ϒ-decays of the 215 keV resonance were measured for the first time. In addition a number of transitions to intermediate states that decay through α-emission were identifed. 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 18O(p,γ)19F reaction rate does not change with respect to the reaction rate reported in the compilations commonly adopted in the extant computations of RGB and AGB stellar models. Nevertheless, our measurements definitely exclude a non-standard scenario for the fluorine nucleosynthesis and a nuclear physics solution for the 18O depletion observed in Group 2 oxygen-rich stardust grains.