Low-energy resonances in the O18 ( p,γ)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 $^{18}\mathrm{O}/^{16}\mathrm{O}$ relative abundance in presolar grains and in stellar atmospheres. This ratio is strongly influenced by the relative strengths of the reactions $^{18}\mathrm{O}(p,\ensuremath{\alpha})^{15}\mathrm{N}$ and $^{18}\mathrm{O}(p,\ensuremath{\gamma})^{19}\mathrm{F}$ in low-mass AGB stars. While the former channel has been the focus of a large number of measurements, the $(p,\ensuremath{\gamma})$ 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 $^{19}\mathrm{F}$ as populated through the reaction $^{18}\mathrm{O}(p,\ensuremath{\gamma})^{19}\mathrm{F}$ to better determine their influence on the astrophysical reaction rate, and more generally to improve the understanding of the nuclear structure of $^{19}\mathrm{F}$.Method: Branchings and resonance strengths were measured in the proton energy range ${E}_{\mathrm{p}}^{\mathrm{lab}}=150--400\phantom{\rule{4pt}{0ex}}\mathrm{keV}$, 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 $\ensuremath{\gamma}$ branchings and strengths was improved for all four resonances in the studied energy range; many new transitions were observed in the case of the $334\phantom{\rule{0.16em}{0ex}}\mathrm{keV}$ resonance, and individual $\ensuremath{\gamma}$ decays of the $215\phantom{\rule{0.16em}{0ex}}\mathrm{keV}$ resonance were measured for the first time. In addition a number of transitions to intermediate states that decay through $\ensuremath{\alpha}$ 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 $^{18}\mathrm{O}(p,\ensuremath{\gamma})^{19}\mathrm{F}$ 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 $^{18}\mathrm{O}$ depletion observed in Group 2 oxygen-rich stardust grains.