First Direct Measurement of the 64.5 keV Resonance Strength in the O 17 (p,γ) F 18 Reaction

The CNO cycle is one of the most important nuclear energy sources in stars. At temperatures of hydrostatic H-burning (20 MK<80 MK) the O17(p,γ)F18 reaction rate is dominated by the poorly constrained 64.5 keV resonance. Here, we report on the first direct measurements of its resonance strength and of the direct capture contribution at 142 keV, performed with a new high sensitivity setup at LUNA. The present resonance strength of ωγ(p,γ)bare=(30±6stat±2syst) peV is about a factor of 2 higher than the values in literature, leading to a Γpbare=(34±7stat±3syst) neV, in agreement with the LUNA result from the (p,α) channel. Such agreement strengthens our understanding of the oxygen isotopic ratios measured in red giant stars and in O-rich presolar grains.