Indirect Measurements of n- and p-Induced Reactions of Astrophysical Interest on Oxygen Isotopes

Observations of abundances and isotopic ratio determinations in stars yield powerful constraints on stellar models. In particular, the oxygen isotopic ratios are of particular interest because they are affected not only by nucleosynthesis but also by mixing processes, which are not very well-understood yet. This review is focused on the measurements via the Trojan Horse Method (THM) that have been carried out to investigate the low-energy cross sections of proton and neutron-induced reactions on O-17 as well as the proton-induced reaction on O-18, overcoming extrapolation procedures and enhancement effects due to electron screening. The (p,alpha) reactions induced on these oxygen isotopes are of paramount importance for the nucleosynthesis in a number of stellar sites, including red giants (RGs), asymptotic giant branch (AGB) stars, massive stars, and classical novae. In detail, the indirect measurement of the low-energy region of O-17(p,alpha)N-14 was performed. The strength of the narrow resonance at 65 keV was evaluated, and it was used to renormalize the corresponding resonance strength in the O-17+p radiative capture channel. The reaction rate was then evaluated for both the O-17(p,alpha)N-14 and the O-17(p,gamma)F-18 reactions, and significant differences of 30 and 20% with respect the literature data were found, respectively, in the temperature range relevant for RG, AGB, and massive stars nucleosynthesis. Regarding the O-18(p,alpha)N-15 reaction, the strength of the 20 keV resonance was extracted, which is the main contribution to the reaction rate for astrophysical applications. This approach has allowed us to improve the data accuracy of a factor 8.5, as it is based on the measured strength instead of educated guesses or spectroscopic measurements. Finally, the O-17(n,alpha)C-14 reaction was studied because of its role during the s-process nucleosynthesis as a possible neutron poison reaction. This study represents the extension of THM to resonant neutron-induced reactions. In this measurement, the subthreshold level centered at -7 keV in the center-of-mass system, corresponding to the 8.039 MeV O-18 excited level, was observed. Moreover, the THM measurements showed a clear agreement with the available direct measurements and the additional contribution of the 8.121 MeV O-18 level, strongly suppressed in direct measurements because of its l = 3 angular momentum. The contributions of those levels to the total reaction rate were than evaluated for future astrophysical applications.