New Cross Section Measurements Relevant to the CNO Nucleosynthesis

Abstract

The reaction rates of nuclear processes in static hydrogen burning cannot be measured directly at the relevant stellar energies. In the usual procedure, the nuclear reactions are studied therefore over a wide range of beam energies and as low as possible in energy. These results are then extrapolated to stellar energies with the guidance of theoretical and other considerations. This procedure will be illustrated in the first example, the ¹⁵N(p, 8) ¹⁶O reaction. However, the stellar reaction rates obtained through this extrapolation represent only lower limits if states exist in the compound nucleus near the proton threshold, which can alterate significantly the extrapolated reaction rates. The observed cross sections at higher beam energies are usually insensitive to the contribution of such threshold states. One has therefore to calculate the reaction rate of these threshold states, where all necessary parameters in the theoretical estimate (e.g. Breit-Wigner-formalism) have to be determined by experiments.Such a case will be described in the second example, the hydrogen burning of ¹⁷O.