Effects of uncertainties of the 22Ne(α, n)25Mg and 13C(α, n)16O reaction rates in the s-process yields Original Research Article

In the low mass star AGB phase the region between H shell and He shell (He-intershell) is affected by brief convective instabilities (Thermal Pulses), due to trigger of the He burning in the He shell. In this conditions the neutron source 22Ne(α, n)25Mg is marginally activated (releasing a short neuton burst with a peak neutron density nn≤1010 n cm−3), and may change isotopic ratios near s-path branchings sensitive to the peak neutron density. For each element we analyse the isotopic ratios obtained in the AGB nucleosynthesis model using three different 22Ne(α, n)25Mg rates: first,with the "standard rate", evaluated in [F. Käppeler et al., Astrophys. J. 437, 396 (1994)]; second, with the standard rate multiplied by a factor of two, and third, the standard rate divided by a factor of two. This factor is close to the present uncertainty factor of the 22Ne(α, n)25Mg rate. The same analysis is presented for 13C(α, n)16O reaction rate, the primary neutron source in the He-intershell, which works in radiative conditions in a thin region in the top layers of the He-intershell (13C-pocket) during interpulse periods. In this case the standard rate is evaluated in [G.R. Caughlan and W.A. Fowler, Atomic Data Nucl. Data Tables 40, 283 (1988)].