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The reaction 13C(alpha,n) is considered to be the main source of neutrons for the s process in asymptotic giant branch stars. At low energies, the cross section is dominated by the 1/2+ 6.356 MeV subthreshold resonance in (17)O whose contribution at stellar temperatures is uncertain by a factor of 10. In this work, we performed the most precise determination of the low-energy astrophysical S factor using the indirect asymptotic normalization (ANC) technique. The alpha-particle ANC for the subthreshold state has been measured using the sub-Coulomb alpha-transfer reaction ((6)Li,d). Using the determined ANC, we calculated S(0), which turns out to be an order of magnitude smaller than in the nuclear astrophysics compilation of reaction rates.

Excited states in $^{20}\mathrm{O}$ were populated in the reaction $^{10}\mathrm{Be}(^{14}\mathrm{C},\ensuremath{\alpha})$ at Florida State University (FSU). Charged particles were detected with a particle telescope consisting of 4 annularly segmented Si surface barrier detectors and $\ensuremath{\gamma}$ radiation was detected with the FSU $\ensuremath{\gamma}$ detector array. Five new states were observed below 6 MeV from the $\ensuremath{\alpha}\mathrm{\text{\ensuremath{-}}}\ensuremath{\gamma}$ and $\ensuremath{\alpha}\mathrm{\text{\ensuremath{-}}}\ensuremath{\gamma}\mathrm{\text{\ensuremath{-}}}\ensuremath{\gamma}$ coincidence data. Shell model calculations suggest that most of the newly observed states are core-excited $1p\mathrm{\text{\ensuremath{-}}}1h$ excitations across the $N=Z=8$ shell gap. Comparisons between experimental data and calculations for the neutron-rich O and F isotopes imply a steady reduction of the $p\mathrm{\text{\ensuremath{-}}}sd$ shell gap as neutrons are added.

Excited states in 20O were populated in the reaction 10Be(14C,alpha) at Florida State University (FSU). Charged particles were detected with a particle telescope consisting of 4 annularly segmented Si surface barrier detectors and gamma radiation was detected with the FSU gamma detector array. Five new states were observed below 6 MeV from the alpha-gamma and alpha-gamma-gamma coincidence data. Shell model calculations suggest that most of the newly observed states are core-excited 1p-1h excitations across the N=Z=8 shell gap. Comparisons between experimental data and calculations for the neutron-rich O and F isotopes imply a steady reduction of the p-sd shell gap as neutrons are added.