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.

Abstract. The structure of the18 O nucleus at excitation energies above the α decay threshold was studied using 14 C + α resonance elastic scattering. A number of states with large α reduced widths have been observed, indicating that the α-cluster degree of freedom plays an important role in this N = Z nucleus. A 0 + state with an α reduced width exceeding the single-particle limit was identified at an excitation energy of 9.9 ± 0.3 MeV. We discuss evidence that states of this kind are common in light nuclei and give possible explanations of this feature. PACS. 21.10.Gv Nucleon distributions and halo features -24.30.-v Resonance reactions -25.55.Ci Elastic and inelastic scatteringAfter the discovery of the neutron it was understood that α-particles cannot exist in the bulk of a nucleus at normal nuclear density due to the need for antisymmetrization over all nucleons. Still, α-clustering manifests itself in light 4N nuclei such as 8 Be, 12 C, 16 O, and 20 Ne through the existence of twin quasi-rotational bands of states with alternating parities and α-particle reduced widths which are close to the single-particle limit (see the recent review by M. Freer [1] and references therein).There are different approaches that attempt to describe the shell model and the α-cluster structure of nuclei on an equal footing in order to shed light on the interplay between the α-cluster and single-particle degrees of freedom [1]. Data on the nucleon decay of α-cluster states would be instrumental for such efforts. However, this data is practically absent due to the much higher nucleon decay thresholds in comparison with the thresholds for the α decay in the 4N nuclei. It is more promising to observe the nucleon decay from the α-cluster states in N = Z nuclei where the nucleon and α-particle thresholds are close to each other. The study of non-self-conjugate nuclei has an advantage in that one can investigate α-cluster states in mirror systems and use the Coulomb shift to extract information on the relationship between the cluster and single-particle degrees of freedom. Unfortunately, the data on the α-cluster structure of N = Z nuclei are generally very limited.

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New14 N(d,p) angular distribution data were taken at a deuteron bombarding energy of 16 MeV to locate all narrow peaks up to 15 MeV in excitation. A new shell model calculation is able to reproduce all levels in 15 N up to 11.5 MeV and is used to characterize a narrow level at 11.236 MeV and to provide a map of single particle strengths. These results confirm the degeneracy of these orbitals and that the 15 N− 15 O nuclei are where the transition between the 2s1 /2 lying below the 1d5 /2 to lying above it, takes place. The 1d3 /2 single particle strength is estimated to be centered around 14 MeV in these nuclei.

The separation between single particle levels in nuclei plays the dominant role in determining the location of the neutron drip line. The separation also provides a test of current crossed shell model interactions if the experimental data is such that multiple shells are involved. The present work uses the 14 N(d, p) 15 N reaction to extract the 2s 1/2 , and 1d 5/2 total neutron single particle strengths and then compares these results with a shell model calculation using a p-sd crossed shell interaction to identify the J π of all levels in 15 N up to 12.8 MeV in excitation.