Clustering in ¹⁸O and ANC Measurements Using (⁶Li,D) Reactions

Clustering in atomic nuclei is an interesting phenomenon that has been studied extensively in the past. However, most studies were related to clustering in α-like 4N nuclei. This work is focused on clustering in N≠Z non-self-conjugate nuclei. Two experimental approaches are applied. Properties of resonances above the α-decay threshold in 18O are studied using resonance elastic scattering of α-particles on 14C. Asymptotic Normalization Coefficients (ANCs) of near-threshold resonances in 16O, 17O and 20Ne are measured using sub-Coulomb α-transfer (6Li,d) reaction. To investigate the α-cluster states of 18O the α+14C elastic scattering reaction was studied using the Thick Target Inverse Kinematics (TTIK) technique. An exhaustive analysis of the 14C+α excitation functions using multi-channel, multi-level R-matrix approach has been performed. This analysis led to a more complete picture of α-cluster states in 18O. The excitation function was studied in an excitation energy range of 8 MeV-15 MeV. The most remarkable feature found in this analysis is the presence of states with high degree of clustering throughout the whole excitation energy range, including two states with pure α-cluster configuration. To provide a more detailed description of these two broad states, potential model calculations were preformed. These calculations reproduced rather well the excitation energy and width of these states. The Asymptotic Normalization Coefficient (ANC) technique can be used as an effective method to determine the astrophysical S-factor. Combination of the sub-Coulomb α-transfer reaction and application of the ANC technique in the analysis of the experimental data practically eliminates dependence of the result on model parameters, making this approach a very valuable tool for studies of astrophysically important reaction rates. In this study we report the ANC measurements of near threshold states for the reactions: 16O(6Li,d20Ne, 13C(6Li,d)17O and 12C(6Li,d)16O. Two astrophysically important reactions, 13C(α,n) and 12C(α,γ) were studied. The 13C(α,n) reaction is considered to be the main source of neutrons for the s-process in Asymptotic Giant Branch (AGB) stars and the 12C(α,γ) reaction is often regarded as the "holy grail"of nuclear astrophysics, because it enters as a crucial parameter for so many stellar processes. The ANCs of the near α-threshold states were used to calculate astrophysical S-factors and corresponding reaction rates.