Cluster Structure of Oxygen Isotopes

The alpha-cluster structure of two oxygen isotopes, 17O and 18O, was studied using two experimental techniques. The first technique measured resonance elastic scattering of alpha particles and the second used the direct alpha-transfer reactions (6Li,d) and (7Li,t) to determine resonance properties. Motivation for this study was two-fold. First, the alpha-cluster structure of N not equal Z nuclei is poorly known and is a subject of intense theoretical discussion . Historically, the alpha-particle model of the atomic nucleus was the leading model of nuclear structure. As it became clear that nuclei consist of protons and neutrons this model was replaced. The Pauli principle forbids nucleons from different alpha particles to be in the same state since the total wave function of an atomic nucleus must be antisymmetric. However, alpha clusters have been used to explain various nuclear effects including quasi-rotational bands of states with large alpha-particle widths which were observed in light 4N nuclei, 8Be, 12C, 16O and so on. Included here is a report on the observation of the alpha-cluster structure in the N not equal Z nucleus 18O. We measured the alpha-cluster structure of 18O using the Thick Target Inverse Kinematics (TTIK) technique . We found that 18O has a very elaborate alpha-cluster structure, including two unusual states with alpha widths larger than the single particle limit (the Wigner limit ). A comparison of the observed 18O alpha-cluster structure with the predictions of modern theoretical approaches is given. The peculiar nature of the two very broad states is discussed. Second, the alpha-cluster structure of near alpha-threshold excited states in 17O and 18O plays a crucial role in the field of nuclear astrophysics as it determines the rates of the 13C(alpha,n) and 14C(alpha,gamma) reactions. These reactions are thought to play important roles in stellar evolution of Asymptotic Giant Branch (AGB) stars. Unfortunately, direct measurement of these reactions is currently impractical. This has led to the development of indirect methods to determine reaction rates. One such method is the Asymptotic Normalization Coefficient (ANC) technique. We determined the alpha-cluster structure of several near alpha-threshold states in 17O and 18O using this ANC technique. Using this alpha-cluster information we were able to determine the 13C(alpha,n) reaction rate and reduce its uncertainty from approximately 300% to 25%. Also we were able to calculate the 14C(alpha,gamma) reaction rate. Accurately knowing the 13C(alpha,n) reaction rate is crucial to the modeling of AGB stars, and a reliable determination of the 14C(alpha,gamma) reaction rate is needed in order to help understand the unexplained abundance of 19F in the universe.