Doctor of Philosophy (PhD)
Physics & Astronomy
Clustering in nuclear systems has broad impacts on all phases of stellar burning, and plays a significant role in our understanding of nucleosynthesis, or how and where nuclei are produced in the universe. The role of alpha particles in particular is extremely important for nuclear astrophysics: 4He was one of the earliest elements produced in the Big Bang, it is one of the most abundant elements in the universe, and helium burning -- in particular, the triple-alpha process -- is one of the most important ``engines'' in stars. To better understand nucleosynthesis and stellar burning, then, it is important to develop theoretical frameworks that can describe clustering for exotic systems of interest, however, modeling nuclear systems with cluster substructures represents a major challenge for ab initio many-particle approaches.
This work presents a new framework for calculating alpha widths, asymptotic normalization coefficients (ANCs), and alpha-capture reaction rates for narrow resonances, using ab initio wave functions. The method considers the overlap between cluster configurations and shell-model states computed within a symmetry-informed no-core shell model framework, the no-core symplectic shell model and the ab initio symmetry-adapted no-core shell model. We validate the theory in the well-studied, highly-clustered 20Ne system. In particular, we calculate the spectroscopic amplitude and alpha partial width for the low-lying excited 1- state at 5.79 MeV in 20Ne, which is the resonance that dominates the alpha capture reaction rate for 4He+16O at astrophysical temperatures. This framework is used to study spectroscopic amplitudes, alpha partial widths, bound-state wave functions, and ANCs for 4He+8Be-->12C, 4He+12C-->16O, 4He+15O-->19Ne, and 4He+16O-->20Ne. We predict the reaction rate for 4He+15O, which is used in a simulation of an X-ray burst to determine the impact on nuclear abundances produced.
Dreyfuss, Alison Constance, "Alpha Capture Reaction Rates for Nucleosynthesis Within an Ab Initio Framework" (2019). LSU Doctoral Dissertations. 5105.