Date of Award

12-2006

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Physics

Major Professor

Mike W. Gudiry

Committee Members

Michael S. Smith, Pengcheng Dai, Chia C. Shih, Lawrence W. Townsend

Abstract

The 18F(p,α)15O reaction plays a crucial role in understanding γ-ray emission from novae. Because of the importance of understanding the 18F + p reactions, a number of studies of the A=19 isobars have been made using stable and exotic beams. The interference effects among J π = 3/2+ resonances in the 18F + p system, however, have never been measured, but they can change the S-factor by a factor of 20 at nova energies. R-matrix calculations indicate that the cross sections above the Ec.m. = 665 keV resonance are sensitive to the interference between the Ec.m. = 8, 38, and 665 keV resonances. In order to study the interference effects, an excitation function for the 1H(18F,α)15O reaction has been measured in the energy range of Ec.m. = 663-877 keV using radioactive 18F beams at the Holifield Radioactive Ion Beam Facility (HRIBF). By measuring the 18F(p,α)15O cross section off resonance and comparing the cross section with theoretical calculations, we provide the first experimental constraints on the interference of 3/2+ resonances.

The 3He(3He,2p)4He reaction is responsible for destruction of 3He in stars, and strongly affects the calculated neutrino luminosity from the sun. Previous measure- ments demonstrated a rise in the 3He(3He,2p)4He S-factor at low energies, which could be due to a low energy resonance in the 3He + 3He (6Be) system, or be due to an electron screening effect. In the 6Be nucleus, however, no excited states have been observed above the first 2+ state at Ex = 1.67 MeV up to 23 MeV. But there is considerable evidence to support the presence of unknown excited states in 6Be. First of all, in the mirror nucleus 6He two excited states have been established below the t + t threshold at Ex = 12.3 MeV. Secondly, a recent measurement at Notre Dame University found tentative evidence for a 6Be level at 9.6 MeV. A search for the missing 6Be levels was performed by studying the d(7Be,t)6Be reaction with the radioactive 7Be beam at Oak Ridge National Laboratory HRIBF. No excited states in 6Be were found; however, we could set the upper limits on the reaction cross section.

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