Study of charge symmetry in A=18 nuclei and application to oxygen-18

Neutron scattering from 18O has been measured in an attempt to verify charge symmetry in the A=18 region to a better precision than previously determined. Neutron elastic and inelastic scattering data are measured and analyzed to determine the neutron matrix element for direct excitation of the first excited 2+ level in 18O. Comparison of this with the proton matrix element of the mirror nucleus 18Ne provides a test of charge symmetry.; Excitation function and angular distribution data for gamma-ray decays of excited levels of 18O are also measured in neutron inelastic scattering. The excitation function consisted of the energy dependence of gamma-ray yields from 5.0 to 8.2 MeV. Analysis of the excitation function is used to calculate the total cross section at 6.0 and 8.5 MeV incident neutron energy, using a novel method to determine that quantity. The angular distributions are useful in determining the lifetimes and decay multipole mixing ratios, which in turn can be used to determine the transition strengths which determine the collectivity of the nuclear levels.; Level energies and transition strengths obtained from shell model calculations for the 18O nucleus are compared to the experimental values in order to ascertain the configuration which dominates the excitations of that nucleus. For the shell model, we used one new effective interaction, labeled usda, and one-well tested interaction, psd/pn. These calculations have succesfully reproduced level energies and transition strengths for some of the low-lying nuclear levels in 18O. The first 3 + level in 18O is of particular interest since the mirror level in 18Ne is of astrophysical importance in nucleosynthesis in novae explosions and X-ray bursts through its role in the 17F(p,gamma) 18Ne reaction. The study of the 3+ level in 18O has shown it to be a nearly pure two-neutron (1d5/2 2s1/2) subshell configuration; this indicates that the s-capture of the proton from the 1d5/2 ground state of 17F into the corresponding level in 18Ne should occur with high probability, as known experimentally.; Keywords: Charge symmetry, transition strengths, lifetime measurements, neutron matrix element, shell model calculations.