Configuration Mixing of Major Shells in Mass-16 and Mass-17 Nuclei

The odd-parity ($T=\frac{1}{2}$) levels of ${\mathrm{O}}^{17}$ are investigated within the framework of the spherical shell model in order to determine the importance of configuration mixing of the ($2s, 1d$) and $1p$ major shells. Particles in the $2s_{\frac{1}{2}}$ and $1d_{\frac{5}{2}}$ subshells and holes in the $1p_{\frac{1}{2}}$ subshell are considered. It is shown that a configuration space which includes both 2-particle-1-hole states and 4-particle-3-hole states is required to achieve good agreement between calculated and experimental energies in the range of excitation from 3.06 to 8.88 MeV. As confirmed by previous work, calculations based on a truncated 2-particle-1-hole space predict excitation energies that are higher by about 2 MeV than the observed values. In addition, the effects of $(2\mathrm{}{s}_{\frac{1}{2}}, 1d_{\frac{5}{2}})-1\mathrm{}{p}_{\frac{1}{2}}$ mixing on the odd- and even-parity ($T=1\mathrm{}$) states of ${\mathrm{N}}^{16}$ above its four lowest levels are evaluated. The consequences of this mixing are a lowering of calculated energies by an average of 2-3 MeV and many changes in the predicted sequence of levels. The significance of these results in relation to the neutron elastic scattering cross section for ${\mathrm{N}}^{15}$ is discussed, and a need for more experimental data on the spins and parities of ${\mathrm{N}}^{16}$ levels is indicated.