Scattering of Alpha Particles by Oxygen. II. Bombarding Energy Range 10 to 19 MeV

Abstract

The ${\mathrm{O}}^{16}(\ensuremath{\alpha}, \ensuremath{\alpha}){\mathrm{O}}^{16}$ and ${\mathrm{O}}^{16}(\ensuremath{\alpha}, {\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{\gamma}){\mathrm{O}}^{16}$ reactions have been studied in the bombarding energy range from 10.0 to 19.0 MeV. Elastic-scattering differential cross sections were measured at eight angles as a function of energy, and a total of thirty-five levels in the compound nucleus ${\mathrm{Ne}}^{20}$ have been observed in the excitation curves. Angular distributions of the elastically scattered $\ensuremath{\alpha}$ particles have been measured at seven energies and each shows a strong dependence of the differential cross section on angle. These were analyzed in terms of the Ackhiezer-Pomeranchuk-Blair-McIntyre (APBM) model and the nuclear optical model. Good theoretical fits to the experimental data were obtained only when contributions from resonances were included explicitly in the APBM-model calculations. Excitation curves for the $\ensuremath{\gamma}$ rays emitted from nuclei excited in inelastic-scattering events have been measured, and the ${\mathrm{Ne}}^{20}$ energy levels found in the $\ensuremath{\gamma}$-ray excitation curves confirm most of the levels found in the elastic-scattering experiment. Thin- and thick-target angular distributions for the 6.13-MeV $\ensuremath{\gamma}$ rays were measured at 15 energies. Thick-target $\ensuremath{\gamma}$-ray angular distributions which average over many resonances could not be simply interpreted in terms of broad levels in ${\mathrm{Ne}}^{20}$. Twenty spin-and-parity assignments to states in ${\mathrm{Ne}}^{20}$ have been made in the analysis of the elastic- and inelastic-scattering data, and of these, 14 are tentative. The levels in ${\mathrm{Ne}}^{20}$ observed here have broader widths, larger spacings, and greater spins than the majority of those reported in the literature for the same excitation energy range. These differences are discussed and a possible explanation is offered. The incorporation of several of the levels found in this work in ${\mathrm{Ne}}^{20}$ rotational bands has been included in Paper I.