High resolution study of the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$${+}^{16}$O system at 15.5 ≤$E_{\mathrm{c}.\mathrm{m}.\mathrm{}}$≤17.0 MeV is reported including excitation functions for elastic scattering, and angle integrated cross sections for ${\alpha }_0$ and ${\alpha }_1$ exit channels and several γ-ray channels. All show correlated resonant structure. Fifteen angular distributions for the ${\alpha }_0$ and ${\alpha }_1$ exit channels are measured in Δ$E_{\mathrm{c}.\mathrm{m}.\mathrm{}}$≃50 keV steps for 15.5≤$E_{\mathrm{c}.\mathrm{m}.\mathrm{}}$≤16.4 MeV in the angular range 17°≤${\theta }_{\mathrm{c}.\mathrm{m}.\mathrm{}}$≤93° in steps of Δ${\theta }_{\mathrm{c}.\mathrm{m}.\mathrm{}}$≃2.5°. Phase shift analysis of the ${\alpha }_0$ angular distribution was carried out. Ambiguities in the extraction of the S matrix elements are removed using the usual techniques with an additional new constraint—the measurement of cross section at selected angles (zeros of $P_l$) arising from a single partial wave. Reaction amplitudes corresponding to a narrow l window around the grazing partial wave are shown to dominate the cross section around ${\theta }_{\mathrm{c}.\mathrm{m}.\mathrm{}}$=90°. The grazing partial wave shows broad nonresonant structure; a specific parametrization of the energy dependence of the nonresonant (background) cross section corresponding to the l window is presented. The interplay of background amplitudes (l window) and resonant amplitudes is emphasized. Resonances in the ${\alpha }_0$ channel are located at $E_{\mathrm{c}.\mathrm{m}.\mathrm{}}$=15.8, 15.9, and 16.1 MeV having $J^{\pi }$${=10\mathrm{}}^{+}$, $8^{+}$, and $8^{+}$, respectively, and intermediate widths (Γ∼70 keV). Spin assignments are obtained via a phase shift analysis and study of excitation functions measured at zeros of Legendre polynomials. Parametrization of the resonance plus background cross section yields the result √${\Gamma }_0$${\Gamma }_{\alpha }$ /Γ≃1.0%. The extracted partial widths are analyzed in terms of the Wigner limit and suggest resonances not of dinuclear structure, but of more complicated structure. Such states, having $J^{\pi }$${=8\mathrm{}}^{+}$, are found to appear systematically at the empirically determined energies: $E_{\mathrm{c}.\mathrm{m}.\mathrm{}}$=4×2.8+N×2.4 MeV (N=0,1,2 , . . .) in many different heavy ion resonant systems; this may suggest that the underlying alpha particle structure of the participant nuclei may play an ex- plicit role in these resonances.

• Received 14 January 1985

DOI:https://doi.org/10.1103/PhysRevC.31.1255