Absolute cross section data at over a thousand energies [$15<\mathrm{}{E}_x\left({}_{}{}^{16}{}_{}{}^{}\mathrm{O}\right)<\mathrm{}22\mathrm{}$ MeV], usually at 18 angles and for as many as four reactions [${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$($\alpha$,${\alpha }_i$)${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ with $0\le i\le 2$ and ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$($\alpha$,$p_0$)${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$], reveal much alpha cluster and other level structure in ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$. A differentially pumped methane gas target allowed laboratory energy resolution better than 10 keV. The author critically discusses problems in past and present data analyses and introduces some new techniques. With the aid of these techniques and a resonance shape fitting program, all of the ${\alpha }_0$ data (plus some of the ${\alpha }_2$ and ($\alpha$,$p_0$) data) were fitted and resonant parameters to over 50 states in ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ were assigned. These levels often correspond to families of excited ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ core states. Clear absence of an $8^{+}$ state at $E_x\left({}_{}{}^{16}{}_{}{}^{}\mathrm{O}\right)\approx 20$ MeV makes questionable early rotational band assignments. Instead evidence is cited supporting Robson's predictions of ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ as a tetrehedral rotor.

NUCLEAR REACTIONS ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$($\alpha$,${\alpha }_i$), ($\alpha$,$p_0$), $E=10.5-20\mathrm{}$ MeV measured $\sigma (E,\theta )$; deduced ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ level parameters; new analysis techniques, resolution 10 keV, ${\theta }_{\mathrm{lab}}=30-170\mathrm{}°$.

NUCLEAR STRUCTURE Alpha cluster structure of ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$; excited core states.

• Received 21 May 1981

DOI:https://doi.org/10.1103/PhysRevC.25.729