Intermediate structure in theO16+O16system

Excitation functions of ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}({}_{}{}^{16}{}_{}{}^{}\mathrm{O},{\alpha }_{0,1}){}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ at eight laboratory angles and angular distributions at twelve energies have been measured in the region $E_{\mathrm{lab}}=20-44\mathrm{}$ MeV, with an experimental arrangement appropriate for studying structures of intermediate width. A detailed statistical analysis of the data (correlation and deviation functions) shows that the region above $E_{\mathrm{lab}}=29\mathrm{}$ MeV is characterized by anomalously high amounts of correlation of intermediate-width structures ($\Gamma \sim 500$ keV). Analysis of the ${\alpha }_0$ angular distributions by means of coherently summed pairs of Legendre polynomials reveals a sharp selectivity of near-grazing angular momenta throughout the energy region studied. This property is also present in the cross sections calculated by the statistical model, so that the experimental angular distributions are generally well reproduced in shape by Hauser-Feshbach calculations. However, peaks at $E_{\mathrm{lab}}=30.4, 31.9, 33.9, 35.9, \mathrm{and} 39.65$ MeV exceed the Hauser-Feshbach cross sections in magnitude by factors of about 2-3. The two most pronounced structures, i.e., those at 30.4 and 31.9 MeV, are found to be dominated by a resonant enhancement of the $J^{\pi }={10}^{+}$ contribution, although the interference with the statistical background is not entirely negligible. The three weaker resonantlike anomalies, at 33.9, 35.9, and 39.65 MeV, also contain enhanced contributions of near-grazing partial waves, $l=12, 12, \mathrm{and} 14$, respectively, which mix strongly with the compound-nucleus background. Thus, none of the reported structures could be identified as an isolated resonance of a single, well-defined angular momentum.