O16(α, 2α)Cg.s.12Reaction Mechanisms near 25 MeV

Measurements of the angular correlation between the two $\alpha$ particles emitted in the reaction ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}(\alpha , 2\alpha ){}_{}{}^{12}{}_{}{}^{}\mathrm{C}_{\mathrm{g}.\mathrm{s}.\mathrm{}}^{}{}_{}{}^{}$ at incident energies near 25 MeV suggest that this reaction can be described in terms of both sequential and one-step knockout processes. The sequential reaction proceeds as inelastic $\alpha$-particle scattering followed by $\alpha$-particle decay of the excited levels of ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ to levels of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$. Only the transitions to the ground state of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ were studied. The most strongly excited levels were those at 11.1 and 13.88 MeV. The strong energy dependence of the cross section for the reaction going through the 13.88-MeV level indicates the existence of a broad state in ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$ with an excitation energy near 24 MeV. Apparently, resonance effects are present in the initial-state interaction at this excitation. Any quasi-elastic scattering appears as a background under the peaks of the sequential reaction. The angular distribution of this background is in limited agreement with the predictions of a modified Born-approximation model which takes into account the final-state interaction between the two $\alpha$ particles.