Structure in the Mg24(O18, C14)Si28 ground-state excitation function

The excitation function of the reaction Mg24(O18, C14)Si28(g.s.) has been measured at the second maximum of the angular distributions (θlab 5 °) from 39- to 65-MeV incident energies. Oscillatory structure in the excitation function is observed with peak-to-valley ratios equal to ∼1.5. The angular distributions measured near the maxima of the excitation function decrease more rapidly toward large angles than squares of Legendre polynomials, suggesting a direct reaction process to be important. Standard distorted-wave Born-approximation calculations do fit the angular distributions quite well, but fail to reproduce the structure in the excitation function.