We have measured inclusive γ-ray production cross sections for ${}_{}{}^3{}_{}{}^{}\mathrm{-}$ and α-induced reactions on various targets in the mass range A=61–181 at projectile energies of 11–27 MeV. Except for α-induced yields on the lightest targets, all reactions show high-energy γ-ray yields 1–3 orders of magnitude larger than can be obtained from statistical model calculations. Angular distributions are strongly forward peaked in the center of mass frame for the highest γ-ray energies, providing model-independent evidence for a nonstatistical reaction mechanism. Calculations of direct radiative capture and semidirect excitation of the giant dipole resonance can qualitatively reproduce the angular distributions but cannot reproduce either the dependence of the γ-ray cross sections on ${\mathit{E}}_{\gamma }$ or the target dependence of the absolute cross sections. The ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$${(}^3$He,${\gamma }_0$${)}^{15}$O reaction was found to change from predominantly statistical to predominantly direct over this bombarding energy range. © 1996 The American Physical Society.

• Received 21 September 1995

DOI:https://doi.org/10.1103/PhysRevC.53.1759