Nonstatistical γ emission in 3He- and α-induced reactions

We have measured inclusive γ-ray production cross sections for 3He- and α-induced reactions on various targets in the mass range A=61 endash 181 at projectile energies of 11 endash 27 MeV. Except for α-induced yields on the lightest targets, all reactions show high-energy γ-ray yields 1 endash 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 Eγ or the target dependence of the absolute cross sections. The 12C(3He,γ0)15O reaction was found to change from predominantly statistical to predominantly direct over this bombarding energy range. copyright 1996 The American Physical Society