High energy gamma rays from complex particle collisions

High energy γ-rays (E/sub γ/ ∼10--30 MeV), from the decay of Giant Dipole Resonances built on excited nuclear states (e.s.-GDRs) are a common feature of all energetic nuclear collisions. Current results indicate that for bombarding energies below 5--6 MeV per nucleon the statistical emission of high energy γ-rays predominates in complex particle collisions. Statistical GDR properties have now been studied over a wide range of mass, energy and angular momentum. Recent results include 1) in light compound nuclei considerably broadended e.s --GDRs are observed, with strengths that indicate relatively pure compound nuclear isospin, 2) in medium mass nuclei (A∼60--80) e.s. -GDRs are found with substantially reduced strength (approx. <0.5 of the classical dipole sum rule) and 3) in rare-earth deformed nuclei a splitting of e.s. -GDRs is apparent, indicating the persistence of deformation at elevated temperature. At higher bombarding energies (6--9 MeV per nucleon) for 3He and 4He strong enhancements are seen for E/sub γ/approx. >15 MeV, due to nonstatistical effects. These enhancements are associated with strong forward-backward asymmetries, arising from (phase coherent) E1-E2 interference, which peaks at energies near the isovector e.s. -GQR