PREEQUILIBRIUM PARTICLE-EMISSION AND THE GIANT-DIPOLE RESONANCE IN SNNUCLEI

Light charged particles from O-18 + Mo-100 reactions at 200 MeV bombar ding energy have been measured in singles and in coincidence with high -energy gamma rays (greater than or equal to 10 MeV) produced primaril y in the decay of the giant-dipole resonance. Particle spectra are ana lyzed in terms of a moving source fit to decompose preequilibrium and evaporative components. Preequilibrium particle multiplicities and ene rgies indicate a reduction in the compound nucleus excitation energy o f approximately 20% and a loss of mass of approximately three mass uni ts relative to complete fusion. The energy lost to preequilibrium emis sion is shown to affect significantly the strength, width, and centroi d energy of the giant-dipole resonance deduced from fits to high-energ y gamma-ray emission spectra. Evaporation residue cross sections were measured for the same reaction from 100 to 217 MeV in order to determi ne the fusionlike event cross section.