Preequilibrium particle emission and the giant-dipole resonance in Sn nuclei

Light charged particles from ¹⁸O+¹⁰⁰Mo reactions at 200 MeV bombarding energy have been measured in singles and in coincidence with high-energy {γ} rays (>~10MeV) produced primarily in the decay of the giant-dipole resonance. Particle spectra are analyzed in terms of a moving source fit to decompose preequilibrium and evaporative components. Preequilibrium particle multiplicities and energies indicate a reduction in the compound nucleus excitation energy of approximately 20% and a loss of mass of approximately three mass units relative to complete fusion. The energy lost to preequilibrium emission is shown to affect significantly the strength, width, and centroid energy of the giant-dipole resonance deduced from fits to high-energy {γ}-ray emission spectra. Evaporation residue cross sections were measured for the same reaction from 100 to 217 MeV in order to determine the fusionlike event cross section.