Dynamical effects on the de-excitation of hot nuclei with A

Abstract

Light charged-particle emission and neutron emission have been measured for the fusion-evaporation and fusion-fission channels in the $^{60}\mathrm{Ni}$${+}^{100}$Mo reaction at 550 and 655 MeV bombarding energies. Temperatures, emission barriers, and multiplicities for the particles detected in coincidence with evaporation residues and fission fragments have been determined. For the evaporation residue data, the ``first-chance'' spectra of particles emitted from the compound nucleus have been isolated and the same initial temperature for the different evaporated particles has been extracted. The inverse level density parameter K=A/a reaches a value of 13.8\ifmmode\pm\else\textpm\fi{}0.7 MeV at ${\mathit{E}}_{\mathrm{th}}$\ensuremath{\simeq}236 MeV. A decrease of the apparent emission barriers for the charged-particle emission at high excitation energy suggests dynamical effects on the de-excitation process. Analysis of the fissionlike events shows a saturation of light particle emission from the fission fragments. Using the average energies and multiplicities of the emitted neutrons, and charged particles, it has been found that for both initial excitation energies, 251 and 293 MeV, scission occurs at an excitation energy \ensuremath{\simeq}140 MeV. For both fusion evaporation and fusion fission, the light charged particles are preferentially emitted during the early part of the de-excitation cascade. Statistical models and dynamical calculations have been used in an attempt to interpret the experimental data.