Particle-to-fission competition at high excitation energies

For application in statistical model evaporation codes, a model is presented which describes the experimental observation of multifold particle emission prior to fission, even from very fissible highly excited composite nuclei. This model is based upon the assumption that collective motion of many nucleons towards the scission point is slow compared to evaporation times for single nucleons. The relative times required for particle evaporation and collective motion towards the scission point are estimated, using as a first-order approximation, the frequencies of the lowest modes of particle vibration and low-frequency β-quadrupole vibration, respectively. Comparisons with data for the systems 12C+182W, 40Ar+154Sm, 19F+181Ta, and 20Ne+165Ho show good agreement with the model calculations. (orig.)