Influence of pre-fission particle emission on fragment angular distributions studied forPb208(16O,f)

Neutrons have been measured in coincidence with fission fragments of the reaction ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ ${(}^{16}$O,f) at bombarding energies ranging between 80 and 130 MeV. Pre-scission and post-scission multiplicities were deduced from the neutron spectra by application of a moving source fit procedure. Particles evaporated before the saddle point configuration of the fissioning nucleus will reduce the nuclear temperature of the transition nucleus. This effect of higher chance fission increases the anisotropies of fission fragment angular distributions compared to the assumption of first chance fission. Compound nuclei with nuclear temperatures considerably smaller than the fission barrier have statistical fission decay times that are much longer than saddle-to-scission times. With these conditions it is plausible that most of the particles evaporated from the fissioning nucleus are emitted before the saddle. Fission fragment angular distributions for ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ ${(}^{16}$O,f) analyzed with the transition state model including the nuclear temperature reduction caused by pre-fission particle emission show ‘‘normal’’ anisotropies at low and near barrier energies.