Measurement and statistical model analysis of pre-fission neutron multiplicities

The average number of neutrons preceding fission (ν_pre) was measured for the compound systems ¹⁶⁸YB, ¹⁷⁸W, ¹⁸⁸Pt, ¹⁹²Pb, ¹⁹⁸Pb, ²⁰⁰Pb, ²¹⁰Po, ²¹³At and ²⁵¹Es formed by reactions induced by ¹⁶O, ¹⁸O, ¹⁹F, ²⁸Si or ³⁰Si projectiles with energies (E) between 4.9 and 7.2 MeV/A In some cases ν_pre is seen to increase with increasing E above a threshold energy (E_th) whereas the statistical model indicates that it should decrease. For a given projectile, this threshold decreases with increasing fissility, becoming equal to the Coulomb barrier around A_CN∼213 for ¹⁶O projectiles. Below E_th the variation of ν_pre with E is consistent with statistical model predictions. The deviations above E_th have been attributed to dissipative effects not included in the model. Extensive statistical model and Xχ² analyses of the pre-fission data below E_th and of fission and fusion excitation function data, previously measured, were made. The diffuseness parameters of the fusion spin-distributions agreed reasonably well with those suggested by the zero-point motion model. The ratios of level densities at saddle and equilibrium deformations (a_f/a_v) were found to be consistent with a value of unity, and the fission barriers (E_f) consistent with the predictions of the finite-range rotating liquid-drop model. However these values for a_f/a_ν and E_f may not represent the true values. Inclusion of dissipation requires higher values, whilst inclusion of the temperature dependence of E_f in statistical model calculations is shown to result in a reduction in the value of a_f/a_ν. Since reliable theoretical calculations are unavailable for either effect the consistency of the data with the finite range fission barriers can only be demonstrated to within 10-15% and values for a_f/a_ν, have an uncertainty of at least 5%.