Measurement and statistical model analysis of pre-fission neutron multiplicities

Abstract

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 Xχ 2 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%.