Measurement and statistical model analysis of pre-fission neutron multiplicities
1988; Elsevier BV; Volume: 483; Issue: 1 Linguagem: Inglês
10.1016/0375-9474(88)90068-1
ISSN1873-1554
AutoresJ.O. Newton, D. J. Hinde, R. J. Charity, J. R. Leigh, J.J.M. Bokhorst, A. Chatterjee, G.S. Foote, S. Ogaza,
Tópico(s)Nuclear Physics and Applications
ResumoThe average number of neutrons preceding fission (νpre) 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 νpre is seen to increase with increasing E above a threshold energy (Eth) 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 ACN∼213 for 16O projectiles. Below Eth the variation of νpre with E is consistent with statistical model predictions. The deviations above Eth have been attributed to dissipative effects not included in the model. Extensive statistical model and Xχ2 analyses of the pre-fission data below Eth 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 (af/av) were found to be consistent with a value of unity, and the fission barriers (Ef) consistent with the predictions of the finite-range rotating liquid-drop model. However these values for af/aν and Ef may not represent the true values. Inclusion of dissipation requires higher values, whilst inclusion of the temperature dependence of Ef in statistical model calculations is shown to result in a reduction in the value of af/aν. 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 af/aν, have an uncertainty of at least 5%.
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