Neutron multiplicity for neutron induced fission of 235U, 238U and 239Pu as a function of neutron energy

Recent developments in the theory and practice of neutron correlation ('coincidence') counting require knowledge of the higher factorial moments of the Pν distribution (the probability that ν neutrons are emitted in a fission) for the case where the fission is induced by bombarding neutrons of more than thermal energies. In contrast to the situation with spontaneous and thermal neutron induced fission, where, with a few exceptions, the Pν is reasonably well known, in the fast neutron energy region almost no information is available concerning the multiplicity beyond the average value (ν), even for the most important nuclides. The reason for this is the difficulty of such experiments, with consequent statistically poor and physically inconsistent results. In the paper, previously unpublished data made available from a fast neutron fission multiplicity experiment have been used to extract usable multiplicity probabilities from otherwise intractable data, using smoothing techniques supplemented by mathematical and physical plausibility arguments. In contrast, it appears that attempts to apply systematics to deducing such parameters as Pν to the required accuracy will not work because of particular differences between even neighbouring nuclides. Thus, despite the difficulties, further experimental work is required, supplemented by data evaluation, if necessary, as in the paper. (author)