Predictions of the Energy Dependence of the Average Yield of Neutrons Per Fission of Isotopes of Thorium, Uranium, and Plutonium

A formalism developed in 1963 for predicting the energy dependence of the average neutron yield per fission, ν̅, is tested against post-1963 experiments. It is shown that the 1963 formalism accounts adequately for src ν̅(E) for uranium isotopes but is inadequate for isotopes of other species. A revised formalism is presented which accounts for the Z dependence of ν̅( E, A, Z) by inclusion of a first-order term in Z. The coefficient of the Z-dependence term is derived from consideration of detailed measurements of ν̅ (E) for 239Pu. The resulting equation is used to calculate ν̅(E, A, Z) for isotopes of plutonium, uranium, thorium, and thermal values of americium isotopes. Uranium-235, -238, and 239Pu are the only isotopes which have detailed measurements of ν̅(E) over a large range in energy made by a single experimental group. The equation predicts these measured values of ν̅(E, A, Z) to better than 0.5% in first moment, and standard deviations better than 1.5% about the central point of the measurements. This suggests that the extended formalism is a useful tool for prediction of ν̅ (E, A, Z) for isotopes having no measurement.