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, (E) for uranium isotopes but is inadequate for isotopes of other species. A revised formalism is presented which accounts for the Z dependence of ( 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 (E) for ²³⁹Pu. The resulting equation is used to calculate (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 (E) over a large range in energy made by a single experimental group. The equation predicts these measured values of (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 (E, A, Z) for isotopes having no measurement.