An Evaluation of the Number of Neutrons Per Fission for the Principal Plutonium, Uranium, and Thorium Isotopes

Experimental values (apart from thermal-neutron data) of the number of prompt neutrons per fission () for ²³²Th, ²³³U, ²³⁴U, ²³⁵U, ²³⁸U, ²³⁹Pu, ²⁴⁰Pu, and ²⁴¹Pu over the energy range from 0 to 15 MeV up to February 1970 are evaluated to derive the best available data, primarily for fast reactor analysis. Limited comments are made on data published subsequently. Data are renormalized where necessary to the latest recommended value (3.756) for for ²⁵²Cf. Many of the existing lower energy data for ²³³U and ²³⁵U indicate a nonlinear energy dependence that would be of significance for fast reactor analysis, but a limited number of recent specific studies on ²³⁵U strongly support linear dependence, and the existence of low-energy structure is not established. The more limited low-energy measurements on ²³⁹Pu indicate little, if any, structure. At energies above several MeV the normally assumed linear energy dependence of is more applicable, but the occurrence of the (n, n′f) and (n, 2nf) reactions in addition to the (n, f) reaction introduces a nonlinear dependence that is generally small but may be of significance for some isotopes. Consideration of the effects of first-. second-, and third-chance fission [(n, f), (n, n′f), and (n, 2nf) reactions] gives a good fit to the single case, ²³⁵U, in which a test can be made and these effects are considered in the evaluation of other isotopes.