Delayed Neutrons from Fissionable Isotopes of Uranium, Plutonium, and Thorium

Abstract

The periods, relative abundances, and absolute yields of delayed neutrons from "fast" fission of six nuclides (${\mathrm{U}}^{235}$, ${\mathrm{U}}^{233}$, ${\mathrm{U}}^{238}$, ${\mathrm{Pu}}^{239}$, ${\mathrm{Pu}}^{240}$, and ${\mathrm{Th}}^{232}$) and thermal fission of three nuclides (${\mathrm{U}}^{235}$, ${\mathrm{U}}^{233}$, and ${\mathrm{Pu}}^{239}$) have been measured. "Godiva," the bare ${\mathrm{U}}^{235}$ metal assembly at Los Alamos, was the neutron source. Six exponential periods were found necessary and sufficient for optimum least-squares fit to the data. Despite evident perturbations, general agreement among delayed-neutron periods was obtained for all nuclides. The absolute total delayed-neutron yield for each nuclide has been measured for fast and thermal fission; a spectral effect was not observed. Representative of general delayed-neutron periods (half-lives) and abundances are the ${\mathrm{U}}^{235}$ fast-fission data: These data have been corroborated in detail by independent period-vs-reactivity measurements on the bare ${\mathrm{U}}^{235}$ assembly. A comparison of the present results with a phenomenological theory of delayed-neutron emission has been made. This treatment has led to the prediction of several new short-period delayed-neutron precursors.