Delayed Neutrons in Plutonium and Uranium Fission

Samples of ${\mathrm{Pu}}^{239}$ and U, enriched in the 235-isotope, were irradiated in the graphite pile at Argonne. The decay of the delayed neutrons following fission was recorded on a magnetic wire recorder. Analysis of the decay data, 18 runs with ${\mathrm{Pu}}^{239}$, and 37 with U, gave the following expression for the decay curves: $A_t=C_{\mathrm{Pu}}({1.2e}^{-0.660t}+{1.1e}^{-0.154t}+{1.0e}^{-0.0309t}+{0.10e}^{-0.0127t}),$ for ${\mathrm{Pu}}^{239}$, and: $A_t=C_U({1.65e}^{-0.613t}+{1.36e}^{-0.156t}+{1.00e}^{-0.0308t}+{0.15e}^{-0.0126t}),\mathrm{for} U^{235}$ when all the periods are irradiated to saturation.