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.

Within the limits of experimental error, the half-lives for ${\mathrm{Pu}}^{239}$ and ${\mathrm{U}}^{235}$ are identical, namely: $(55.0\mathrm{}\pm 0.4) \sec ., (22.5\mathrm{}\pm 0.3) \sec ., (4.45\mathrm{}\pm 0.15) \sec ., \mathrm{and} (1.10\mathrm{}\pm 0.06) \sec .\mathrm{}$ The error limits given were determined from the internal consistency of the various values of the periods, not from the statistical fluctuations in the counting procedure.

The change in the yield of the 55.0-sec. period relative to that of the 22.45-sec. period, in going from ${\mathrm{U}}^{235}$ to ${\mathrm{Pu}}^{239}$, is of the proper sign but small in magnitude as compared to the change predicted from the fission yields for the known mass numbers of these delayed-neutron emitters. A comparison of the relative number of delayed neutrons per fission gives the ratio of the number from ${\mathrm{Pu}}^{239}$ to the number from ${\mathrm{U}}^{235}$ as 0.5.

• Received 19 June 1947

DOI:https://doi.org/10.1103/PhysRev.72.570