DelayedγRays between 2 and 80 μsec afterU235(n,f)andPu239(n,f)

Measurements of the energy spectrum of delayed $\gamma$ rays from the neutron fission of ${\mathrm{Pu}}^{239}$ and ${\mathrm{U}}^{235}$ were performed with a NaI detector at a number of time intervals between 2 and 80 μsec. The data showed three new prominent $\gamma$ rays with energies of 205, 390, and 1330 keV, two unresolved $\gamma$ rays having energies of approximately 110 keV, and the six $\gamma$ rays with energies of 260, 460, 720, 850, 910, and 1250 keV which were observed in previous studies for $t>\mathrm{}50\mathrm{}$ μsec after fission. Absolute intensities of the new $\gamma$ rays were obtained using the normalization factor derived from the relative intensities of the last six $\gamma$ rays and their previously determined absolute intensities. The 205-, 390-, and 1330-keV $\gamma$ rays probably result from the cascade decay of a single isomer which has a half-life of 3.4±0.4 μsec and fission yields of (1.30±0.30) and (0.63±0.20)% per fission of ${\mathrm{Pu}}^{239}$ and ${\mathrm{U}}^{235}$, respectively. Half-lives and yields were also obtained for the unresolved 110-keV photopeaks, but these results are tentative because of uncertainties in the data. The fission yields, cascade $\gamma$ rays, and half-lives of the isomers responsible for early delayed $\gamma$ rays from ${\mathrm{U}}^{235}\mathrm{}(n,f)\mathrm{}$ and ${\mathrm{Pu}}^{239}\mathrm{}(n,f)\mathrm{}$ are summarized, and total intensities and energy emission rates of $\gamma$ rays with energies greater than 140 keV are presented as a function of time after fission over the range from 2 μsec to 25 msec.