Delayed γ Rays between 2 and 80 μsec after U235(n,f) and Pu239(n,f)

Measurements of the energy spectrum of delayed $\ensuremath{\gamma}$ 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 \ensuremath{\mu}sec. The data showed three new prominent $\ensuremath{\gamma}$ rays with energies of 205, 390, and 1330 keV, two unresolved $\ensuremath{\gamma}$ rays having energies of approximately 110 keV, and the six $\ensuremath{\gamma}$ rays with energies of 260, 460, 720, 850, 910, and 1250 keV which were observed in previous studies for $t>50$ \ensuremath{\mu}sec after fission. Absolute intensities of the new $\ensuremath{\gamma}$ rays were obtained using the normalization factor derived from the relative intensities of the last six $\ensuremath{\gamma}$ rays and their previously determined absolute intensities. The 205-, 390-, and 1330-keV $\ensuremath{\gamma}$ rays probably result from the cascade decay of a single isomer which has a half-life of 3.4\ifmmode\pm\else\textpm\fi{}0.4 \ensuremath{\mu}sec and fission yields of (1.30\ifmmode\pm\else\textpm\fi{}0.30) and (0.63\ifmmode\pm\else\textpm\fi{}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 $\ensuremath{\gamma}$ rays, and half-lives of the isomers responsible for early delayed $\ensuremath{\gamma}$ rays from ${\mathrm{U}}^{235}(n,f)$ and ${\mathrm{Pu}}^{239}(n,f)$ are summarized, and total intensities and energy emission rates of $\ensuremath{\gamma}$ rays with energies greater than 140 keV are presented as a function of time after fission over the range from 2 \ensuremath{\mu}sec to 25 msec.