DELAYED GAMMA RAYS BETWEEN 2 AND 80 MUSEC AFTER U235 (N,F) AND PU239 (N,F)

Measurements of the energy spectrum of delayed γ rays from the neutron fission of Pu239 and U235 were performed with a NaI detector at a number of time intervals between 2 and 80 μsec. The data showed three new prominent γ rays with energies of 205, 390, and 1330 keV, two unresolved γ rays having energies of approximately 110 keV, and the six γ rays with energies of 260, 460, 720, 850, 910, and 1250 keV which were observed in previous studies for t>50 μsec after fission. Absolute intensities of the new γ rays were obtained using the normalization factor derived from the relative intensities of the last six γ rays and their previously determined absolute intensities. The 205-, 390-, and 1330-keV γ 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 Pu239 and U235, 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 γ rays, and half-lives of the isomers responsible for early delayed γ rays from U235(n,f) and Pu239(n,f) are summarized, and total intensities and energy emission rates of γ 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. © 1969 The American Physical Society.