Isomeric gamma rays fromU235(n,f)andPu239(n,f)for times less than 1 μsec after fission

Abstract

Measurements of the isomeric $\ensuremath{\gamma}$-ray energy spectra from the thermal-neutron fission of $^{235}\mathrm{U}$ and $^{239}\mathrm{Pu}$ were performed with a Ge(Li) detector for times between 20 nsec and \ensuremath{\sim}1 \ensuremath{\mu}sec after fission. Eighty resolved $\ensuremath{\gamma}$-ray peaks with different energies and half-lives were observed; 37 of these $\ensuremath{\gamma}$-ray peaks had not been seen in previous delayed $\ensuremath{\gamma}$-ray measurements. The fission-fragment mass numbers for many of the $\ensuremath{\gamma}$ rays were determined by comparison of the $\ensuremath{\gamma}$-ray energies and half-lives with the results of previous $^{252}\mathrm{Cf}$ measurements. $\ensuremath{\gamma}$ rays which decay in cascade from the same isomeric state were identified on the basis of mass numbers, $\ensuremath{\gamma}$-ray half-lives, and intensities. Isomeric $\ensuremath{\gamma}$-ray spectra from the spontaneous fission of $^{252}\mathrm{Cf}$ were also measured in the present experiment; the analysis of a number of strong, resolved $\ensuremath{\gamma}$ rays in these data indicated that the present results are consistent with previous $^{252}\mathrm{Cf}$ results, within the sum of the systematic uncertainties of the two experiments. The total energy of the resolved peaks from this experiment, when integrated over all time, is 163 and 164 keV/fission for $^{235}\mathrm{U}$ and $^{239}\mathrm{Pu}$, respectively. Roughly 40% of the total energy of the resolved peaks is from $\ensuremath{\gamma}$ rays in the 1100- to 1340-keV region. Isomers in $_{52}^{134}\mathrm{Te}_{82}$ and $_{54}^{136}\mathrm{Xe}_{82}$ contribute most of these high-energy $\ensuremath{\gamma}$ rays. In the 140- to 1340-keV energy range and 20- to 958-nsec time interval, the energy of the observed continuum of unresolved $\ensuremath{\gamma}$ rays was \ensuremath{\sim}20 and \ensuremath{\sim}24% of the total delayed $\ensuremath{\gamma}$-ray energy for $^{235}\mathrm{U}$ and $^{239}\mathrm{Pu}$, respectively. The energy of the resolved $\ensuremath{\gamma}$ rays from this experiment for $^{235}\mathrm{U}$ and $^{239}\mathrm{Pu}$ is about twice that for the isomers from $^{252}\mathrm{Cf}$ with the same range of half-lives, as observed from previous results. Most of the difference between $^{252}\mathrm{Cf}$ and $^{235}\mathrm{U}$ or $^{239}\mathrm{Pu}$ is due to seven possible $\ensuremath{\gamma}$-ray cascades.NUCLEAR REACTIONS, FISSION $^{235}\mathrm{U}(n,f)$, $^{239}\mathrm{Pu}(n,f)$, and $^{252}\mathrm{Cf}$, measured isomeric $\ensuremath{\gamma}$-ray ${E}_{\ensuremath{\gamma}}$, ${T}_{\frac{1}{2}};20\ensuremath{-}1000$ nsec; measured ${E}_{\ensuremath{\gamma}}$ (total), deduced $A$; Ge(Li) with unfolding; 85-4000 keV.