The $\gamma$ rays emitted from 3 to 2000 nsec after spontaneous fission of ${}_{}{}^{252}{}_{}{}^{}\mathrm{Cf}$ have been studied in detail. Both fission fragments were stopped on Si detectors; a Ge detector was used to detect $\gamma$ rays from the fragments on one Si detector. A time-to-amplitude converter was started on the fission-fragment signal and stopped on the $\gamma$-ray signal. For each event the two fission-fragment kinetic energies, the $\gamma$-ray energy, and the time delay were recorded. The data were then analyzed to obtain the energy, half-life, and intensity of each $\gamma$ ray as well as the mass of the emitting fission fragment. Some 144 $\gamma$ rays were so analyzed, corresponding to more than 80 isomeric states.

$\gamma$ rays were observed from practically all masses. However, the intensity was concentrated in the mass regions near 96, 108, 134, and 146. The energy spectrum consisted of a group of $\gamma$ rays below 500 keV and a group near 1300 keV. The high-energy group associated with masses 134 and 136 dominates the energy intensity after 50 nsec. A strong cascade from a 162-nsec isomeric state is assigned to ${}_{ 52}{}^{134}{}_{}{}^{}\mathrm{Te}_{82}^{}{}_{}{}^{}$, and a 3000-nsec isomeric state to ${}_{ 54}{}^{136}{}_{}{}^{}\mathrm{Xe}_{82}^{}{}_{}{}^{}$. Rotational cascades were not observed, in contradiction with earlier low-resolution work. The observed energies and half-lives can be accounted for by $E1\mathrm{}$, $M1\mathrm{}$, or $E2\mathrm{}$ transitions, either allowed or $K$ forbidden by a few units. The interpretation of these results is that the initially high spins of the fragments have less effect on the delayed $\gamma$ rays than was previously thought.

Fragment kinetic energy distributions were obtained for fissions leading to the emission of a particular $\gamma$ ray. The $\gamma$ ray serves to restrict the events to those having a definite final isotope for one fragment. The average kinetic energy of such events is found to be slightly greater than the average for all fissions yielding the same mass.

• Received 5 June 1970

DOI:https://doi.org/10.1103/PhysRevC.2.1451