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Four-Parameter Measurements of Isomeric Transitions in Cf 252 Fission Fragments

1970; American Institute of Physics; Volume: 2; Issue: 4 Linguagem: Inglês

10.1103/physrevc.2.1451

1538-4497

Walter John, F.W. Guy, Jerome J. Wesolowski,

X-ray Spectroscopy and Fluorescence Analysis

The $\ensuremath{\gamma}$ 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 $\ensuremath{\gamma}$ rays from the fragments on one Si detector. A time-to-amplitude converter was started on the fission-fragment signal and stopped on the $\ensuremath{\gamma}$-ray signal. For each event the two fission-fragment kinetic energies, the $\ensuremath{\gamma}$-ray energy, and the time delay were recorded. The data were then analyzed to obtain the energy, half-life, and intensity of each $\ensuremath{\gamma}$ ray as well as the mass of the emitting fission fragment. Some 144 $\ensuremath{\gamma}$ rays were so analyzed, corresponding to more than 80 isomeric states.$\ensuremath{\gamma}$ 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 $\ensuremath{\gamma}$ 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 $E1$, $M1$, or $E2$ 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 $\ensuremath{\gamma}$ rays than was previously thought.Fragment kinetic energy distributions were obtained for fissions leading to the emission of a particular $\ensuremath{\gamma}$ ray. The $\ensuremath{\gamma}$ 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.