Abstract
A pure class-II state can be defined in terms of a double-humped fission barrier as a state in the second well which has two modes of decay: fission through the outer barrier () and decay () to the ground state in the second well (fission isomer). The purpose of this experiment was to measure the branching ratio () for an almost-pure class-II state in the system, thereby establishing for the first time a direct connection between fission intermediate structure and shape isomerism. A 10-g sample of was bombarded by neutrons from the Livermore 100-MeV linac. Both the energy of a neutron inducing an event and the time relationship between rays detected in a pair of scintillators were stored in a two-dimensional matrix consisting of 7.7× channels. Analysis of subthreshold fission groups at neutron energies of 763 and 1839 eV showed no evidence of a -decay branch to the 33-nsec fission isomer in . From the limit on the branching ratio () obtained from these data, an upper limit of meV was derived. Since theoretical calculations predict meV for , the experiment had a sensitivity which is a factor of 10 lower than should have been necessary to observe the decay to the fission isomer. From the present results and from previous measurements on , we are led to the conclusion that there may exist an additional longer-lived isomer in similar to the other odd-Pu isotopes.
[NUCLEAR REACTIONS , eV; investigated decay to fission isomer in ; deduced limit on ; predicted an additional isomer in .]
- Received 12 October 1973
DOI:https://doi.org/10.1103/PhysRevC.9.1177
©1974 American Physical Society