Investigation of the γ decay of subthreshold-fission resonances of Pu242 to a fission isomeric state

J. C. Browne and C. D. Bowman
Phys. Rev. C 9, 1177 – Published 1 March 1974
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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 (ΓfII) and γ decay (ΓγII) to the ground state in the second well (fission isomer). The purpose of this experiment was to measure the branching ratio (ΓγIIΓfII) for an almost-pure class-II state in the Pu242+n system, thereby establishing for the first time a direct connection between fission intermediate structure and shape isomerism. A 10-g sample of Pu242 was bombarded by neutrons from the Livermore 100-MeV linac. Both the energy of a neutron inducing an event and the time relationship between γ rays detected in a pair of C6D6 scintillators were stored in a two-dimensional matrix consisting of 7.7×105 channels. Analysis of subthreshold fission groups at neutron energies of 763 and 1839 eV showed no evidence of a γ-decay branch to the 33-nsec fission isomer in Pu243. From the limit on the branching ratio (ΓγIIΓfII) obtained from these data, an upper limit of ΓγII<1 meV was derived. Since theoretical calculations predict ΓγII810 meV for Pu243, the experiment had a sensitivity which is a factor of 10 lower than should have been necessary to observe the γ decay to the fission isomer. From the present results and from previous measurements on Pu243, we are led to the conclusion that there may exist an additional longer-lived isomer in Pu243 similar to the other odd-Pu isotopes.

[NUCLEAR REACTIONS Pu242+n, E=4003000 eV; investigated γ decay to fission isomer in Pu243; deduced limit on ΓγIIΓfII; predicted an additional isomer in Pu243.]

  • Received 12 October 1973

DOI:https://doi.org/10.1103/PhysRevC.9.1177

©1974 American Physical Society

Authors & Affiliations

J. C. Browne and C. D. Bowman*

  • Lawrence Livermore Laboratory, University of California, Livermore, California 94550

  • *Now at National Bureau of Standards, Washington, D.C. 20234.

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Issue

Vol. 9, Iss. 3 — March 1974

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