Investigation of theγdecay of subthreshold-fission resonances ofPu242to a fission isomeric state

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 (${\ensuremath{\Gamma}}_{f\mathrm{II}}$) and $\ensuremath{\gamma}$ decay (${\ensuremath{\Gamma}}_{\ensuremath{\gamma}\mathrm{II}}$) to the ground state in the second well (fission isomer). The purpose of this experiment was to measure the branching ratio ($\frac{{\ensuremath{\Gamma}}_{\ensuremath{\gamma}\mathrm{II}}}{{\ensuremath{\Gamma}}_{f\mathrm{II}}}$) for an almost-pure class-II state in the $^{242}\mathrm{Pu}+n$ system, thereby establishing for the first time a direct connection between fission intermediate structure and shape isomerism. A 10-g sample of $^{242}\mathrm{Pu}$ was bombarded by neutrons from the Livermore 100-MeV linac. Both the energy of a neutron inducing an event and the time relationship between $\ensuremath{\gamma}$ rays detected in a pair of ${\mathrm{C}}_{6}$${\mathrm{D}}_{6}$ scintillators were stored in a two-dimensional matrix consisting of 7.7\ifmmode\times\else\texttimes\fi{}${10}^{5}$ channels. Analysis of subthreshold fission groups at neutron energies of 763 and 1839 eV showed no evidence of a $\ensuremath{\gamma}$-decay branch to the 33-nsec fission isomer in $^{243}\mathrm{Pu}$. From the limit on the branching ratio ($\frac{{\ensuremath{\Gamma}}_{\ensuremath{\gamma}\mathrm{II}}}{{\ensuremath{\Gamma}}_{f\mathrm{II}}}$) obtained from these data, an upper limit of ${\ensuremath{\Gamma}}_{\ensuremath{\gamma}\mathrm{II}}<1$ meV was derived. Since theoretical calculations predict ${\ensuremath{\Gamma}}_{\ensuremath{\gamma}\mathrm{II}}\ensuremath{\approx}8\ensuremath{-}10$ meV for $^{243}\mathrm{Pu}$, the experiment had a sensitivity which is a factor of 10 lower than should have been necessary to observe the $\ensuremath{\gamma}$ decay to the fission isomer. From the present results and from previous measurements on $^{243}\mathrm{Pu}$, we are led to the conclusion that there may exist an additional longer-lived isomer in $^{243}\mathrm{Pu}$ similar to the other odd-Pu isotopes.[NUCLEAR REACTIONS $^{242}\mathrm{Pu}+n$, $E=400\ensuremath{-}3000$ eV; investigated $\ensuremath{\gamma}$ decay to fission isomer in $^{243}\mathrm{Pu}$; deduced limit on $\frac{{\ensuremath{\Gamma}}_{\ensuremath{\gamma}\mathrm{II}}}{{\ensuremath{\Gamma}}_{f\mathrm{II}}}$; predicted an additional isomer in $^{243}\mathrm{Pu}$.]