The electrofission cross section of ${}^{239}\mathrm{Pu}\mathrm{}(e,f)\mathrm{}$ is measured between 7 and 12 MeV. The data are analyzed by means of the virtual photon formalism, assuming that $E1,$ $E2\mathrm{}$ $\mathrm{}(T=0\mathrm{}),$ and $M1\mathrm{}$ transitions are involved. Using known estimates for the $E1\mathrm{}$ and $E2\mathrm{}$ $\mathrm{}(T=0\mathrm{})\mathrm{}$ fission strengths, it is deduced an $M1\mathrm{}$ fission strength of $19\pm 4{\mu }_N^2$ concentrated near the fission barrier (between 5.4 and 5.8 MeV). The levels of the ${}^{239}\mathrm{Pu}$ transition nucleus are theoretically obtained; a bunch of positive-parity levels shows up between 5.5 and 5.9 MeV, which might well be associated with the deduced $M1\mathrm{}$ strength, since the $E2\mathrm{}$ strength is negligible in this energy interval.

• Received 18 June 1996

DOI:https://doi.org/10.1103/PhysRevC.55.2471