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We have directly measured, for the first time, the neutron-fission branching ratio, {Gamma}{sub {ital n}}/{Gamma}{sub {ital f}}, of an actinide nucleus ({sup 238}U) excited in inelastic scattering as a function of excitation energy. This was accomplished using a {approx}4{pi} array of fission counters to separate fission neutrons from decay neutrons in the simultaneous coincidence experiments ({alpha},{alpha}{prime}{ital f}) and ({alpha},{alpha}{prime}{ital n{bar f}}). The measured branching ratio agrees well with previous photoneutron data. In a separate experiment, we have measured the angular distribution of alpha particles scattered from {sup 238}U in coincidence with fission decay ({alpha},{alpha}{prime}{ital f}). This provides us with an estimate of the cross section (in the fission channel) for exciting the isosca- lar giant monopole and giant quadrupole resonances. This estimate, combined with the {Gamma}{sub {ital n}}/{Gamma}{sub {ital f}} measurement, allows us to place bounds on the neutron-fission branching ratio for the isoscalar giant monopole and giant quadrupole resonances themselves.

We have measured the coincidence cross sections /sup 238/U(e,e'f) at three values of momentum transfer (qroughly-equal0.26, 0.40, and 0.55 fm/sup -1/) over the range from 5 to 23 MeV in excitation energy; these values of q correspond approximately to the first maxima of the electric dipole, quadrupole/monopole, and octupole form factors, respectively. The angular distribution of fission fragments is peaked along the direction of q near fission threshold, indicating the dominance of K -- 0 states, but becomes isotropic above 10 MeV. We have integrated our cross sections over fission angle and compared them to recent calculations of multipole strength derived in the quasiparticle random phase approximation. Using model-dependent form factors generated from the Tassie model and the quasiparticle random phase approximation, we have extracted strength functions for the lowest electric multipoles. The deduced E1 strength agrees well with photofission results for reasonable choices of the model form factors. The E2/E0 distribution displays resonant structures at 10 and 13 MeV which agree in position with the collective E2 and E0 strength, respectively, predicted in the quasiparticle random phase approximation. If we make such an assignment for the multipolarity of these two structures, the observed E0 strength agrees with the theoreticalmore » prediction, whereas the E2 strength is low by a factor of 2-3. Higher order multipoles also contribute substantially to the cross section at our highest value of q.« less