Analyses of fast neutron inelastic scattering cross sections to higher (vibrational) states of 232Th and 238U. I. Standard formalism

Angle-integrated cross-section data in the form of excitation functions from threshold to 2.5 MeV incident (lab) energy, as measured and calculated, are presented for inelastic scattering of fast neutrons on 232Th and 238U, proceeding to higher collective (quadrupole and octupole vibrational) states of these deformed actinide nuclei. The experimental (n,n') data were for the most part obtained from corrected γ-production (n,n'γ) yields. The theoretical data were derived from an incoherent sum of the compound-nuclear and strong-coupling direct-interaction cross sections, computed using the Bruyeres set of optical potential and deformation parameters. The relative coupling strength was the sole adjustable parameter. Provision was made for the effect of Moldauer level-width fluctuations and for competing neutron exit channels (but not for radiative capture, charged-particle, of fission competition); the coupled-channels computations permitted up to six levels to be coupled simultaneously. The ensuing ''standard'' theoretical level excitation functions matched the experimental data fairly closely in most instances, confirming the viability of this conventional method of analysis. However, indications of potentially still better fits were provided by preliminary calculations employing the unified statistical S-matrix formalism of Weidenmueller et al. As illustrated for (n,n') scattering to the lowest members of the K = 0- octupole band in 232Th and 238U, the latter approach reproduces experimental trends closely when the relative coupling strength is optimized. A subsequent paper will present detailed results of the unified analyses and comparisons with evaluated neutron data-file excitation functions for these actinide target nuclei