Analyses of fast neutron inelastic scattering cross sections to higher (vibrational) states ofTh232andU238. II. Intrinsic unified formalism

Abstract

From the standpoint of nuclear theory, the consistency of the unified (statistical $S$ matrix) formalism, even in its most basic form as treated in this paper, represents a fundamental improvement over the standard approach involving the incoherent addition of compound-nucleus and direct-interaction contributions to a net angle-integrated cross section. A previous paper dealt with standard analyses of $^{232}\mathrm{Th}(n, {n}^{\ensuremath{'}})$ and $^{238}\mathrm{U}(n, {n}^{\ensuremath{'}})$ level excitation functions from threshold to ${E}_{n}=2.5$ MeV, and foreshadowed the successful application of the unified formalism in illustrative analyses of ($n, {n}^{\ensuremath{'}}$) scattering to triads of $K={0}^{\ensuremath{-}}$ octupole vibrational states in each of the above nuclei. For these deformed actinide nuclei, a coupled-channels treatment is appropriate: The unified approach intrinsically entails this as a central feature. This paper describes the results of such unified computations for the $^{232}\mathrm{Th}$ and $^{238}\mathrm{U}$ scattering cross sections to each of 21 principal collective (quadrupole and octupole vibrational) states in $^{232}\mathrm{Th}$ up to ${E}^{*}=1208.7$ keV, and to 17 such levels in $^{238}\mathrm{U}$ up to ${E}^{*}=1269.4$ keV, using the relative coupling strengths as the only adjustable parameter. When compared with the measured data, the unified fits were generally of equivalent or superior quality to those derived from the standard treatment, with the same (Bruy\`eres) optical potential parameters employed throughout. Comparisons with (composite) evaluated neutron data file values revelas discrepancies which indicate an evident need for reevaluation. Corrections to the basic unified data will be considered in a subsequent paper.