Pre-equilibrium processes in nuclear reaction theory: the state of the art and beyond

Abstract

We have made a detailed comparative study of many existing programs in order to assess their usefulness for the present purpose. We begin by summarizing the essential ideas of the exciton model and the various formalisms that have been developed to enable the preequilibrium cross-sections to be calculated. In the work we describe here we use the theory of Feshbach, Kerman, and Koonin (1980) because it is applicable to both multistep compound and multistep direct processes and has been developed to the stage of numerical calculations. This theory distinguishes between two types of pre-equilibrium reactions, the multistep compound and the multistep direct. In the multistep compound reactions there is at least one stage in the reaction when all particles are bound, whereas in the multistep direct there is always at least one particle in the continuum. Multistep compound reactions predominate at lower energies and are defined to give cross-sections symmetrical about 90/sup 0/, whereas multistep direct reactions become increasingly important as the energy increases and give cross-sections peaked in the forward direction. At the energies of interest here both the multistep compound and the multistep direct processes are important. The formalisms for these processes are described applied to analyse the inelastic scattering of neutrons by /sup 59/Co and /sup 93/Nb. The final section is devoted to a detailed comparison between the exciton and quantum-mechanical theories, and some conclusions are drawn concerning their application to data evaluation.