Phenomenology of continuum angular distributions. I. Systematics and parametrization

A large number of experimental angular distributions have been studied for particles emitted into the continuum in preequilibrium nuclear reactions in order to study their systematics. For pure multistep direct reactions it has been found that to first order the shapes of these angular distributions are determined by the energy of the outgoing particle. In addition, there is evidence that the division of the cross section into multistep direct and multistep compound components with the latter having a symmetric angular distribution is a meaningful one. The shapes of the angular distributions can be described in terms of Legendre polynomials. The polynomial coefficients are given by simple phenomenological relations involving the energy of the outgoing particle. The same reduced coefficients are used for multistep direct and multistep compound processes except that only even order polynomials are considered in the latter case. The present formulation has been shown to have significant predictive ability for light ion reactions.