Reaction rates for equilibrium - and non-equilibrium systems

It is proved that the classical concept of cross section is valid in equilibrium systems with elastic Coulomb interactions. The formula for the reaction rate in this system is formally generalized to a non-equilibrium system with general binary interaction were the validity is uncertain. The sixfold velocity space integral in the formula (sigma v), called reaction formula or reactivity is reduced to lower dimensions in case of certain symmetries and further processed in preparation for numerical evaluation. The necessary assumptions and approximations as well as coordinate transformations are shown explicitely. Possible applications are to elastic and inelastic nuclear reactions and to plasma physics. (G.Q.)