On asymptotic power corrections to differential fluxes and   generalization of optical theorem for potential scattering
  In a wide class of potentials the exact asymptotic dependence on finite distance R from scattering center is established for outgoing differential flux. It is shown how this dependence is eliminated by integration over solid angle for total flux, unitarity relation, and optical theorem. Thus, their applicability domain extends naturally to the finite R. 
S.E. Korenblit and A.V. Sinitskaya
quant-ph