On the singular structure of the uncollided and first-collided components of the Greenʹs function

Explicit expressions are given for the uncollided and first-collided fluxes originated from a singular source in a finite heterogeneous medium with vacuum boundary conditions. The analysis of the singular behavior of the first-collided flux shows a known discontinuity in angle, but also a discontinuous limit for angular directions parallel or antiparallel to that of the original source that can be important for strongly forward scattering.