Slowing-down Process in the Infinite Medium

This problem was previously encountered in the case of the infinite-medium slowing-down process for a multiplying material. In that instance we identified the source term for the thermal neutrons as the slowing-down density at thermal energy g(i th) [cf. Eq. (4.252)]. In the present case we require a somewhat more generalized function which takes into account the spatial distribution of the slowing-down density. Thus a suitable description for the source term in the diffusion equation for thermal neutrons would be... 

S A point source in an infinite medium is emitting neutrons of zero lethargy isotropically at a constant rate in time. Assume that the slowing-down process may be described by means of the Fermi age model once the first collision has occurred. Thus at points of first collision, the neutrons enter a continuous slowing-down process. 

---