Analysis of diffusion length problem in the presence of large absorption

The propagation of a thermal neutron beam into a water assembly containing various concentrations of Cd has been studied by solving the diffusion equation in the multigroup formalism. The Nelkin model has been used for the scattering kernel. It is observed that for very high Cd concentrations the experimentalist measures a pseudomode which dominates at. small distances from the source. This pseudomode decays very slowly to reach the asymptotic distribution at very large distances from the thermal neutron source and at such large distances the neutron density would become so small that it would be difficult to detect these neutrons.