Influence of the grain size on the effective absorption cross-section of thermal neutrons in a medium containing highly absorbing centres

The thermal neutron absorption is considered in a medium which contains highly absorbing grains. The theoretical approach is based on a definition of the effective macroscopic absorption cross-section for such a medium. The parameter of the grain effect is defined as the ratio of the effective cross-section to the cross-section of a corresponding homogeneous material. It is generally obtained as a function of the ratio of the absorption cross-sections of components, of the grain size, and of the contributions of the components. A further analysis is performed for the grain size effect when the two other parameters are fixed. The presented approach is applied to an interpretation of experiments on Plexiglas models containing silver grains. A good agreement of the experimental and theoretical results is obtained when relevant effective energy-dependent cross-sections are weighted by the thermal neutron energy distribution.