A pulsed measurement of the effective thermal neutron absorption cross-section of a heterogeneous medium

The material heterogeneity for the thermal neutron absorption has been experimentally investigated. Silver grains of different sizes in a Plexiglas matrix have been used as models of the heterogeneous material. Three silver-in-Plexiglas models have been designed and constructed in order to create a controlled lattice of absorbing centres. The grains are small cylinders of the height equal to the diameter (4, 6 and 10 mm) and their total volume content φ=0.05 in all the models is constant. These models allow the investigation of an isolated effect of the grain size only on the neutron absorption. The effective thermal neutron absorption cross-section has been measured. The results strongly depend on the silver grain size and significantly differ from the cross-section of a corresponding homogeneous material. The ratio of the absorption cross-section of the heterogeneous to homogeneous material has reached 0.34 for the largest silver grains and 0.66 for the smallest ones.