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 expe rimentally investigated. Silver grains of different sizes in a Plexiglas ma trix have been used as models of the heterogeneous material. Three silver-i n-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 co ntent phi = 0.05 in all the models is constant. These models allow the inve stigation of an isolated effect of the grain size only on the neutron absor ption. The effective thermal neutron absorption cross-section has been meas ured. The results strongly depend on the silver grain size and significantl y differ from the cross-section of a corresponding homogeneous material. Th e 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 s mallest ones. (C) 2001 Elsevier Science Ltd. All rights reserved.