ACCOUNTING FOR THE THERMAL-NEUTRON FLUX DEPRESSION IN VOLUMINOUS SAMPLES FOR INSTRUMENTAL NEUTRON-ACTIVATION ANALYSIS

At the Delft University of Technology Interfaculty Reactor Institute, a facility has been installed to irradiate cylindrical samples with di ameters up to 15 cm and weights up to 50 kg for instrumental neutron a ctivation analysis (INAA) purposes. To be able to do quantitative INAA on voluminous samples, it is necessary to correct for gamma-ray absor ption, gamma-ray scattering, neutron absorption, and neutron scatterin g in the sample. The neutron absorption and the neutron scattering are discussed. An analytical solution is obtained for the diffusion equat ion in the geometry of the irradiation facility. For samples with know n composition, the neutron flux-as a function of position in the sampl e - can be calculated directly. Those of unknown composition require a dditional flux measurements on which least-squares fitting must be don e to obtain both the thermal neutron diffusion coefficient D(s) and th e diffusion length L(s) of the sample. Experiments are performed to te st the theory.