On the Experimental Application of the Neutron-Wave Technique to Thermal-Neutron Systems

A technique is presented for conducting neutron-wave experiments in thermal-neutron systems using either a sinusoidally modulated or a pulsed source of thermal neutrons. A neutron source suitable for this experiment is described, data-accumulation criteria are presented, and the electrical systems used for collection are discussed. A specific experiment with graphite is reported and the discussion of data analysis is carried through the experimental determination of this system's dispersion law. It is found that, in general, a pulsed source of thermal neutrons is more suitable for neutron-wave experimentation than a sinusoidally modulated source. Confirmation is given to the theoretical prediction that diffusion and thermalization parameters can be measured by this technique over a relatively wide range of frequency without significant higher space- and energy-mode contamination. The values we obtained for the diffusion and thermalization parameters for graphite of density 1.60 g/cm³ were α₀ ± 91. ± 1 sec⁻¹, D₀ = (2.16 ± 0.01) × 10⁵ (cm² sec⁻¹), C₀ = (39. ± 2) × 10⁵ (cm⁴ sec₋₁), and F₀ = (12. ± 2) × 10⁷ (cm⁶ sec⁻¹).