Measurements of Neutron Fluxes in Strongly Absorbing Gaseous Media

Spatial neutron flux distributions have been measured in large volumes (450 liters) of several gaseous media. Measurements were made in nearly pure CO₂ having a σ_s/σ_c ratio of ∼1200 and in mixtures of BF₃ and CO₂ with σ_s/σ_c ratios down to ∼0.02. Thus, distributions were obtained in an almost purely scattering and purely absorbing medium. The experimental results have been processed to represent the flux distribution in a semi-infinite medium of identical composition. This required removal of the leakage component, a correction made possible because the neutron mean-free-path of the measured media differed by several orders of magnitude. In effect, distributions determined almost entirely by neutron leakage and distributions determined almost entirely by neutron capture were directly measured. The experimental results were compared with transport theory calculational models by using several assumptions concerning the angular distribution of the neutron source flux. Satisfactory agreement was achieved between the experimental and analytical results when a purely collimated incident flux was assumed.