Spatial Distribution of Thermal Neutrons from an N¹⁷ Source in Water

The thermal neutron flux in a water medium from a source disk containing water irradiated in the Materials Testing Reactor has been measured using a BF₃ detector. Neutrons from N¹⁷ and photoneutrons from N¹⁶ gamma rays have been observed. The radial distribution of thermal neutrons from an isotropic point source of N¹⁷ neutrons in a water medium out to 35 cm has been deduced, and has been compared with a theoretical calculation of the distribution based on Holte's values of the collision density at 1 ev from a point isotropic source of 1-Mev neutrons. From the results of this experiment and Holte's distribution, the square of the diffusion length plus the age from 1 ev to thermal energies, has been calculated to be 8.2 cm². This value is in reasonable agreement with previously reported measured values.