Slowing Down of Fast Neutrons in Water

The migration area of thermal neutrons and the age of In-resonance neutrons in water have been obtained with T-d neutrons. Taking into account the angular dependence of the emitted neutron energy and yield in the T-d reaction, the average source energy was taken to be 14.2 MeV. By means of “Cd difference” method and a small (13mmφ×50mm) BF3 counter, the spatial distribution of thermal neutrons in a large water tank was measured at the 0°, ±45° and ±80° direction along the radial distance, from the source to the point of 110cm. Then, the migration area M2 was obtained from the spatial distribution of thermal neutrons in each direction. The spatial distribution of In-resonance neutrons also was measured by In foils with Cd covers to the point of 55cm at the same directions. And the neutron age τln of In-resonance energy (1.44 eV) was obtained, using the same relaxation length in large distance as the thermal neutron distribution. The values of migration area M2 and neutron age τln are determined as 176±5cm2 and 167±8cm2, respectively. The slowing down length Ls (=τln1/2) to In-resonance energy is 12.9±0.3cm, and in agreement with Volkin's curve, which was calculatad assuming the anisotropic nature of the elastic scattering by oxygen.