Direct observations of the time variation of slow neutron density in a large volume of water during and after irradiation by neutrons from the D - D reaction have been made. An exponential growth and decay is found. From an analysis of these data and data on the spatial density distribution which determines the effect of diffusion, the mean life of neutrons in water was found to be 205 ± 10 microseconds. This leads to a cross section of 0.33 × ${10}^{-24\mathrm{}}$ ${\mathrm{cm}}^2$ for the capture of slow neutrons by hydrogen.

• Received 22 September 1941

DOI:https://doi.org/10.1103/PhysRev.61.152