The Production and Absorption of Thermal Energy Neutrons

From measurements of the number of neutrons reduced to thermal energies in successive cylindrical layers of water the rate of absorption of the thermal energy neutrons in the water, corrected for the rate of production, was calculated. The total number of neutrons emitted from a Rn-Be source was estimated to be of the order of 15,000/sec./mc. Similar experiments with C, Al, Cu, Fe and Si${\mathrm{O}}_2$ were performed, in each case some slowing down of the neutrons from the source being observed. The angular distribution of slow neutrons emerging from the surface of paraffin spheres was found to be more concentrated along the normal than a cosine law distribution. A description is given of the construction of a source arrangement concentrating the slow neutrons coming from it into a roughly collimated beam. Measurement of the absorption of neutrons having slightly greater than thermal energy proved that Hg is nearly transparent while Rh and Ag showed less absorption than for thermal neutrons. Experiments with a neutron source cooled to liquid-air temperature show increased absorption for H, Li, B, Rh, Ag, I, Gd and Hg and practically no change for Sm and Cd. The construction and operation of mechanical velocity selectors for slow neutrons is discussed and results are given showing that the neutrons from the source used were in thermal equilibrium with it.