A Precise Determination of the Slow Neutron Cross Section of the Free Proton

The total neutron cross sections of H₂, n-butane, and water have been measured with the Columbia slow neutron velocity spectrometer in the energy range 0. 8 to 15 ev. Analysis of these data gives a value of 20. 360. 10 barns for the free proton cross section. In this energy interval the measurements confirm Placzek's theoretical prediction of the variation of the H₂ cross section. The technique of making absolute transmission measurements and the corrections for multiple scattering are discussed. Measurements on H₂ for neutron energies above 15 ev show a constant cross section to several thousand ev. Measurements on fused quartz in the energy interval 0. 003 ev to 10 ev show interference effects.