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

Abstract

The total neutron cross sections of ${\mathrm{H}}_{2}$, $n$-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 20.36\ifmmode\pm\else\textpm\fi{}0.10 barns for the free proton cross section. In this energy interval the measurements confirm Placzek's theoretical prediction of the variation of the ${\mathrm{H}}_{2}$ cross section. The technique of making absolute transmission measurements and the corrections for multiple scattering are discussed. Measurements on ${\mathrm{H}}_{2}$ 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.