The Totaln−pScattering Cross Section at 4.75 Mev

Attenuation measurements in good geometry on polyethylene and graphite scatterers with transmissions from 0.30 to 0.65 have been made. A thin-walled gas target was used, producing forward $d-d$ neutrons at a mean energy of 4.749±0.009 Mev. Care was taken to minimize uncertainties arising from neutrons of other energies, from unshadowed background, and from impurities in the samples. The geometry was such that multiple scattering-in contributed no more than 0.12 percent to the total uncertainty. Checks against rate-dependent sensitivity of the detector revealed no effects greater than 0.15 percent. The total $n-p$ scattering cross section deduced from the measurements is 1.690±${0.006}_6$ barns, where the probable error has been increased from the statistical error of ${0.002}_4$ barn to include the energy uncertainty and estimates of possible systematic effects. Values of ${r_{0s}}^{\mathrm{}(n-p)\mathrm{}}$, the singlet effective range of the $n-p$ interaction, obtained from this result for several potential shapes, are compared with corresponding values of ${r_{0s}}^{\mathrm{}(p-p)\mathrm{}}$. The validity of the hypothesis of charge independence is found to depend on the potential shape assumed.