Measurement of the Neutron-Proton and Neutron-Carbon Total Cross Section from 150 to 800 keV

There have been very few measurements of the total cross section for $np$ scattering below 500 keV. In order to differentiate among $NN$ potential models, improved cross section data between 20 and 600 keV are required. We measured the $np$ and $n$C total cross sections in this energy region by transmission; a collimated neutron beam was passed through CH$_2$ and C samples and transmitted neutrons were detected with a BC501 liquid scintillator. Cross sections were obtained between 150 and 800 keV by taking the ratios of normalized neutron yields with the samples in the beam and with no sample in the beam. The parameters resulting from fitting effective range theory to the data for $np$ scattering are in good agreement with parameters determined from previous fits.