The $n-p$ total cross section at 0.4926 MeV has been measured to be 6.202 b ±0.18%. This measurement when combined with the latest results at zero energy, determines the $n-p$ singlet effective range to be (2.46±0.12)×${10}^{-13\mathrm{}}$ cm. The uncertainty has been decreased by a factor of 2 from the best previous determination. The systematic uncertainty due to shape dependence effects is less than ±0.04×${10}^{-13\mathrm{}}$ cm. The total $n-p$ cross section at 3.205 MeV has been measured as 2.206 b ±0.31%. This preliminary value was obtained in the process of measuring rate dependence effects to correct the $n-p$ cross section at 0.4926 MeV for count rate dependence. The use of resonant ($n, p$) and ($n, \alpha$) reactions for neutron detection in high-pressure gas scintillators is the most novel feature of the experimental method. An analysis is made of the propagation of the uncertainties in the various independent low-energy experiments into uncertainties in the calculated shape-independent cross section. The results of the analysis are presented as plots of percent cross section uncertainty versus neutron lab energy.

• Received 5 July 1962

DOI:https://doi.org/10.1103/PhysRev.129.324