Measurement of the Neutron-Proton Cross Section at 1.0 and 2.5 Mev

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The total cross section of hydrogen for 1.0- and 2.5-Mev neutrons has been determined by measuring the neutron transmission of samples of 2-2-4 trimethylpentane and graphite. Electrostatically analyzed protons from an electrostatic generator were used to bombard thin targets of tritium absorbed in zirconium to produce 2.5-Mev neutrons, and thin targets of Li₂O to produce 1.0-Mev neutrons. A gas-filled recoil counter served as neutron detector. For both cross-section determinations the geometry of the measurement was such that neutrons scattered from the samples at angles greater than 4.2^∘{} in the laboratory system were not detected. A pulse-height discriminator, biased to reject pulses from low-energy neutrons, reduced the background from room-scattered neutrons to less than one percent and eliminated effects of low-energy neutron groups from the Li⁷(p, n) reaction and the O¹⁸(p, n) reaction. Neutron energy spreads, primarily caused by the finite target thicknesses, were determined by measuring the widths of narrow neutron-scattering resonances in sulfur and carbon. The results of the cross-section measurements are: σ=2.525±0.009×10^-24 cm² at a neutron energy of 2.540 Mev, and σ=4.228±0.018×10^-24 cm² at a neutron energy of 1.005 Mev. These values, together with the values of aₜ, aₛ, and ρₜ given by Burgy, Ringo, and Hughes yield values of the singlet effective range in the shape independent approximation of 2.48±{}0.20×{}10^-13 cm from the 2.5-Mev measurement, and 2.56±{}0.25×{}10^-13 cm from the 1.0-Mev observations. These effective ranges are consistent with the proton-proton scattering data and the hypothesis of the charge independence of nuclear forces.