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

Abstract

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 ${\mathrm{Li}}_{2}$O 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\ifmmode^\circ\else\textdegree\fi{} 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 ${\mathrm{Li}}^{7}(p, n)$ reaction and the ${\mathrm{O}}^{18}(p, n)$ 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: $\ensuremath{\sigma}=2.525\ifmmode\pm\else\textpm\fi{}0.009\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}24}$ ${\mathrm{cm}}^{2}$ at a neutron energy of 2.540 Mev, and $\ensuremath{\sigma}=4.228\ifmmode\pm\else\textpm\fi{}0.018\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}24}$ ${\mathrm{cm}}^{2}$ at a neutron energy of 1.005 Mev. These values, together with the values of ${a}_{t}$, ${a}_{s}$, and ${\ensuremath{\rho}}_{t}$ given by Burgy, Ringo, and Hughes yield values of the singlet effective range in the shape independent approximation of 2.48\ifmmode\pm\else\textpm\fi{}0.20\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}13}$ cm from the 2.5-Mev measurement, and 2.56\ifmmode\pm\else\textpm\fi{}0.25\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}13}$ 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.