Analyzing power measurements for n-p scattering between 13.5 and 16.9 MeV

The analyzing power A_γ(θ) for neutron-proton scattering has been measured for θ = 90°(c.m.) from 13.5 to 16.9 MeV and from θ = 50° to 145°(c.m.) at 16.9 MeV. Extensive Monte Carlo calculations have been made to correct for multiple scattering effects. Overall uncertainties are about ± 0.002. All the A_γ(θ) data, but primarily those at 16.9 MeV, disagree with predictons based on the phase-shift sets which have been derived previously by way of global analyses of nucleon-nucleon scattering data. Data for the product σ(θ)A_γ(θ) have been fitted with an expansion of the form (sin θ)($\text{a}{}_0\text{+ a}{}_1\text{cos}\text{θ + a}{}_2\text{cos}{}^2\text{θ)}$. For the first time the need for a non-zero a₂ has been illustrated for energies below 20 MeV. This parameter is shown to be related to the nucleon-nucleon F-state spin-orbit phase parameter. In addition, the P, D, and F spin-orbit phase parameter values derived from the present data differ significantly from the ones based on the Yale-IV and Liver-more-X global analyses. The derived D and F spin-orbit phase parameters also differ from those obtained in the recent analysis of nucleon-nucleon scattering data by Arndt et al.