A Study of the 2.8-keV Neutron Resonance of Sodium and an Analysis of the Total Neutron Cross Section up to 50 keV

The total neutron cross section of sodium has been measured in the vicinity of the 2.8-keV resonance with the high-resolution time-of-flight spectrometer associated with the Yale University 70-MeV Electron Linear Accelerator. The spin of the resonance is unambiguously identified to be J = 1 . A least-squares analysis of the cross section has been carried out up to an energy of 50 keV using a model that takes into account the effects of local and distant levels. The observed total cross section is well described throughout the entire range with a spin-independent interaction radius of 5.8 fm and with reasonable values of the R functions (distant level effects) for both spin states. The resonance energy, the neutron width, and the effective nuclear radii derived from the analysis are, respectively, E_R = 2805 ± 30 eV, Γ_nR = 376 ± 15 eV, a_{J =1} = 5.3 fm, and a_J=2(E) = 5.7 + [2 × 10⁸/(E + 18500)²] fm.