Neutron Resonances in the kev Region: Differential Scattering Cross Sections

The ratio of the differential elastic neutron scattering cross sections for scattering by 180° and 90°, $R=\frac{{\sigma }_{\mathrm{nn}}\left(180\mathrm{}°\right)}{{\sigma }_{\mathrm{nn}}\left(90\mathrm{}°\right)}$, has been measured as a function of neutron energy between approximately 30 and 110 kev for F, Na, Mg, Al, K, V, Cr, Cu, Zn, As, Se, Sr, Zr, Nb, Mo, Ag, and Cd. When plotted against atomic weight, the average ratios $\overline{R}$ (averaged over the energy range of each element) exhibit peaks at $A\approx 90$ and $A\approx 25$, in qualitative agreement with the predictions of the complex square well potential model of the nucleus. However, the experimental ratios are in general larger than those predicted by the theory. Scattering from the lighter elements was also analyzed by single-level resonance theory and resulted in the following assignments: resonance in Al at 88 kev, $J=3^{+}$; Mg (85 kev), ${\frac{3}{2}}^{-}$; Na (53 kev), $3^{-}$; F (99 kev), $1^{-}$; F (49 kev), $1^{-}$; F (27 kev), $1^{-}$ or $2^{-}$. These assignments are in good agreement with those obtained from the latest transmission measurements.