Collision Cross Sections for 25-Mev Neutrons

Abstract

Collision cross sections of a number of elements for high energy neutrons have been measured. Neutrons with a maximum energy of 25.4 Mev were obtained by bombarding lithium with 10.2 Mev deuterons. The reaction ${\mathrm{C}}^{12}(n,2n){\mathrm{C}}^{11}$, which has a measured threshold energy of approximately 21 Mev, was used as an energy sensitive detector for the transmission measurements. The cross section obtained for the neutron-proton collision process was 0.39 \ifmmode\pm\else\textpm\fi{}0.03\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}24}$ ${\mathrm{cm}}^{2}$. This is higher than the cross section calculated for $s$-scattering (0.35\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}24}$ ${\mathrm{cm}}^{2}$), but agrees well with the value of 0.40\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}24}$ ${\mathrm{cm}}^{2}$ predicted by the symmetrical meson theory of Rarita and Schwinger. Measurements on other nucleii ranging from carbon to mercury show that the collision radius is given by ${R}^{\ensuremath{'}}=b+{r}_{0}{A}^{\frac{1}{3}}$, with $b=1.7\ifmmode\pm\else\textpm\fi{}0.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}$ cm and ${r}_{0}=1.22\ifmmode\pm\else\textpm\fi{}0.15\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}$ cm. These measurements are in good agreement with the inelastic cross-section measurements of Grahame and Seaborg. The value of ${r}_{0}$ is somewhat lower than the values deduced from $p\ensuremath{-}n$ reactions, Coulomb energies, and $\ensuremath{\alpha}$-particle decay.