Collision Cross Sections for 25-Mev Neutrons

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 ${\mathrm{C}}^{12}\mathrm{}(n,2n)\mathrm{}{\mathrm{C}}^{11}$, 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 ±0.03×${10}^{-24\mathrm{}}$ ${\mathrm{cm}}^2$. This is higher than the cross section calculated for $s$-scattering (0.35×${10}^{-24\mathrm{}}$ ${\mathrm{cm}}^2$), but agrees well with the value of 0.40×${10}^{-24\mathrm{}}$ ${\mathrm{cm}}^2$ 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 $R^{\prime }=b+r_0{A}^{\frac{1}{3}}$, with $b=1.7\mathrm{}\pm 0.4\times {10}^{-13\mathrm{}}$ cm and $r_0=1.22\mathrm{}\pm 0.15\times {10}^{-13\mathrm{}}$ 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 $p-n$ reactions, Coulomb energies, and $\alpha$-particle decay.