Nonelastic Scattering Cross Sections for Fast Neutrons

Measurements have been made of the nonelastic cross sections of neutrons in Be, C, Al, Ti, Cr, Fe, Ni, Cu, Ag, Sn, Pb, and Bi. These cross sections include all processes except elastic scattering. The experiments have been carried out with monoenergetic neutrons of 3.5, 4.7, 7.1, 12.7, and 14.1 Mev. The neutrons were produced with a Van de Graaff accelerator using the nuclear reactions: ${\mathrm{T}}^3(p, n){\mathrm{He}}^3$, $\mathrm{D}(d, n){\mathrm{He}}^3$, and ${\mathrm{T}}^3(d, n){\mathrm{He}}^4$. Cross sections were determined from transmissions through spherical shells which were 3.8 cm in radius and 2 cm thick. A new type of neutron detector which gives energy information about neutrons and is insensitive to $\gamma$ radiation was placed at the center of the spherical shell and was used as a biased detector of neutrons. The results of these experiments show that the nonelastic neutron cross section for 12.7- and 14.1-Mev neutrons is nearly geometrical and is closely approximated by $\sigma =\pi {(R+\lambda )}^2$, where $R=1.4\mathrm{}\times {10}^{-13\mathrm{}}{A}^{\frac{1}{3}}$. The nonelastic cross sections are roughly constant in the energy range 5 to 14 Mev; below about 5 Mev the cross sections begin to decrease.