The Scattering of Fast Neutrons by Bismuth and Lead

Using good geometry, homogeneous fast neutrons of energy 4.3 Mev, supplied by the D-D reaction in the Bartol Van de Graaff statitron, have been scattered in bismuth and lead. Using binocular microscopes and Eastman NTA plates, fifty thousand fields of view were examined to obtain 5000 acceptable recoil proton tracks. The energy spectrum of the scattered neutrons was studied to establish the presence or absence of any inelastic groups. From a consideration of gross effects, it is concluded that no inelastic group exists in the bismuth data with an intensity greater than five percent of the elastically scattered neutrons, or that no level in Bi²⁰⁹ lying between the ground state and 3.4 Mev is excited by a neutron group of more than five percent of the intensity of the elastic group. Appreciable inelastic scattering was noted in naturally occurring lead, the inelastic scattering cross section being less than half the elastic cross section. The energy distribution of the inelastically scattered neutrons is interpreted to indicate an energy level in lead in the vicinity of 3.3 Mev. This level is consistent with radioactivity measurements which show that most of the beta-disintegrations of ThC^''β→*Pb²⁰⁸ lead to energy levels in the vicinity of 3.2 Mev, and with Q-values of the reaction Pb²⁰⁷(d,p)Pb²⁰⁸ which disclose an energy level at 3.45 Mev in the nucleus Pb²⁰⁸.