Nuclear Cross Sections for 95-Mev Neutrons

Abstract

The total cross sections of twelve different elements were measured using the neutron beam from the 184-inch cyclotron operating with deuterons. Bismuth fission ionization chambers were employed as both monitor and detector in conventional good geometry attenuation measurements in the neutron flux emerging from the three-inch diameter collimating port in the 10-foot thick concrete shielding. The mean energy of detection of the neutrons in this experiment is estimated to be 95 Mev.Measurements were also made with a monitor and detector placed inside the concrete shielding where we could obtain an intense neutron flux over a large area. Attenuators of four different elements were placed in front of the detector in a poor geometry arrangement so that attenuation was due essentially to inelastic collisions which degrade the neutron energy below the fission threshold. A second detector was placed outside the concrete shielding in the collimated neutron beam in line with the neutron source, absorber, and first detector. Attenuation in it is caused by both inelastic and elastic scattering. By this arrangement the ratio of inelastic to total cross section can be determined directly in one experiment.The nuclear radii as calculated from the observed cross sections using the theory of the transparent nucleus vary as 1.38\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}13}$ A\textonehalf{} cm. In this energy range the ratios of the inelastic to total cross sections are all less than one-half.