The calculation of neutron transport in air requires an accurate knowledge of the neutron cross sections for nitrogen and oxygen. Existing direct measurement of the total elastic cross section for nitrogen is in marked disagreement with the value deduced by taking the difference between the total cross section and the sum of all partial non-elastic cross sections. This discrepancy in the available data indicates an error in the total-elastic measurement of 30% or in the total non-elastic cross section of 50%. A technique to measure differential cross sections for the small-angle (3 degrees to 15 degrees) scattering of fast neutrons has been developed and applied to a study of the scattering of 7.55 and 9.5 MeV neutrons from N, O, and C. Preliminary analysis of measurements obtained thus far show that the forward angle elastic scattering in nitrogen (theta

The measurement of the small-angle scattering of 5-15 MeV neutrons from low-A nuclei requires an experimental technique of high sensitivity. In order to determine the most suitable technique, various scattering geometries are studied and compared. The experimental approach found optimum involves the use of an unusual scattering geometry for which a four-fold collimator-detector array with fixed collimating channels has been especially designed and constructed. The principles of measurements in this geometry are developed and discussed in detail. From performance tests with the collimator-detector array, the acceptances of the collimating channels and the overall experimental sensitivity are determined. Preliminary measurements are presented of the 7.55 MeV elastic cross section of 0, Al, Fe, Sn, and Pb for neutron scattering in the angular range 2.5 to 15 degrees. (Author).

The calculation of neutron transport in air requires an accurate knowledge of the neutron cross sections for nitrogen and oxygen. Existing direct measurements of the total elastic cross section for nitrogen in the energy range from 6 to 10 MeV are in marked disagreement with the value deduced by taking the difference between the total cross section and the sum of all partial non-elastic cross sections. This discrepancy in the data previously available indicates an error in the total elastic measurement of approximately 30 percent or a 50 percent error in the total non-elastic cross section. A technique to measure differential cross sections for the small-angle (2 degrees to 15 degrees) scattering of fast neutrons has been developed and applied to a study of the scattering of 7.55 and 9.50 MeV neutrons from C, N, and O. The results obtained for nitrogen at 7.55 MeV yield differential cross sections 20 percent to 40 percent higher (at forward angles) than would be inferred from previous measurements and shows that at least 25 percent of the missing cross section should be assigned to the total elastic cross section. (Author).