Book Description

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).