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The design and building of reactors requires a knowledge of the interaction cross sections of neutrons with different materials over a broad energy range. Interactions between neutrons and nuclei (radiative capture, fission, scattering) are viewed in the light of current concepts as processes involving the formation of a compound nucleus and displaying a clearly pronounced resonance response: the interaction increases sharply at certain neutron energies. The position and quantitative value of the resonance levels still defy theoretical prediction, and direct measurements of the cross sections concerned are consequently the sole sources of the required information.

Values of ? and total neutron cross sections of the nuclides U236 and Pu239 were measured on a neutron spectrometer, using a pulsed cyclotron beam [1]. Energy relations of the cross sections for fission and radiative capture in the neutron energy region from thermal energies up to 20 eV for U235 and from 5 to 100 eV for Pu239 were obtained, The parameters of an appreciable number of resonances were determined.

Conclusion The development of techniques for measuring cross-sections has permitted a considerable extension of the range of energies over which these measurements can be carried out with a resolution that is sufficiently high for analysis; simultaneously, the range of nuclei studied has been extended.