HYDROGEN EFFECTS IN NEUTRON-ACTIVATION ANALYSIS

Perturbations generated in two neutron fields, with different energy distributions, by scattering on hydrogen atoms are reported and discussed, as well as the influence of various parameters. The thermal neutron capture rates of several nuclei can be enhanced by about 18% in solutions in light water as compared to heavy water, for 20 cm3 cylindrical containers. This effect appears to be independent of the initial distribution of the epithermal neutrons, and can be accounted for by the scattering of non-Maxwellian, quasi thermalized neutrons to lower energies. The enhancement ratio is found to be about the same for a variety of nuclei (B, Hg, Gd, Eu, Sm and Cd), in spite of large differences in the variations of their cross sections with neutron energy. This can be explained by assuming that the incoming fluxes present a depression at the level of the Cd resonance, at about 0.18 eV.