Fluctuations in Neutron Total Cross Sections

Abstract

The total cross sections of 18 elements from Mg to Bi were measured with energy spreads of 20 or 50 keV and a statistical accuracy of 1% as a function of neutron energy in the energy range from 4.5 to 7.5 MeV. No deviations from a smooth energy dependence were observed in the cross sections of the 10 elements from Co to Ta. For the remaining elements fluctuations were observed, the amplitudes of which decreased with neutron energy. For these elements, measurements were extended to higher energies until no fluctuations were observable. In addition, for Pb and Bi the energy interval from 2.5 to 4.5 MeV was investigated. The fluctuations could be accounted for by fluctuations in widths and spacings of compound-nucleus levels. It was not necessary to assume intermediate structure. Although the conditions for the Ericson theory of fluctuations may not be satisfied for all the measurements, this theory also accounts for the observed fluctuations at the higher energies.