Measurements of the fission cross-sections of 239Pu and 241Pu relative to that of 235U in the neutron energy range 0.016 to 0.55 eV

Measurements have been made to an accuracy of ±2% of the ratios of the fission cross-sections of 239Pu and 241Pu to that of 235U in the energy range 0.016 to 0.55 eV and also in two thermalized fields of neutrons. The monoenergetic neutrons were obtained from a crystal spectrometer on the AWRE reactor HERALD. At the three lowest neutron energies a velocity selector was used to remove neutrons diffracted into the second and higher orders. At the other energies resonance filters were used. One thermal neutron field was an extracted beam from the graphite thermal column and its spectrum was measured by a time-of-flight method. The other thermal neutron field was in a cavity inside the thermal column. Some discrepancies between the present results and previous measurements are discussed. Corrections and errors due to the neutron spectra of both the monoenergetic neutrons and the thermal beam and errors introduced by beam non-uniformity are described. (author)