Measurement of the fast-neutron fission cross sections of231Pa and243Am

The authors measure the fission cross section ratios Pa 231/Pu 239 and Am 243/Pu 239 in the energy range E /sub n/ = 0.135-7.4 MeV. The Am 243 fission cross section is measured because of the considerable (up to 30%) divergence of the results in the plateau region (E /sub n/ = 2-5 MeV). The measurements were performed with neutrons from the Li 7 (p,n) Be 7, T (p, n) He 3, and D (d, n) He 3 reactions at electrostatic accelerators. Solid targets of lithium fluoride or scandium hydride were used. Fission fragments were recorded by an ionization chamber. The fission cross section ratios Pa 231/Pu 239 and Am 243/Pu 239 were measured simultaneously by using two back-to-back ionization chambers. The results of the measurements are listed and the total error shown is the rms sum of all the uncertainties determined. A table shows the structure of the characteristic corrections and errors of the measurements performed at 2.5 MeV.