NEUTRON-INDUCED FISSION CROSS-SECTION RATIOS FOR PU-239, PU-240, PU-242, AND PU-244 RELATIVE TO U-235 FROM 0.5 TO 400 MEV

The fission cross-section ratios for isotopic targets of Pu-240, Pu-24 2, and Pu-244 relative to U-235 are measured for neutron energies from 0.5 to 400 MeV and for Pu-239 relative to U-235 for energies from 0.8 5 to 62 MeV. A multiple-plate gas ionization detector was used to meas ure simultaneously the fission rate for each of the isotopic targets. The neutron energies were determined by the time-of-flight technique o n a 20-m flight path at the Weapons Neutron Research white neutron sou rce at the Las Alamos National Laboratory Neutron Science Center. Unce rtainties are <4% for energies <50 MeV. This measurement provides the capability to resolve discrepancies among previous measurements for th ese isotopes over this energy range and are the first measurements for mast of these isotopes for energies >30 MeV. The results are compared with previous measurements and to ENDF/B-VI.