Preview of a measurement of the neutron-induced fission cross section of 241Pu relative to 235U from 0.1 to 30 MeV

The ratio of the 241Pu to 235U neutron-induced fission cross sections was measured with fission ionization chambers at the LLL 100-MeV electron linear accelerator. The time-of-flight technique was used to measure the cross section ratio as a function of neutron energy from 0.001 to 30 MeV. The continuous energy spectrum of the neutron source allowed us to cover the entire energy range in one measurement. Pulse height distributions were also measured as functions of neutron energy over broad energy bands. Based on these measurements, the energy variations in the efficiency were estimated to be less than 0.75 percent. The threshold cross section method was used to normalize the ratio in the MeV energy range, independent of other cross section measurements. The ratio was also taken to thermal neutron energy where it was normalized to evaluated thermal fission cross section ratios. Good agreement was found when the two means of normalization were compared. Typical energy resolution is 5 percent at 20.0, 1.0, and 0.1 MeV. Most of the data have counting uncertainties smaller than 4 percent. Systematic errors are discussed, and current results are compared with previous measurements. Tables of data are included. (1 figure) (U.S.)