Book Description

The ratio of the neutron-induced fission cross sections of 241Pu relative to that of 235U 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 energy over broad energy bands. These data were used to estimate the energy variation in the efficiency to be less than 0.75 percent. The threshold cross-section method normalized the ratio independent of other cross-section measurements to 1.268 +- 0.022 in the interval 1.75 to 4.00 MeV. The ratio was also taken to thermal neutron energy where it was normalized to evaluated thermal fission cross sections and the resulting ratio in the 1.75- to 4.00-MeV interval was 1.242 +- 0.021. Corrected for the impurities in the 241Pu sample, the average of these two values resulted in a $sup 241$Pu:235U fission cross-section ratio of 1.251 +- 0.016 for the normalization interval. 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, expressed as a standard deviation. Systematic errors are discussed, and current results are compared with previous measurements. Tables of our data are included. (9 tables, 8 figures) (auth).