Spectral indices for BIG-10: spectrum-averaged fission cross section ratios for 235U, 238U, 239Pu, 237Np

Fission ratios were observed for back-to-back deposits of the isotopes 235U, 238U, 239Pu, and 237Np. The fission counts were recorded by means of a dual triple-scaler counting system which was compared with a PDP-8 computer-based analyzer system using LASL-built live-timing circuitry. An rms discrepancy of 0.2% was obtained between the two independent systems. In addition, a check of the NBS dead-time corrections was made by the variance-to-mean method using LASL multiscaling hardware. The NBS dead-time corrections were found to agree with the variance-to-mean results within +-0.2% over the count rate range used in the present measurements. The fission cross section ratios measured were as follows: 1.000: 0.0366 +- 1.8%: 1.198 +- 1.5%: 0.314 +- 2.1% for 235U, 238U, 239Pu, 237Np, respectively. By application of calculated perturbation corrections, the following fission cross section ratios were obtained for the unperturbed BIG-10: 1.000: 0.0372 +- 1.9%: 1.198 +- 1.5%: 0.317 +- 2.2% for 235U, 238U, 239Pu, 237Np, respectively. For comparison, these cross section ratios were calculated from ENDF/B-IV cross section data using an 87 group computed spectrum for BIG-10. The ratios of the observed spectral indices to the calculated spectral indices were 1.106, 1.021, and 0.985 for the cross section ratios of 238U/235U, 239Pu/235U, and 237Np/235U, respectively