An absolute determination of the 235U fission cross section at 964 keV

Abstract

An absolute measurement of the 235U fission cross section has been carried out using a S4Na-Be photoneutron source with median neutron energy of 964 keV. A symmetric two-foil experiment was set up to measure the fission rate in a low-albedo laboratory, and variations in the sourceto-foil spacing used to determine the room background. Fission fragments passing through a limited solid angle aperture were recorded from each foil by solid state tracketch techniques. The photoneutron source was calibrated after each run using the manganese bath method and the secondary national standard source NBS-II. A computed neutron source spectrum with 32 keV FWHM was derived by the Monte Carlo method and used in reducing the data to a cross section at 964 keV. The final value of 1.21 ~ 0.025 barns is absolute in that, except for small corrections, its determination was independent of any other cross section data.

Fission cross-sections of 235U and 239Pu averaged over 252Cf neutron spectrum

“…The source strength was corrected for decay using a half-life of 2.638 yr. For comparison with differential cross-section data, the Grundl and Eisenhauer (1975) evaluation of the 252Cf spectra was used. As discussed by Gilliam and Knoll (1975), the source strength of NBS-II was taken as 1.174 x lO° sec 1 ~__ 0.5%, referred to June 1972.…”

“…The Michigan manganese bath facility has been previously described in detail (Gilliam, 1975;Robertson et al, 1975) Small corrections applied to the manganese calibration results include bulk penetration of the bath. A long counter was chosen for this measurement for its flat response over the measured energy range.…”

“…The masses of the U30 2 deposits were determined to be 8.16rag and 6.25mg by the Isotope Target Laboratory at ORNL. The uncertainty was taken as 0.50% for reasons discussed by Gilliam and Knoll (1975). The plutonium deposit was alpha counted relative to the NBS foil standard, 49K-4-1, whose isotopic composition was identical to the foils used in these measurements.…”

Absolute measurements of 235U and 239Pu fission cross-sections with photoneutron sources

“…The basic experimental method has been described previously by Gilliam and Knoll (1975). Modifications in that procedure are pointed out below.…”

“…The weighted average of the saturated activity over the entire cycle had a statistical precision of about 0.03%, and repeated counts with the NBS-II reference source before and after each fission rate measurement showed reproducibility of better than 0.1%. As discussed by Gilliam and Knoll (1975), the source strength of NBS-II was taken as 1.174 x 10 6 sec -1 q-0.5~o , referred to June 1972.…”