Monoenergetic photon-induced fission cross-section ratio measurements for U 235, U 238, and Pu 239 from 9.0 to 17.0 MeV
© 2018 American Physical Society. Background: High-accuracy data are needed to advance microscopic descriptions of the nuclear fission process, to improve the predictive precision of phenomenological models, and for applications in nuclear energy and homeland security. Purpose: The main goal of this work is to provide high-accuracy cross-section data for photofission of U235, U238, and Pu239 in the energy range from the fission threshold to the high-energy tail of the giant dipole resonance. These new data should contribute significantly to the reduction of the systematic uncertainty in evaluated photofission cross-section databases. Method: Cross-section ratios for photofission of the "big three" actinide nuclei were measured using a quasimonoenergetic photon beam and dual-fission chambers. The measurements were performed at the HIγS facility using a photon beam produced from Compton backscattering of free-electron laser light. The dual-fission chamber enabled simultaneous counting of fission events from two targets. This method allows for cross-section ratio measurements with very small systematic errors. The largest source of systematic error in the determination of the cross-section ratios is the uncertainty in the ratio of the target thicknesses. Results: We report photon-induced fission cross-section ratios for σ[U235(γ,f)/U238(γ,f)], σ[U235(γ,f)/Pu239(γ,f)], and σ[U238(γ,f)/Pu239(γ,f)] at photon energies from 9.0-17.0 MeV. More than 20 data points were measured for each ratio, and the systematic uncertainties are less than 2%. Conclusion: The present photofission cross-section ratio data sets are compared to ratios computed from previous measurements, to the corresponding neutron-induced fission cross-section ratios, and to ratios computed from evaluated databases. The data obtained in this work for σ[U235(γ,f)/U238(γ,f)] are consistent with the existing data, while for σ[U235(γ,f)/Pu239(γ,f)] and σ[U238(γ,f)/Pu239(γ,f)] the present data are systematically lower.
Krishichayan, ; Finch, SW; Howell, CR; Tonchev, AP; Tornow, W
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