Theory in Evaluation of Actinide Fission and Capture Cross Sections
Theory in Evaluation of Actinide Fission and Capture Cross Sections

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Published: 2004

Total Pages: 7

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The authors discuss the possibilities and limitations of the use of theory as a tool in the evaluation of actinide fission and capture cross-sections. They consider especially the target 235U as an example. They emphasize the roles of intermediate structure in the fission cross-section and of level width fluctuations in both intermediate structure and fine structure, noting that these lead to a breakdown of Hauser-Feshbach theory at sub-barrier and near barrier energies. At higher energies (where fluctuation-averaged Hauser-Feshbach theory is applicable) semi-quantitative and intuitive representations of transition state spectra and barrier level density functions have to be tested against experimental data wherever these are available. Adjustment of the fission cross-section against inelastic scattering to the much better known levels of the residual nucleus should then lead to a fairly sound estimate of the capture cross-section. They compare such estimates with evaluated and experimental data for 235U.

The 237U(n, F) Cross Section
The 237U(n, F) Cross Section

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Published: 2003

Total Pages: 10

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The purpose of this note is to combine existing information on the 237U(n, f) cross section to determine if some consistency can be obtained for the neutron induced fission excitation of 237U. The neutron induced fission cross section of the 6.8 day 237U was measured directly by McNally et al. in 1968 using the Pommard nuclear device test. At the same time critical assembly measurements were done at Los Alamos using the Flattop assembly. A previous measurement was also made at LASL in 1954 with two different neutron sources, each peaked near 200 keV. The results were 0.66 " 0.10 b and 0.70 " 0.07 b for the (n, f) cross section. More recently Younes and Britt have reanalyzed direct reaction charged particle data of Cramer and Britt that had determined the fission probability of the 238U compound nucleus as a function of nuclear excitation energy. They have combined fission probabilities with calculated neutron absorption cross sections, including corrections for the differences in angular momentum between the direct and neutron induced reactions. From this analysis they have extracted equivalent 237U(n, f) cross sections. The technique for extracting surrogate (n, f) cross sections from (t, pf) data has been demonstrated in a recent publication for the test case 235U(n, f). In addition to this experimental information, Lynn and Hayes have recently done a new theoretical study of the fission cross sections for a series of isotopes in this region. A summary plot of the data is shown in Fig. 1. Below 0.5 MeV the McNally, Cowan, and Younes-Britt results are in reasonable agreement. The average cross section in the Younes-Britt results, for En = 0.1 to 0.4 MeV, is 0.80 times the McNally values which is well within the errors of the McNally experiment. Above 0.5 MeV the McNally results diverge toward higher values. It should be noted that this divergence begins approximately at the 237Np threshold and that 237Np is the daughter of the 6.8 day 237U decay.