Evaluation of the resonance region for 58Fe

An evaluation of the resonance parameters for neutrons incident on 58Fe has been carried out in the energy region of resolved resonances. The recommended parameters given in this report reproduce the measured differential cross section and the recommended thermal capture cross section to within the published errors and more closely than the present JEF-2.2 evaluated file. The spins of the resonances with neutron widths greater than the radiation widths could be derived from the observed capture areas by assuming that the average radiation width of 0.2446 eV did not vary greatly with spin or momentum. Spins and momentum of the remaining resonances were allocated in a random fashion to make up the expected 2J + 1 spacing distribution and the strength functions. The expected 1/v dependence of the capture cross section below the first positive resonance is accomplished by the addition of several negative energy resonances. The radiation width of the first bound level was adjusted to reproduce the evaluated thermal capture cross section of 1.3143 barns and its neutron width adjusted to fit the measured total cross section in the region above ∼ 50 eV. These recommended parameters should only be used to calculate cross sections up to 155 keV. Between 155 keV and 2 MeV the averaged cross section given in Appendix 2 could be used, and above 2 MeV the recommended values given in the present JEF-2.2 file could be adopted. One of the problems with this evaluation and previous ones is that ∼ 45% of the calculated value of the total resonance integral of 1.2817 ± 0.0277 barns comes from the first two resonances for which there is effectively only one transmission measurement by Garg et al [Gar-78a] and some preliminary capture measurements by Borella [Bo-04]. New measurements on enriched or natural iron are needed to solve this problem. Another is that there is a correlation between the radiation widths and the neutron widths. This correlation may be genuine or may be due to the detection of scattered neutrons in the γ-ray detectors, and needs investigating. (author)