Systematic effects on cross section data derived from reaction rates at a cold neutron beam

The methodology to derive cross section data from measurements in a cold neutron beam was studied. Mostly, capture cross sections at thermal energy are derived relative to a standard cross section, e.g. the cross section of the 1H(n,g), 14N(n,g), or 197Au(n,g) reaction, and proportionality between the standard and the measured cross section, evaluated at different energies in the sub-thermal region, is often assumed. Due to this assumption the derived capture cross section at thermal energy can be biased by more than 10%. Evidently the bias depends on how much the energy dependence of the cross section deviates from a direct proportionality with the inverse of the neutron speed. The effect is reduced in case the cross section is not derived at thermal energy but at an energy close to the average energy of the cold neutron beam. Nevertheless, it is demonstrated that the bias can only be avoided in case the energy dependence of the cross section is known and proper correction factors are applied. In some cases the results are also biased when the attenuation of the neutron beam within the sample is neglected in the analysis. Some of the cross section data reported in the literature suffer from such bias effects. Hence, the results have to be corrected using the correction factors presented in this paper.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard