Determination of the neutron spectrum shape parameter α in k0 NAA methodology using covariance analysis

The k0 method for quantitative reactor neutron activation analysis (NAA) has been applied in several laboratories for the determination of multi-elemental concentrations in different materials. The general formula that yields the concentration value can be divided in two parts: one involving detection parameters and the other involving irradiation parameters. A rigorous uncertainty calculation must take into account the correlations between each of these parameters. The Nuclear Metrology Laboratory at IPEN has a research program intended to develop a methodology applying covariance analysis in order to obtain the overall uncertainty in the concentrations of different elements in a given sample, and the correlation between each pair of them. The present paper concentrates in the determination of the neutron spectrum shape factor α by two methods: Cd-covered and Cd-ratio, using experimental data obtained in the IEA-R1 research reactor. The final values for α were: (0.001±0.018) and (0.001±0.019) for the Cd-covered and Cd-ratio methods, respectively, in good agreement with each other.