Critical evaluation of the effective resonance energy concept and the epithermal neutron spectrum shape-factorα for reactor NAA purposes

AbstractAbstract

The epithermal reactor neutron spectrum shape-factorα, and the associated effective resonance energy$\overline E _r$ for a given nuclide, are examined theoretically in great detail. First the necessity, meaning and importance of the choice of a reference neutron energy in a non-ideal spectrum (α≠0) are explained. Next, the definition and practical calculation of$\overline E _r$ are discussed, showing that the relation between the reasonance integrals in ideal and non-ideal spectra cannot be described adequately by two independent parameters$\overline E _r$ and α. The exact meaning of the logarithmic expression defining an$\overline E _r$ independent ofα is clarified, its limits of validity are established as a function ofα and, as a result, it follows that relatively large systematic errors on$\overline E _r$ can be introduced by its use. It is shown that$\overline E _r$ is dominated by the first lowest resonance energies for a given nuclide, making its vulnerability to literature updates of resonance parameters almost equal to those of individual resonances. The effect, on the epithermal and total activation of specific nuclides, of large systematic and statistical errors on$\overline E _r$ andα, is calculated for a series of nuclides (different I₀/σ₀ and$\overline E _r$) and irradiation facilities (differentα and Φ_thΦ_e), and represented graphically.

Finally, the effect of important errors onα and$\overline E _r$ is calculated for final NAA results in terms of concentrations, botained by a comparator technique based on the¹⁹⁷Au reference nuclide. Conclusions are drawn concerning the impact of the foregoing on the usefulness of comparator type reactor NAA as an alternative to classical NAA using multi-element standards.