Neutron activation analysis in the determination of very long-lived radionuclides

This article describes the use of neutron activation analysis (NAA) for the quantitative determination of very long-lived radionuclides in the environment and nuclear waste. Because of its sensitivity and isotope specificity NAA is used for the determination of a few radionuclides with half-lives of the order of 10⁴ years or longer. The detection limits for some nuclides are lower than those obtainable with most kinds of mass spectrometry. The method has a substantial freedom from systematic errors.

The benefit of NAA, compared with direct radiometric measurement of the parent nuclide, is based on the fact that neutron activation may produce radionuclides with shorter half-lives and higher specific activities than those of the parent nuclides. An especially significant improvement is obtained when a γ-emitting nuclide is produced from a nuclide which only emits β-particles.

There are a few long-lived radionuclides which need to be determined because they are produced in nuclear fuel, and may pose a hazard in the final disposal of nuclear waste. Of these possible applications, the determination of ⁹⁹Tc, ¹²⁹I, ¹³⁵Cs and ²³⁷Np is described here. NAA has proven to be the most sensitive method to determine ¹²⁹I and a useful method to determine ²³⁷Np in environmental samples. Therefore, NAA can be recommended as an analytical method for these nuclides, when a research reactor is available.