Neutron Activation Analysis
Neutron activation analysis (NAA) stands at the forefront of techniques for the quantitative multi-element analysis of major, minor, trace and rare elements. NAA begins with neutron bombardment of a sample to convert stable isotopes to radioactive isotopes (e.g., natural sodium [23Na] is converted to radioactive sodium [24Na]). The usual procedure involves placing the samples to be analyzed plus a number of suitable standards into the neutron field produced by a neutron source. Radioisotopes created during the irradiation will decay with time. A portion of the energy released during decay is often in the form of gamma radiation, which is capable of traveling out of the sample. The gamma rays possess unique energies that are characteristic of the radioisotope undergoing decay. Gamma rays detected at a particular energy are usually indicative of a specific radionuclide's presence.
Using 24Na as an example, when it decays to stable 24Mg, gamma rays having energies of 1368.53 and 2754.09 kilo-electron volts (keV) are released. If these gamma rays enter a suitable detector, their energy can be converted to an electrical signal that is processed as a count in an energy spectrum. The accumulation of gamma counts at a particular energy will generate a curve, the area of which is proportional to the radioactivity of the characteristic radionuclide. By irradiating and counting standards containing known amounts of various elements, it is possible to establish a relationship between the radioactivity of the standard and the radioactivity of the sample, which in turn allows the researcher to determine the abundance of a particular element or elements.
Data reduction of gamma ray spectra yields the concentrations of various elements in samples being studied. With sequential instrumental neutron activation analysis it is possible to measure quantitatively about 60 elements in small samples (5 to 100 mg). Neutron activation analysis sensitivities and accuracy are dependent on the concentration of a particular element and radionuclide parameters (i.e., parent isotope abundance, neutron cross-section, half-life, and gamma ray abundance). Element sensitivities vary from 10-3 to 10-10 grams per gram of sample. Accuracy of an neutron activation analysis determination is usually between two and ten percent of the reported value, depending on the element analyzed and its concentration in the sample.
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The following table gives a list of elements that may be quantitatively analyzed using neutron activation analysis: