Other meanings of Neutron activation analysis
Analytical chemistry
Neutron activation analysis is an analytical technique for determining elemental composition by irradiating samples with neutrons. Nuclear reactions make some target atoms radioactive, and the characteristic gamma rays emitted during decay reveal which elements are present and, from their intensities, in what amounts.1 The method can measure trace concentrations without dissolving or otherwise chemically processing a specimen, making it valuable in geology, archaeology, environmental science, materials research, and forensic examination.
Neutron activation analysis identifies elements through neutron-induced nuclear reactions and the radioactive decay that follows. A sample is exposed to a neutron flux, usually in a research reactor, where a nucleus may capture a neutron and become an unstable isotope. Its decay produces gamma rays at energies characteristic of the nuclide; comparing those energies with reference data identifies the element, while peak areas provide a concentration estimate.
The method grew from the discovery of neutron-induced radioactivity in the 1930s and the development of nuclear reactors and gamma-ray spectrometers. It is fundamentally different from ordinary chemical spectroscopy because the signal comes from the nucleus rather than from electrons. Some activation products have half-lives of seconds, whereas others persist for years, so irradiation and counting schedules are selected around the relevant isotopes.
Accurate neutron activation analysis depends on controlled irradiation, decay, and gamma-ray counting. Analysts weigh and seal a sample, expose it to a calibrated neutron field, allow interfering short-lived activities to decay when necessary, and measure the emitted spectrum with a high-resolution detector, commonly a high-purity germanium detector. A comparator or certified reference material helps correct for neutron flux, geometry, detector efficiency, decay, and counting time.
Two broad modes are common. Instrumental NAA measures the activated specimen directly and usually avoids chemical separation. Radiochemical NAA adds a separation step after irradiation to isolate an element or reduce spectral interference, improving selectivity at the cost of labor and possible contamination. Quantification requires nuclear data such as half-life, gamma-ray emission probability, and neutron-capture cross section.
Neutron activation analysis is especially useful when many elements must be measured at trace or ultratrace concentrations in a small, solid, and chemically complex sample. It has supported provenance studies of pottery and obsidian, measurements of geological materials and meteorites, determinations of pollutants in biological and environmental samples, and characterization of high-purity materials.1
Its strengths include multielement capability, good selectivity, relatively little sample preparation, and the ability to analyze objects without dissolving them. Limitations include access to a neutron source, radioactive waste and radiation-safety requirements, activation of the sample matrix, and spectral overlaps. Elements with unfavorable nuclear properties may be difficult to determine, while major constituents can produce intense activities that obscure trace signals. Hydrogen-rich or strongly absorbing matrices can also make neutron-flux corrections important.
Neutron activation analysis also includes specialized forms that extend beyond conventional reactor counting. Prompt gamma-ray neutron activation analysis detects gamma rays during irradiation, allowing measurements of elements whose induced products are too short-lived for ordinary post-irradiation counting; it can also support bulk measurements of hydrogen, boron, chlorine, and other elements in selected settings.2
Neutron beams can be shaped or transported to examine large objects, and small samples may be analyzed with neutron generators rather than reactors, though neutron energy and flux affect sensitivity. The technique is often regarded as minimally destructive rather than universally nondestructive: irradiation can create long-lived radionuclides, and high fluxes may alter minerals, polymers, or biological material. Careful blank measurements, certified standards, spectral libraries, and decay corrections are therefore central to defensible results.
Results depend on irradiation conditions, nuclear decay data, detector calibration, and appropriate reference materials; terminology and reporting conventions vary among laboratories.
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