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Neutron activation analysis (NAA) is a very sensitive and accurate method of determining the elements present in a sample of material. The sample is targeted with neutrons from a radioactive source. This causes many of the elements present to emit gamma rays at specific frequencies, from which they can be identified. Around 65 different elements can be detected in this way. It is one of the most useful scientific techniques for investigating the elemental composition of samples and has many applications in analytical chemistry, geology, forensic science and other areas.
When a neutron hits the nucleus of an atom, it is often absorbed, forming a heavier isotope and emitting a gamma ray. In many cases, these isotopes are unstable and will decay into another, lighter, isotope after a short delay, emitting one or more gamma rays at energies that are characteristic for that isotope. For example, the most common isotope of sodium — sodium-23 — can absorb a neutron, forming the unstable isotope sodium-24, which then decays into magnesium-24, emitting two gamma rays at specific energies. By measuring the energies of the gamma rays and the amount emitted, the elements present and their abundance within the sample can both be determined. The initial gamma ray, emitted immediately when the neutron is absorbed is known as the prompt gamma ray, but it is usually the delayed gamma rays that are measured.
Neutron activation analysis is a very sensitive technique. It can detect elements at one part per million or less, and in some cases, down to one part per billion. The method is also very versatile, in that it can analyze samples in solid, liquid and gas forms and can handle sample sizes down to 0.000035 ounces (0.001 grams).
The neutron source is sometimes known as a neutron howitzer. When some light elements are subjected to alpha particles, their nuclei emit neutrons. The element beryllium is particularly suitable for this purpose. By mixing beryllium with a source of alpha particles, such as plutonium 239 or radium 226, a strong source of neutrons can be created. This can be encased in suitable radiation shielding, but with an opening where the neutrons can emerge.
Nuclear reactors are also used as neutron sources. In the US, in Oak Ridge, Tennessee, the High Flux Isotope Reactor (HFIR) provides a source of neutrons at Oak Ridge National Laboratory, making it a major center for neutron activation analysis. Radioactive elements that produce neutrons through nuclear fission, for example californium-252, can also be used on a smaller scale, allowing desktop-sized neutron sources to be used.
Neutron activation analysis has a wide range of applications. It can be used in manufacturing industry to detect impurities in metals, in biology to investigate the metabolism of trace elements, in geology to analyze rock and soil samples and in forensic science to obtain crucial information from crime scene samples. One well-known specific example of neutron activation analysis in action is the finding that all the bullet fragments from the John F. Kennedy assassination scene came from the same two bullets, fired from the same gun. Another example was the discovery of a layer of iridium-rich sediment at the boundary between the cretaceous and tertiary geological periods, indicating a major meteorite impact that more or less coincided with a mass extinction event that marked the demise of the dinosaurs.