What’s neutron activation analysis?

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Neutron Activation Analysis (NAA) is a very sensitive and accurate method for determining the elements present in a material sample. The sample is targeted with neutrons from a radioactive source. This causes many of the elements present to emit gamma rays at specific frequencies, by which they can be identified. In this way it is possible to detect about 65 different elements. 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 strikes 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 quantity emitted, it is possible to determine both the elements present and their abundance within the sample. The initial gamma ray, emitted immediately as the neutron is absorbed, is known as the immediate 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 as low as one part per billion. The method is also very versatile, as it can analyze samples in solid, liquid and gaseous forms and can handle sample sizes up 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 an alpha particle source, such as plutonium 239 or radium 226, a strong neutron source can be created. This may be enclosed in adequate radiation shielding, but with an opening from which neutrons can emerge.

Nuclear reactors are also used as neutron sources. In the United States, the High Flux Isotope Reactor (HFIR) at Oak Ridge, Tennessee provides a source of neutrons to Oak Ridge National Laboratory, making it a major center for the analysis of neutron activation. Radioactive elements that produce neutrons through nuclear fission, such as 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 study 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. A well-known specific example of analyzing neutron activation in action is the discovery that all of the bullet fragments from the scene of the assassination of John F. Kennedy came from the same two bullets, fired from the same gun. Another example was the discovery of an iridium-rich sediment layer 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 disappearance of the dinosaurs.

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