Neutron activation analysis (NAA) was discovered and cited in research by George de Hevesy and Hilde Levi in 1936. It is a non-destructive method of determining the elemental composition and concentration in a material sample, including major, minor, and trace amounts.
NAA is usually used with small samples of material, generally 100-400 mg at minimum, though smaller sizes may be used in certain situations. It is also possible in some situations to test intact objects if they are small. NAA is also efficient, simultaneously detecting and quantifying multiple elements in concentrations as small as 0.03 ng to 4 μg. It is highly sensitive with error rates within 2-5% relative standard deviation, so it is considered very reliable for a range of applications.
Another advantage of this method is that it can be performed in the field with portable equipment using neutrons emitted from isotopes like Cf-252. This is because it results in less gamma radiation and lower remaining radioactivity in the sample. It is also more cost effective than performing NAA at a reactor facility.
How Neutron Activation Analysis Works
A sample of the material to be tested must be encapsulated in a vial, often made of quartz or polyethylene, then encapsulated again in a metal capsule. Both the sample and a standard sample are placed in a reactor (or exposed to neutron radiation by other means outside of a reactor) and irradiated with neutrons for a predetermined time period, depending on the suspected composition of the sample and the elements sought.
Bombarding the sample and standard with neutrons allows them to penetrate the material so that the neutrons can be captured by their nuclei. As the neutrons are captured, the nuclei produce unstable radioactive isotopes (radionuclides), which emit beta particles and gamma rays as they decay. Each element produces a unique gamma ray spectral signature, which is studied to determine the elements present in the sample. Further, the rate at which gamma rays are emitted from an element is directly proportional to the concentration of that element in the sample.
Spectral analysis is most commonly performed with a semiconductor based germanium detector to find the photopeaks of gamma ray energies that stand out, though a scintillation based detector with thallium-doped sodium iodide is another option. The detector works in conjunction with a multichannel analyzer and computer software.
The ability to detect elements is affected by factors including the stability of the isotopes produced, their half lives, and the amount of radiation they emit. Some elements are far more difficult to detect than others, based on the number of neutrons needed to produce a radionuclide of that element, and the probability that neutrons will interact with their nuclei at all, called capture cross section relationship. The best chance of interaction is often with low energy neutrons, also called slow or thermal neutrons, but some elements cannot be detected through NAA at all. About 70% of the elements have properties suitable for measurement by NAA.
Types of NAA
There are multiple types of NAA procedures, each suited to specific applications, types of samples, elements, neutron capture cross-section sizes, and settings. Four common types are:
- INAA (Instrumentation NAA) – Conducted with instrumentation that analyzes the gamma spectrum emitted
- RNAA (Radiochemical NAA) – Samples are irradiated and then chemically separated before analysis
- PGNAA (Prompt Gamma NAA) – This method has a brief irradiation time and spectra are measured during irradiation. The process takes seconds or minutes and is used with isotopes that decay rapidly, have high neutron capture cross sections, only produce stable isotopes, or elements with weak gamma ray intensity. Slow (i.e., thermal) neutrons with low kinetic energy are used.
- DGNAA (Delayed Gamma NAA) – This method uses longer irradiation times and measures specta hours, days, or longer afterward.
NAA Neutron Source Options
Multiple options for reliable production of neutrons, and choice depends on factors like availability of a reactor, material being sampled and suspected composition, stability and rate of decay of isotopes produced, and testing location. Common neutron sources for NAA applications include:
- Uranium fission in a reactor
- Cf-252 actinoid – weaker than reactor but only need small amount so portable for field work
- Alpha source like americanum or radium with beryllium that react to become a neutron source
- DD or DT neutron generator – deuterium and tritium (ions of hydrogen) are accelerated and fuse, producing neutrons
Some Common Applications for NAA
NAA is used in many industries to identify the composition of samples. Some examples include:
- Determining the activity level of radioactive samples
- Analyzing artwork to identify paint and media composition
- Analyzing historical artifacts to determine geographic origins (e.g., clay, pottery, stone, metal)
- Detecting the presence of certain contaminants or substances in plant and animal tissues and products
- Studying soil and rock composition (e.g., checking for the presence of bromide when tracking fertilizers and pesticides in soil)
- Identifying rare earth elements and trace elements, and locating ore deposits
- Creation of measurement standards in the semiconductor industry
- Analyzing crude oil samples
- Analyzing water samples for contaminants and trace elements
- Studying ice core dust for contaminants
- Testing animal and plant tissue and material samples
- Forensics and criminal investigation
- Testing for contamination of oils, lipids
- Identifying trace amounts of heavy metals and other toxic elements in salts
Trust Frontier for NAA Neutron Sources and More
Frontier Technology Corporation has established itself as the global leader in the supply and shipment of californium-252 (Cf-252). We design and manufacture standard and custom NIST traceable californium-252 sources for a variety of NAA applications across a range of industries. All sources are approved for licensing purposes by the U.S. Nuclear Regulatory Commission (USNRC) for general use. Shipping is available to over 200 countries, excluding embargo list countries.
We also offer Type-A shipping containers, custom neutron shielding walls, nuclear reactor start-up rods, and much more. Contact us to learn more about our products and services, or request a quote.