NEUTRON ACTIVATION ANALYSIS

METHOD DESCRIPTION


Activation Analysis is a nuclear method of determining the concentrations of elements in a wide variety of materials. The sample is first made radioactive by bombardment with suitable nuclear particles, then the radioactive isotopes created are identified and the element concentrations are determined by the gamma-rays they emit. The most common particle employed is the Neutron and the method is then known as NEUTRON ACTIVATION ANALYSIS (NAA). NAA is capable of detecting many elements at extremely low concentrations.

The sample is first weighted into a plastic or quartz container and sealed. After this step, it is difficult to contaminate the material. It is then placed near the core of a Nuclear Reactor and irradiated for a suitable period of time. We think of a nuclear reactor as a neutron source. Therefore, placing the sample near the core results in "soaking" the sample in neutrons. If a neutron approaches the nucleus of an atom, it may be absorbed. When this happens, the element will become a different isotope of the same element. Sometimes this "new" isotope is unstable (radioactive) and decays by emitting a gamma ray(s). Approximately one trillion neutrons pass through every square centimeter of the sample every second during the irradiation.

Since neutrons "activate" the nucleus of an atom, not the electron shell, this method "sees" the total elemental content, regardless of oxidation state, chemical form or physical location. Neutrons have no charge and will pass through most materials without difficulty. Therefore the center of the sample becomes just as radioactive as the surface (few matrix problems).

After the irradiation the sample is allowed to decay and then it is "counted" using Lithium drifted Germanium or High Purity Germanium detectors looking for gamma rays. Gamma rays are very penetrating, so the gamma rays emitted from the center of the sample do reach the detector (again, few matrix problems). The resulting gamma-ray spectra looks something like a gas chromatograph spectra with "peaks" at different "retention times". The position of each peak determines the energy of the gamma ray (identifying the responsible element), and the area under the peak is proportional to its concentration. Final results are obtained after correcting for decay, sample size, counting time and irradiation time.

The comparator standard approach is normally employed with this method. A "standard" is irradiated and counted along with the sample(s). This standard contains a known amount of the element(s) to be determined. Since matrix problems are almost unknown, the standard does not have to be similar to the sample(s).

There are two matrix effects that can produce low results. They have been detected in about one percent of the samples we have tested. If a sample has a high concentration of an element with a large cross section, neutrons may be absorbed near the surface of the sample making the sample less radioactive than it theoretically should be. About a dozen elements have large cross sections ranging from a few hundred to tens of thousand of barns. The second effect is density. If a sample has a density of 5 or more, gamma rays may have difficulty reaching the detector (we use lead bricks for shielding). Corrections can be made for both these problems.

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General Activation Analysis, Inc.
1011 Elmview Drive
Encinitas, CA 92024
760/753-2858
USA
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