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Neutron activation analysis stands at the forefront of techniques for the quantitative multi-element analysis of major, minor, trace and rare elements. The principle involved in neutron activation analysis consists of first irradiating a sample with neutrons in a nuclear reactor (such as the Oregon State University TRIGA Reactor) to produce specific radionuclides. After the irradiation, the characteristic gamma rays emitted by the decaying radionuclides are quantitatively measured by suitable semiconductor radiation detectors, and the gamma rays detected at a particular energy are usually indicative of a specific radionuclide's presence. Data reduction of gamma ray spectra by means of a computer then yields the concentrations of various elements in samples being studied. With sequential instrumental neutron activation analysis it is possible to measure quantitatively about 35 elements in small samples (5 to 100 mg), and for activatable elements, the lower limit of detection is on the order of parts per million or parts per billion depending on the element.
The Radiation Center's neutron activation analysis laboratory has analyzed for the major, minor, and trace element content in many thousands of samples covering essentially the complete spectrum of material types and involving virtually every scientific and technical field. While some researchers perform their own neutron activation analysis sample counting using their own or Radiation Center equipment, the Radiation Center also provides a complete neutron activation analysis service for researchers and others who may require it. This includes sample preparation, sequential irradiation and counting, data reduction and analysis, and report preparation.
Methods & Detection Limits
Neutron activation analysis of a sample begins with neutron bombardment of a target to convert stable isotopes in the sample to radioactive isotopes (e.g., natural sodium [23Na] is converted to radioactive sodium [24Na]). The usual procedure involves placing the samples to be analyzed plus a number of suitable standards into the neutron field produced by a research nuclear reactor. Following the irradiation (neutron bombardment), the samples are analyzed using an appropriate gamma ray detector system. The principle of the analysis is straightforward, as the radioisotopes created during the irradiation will decay with time causing the release of energy. A portion of the energy released during decay is often in the form of gamma radiation, which is capable of traveling out of the sample. The gamma rays possess unique energies that are characteristic of the radioisotope undergoing decay.
Using 24Na as an example, when it decays to stable 24Mg, gamma rays having energies of 1368.53 and 2754.09 kilo-electron volts (keV) are released. If these gamma rays enter a suitable detector, their energy can be converted to an electrical signal that is processed as a count in an energy spectrum. The accumulation of gamma counts at a particular energy will generate a curve, the area of which is proportional to the radioactivity of the characteristic radionuclide. By irradiating and counting standards containing known amounts of various elements, it is possible to establish a relationship between the radioactivity of the standard and the radioactivity of the sample, which in turn allows the researcher to determine the abundance of a particular element or elements.
Neutron activation analysis sensitivities and accuracy are dependent on the concentration of a particular element and radionuclide parameters (i.e., parent isotope abundance, neutron cross-section, half-life, and gamma ray abundance). Element sensitivities vary from 10-3 to 10-10 grams per gram of sample. Accuracy of an neutron activation analysis determination is usually between two and ten percent of the reported value, depending on the element analyzed and its concentration in the sample.
The range of samples amenable to analysis by neutron activation analysis is nearly limitless. In addition, an experienced Radiation Center staff member is available to customize an neutron activation analysis program to meet your specific analytical needs.